Xylem development and hydraulic conductance in sun and shade shoots of grapevine (Vitis vinifera L.): evidence that low light uncouples water transport capacity from leaf area

Planta (1993)190:393-406 P l r m m

�9 Springer-Verlag 1993

Xylem development and hydraulic conductance in sun and shade shoots of grapevine (Vitis vinifera L.): evidence that low light uncouples water transport capacity from leaf area Hans R. Schultz*, Mark A. Matthews

Department of Viticulture and Enology, University of California, Davis, CA 95616, USA

Received: 18 August 1992/Accepted: 30 October 1992

Abstract. Morphology, water relations, and xylem anatomy of high-light (sun)- and low-light (shade)-grown Vitis vinifera L. shoots were studied to determine the effects of shading on the hydraulic conductance of the pathway for water flow from the roots to the leaves. Shade shoots developed leaf area ratios (leaf area: plant dry weight) that were nearly threefold greater than sun shoots. Wa- ter-potential gradients (A ~U.m 1) in the shoot xylem ac- counted for most of the A 7/. m - 1 between soil and shoot apex at low and high transpiration rates in both sun and shade shoots, but the gradients were two- to fourfold greater in shade-grown plants. Low light reduced xylem conduit number in petioles, but had an additional slight effect on conduit diameter in internodes. The hydraulic conductance per unit length (Kh) and the specific hydraulic conductivity (ks, i.e. Kh per xylem cross-sectional area) of internodes, leaf petioles, and leaf laminae at different developmental stages (leaf plastochron index was calculated from measurements of water potential and water flow in intact plants, from flow through excised organs, and from vessel and tracheid lumen diameters according to Hagen-Poiseuille's equation. For all methods and conductance parameters, the propensity to transport water to sink leaves was severalfold greater in internodes than in petioles. The Kh and k~ increased logarithmically until growth ceased, independent of treatment and mea- surement method, and increased further in pressurized- flow experiments and Hagen-Poiseuille predictions. However, the increase was less in shade internodes than in sun internodes. Mature internodes of shade-grown

* Present address: Institut National de la Recherche Agronomique, Station de la Recherches Viticoles, Domaine du Chapitre, F-34750 Villeneuve-les-Maguelonne, France

Abbreviations: Ac = total cross-sectional area (internodes, petioles, leaf laminae); Ax=xylem cross-sectional area; HV=Huber value; Kh = hydraulic conductance per unit length; k~ = specific hydraulic conductivity; LPI = leaf plastochron index; LSC = leaf specific conductivity; 7~=water potential; A~=water-potential gradient; q = volume flow of water per unit time

Correspondence to: M.A. Matthews

plants had a two- to fourfold reduced Kh and significantly lower k s than sun internodes. Except very early in development, leaf lamina conductance and k s from shade- grown plants was also reduced. The strong reduction in Kh with only a slight reduction in leaf area (17% of sun shoots) in the shade shoots indicated a decoupling of water-transport capacity from the transpirational surface supplied by that capacity. This decoupling resulted in strongly reduced leaf specific conductivities and Huber values for both internodes and petioles, which may increase the likelihood of cavitation under conditions of high evaporative demand or soil drought.

Key words: Water transport - Hagen-Poiseuille equation - Leaf specific conductivity - Huber value Leaf plastochron index - Vitis (water transport)

Introduction

In most terrestrial plants, large amounts of water must be transported for evaporative cooling, growth, and sol- ute transport. Plant organs and tissues vary in their ability to conduct water, and environmental factors, e.g., nutrient (Radin and Matthews 1989) and water status (Boyer 1985; Fiscus et al. 1983), are important in determining the hydraulic conductance of whole plants. Al- though it has long been recognized that light has substantial effects on plant morphology and anatomy (Schuster 1908; Wylie 1951), it has not been established whether these effects have direct consequences for partial or whole-plant capacity to conduct water. There is indirect evidence that light can have such an effect. First, plant species adapted to natural shade habitats have less effec- tive conducting systems (Berger 1931; Kfippers 1984) and second, the functional relationship between leaf or needle area and the hydroactive xylem area can change along light gradients within plant canopies (Oren et al. 1986). There is, however, no quantitative information

394 H.R. Schultz and M.A. Matthews: Light and hydraulic conductance in grape shoots

a v a i l a b l e t h a t i nd i ca t e s w h e t h e r the c o n d u c t i v e ab i l i ty o f d i f f e ren t p l a n t pa r t s is sens i t ive to l ight .

T h e r e f o r e , e x p e r i m e n t s w e r e c o n d u c t e d to : (i) de te r - m i n e w h e t h e r l ow l ight r educes h y d r a u l i c c o n d u c t a n c e o f d i f fe ren t o r g a n s o f the s a m e spec ies ; (ii) i n v e s t i g a t e x y l e m a n a t o m y o f h i g h - l i g h t (sun)- a n d low l igh t ( s h a d e ) - g r o w n p l a n t s ; (iii) q u a n t i f y c h a n g e s in h y d r a u l i c c o n d u c t a n c e ( K h ) a n d specif ic h y d r a u l i c c o n d u c t i v i t y (ks) a t d i f fe ren t d e v e l o p m e n t a l s t ages o f sun a n d s h a d e p l a n t s ; (iv) de te r - m i n e the d e g r e e o f c o u p l i n g b e t w e e n the ab i l i ty to c o n - d u c t w a t e r a n d the l e a f a r e a s u p p l i e d as i n f l uenced by d e v e l o p m e n t a l s t age a n d l igh t t r e a t m e n t s .

Material and methods

rected for the heat of respiration (Barrs 1965). Leaves were rinsed with distilled water to eliminate salts which may have accumulated on the surface, blotted dry with paper tissue, and allowed to dry for at least 30 min before beginning an experiment. Leaf discs (3.62 cm 2) were sampled and placed into the psychrometer cup with the abaxial surface facing up.

To quantify aspects of the water-conductance network, local values along the pathway for water within the plant were determined from ~ values of bagged leaves equilibrated with the stem xylem (Begg and Turner 1970; Schultz and Matthews 1988a). This procedure completely inhibited leaf expansion prior to ~ determinations (Schultz 1990) and eliminated potential errors in gt data caused by wall relaxation (Cosgrove et al. 1984). In order to sample bagged leaves, the aluminum foil was first removed and then discs were punched out through the plastic bag and the leaf. The plastic was removed and the leaf sample inserted immediately into the psychrometer cup. The bag was then resealed and the aluminum foil replaced.

Growth conditions. Grapevine (Vitis vinifera L. cv. White Riesling) plants were grown from dormant rootings (Viticulture Field Sta- tion, University of California, Davis) in 8-L pots containing a soil :peat :perlite mixture (1:3:3) in a controlled environment (30/ 20:t: 1 ~ C, 50/90• 10% relative humidity, 13 h photoperiod). One week after bud break (two to three weeks after planting), plants were thinned to one or two shoots and inflorescences removed. All plants were watered daily to saturation and fertilized with half- strength Hoagland solution two times per week. Shoots were trained horizontally to ensure uniform illumination. Two light treatments were imposed throughout the growth period. Half the plants were grown under a 20% neutral shade screen (shade plants, 150-170 pmol photons ' m -2- s - l ) , the other half was grown at full irradiance (sun plants, 900-1000 gmol photons "m -2- s -1 from cool-white fluorescent bulbs) within the same growth chamber. The ratio of red to far-red light (660/730 nm measured with an SKR 100 Ratio Sensor; Skye Instruments, Isle of Skye, UK) was 1.65 (sunlight approx. 1.0) and was unaltered by the screen. Air temperature underneath the screen was less than 1.5 ~ C different from the full light treatment throughout the experiments.

Growth measurements. Growth was measured as increases in shoot internode and leaf length with a hand-held micrometer. Measure- ments were performed at the beginning of the photoperiod and (during one experiment) at various other times when sampling for water-potential (~) determinations. Additional measurements of leaf thickness, and internode and petiole diameters were also conducted. Leaf thickness was measured between the main side veins arising from the midrib. Internode and petiole diameters were determined at their centers on the flat (ventral) side. A relationship between leaf length and leaf area was established in order to nonde- structively estimate leaf area per plant from measurements of leaf length. Since no significant difference was found for sun (n= 102) and shade (n = 99) leaves, a single third-order regression was fitted to the data (r z= 0.94; Schultz 1990).

Plant organs sampled for measurements of dry-weight distribution were oven-dried at 60 ~ C until no further decrease in weight was detected.

Plastochron index and leaf plastochron index. The plastochron index and leaf plastochron index (LPI) were calculated as described previously (Schultz and Matthews 1988b) to quantify physiological age and used to make comparisons at the same developmental stage. The plastochron concept has been shown to be a valid basis for comparison of sun and shade plant development of grape, since leaf number and rate of appearance are independent of irradiance in the range used here (Schultz 1992).

Plant water status. Tissue ~ was determined by isopiestic thermocouple psychrometry (Boyer and Knipling 1965) using sample chambers coated with melted and resolidified petrolatum and cor-

Transpiration and hydraulic conductance. In some experiments, transpirational flux was manipulated by varying incident radiation (0-1000 gmol photons m z �9 s-1) and vapor-pressure deficit of the air (11-31 mbar - bar- 1). The rate of plant transpiration was determined gravimetrically or with a diffusion porometer (Li-1600; Li- Cor Inc., Lincoln, Neb., USA). The latter was used to determine the rate of transpiration of all leaves larger than 9 cm / on a shoot. The total volume of water flow per unit time, q (m3-s - 1), through a shoot segment was determined by summing the amounts of water vapor leaving the shoot distal to that segment. The hydraulic conductance per unit path length, Kh (m 4- M P a - l ' s 1), was then calculated relating q to the driving force, i.e., the water-potential gradient, A~/Ax (MPa-m-1) , across the segment:

Kh - q (Eq. 1) A~/Ax

Equation 1 was used for calculations of Kh from experiments on intact plants and on excised shoot internodes and petioles.

For the latter experiments, internodes and petioles were excised under water about 0.3 cm from the nodes and the junction into the leaf lamina (petioles). The samples were sealed into a pressure chamber filled with degassed and deionized water, and pressurized at 0.02, 0.05, 0.10, 0.20 and 0.30 MPa. Flow was measured by collecting the exudate with a preweighed capillary vial containing cotton wool placed over the protruding sample to minimize evapo- ration. Steady flow rates ( • were attained within 10-30 s, depending upon sample size, and could be maintained for at least 10 min.

To account for differences between treatments in the amount of xylem produced, the specific conductivity of internodes and petioles, k s (m 2 �9 MPa-1 "s-1), was calculated by dividing Kh by the xylem cross-sectional area (Ax, m2):

Kh ks - (Eq. 2)

Ax

Leaf specific conductivity, LSC (m -2 - MPa- 1. s- 1), was calculated according to Zimmermann (1978) and Salleo et al. (1982a). The LSC denotes the volume flow rate of water through a shoot section or a petiole in relation to the pressure gradient imposed to generate the flow and the leaf area supplied by that section or petiole, and was calculated from:

Kh LSC - (Eq. 3)

AL

where A L (m 2) is the supplied leaf area. The investment into xylem development relative to the leaf area

supplied with water was estimated by calculating the Huber values, HV (m 2 �9 m - 2), of internodes and petioles:

HV - Ax (Eq. 4) AL

H.R. Schultz and M.A. Matthews: Light and hydraulic conductance in grape shoots 395

X .<

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B

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Internodes

~~,o Sun o Shade �9

o

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I I I 1 I I

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Lea f P t a s t o c h r o n I n d e x Fig. IA, B. Ratio of total cross-sectional area, Ac, to xylem cross- sectional area, Ax, as a function of leaf plastochron index for internodes (A) and petioles (B) of sun- and shade-grown plants of Vitis vinifera. Symbols indicate measured values. Lines in A are regressions based on Eq. 6; sun, AJA~= 8.63 - EXP (-~ " LPl)_ 2.52, Rz=0.97; shade, AJAx=4.37' EXP {-~176 LP~)_ 2.28, R2=0.97. Line in B is a third-order polynomial; AdAx = 11.63-1.08-LPI + 7.65 �9 10 2. LPi 2 _ 1.44' 10 .3 - LPI 3, R2=0.98

To estimate the water-conducting ability of the leaf lamina, whole leaves including the petioles were excised under degassed water and rehydrated in complete darkness in a humid box at 15-20 ~ C for 3 h. The leaves were inserted into a water-filled pressure chamber with the lamina exposed. Since q, vascularization, and lamina width vary substantially along a grape leaf (Silk 1983), conductance was estimated for three different positions on the lamina. At first, approx. 25 % of the total lamina length was excised from the tip with a razor blade. Water was then forced through the petiole into the lamina at the same low pressure intervals used for internodes and petioles. Exudate was collected across the whole width of the cut edge and weighed. This process was repeated when 50% and 75 % of the lamina length had been excised. All measurements were conducted at 25 ~ C to minimize temperature effects on conductance, and under reduced light to keep transpiration low. The k s was then calculated according to Eq. 3, but water flow was based on total cross-sectional area, Ar (m2), of the cut leaf edge instead of xylem cross-sectional area.

Anatomical measurements and Haoen-Poiseuille predictions. For comparing water-potential gradients and "pressurized flow" results with xylem anatomy, thin sections (15 lain) were prepared from paraffin-embedded samples of internodes and petioles. A total of 83 cross-sections with a minimum of 3 sub-samples per internode and petiole were prepared and subsequently analysed. Sections were stained with safranin and methylene blue (Gerlach 1969). Total xylem area (including the xylem parenchyma) and the number of vessels and tracheids and their lumen diameters were determined from enlargements of light-microscope photomicrographs. The latter were estimated as the diameter of the largest circle that would

fit within a vessel or tracheid lumen. Hydraulic conductance per unit length was then predicted using the Hagen-Poiseuille equation (Zimmermann 1983; Gibson et al. 1984), which describes laminar flow through ideal capillaries:

g ~ d~ Kh - i-o (Eq. 5)

12811

where di (m) is the diameter of the i m vessel or tracheid in a cross-section and 11 is the viscosity of water (MPa �9 s) taken at 25 ~ C. The k s was calculated as in Eq. 3.

In order to estimate Ax non-destructively for internodes and petioles in the intact plant experiments, the ratios of total cross- sectional area (At), as determined from diameter measurements, to Ax, as determined from photomicrographs, were established as a function of LPI for sun and shade organs (Fig. 1). For internodes an exponential equation of the type:

AJAx = (AJAx)re, x - e (-k*LP1) + (AJA~)mi ~ (Eq. 6)

was fitted to the data, where (AJA0m,x and (AjA0mi~ are maximum and minimum ratios with respect to LPI, and k is a coefficient. Parameter estimates for Eq. 6 were obtained by least-square non- linear regression analyses using the program PROC NLIN of SAS (SAS Institute 1987).

The AJAx ratio decreased exponentially with increasing LPI approaching similar lower plateau values of 2.3 and 2.5 for shade and sun plants, respectively (Fig. IA). Internodes from shade plants had lower ratios than internodes from sun plants up to an LPI of approx. 6 (Fig. 1A). For petioles, a single third-order regression was applied to the combined data from sun and shade plants, as there was no obvious difference between treatments at any developmental stage (Fig. 1B).

Results

Shading reduced total a b o v e - g r o u n d dry mat te r greatly (70%) bu t leaf area no t significantly (17%, Table 1). Dry-weight a l locat ion to leaves of shade plants was 9% greater, whereas a l locat ion to stem tissue was 10% less than in sun p lants after a growth per iod of four weeks (Table 1). Accordingly , leaf area rat io (cm 2 leaf area/g p lan t dry weight) was 2.7 t imes greater a nd leaf weight rat io (g leaf dry weight/g p lan t dry weight) was a b o u t 20% greater in shade p lants compared with sun p lants (Table 1). However, the acc l imat ion to low light was no t restricted to dry-weight a l locat ion a m o n g shoot organs because specific leaf area (cm2/g leaf dry weight) was increased a bou t 100% by the low-light e n v i r o n m e n t (Table 1). These da ta indicated that shade p lants accli- mated by invest ing p ropor t iona l ly more into light intercept ion by leaves t han suppor t ing structures. The decreased inves tment in stems a nd petioles of shade plants was also appa ren t f rom length and d iameter measurements on in ternodes a nd petioles (Table 1).

The 44% and 39% reduc t ion in diameters of internodes and petioles (Table 1) ma y indicate altered capacity for water t ranspor t in shade plants , since diameters of stems of trees a nd herbaceous p lants have long been related to hydraul ic archi tecture (Huber 1928). Therefore, we used several approaches to est imate hydraul ic conduc tance t h r o u g h o u t the shoot using in tact plants , excised internodes , petioles, a nd leaf laminae , and pho tomic rographs of s tem and petiole cross-sections.


Table 1. Some morphological features and dry-weight partitioning (percent of new growth) among different organs of sun- and shade- grown plants. Means are from fully grown internodes (including nodal portions) (LPI > 4), petioles and leaves (LPI > 7). Plants were

at plastochron index 28-30 at time of sampling. Data are means of three replicates+SD (one-factor analysis of variance). Total dry weight was 17.5-+2.7 g for the sun plants and 5.2-+ 1.3 for the shade plants

Sun Shade Significance

Internode length (cm) lnternode diameter (cm) Petiole length (cm) Petiole diameter (cm) Leaf area (cm 2)

Dry-weight partitioning Stem Leaf laminae Petioles Tendrils Leaf area ratio (cm 2 �9 g 1) Leaf weight ratio (g - g 1) Specific leaf area (cm- Z/g)

7.41___ 1.03 4.91-+ 0.49 * 0.46+ 0.05 0.26+_ 0.02 ** 4.83--+ 0.47 3.86+ 0.32 * 0.23-+ 0.02 0.14-+ 0.01 **

84.48-+ 8.38 70.15_+ 6.12 ns

46.58_+ 2.18 36.74-+ 2.48 ** 42.87_+ 2.66 51.40-+ 1.62 * 4.75-+ 0.14 5.94_+ 0.02 ** 5.75-+ 0.36 6.17+ 0.96 ns

90.65+ 3.18 242.83+19.2 ** 0.43-+ 0.03 0.51+ 0.02 *

213.50-+ 15.1 469.69-+26.2 **

** P<O.O1, * P<O.05, and ns, not significant P>O.05

First, conduc tance was est imated for the soil-plant con t i nuum and for intact stems f rom the relationship o f the flow rate o f water, q, to the potential energy gradient, A~r driving that flow. W h e n transpirat ional flux was manipula ted by varying evaporat ive demand, bo th sun and shade plants exhibited the c o m m o n response o f increasing slope (Passioura 1984; Boyer 1985) i.e. conductance, as flow increased (Fig. 2). The flow th rough the soil-plant con t i nuum was much greater th rough sun plants than shade plants o f similar node number , indicating a greater resistance to water flow in the shade-plant system (Fig. 2A). F o r example, at a A~t o f 0.6 M P a �9 m - 1 the flow rate in sun plants approached 8 - 1 0 -9 (m 3 �9 s - 1), whereas for the same gradient the flow rate in shade plants was less than 2 . 10 -9 (m3-s -1 ) . Similar differences in the q vs. A~ relationship were observed when only the gradient within the stem (basal shoot in ternode to shoot apex) was considered, indicating that the reduct ion in hydraul ic conduc tance caused by low light was confined primari ly to the shoot (Fig. 2B). The hydraul ic resistance f rom the soil to the basal stem internode was small and similar for sun and shade plants as indicated by an approx imate 0.2 M P a displacement o f the flow curves toward a greater A~ in bo th cases (cf. Fig. 2A, B).

In a second test o f whether the light envi ronment af- fected water t ranspor t in the stem, the q-Atg relationships were determined for individual excised internodes. The non-l ineari ty indicated by the soil-plant and intact shoot systems (Fig. 2A, B) was observed only in the younges t internodes (LPI 0.2-1.4) (cf. Fig. 3A, B, C). As a reference point , complete in ternode expansion is achieved between LPI 4 and 5. The pressure-flow rela- t ionship o f older internodes can be extrapolated th rough the origin (Fig. 3B, C), whereas that o f the young internodes intercepts the x-axis between 0.15 and 0.25 M P a . m -1 (Fig. 3A), which is approximate ly the same intercept observed for whole stems f rom the intact-plant experiments (Fig. 2B). This and the increased K h with

15

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0.0 0.5 1.0 1.5 Water ?otentiaE Gradient, AI~ (MPa.m - 1 )

Fig. 2A, B. Total water flow (q), at different A~ in the soil-plant continuum (soil-shoot apex; A) and the stem xylem (basal internode to shoot apex; B) of sun and shade plants of V. vinifera. Data are from a minimum of eight different plants per treatment. Water flow was measured as transpiration gravimetrically and by diffusion porometry. Lines were drawn by hand

internode ontogeny may reflect the presence o f terminal "ends" o f vessels in young internodes and nodes (Salleo et al. 1984). Hydraul ic conduc tance o f excised young internodes did not differ for sun and shade plants (Fig. 3A). However , inhibited capaci ty for water trans-


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Leaf P l a s t o c h r o n Index Fig. 4A-D, Hydraulic conductance per unit length, Kh, (A, B), and specific hydraulic conductivity, k,, (C, D), of internodes at different locations on the shoots of V. vinifera plants. The Kh was calculated, A, from measurements of q and A~/Ax on intact plants (IP) and from vessel and tracheid lumen diameters using the Hagen- Poiseuille equation (HP) and, B, from pressure-flow (at 0,1 MPa) relationships of excised internodes (PF). For calculations of k~, xylem cross-sectional area was determined from photomicrographs for the HP (C), and PF (D) methods, and estimated from internode diameter measurements using Eq. 6 for the IP (C) method�9 Hor- izontal lines were drawn to facilitate comparison between protocols

port was apparent by LPI 6 (Fig. 3B). For older internodes, the effect of shade on internode Kh was dramatic at all pressures tested (Fig. 3C).

The intact plant and excised-internode assays of q-A~t indicated a developmental change in Kh and that this change was altered under low-light conditions. In order to confirm the effect of low light on internode Kh and to determine the specific developmental stage at which Kh was altered, Kh was estimated from: (i) separate experiments on intact plants, measuring q and A~/Ax at many LPIs; (ii) pressure-flow relationships of excised internodes applying only a 0.1-MPa pressure gradient; and (iii) vessel and tracheid-lumen diameters of the excised internodes using the Hagen-Poiseuille equation (Eq. 5). To account for differences in xylem development between the treatments, the specific hydraulic conductivity, ks (Kh/Ax), was also calculated. For the intact-plant experiments, Ax as a function of LPI was estimated from

the ratio AJAx using Eq. 6, and determining Ao non- destructively from internode diameter measurements (see Fig. 1A).

The Kh estimated by all three methods increased several orders of magnitude during development, but the increase was always greater in sun plants (Fig. 4A, B). When determined at an applied pressure gradient (Ap) of 0.1 MPa �9 m- i , Kh of excised internodes increased by a factor of 105 during development from LPI - 2.0 to LPI + 18.0 (Fig. 4B). All three methods revealed LPI 3 4 (~ 2 d before growth ceases) as the developmental stage where internodes from sun and shade plants started to show differences in their ability to conduct water. In internodes from sun plants, Kh increased up to LP118, whereas those from shade plants showed no more significant increase at a LPI larger than 9 (Fig. 4A, B). Thus, the difference in Kh between treatments increased during development, reaching a factor of about three for the

398 H.R. Schultz and M.A. Manhews: Light and hydraulic conductance in grape shoots

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Leaf P lastochron Index Fig. 5A-C. Dependence of leaf specific conductivity (LSC) on internode position on shoots from sun and shade plants of V. vinifera. The LSC was calculated by relating Kh of a stem section to the leaf surface distal to that section. The data were obtained from pressure- flow experiments (A), by using the Hagen-Poiseuille equation (B), and from intact-plant experiments (C). Data are from three plants per treatment

m o s t basa l i n t e rnodes regardless o f the m e t h o d e m p l o y e d for ca lcu la t ion .

Ca l cu l a t i ons o f ks showed s imi lar deve lopmen ta l and t r e a t m e n t differences as K h (Fig. 4C, D), a l t hough t rea t - m e n t differences were smaller . F r o m Hagen-Po iseu i l l e ca lcu la t ions and pressure- f low exper iments s ignif icant ( P < 0 . 0 1 , one - f ac to r ana lys is o f var iance) shade effects were on ly a p p a r e n t a t LPIs la rger than 12 (Fig. 4C, D). This i nd i ca t ed tha t for mos t o f d e v e l o p m e n t the decrease in K h caused by low l ight was due to r educed xylem f o r ma t ion . Howeve r , shade in t e rnodes o lder than LPI 12 h a d lower k s va lues than sun in te rnodes , ind ica t ing tha t e i ther the n u m b e r o f condu i t s per uni t s tem cross-sec t ion or the ave rage condu i t d i a m e t e r was also reduced.

The c o m b i n a t i o n o f a large r educ t ion in K h o f internodes a n d smal l r educ t ion in leaf a rea o f shade p lan t s

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1 0 - 5 L - 4 0 4 8 12 16 20 24 28

Leaf P l c s t o c h r o n Index Fig. 6. Huber values (HV) of internodes as a function of leaf plastochron index for sun- and shade-grown plants of V. vinifera. The HVs were calculated as the ratio of xylem cross-sectional area to supplied leaf area. Data are from experiments employing all three methods for Kh determination. Lines are fitted third-order regressions; sun, HV= - 3.28-0.14" LPI+ 9.03 �9 10 -3 �9 LPI2-2.31 �9 10 -4 �9 LPI 3, R2=0.88; shade, HV= -3.284).22. LPI+ 1.21 �9 10 -2 - LPI 2 -2.13 �9 10 -4 - LPI 3, R2= 0.94. Symbols are as in Fig. 4

Table 2. Diurnal pattern of the water-potential gradient, Aq j, in the shoot xylem and the basal leaf water potential (~lJbase_leaf) in a controlled environment and in the field. Water potential in the growth-chamber experiment was determined by thermocouple psychrometry and pressure-chamber technique. Field data were obtained with the pressure chamber. Leaf temperatures and leaf- air water vapor pressure differences were 23-32.5~ and 8.9 34.7 mbar �9 bar- 1, respectively for the growth-chamber experiment and 21 33 ~ C and 11.(~30.2 mbar - bar- 1, respectively for the field experiment. The experiments were conducted 4-5.3.1987 (growth chamber), UC Davis, USA and 14-15.6.1988 (field), For- schungsanstalt Geisenheim, FRG. Data are means of two or three determinations from four plants per treatment (growth chamber), and from eight shoots on three different plants per treatment (field)

Time of Day Measure- Sun Shade

ment Field Chamber Field Chamber

Morning Aq s 0.37 0.30 0.31 0.41 (7:30 a.m.) ttJbase.lea f -0.43 -0.69 -0.30 -0.63 Midday A~ 0.46 0,32 0.54 0.45 (2:00 p.m.) ~I/base.lea f 4).64 -0,80 4).54 4).68 Evening AqJ 0.13 0,34 0.15 0.49 ( 7 : 3 0 p . m . ) q/base-leaf 4).58 -0.82 -0.41 -0.58

(Table 1) resul ted in large differences in LSC be tween sun and shade plants , i ndependen t o f the m e t h o d s e m p l o y e d (Fig. 5). Aga in , differences s ta r ted to emerge at LPI 3 -4 and were largest be tween LPI 6 and 18, when LSC o f sun shoots de t e rmined f rom pressure-f low exper iments and Hagen-Po iseu i l l e ca lcu la t ions was a b o u t three t imes tha t o f shade shoots (Fig. 5A, B). The LSC de t e rmined f rom in t ac t -p l an t measu remen t s was rough ly one - fou r th those es t imated by the o ther me thods , bu t differences be tween sun and shade were o f s imi lar m a g n i t u d e (Fig. 5C).


o3 03

50 C ) L ~ 4"0

_. o 20

rY " J 10

0 _d 50 . _

~ -6 40 IZ3 4- '

0 ~ 30

CA N,---

E O e 20 13" ~-~ L

u_ 0 co g}

150 C)

0) 100 n >,

r- 50

D- (9

b_ 0

0

�9 i

l< . , ( ,

. f f D :

,\ p "

% .

[ B , J

11 , ,

40 80

LPI 2 - 2 . 5 l n t e r n o d e s Sun o Shade �9

i i ~ l l

i i ~ I i

�9 . i . . . . .

120 160 0

LPI 5 - 6

.-*"'o'4

. . . . : : --_

E

Q ~ 1 7 6 ~

H

i<;<.~ . . . . . . . . . .

40 80 120 160 0

Conduit Diometer (r Fig. 7A-I. Absolute (G, H, I) and relative frequency (D, E, F) of conduits of different diameter from internodes at different developmental stages. Data were pooled into conduit-diameter classes of 10 gm for the range from 0-180 gm. Relative conductance (i.e.,

C LPI 1 9 - 2 2

. . / % \ e'" / " o o o ,,~o..o-r o.. \ / \ / \

F

q

\ / �9

. s . . . . . . 40 80 120 160

percent contribution of a diameter class to total internode conductance, A, B, C) was calculated according to Eq. 5. Data are single or averages of two samples from three or four different plants per treatment

The lower LSC yet similar k s values for most internodes from shade plants compared with sun plants should be explained by reduced HVs, since LSC is equal to the product of HV and ks (Tyree and Ewers 1991). For both treatments, HVs decreased almost exponentially with increasing LPI but faster for the shade internodes, approaching minimum: values at LPI 12 for shade and LPI 22 for sun plants, respectively (Fig. 6). This rapid decrease suggests a strong apical control of development in grape shoots (Tyree and Ewers 1991). The HVs were consistent with the LSC results in that starting at LPI 3-4, HVs from sun plants were up to three times larger than for shade plants (Fig. 6). These results clearly de- monstrate that stems from shade plants are less opti- mized than those of sun plants for high transpiration rates. Moreover, the results presented in Figs. 2-5 also indicate that even under low transpirational demand a substantial A~g in the shoot xylem is needed to compen- sate for the low Kh and k s.

To determine the transient changes of Atg in the shoot axis during a simulated day, simultaneous changes in radiation, temperature, humidity, shoot xylem and basal leaf ~g were measured diurnally. Each plant type was measured under its environmental growth conditions (i.e., shade-grown plants in a low-light and sun-grown

plants in a high-light environment) in the growth chamber and in the field. The A~g in the shoot xylem was not much different between sun and shade shoots (Table 2), although the maximum rate of transpiration differed greatly. Sun shoots transpired at a rate two times that of the shade plants in the growth chamber (8-10 cm 3 �9 s-1 compared with 4-5 cm 3 �9 s - t ) and about four times that of shade plants in the field (18 cm 3 �9 s-1 compared with 4 cm a �9 s-1). At the same time, basal leaf % which is commonly measured when sun and shade leaves or plants are compared (Larcher 1984), was higher for shade than sun leaves (Table 2).

The reduced Kh and ks in shade plants indicated differences in number and-or diameter of xylem conduits. The diameters of vessel and tracheid lumina were determined from photomicrographs of internodes. These observations and subsequent Hagen-Poiseuille analysis il- lustrated the anatomical differences in xylem at various stages of development (Fig. 7). At an LPI between 2.0 and 2.5, there was no significant difference between sun and shade internodes in either the total amount of vessels and tracheids (between 235-340), or in their relative and absolute diameter distributions (Fig. 7D, G). Most of the conduction at this stage was through conduits between 30 and 50 l~m in diameter (Fig. 7A). These data confirm


I 09

I C~

EL

E c-

,x/

1 0 - 7 I I I l I I

Petioles A

1 0 - 8

10 -9

10 -10

10-11

10 -12 , - 5 0

~/~ OU ~ �9 i,'qz:i

W �9

10-1

I 0 - 2 1 T - - -

I

EL 1 0 - 5

8 1 0 - 4

O9 IP PF HP S u n o A []

Shude �9 �9 �9 , . . . . 10 -5 5 10 15 20 25 50 - 5

I i i i i i

B

O ~ TT

f . . I I I I I I

0 5 10 15 20 25 50

LeGf P IGs tochron I Fig. 8A, B. Hydraulic conductance per unit length, Kh, (A), and specific hydraulic conductivity, k~, (B) of petioles at different locations on shoots of sun- and shade-grown plants of V. vin(fera. The Kh was calculated from measurements of q and AWAx on intact plants (IP), from pressureflow relationships (at 0.1 MPa) of excised petioles (PF), and from vessel and tracheid lumen diameters using

ndex the Hagen-Poiseuille equation (HP). Xylem cross-sectional areas for calculation of k~ were determined from photomicrographs for the HP and PF methods, and estimated from petiole diameter measurements for the IP shown in Fig. 1. Each datum point represents the mean 4- SE of two to four plants. Hagen-Poiseuille data are single values. Lines were drawn by hand

those on Kh and ks values obtained with other techniques and shown earlier, where no distinction between the conductance of sun and shade internodes was apparent up to LPI 3-4 (Fig. 4).

As internodes matured to LPI 5-6, the number of vessels and tracheids increased more in sun (484+ 106, n = 4) than in shade plants (372 + 38) but the differences were not significant. Internodes from shade plants had fewer conduits in the ranges of 30 50 ~tm and 60-100 ~tm (Fig. 7H). The relative distributions of conduit diameters roughly reflected those of the absolute numbers (Fig. 7E, H). At this stage of development, most water conduction occurred through a relatively small number of wider conduits ( > 60 ~tm) for sun plants, whereas the bulk of conduction of shade internodes occurred through conduits between 45 and 65 ~m in diameter (Fig. 7B).

Basal internodes (LPI 19-22) of sun plants had about twice the cross-sectional area, about twice as many conduits (675 + 104 compared with 353 + 24; n = 3, significant at P<0.01) , and more conduits in every size class (Fig. 7I) compared with basal internodes of shade plants. The internodes from sun plants included some conduits larger than 90 pm, whereas shade plants had none (Fig. 7F). Again most of the conductance of sun plants was through the small number of large conduits (Fig. 7C), demonstrating the large effect of vessel or tracheid diameter on conductance. For basal shade internodes, the absolute number of conduits did not change significantly in comparison with the earlier developmental stage (cf. Fig. 7H, I). However, the diameter of basal conduits had increased (Fig. 7E, F), and this shifted the range of conduits supporting most of the conduction to the 60- to 90-~tm range (Fig. 7C). The main effect of low

light was on conduit number in internodes, but the smaller effect on the frequency of conduits of large diameter had a great impact on total Kh and probably caused the ks of these internodes to differ (cf. Fig. 4C, D).

Sample calculations from photomicrographs of internode cross-sections showed that the number of vessels and tracheids per unit xylem area as well as the fraction of total xylem area occupied by conduit lumen were doubled for very young shade internodes (445 -mm -2 compared with 2 0 0 - m m -2 and 24% compared with 10.1% for shade and sun plants, respectively, significant at P < 0.01). This was apparent at a developmental stage (LPI 2) where no differences in either absolute conduit number or size could be detected (Fig. 7A). Thus, conduit density was twofold greater in shade internodes because the development of xylem parenchyma was inhibited early in internode development. The same relationship, although less pronounced, was found at LPIs 5-8 (250 �9 mm -2 vs. 120 - mm -2, and 21.5% to 13.5%, for shade and sun plants, respectively) but was absent in the basal internodes (100-mm -2 vs. 80" mm -2 and 13.3% to 13.5%).

Petiole Kh and k s were estimated using the same methods applied in investigations on internodes. There was a distinct pattern of Kh along the shoot, increasing from the youngest petioles (LPI 0) to a maximum at LPI 4-8 and then declining slightly towards the basal leaves (Fig. 8A). This pattern was evident regardless of the treatment and the method used to determine Kh, although the Kh predicted by the Hagen-Poiseuille equation was about 10-15 times the Kh calculated from pressure-flow relationships of excised petioles, and 15 20 times the Kh calculated from data of intact plants


10 -5 10-3

I 10-6 0 3

I u

10 - 7

c-i E

O 1 0 - 8 0'3 _.J

i i i i i i

A

no 1 0 - 4

/•m-E "n

T

0 ~6~ v v

,,/J I I I I I 1

0 5 10 15 20 25 30

i i i i ~ i

B

1 0 - 9 1 0 - 5 , , , , , , - 5 - 5 0 5 10 15 20 25 30

Leaf P las tochron index Fig. 9A, B. Leaf specific conductivity (LSC) (A) and Huber values Poiseuille equation. Xylem cross-sectional areas were determined (HV) (B) of petioles as a function of LPI for sun and shade plants, from photomicrographs or estimated from petiole diameter meas- The LSC and HVs were calculated by relating the Kh and the xylem urements as shown in Fig. 1. Symbols are as in Fig. 4. Each datum cross-sectional area, respectively, of a petiole to the leaf surface point represents the mean-t-SE of two to four plants. Hagen- supplied by that petiole. Data are from pressure-flow and intact- Poiseuille data are single values. Lines were drawn by hand plant experiments, and from calculations based on the Hagen-

(Fig. 8A). Differences in Kh between sun and shade petioles became evident between LPI 2 and 3. The average Kh of mature sun petioles was 2 to 3.5 times that of shade petioles when calculated from Hagen-Poiseuill- o's equation or pressure-flow data (Fig. 8A).

The differences between treatments disappeared for all methods when the ability to conduct water was ex- pressed as ks (Fig. 8B). Also, in contrast to Kh, ks did not decrease in older petioles (cf. Fig. 8A, B), yet the overall pattern of k~ as a function of LPI was similar to Kh. This indicated that the observed reductions in Kh were solely related to a reduction in xylem area in the shade petioles with no effect on conduit number or average conduit diameter per unit A,. The Hagen-Poiseuille prediction of maximum Kh was 50 times and maximum ks of both organs was five (sun) and two times (shade) lower for petioles than internodes (cf. Fig. 4C, Fig. 8B), indicating that petioles are a barrier for water moving from the soil to the leaves.

Again, the strong reduction in Kh of shade petioles with no significant reduction in average leaf area (Table 1), indicated lower LSC values, and in accordance with the similarity in k~ between light treatments, also lower HVs for shade petioles. This was confirmed by LSC and HV calculations showing a two to fourfold reduction for shade petioles older than about LPI 5 regardless of the method employed (Fig. 9A, B). Both LSC and HV showed a distinct pattern with development, with LSC basically reflecting the change in Kh with LPI (cf. Fig. 8, Fig. 9A) and HV declining rapidly with increasing LPI to approach approximately constant values at LPI 5 (Fig. 9B).

Anatomical analyses of petiole sections revealed a reduction in total xylem cross-sectional area of petioles of shade plants similar to that found for internodes. However, for petioles the sole effect of low light was a reduction in the number of conduits (Fig. 10). Differ ences between petioles of sun and shade plants were already visible at LPI 1-2.5 which compares well with first- observed differences in measured total Kh (Fig. 10A). At this stage, sun petioles already had twice the number of vessels and tracheids than shade petioles, 319 4-26 compared with 140 4- 12 (n = 3, significant at P < 0.01), mainly in the range of 10 to 30 txm (Fig. 10G). However, frequency distribution and relative conductance were not significantly differen t (Fig. 10A, D), which was also true for older petioles (Fig. 10B, C, E, F). Although conduits increased slightly in diameter during further develop: ment (Fig. 10E, F, H, I), most of the conduction occurred through conduits 30-50 gm in diameter at all stages of development tested (Fig. 10A, B, C). Total number of conduits was highest at LPI 5-8 (349 :t: 47 as compared with 1774- 17,,:n=3, significant :at P<0.01) for~both treatments, which coincides with highest measured and predicted Kh and ks (Fig. 8).

Leaves of plants from natural shade habitats (Schus- ter 1908; Bj6rkman et al. 1972) and leaves from shaded canopy zones on the same plant (Schuster 1908; Wylie 1'951) have reduced vascularization. We therefore investigated the hydraulic properties of the lamina. Because Vascularization differs along grape leaves (Silk 1983), measurements of ks, i.e. hydraulic conductance per unit length divided by the total cross-sectional area of the lamina, were conducted on different positions along the


o (D

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o F-

0

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~ [ : ~

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K_

40

30

20

10

0

30 .t.

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G 8 0 o

o / \ ~

4O

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A �9 LPI 1 - '2.5

Petioles 6"/i~. Sun :

D E

LPI 5 - 8

t

". .~.

"" V -~

H

20 40 60 0

C o n d u i t D i a m e t e r ( f fm)

"f••i 20 ' -22 C " LPI

I

20 40 60

Fig. 10A-I. Absolute (G, H, I) and relative frequency (D, E, F) of conduits of different diameter of petioles at different developmental stages. Conduits were grouped into classes at 5-pm intervals. Rela-

tive conductance (A, B, C) was calculated according to Hagen- Poiseuille's equation (Eq. 5). Data are means of two samples per developmental stage from two to four different plants per treatment

150

100 r'--

I o

.~. 50

1 0 3

�9 -- 0 t ~D

EL

I00

E m 50

A S u n o S h a d e �9

LPI 1 - 3

C LPI 12 -15

l

B

i

D

LPI 6 - 8

T

" 0 I I I 1 I i

LPI 19-21

1 ~ 0 ~

0 20 40 60 80 0 20 40 60 80 1 O0

N o r m a l i z e d D i s t a n c e A long Lea f L a m i n a (% of To ta l L e n g t h )

Fig. I l A - D . Specific hydraulic conductivity (ks) at different positions on laminae of sun- and shade- grown leaves at different developmental stages, Each datum point is the mean • SE from leaves of three or four plants

lamina of leaves of different developmental age. Very young leaves (LPI 1-3) showed a very low ks at any point on the lamina regardless of the light treatment (Fig. 11A). At LPI 6-8, ks had increased about threefold, and sun leaves had a higher ks than shade leaves. From

this stage on, which coincides with the stage where leaves are nearly fully grown, there was no further increase in ks of shade leaves (Fig. 11 B-D). Sun leaves had highest ks at LPI 12 15 with values about four times those of shade leaves at any position on the lamina (Fig. 11C). In


10--5 10--2

l co 10-6

I D

0_ 0_ 7

c4 E

o 10-8 u-) __l

A Internodes

/ Petioles

t

(D

[3

-(3

:]2

1 0 - 5

1 0 - 4

~ B Sun Shade - -

1 ' ~ tnternodes

)e \

Pet io les ~ - ~ _ _ \ "

1~q-~ I , I I I , 10-J ,

- 5 0 5 10 15 20 25 30 - 5 0 I I i I t

5 10 15 2O 25 30

L e a f P l u s t o c h r o n i n d e x

Fig. 12A, B. Leaf specific conductivities (,4,) and Huber values (B) for internodes and petioles of sun and shade plants of V. vinifera at different developmental stages. Curves were obtained from data in Figs. 5, 6, and 9. Data on LSC were obtained from pressurized-flow and intact- plant experiments; Huber values are from all three methods

basal leaves, ks had decreased again to values comparable to those obtained at LPI 6-8 (Fig. 11D).

To illustrate the differences in LSC and HV between treatments, plant organs, and positions on the shoot, the relationships obtained from pressure-flow experiments on internodes and petioles were plotted in one figure (Fig. 12). At all later developmental stages LSC and HVs of shade organs were substantially reduced. Figure 12 shows that with the change in flow path from internodes to petioles, LSC was reduced, whereas HVs for most of development were similar for both organs. This indicates that the capacity to conduct water per unit xylem cross- sectional area is larger in internodes than in petioles.

D i s c u s s i o n

Morphology. Grape behaved as a shade-tolerant species by exhibiting significant plasticity in the allocation of dry matter in the shoot (Table 1). This reorganization in the priority structure for dry-weight allocation from stems to leaves is well known for shade-tolerant species (e.g. Fitter and Hay 1983). Generally, these species exhibit changes in leaf area ratio and specific leaf weight very similar to those observed with grape (Table 1). Increased leaf area ratio and specific leaf weight are indications of an increased investment of energy into the enlargement of the light-intercepting surface and can, as such, be regarded beneficial for plant survival under low-light conditions (Fitter and Hay 1983). The observed increase in leaf weight ratio (Table 1) can be interpreted similarly, although this parameter is not generally considered re- sponsive to low light (Fitter and Ashmore 1974). Pro- nounced changes in plant morphology were also apparent from the differences in diameters of internodes and petioles between sun and shade plants. This and field observations of similar A~ along sun and shade shoots (Table 2) indicated that morphological changes were

related to anatomical changes in the functional structures of the xylem of internodes and petioles.

Anatomy. Xylem differentiation and its response to shade treatments were dependent upon stage of development and differed in internodes and petioles. In internodes, we observed a substantial decrease in the ratio of A~ to Ax with aging (Fig. 1A), indicating that xylem development occurs phase-delayed with respect to the structural development of the stem. Stable and similar ratios for sun and shade plants were reached at LPIs> 5, but shade internodes had lower ratios at smaller LPIs (Fig. 1A). This may be due to the reduction in dry-matter allocation to the developing shade apex which affects the formation of structural components related to the mechanical support of the stem, such as the cholenchyma (apparent from photomicrographs), to a larger extent than the formation of the xylem itself. Such an effect was not found for petioles, where the ratio AJAx was similar for sun and shade plants, generally much higher than for internodes, and changed only moderately with development (Fig. 1B).

We also observed the common basipetal decrease in conduit density (Aloni 1987) in both sun and shade shoots. The density of conduits as well as the fraction of total xylem area occupied by conduit lumen were doubled for very young shade internodes, but the differences between treatments decreased with increasing age, indicating a specific inhibition of the development of the xylem parenchyma early during internode growth.

The approximately 300 conduits present in internodes and petioles at the earliest stages tested (~ LPI 2) more than doubled by LPI 18-20 in internodes, but stayed almost constant throughout development in petioles (Figs. 7, 10). Concomitant developmental changes in diameter occurred in both petioles and internodes. Shade reduced the number and diameter of conduits in mature internodes but only the number in petioles (Figs. 7, 10). Conduit density of shade petioles was reduced, not in-


creased as in shade internodes, indicating a reduction in the formation of conduits relative to parenchyma cells.

The results showed clearly that shade reduced the capacity for water transport in shoot organs and that this reduction occurs via altered activity of the vascular cam- bium. Although some morphological responses of woody plants to shade are mediated by phytochrome, the red: far red ratio was unaltered by the neutral shade used in our experiments. The reduction in both conduit number and density indicates that light acted primarily, if not exclusively, on cell division and initiation rate of vascular elements in the petioles, whereas there was an additional important effect on vascular-size distribution in internodes (Fig. 7). These shade effects are possibly caused by changes in the distribution of growth regulators down the shoot (Aloni 1987; Zimmermann and Brown 1971).

Estimates of Kh and ks from A~, q and xylem anatomy. The changes in xylem differentiation that occurred during development and in response to shade were reflected in estimates of Kh and k s by three independent methods. For all methods and both types of plant, Kh and ks of internodes, petioles, and leaf laminae increased approximately logarithmically from LPI 0 to LPI 4-5, and more slowly thereafter (Figs. 4, 8, 11). During development, differences in Kh between sun and shade plants became significant when organ growth ceased. An average three- to fourfold reduction in Kh was observed in mature shade plant parts compared with plants grown in high light. In contrast, differences in k~ between sun and shade plants were only apparent for leaf laminae and internodes of LPI> 12 (Figs. 4, 11). This indicates that in shade plants the reduction in Kh in petioles and internodes < LPI 12 was directly related to the reduced xylem area, and that only in older internodes and possibly leaf laminae (we do not have any information on Ax of leaves) was this relationship broken. Here, the larger ks of sun plant organs was probably caused by a larger average conduit diameter per unit Ax (Fig. 7), since between light treatments internodes of this age did not differ in conduit density. However, we can not exclude other possible causes for a reduced ks in shade internodes, such as lower pit membrane porosities or reduced conduit lengths (Tyree and Ewers 1991).

It should be possible to estimate the conductance using excised internodes and petioles (Zimmermann 1983). Although care was taken to use low pressure gradients, water movement outside the xylem conduits may have contributed to the higher Kh estimated from some excised internodes compared to the intact plant in the present work (Fig. 4). However, the discrepancy may also be related to the development of hydraulic bot- tlenecks or constriction zones in the stem (Zimmermann 1983).

Salleo et al. (1982b) demonstrated the existence of constriction zones in fully grown nodes (at the end of the season) of Vitis vinifera L. Internode Kh from intact plants in this study compared well with pressurized-flow data up to an LPI of about 4, after which the Kh from excised internodes was much higher (Fig. 4). The differences in Kh obtained by the two methods at LPIs > 4

may have occurred because the excised internodes did not include the nodal section. Constriction zones have been interpreted as an adaptation protecting against cavitation (Zimmermann and Milburn 1982), but the further reduction in conductance caused by the presence of these zones may represent a hazard for the shade stems under adverse environmental conditions such as drought.

When Kh was calculated from the cross-sectional area of conduits using Eq. 5, large increases with development and decreases with shade treatments were observed, similar to the Kh estimates by intact plant and pressure-flow methods (Figs. 4, 7). The absolute conduit numbers re- ported in this study for internodes and petioles may contain errors because it was often difficult to distinguish between small tracheids and parenchyma cells in the xylem. However, since diameters of those cells were ex- tremely small (< 5 ~tm), the effect on calculated Kh would be negligible. Correcting the Hagen-Poiseuille Kh for the actual elliptical shape of the conduits using a correction factor for the reduced effectiveness of non- circular transections (Calkin et al. 1986) resulted in only slightly lower Kh-values (between 1.5 and 5.4%). There were no obvious differences in shape of the conduits between petioles and internodes and between sun- and shade-grown plants.

Petiole Kh and ks data from intact-plant and pressurized-flow experiments agreed rather well, but were both much less than Kh predicted by the Hagen- Poiseuille equation (Fig. 8). As with internodes, the differences may have been caused by constriction zones. These have been demonstrated to be present in junctions from petioles to leaf traces in Populus deltoides and between twigs and petioles of several Acer genera (Larson and Isebrands 1978; Zimmermann 1978). In the present study however, their existence was not proved in pressurized-flow experiments where both petiole junctions, the one into the leaf and the one into the shoot node, were left on the petiole and then successively removed. The discrepancies between measured Kh and ks and Hagen-Poiseuille Kh and k s may also indicate that a substantial portion of the conduits in petioles were not functioning.

Localization offlow resistances. Both plant types showed an apparent non-linear relationship between A~ and q in the intact plant, indicating an increasing Kh with increasing q. This is in agreement with the bulk of experimental data obtained at high leaf ~ and low A~, and where growth appeared to be involved (Boyer 1985). The observed non-linearity in the present study may reflect a similar transition from water flow for growth to water flow for transpiration in stems as suggested by Boyer for leaves (1974, 1985). The pressurized-flow experiments on excised internodes at different developmental stages support this hypothesis. The minimum pressure gradient (Ap) required to generate water movement through rapidly growing internodes (similar growth rates were observed for sun and shade internodes) after excision was equivalent to the A~ necessary to generate the same amount of water flow through stems of intact plants (cf. Fig. 2B, and Fig. 3A). In contrast, fully grown internodes


(LPI > 4) required only a minimal Ap to cause significant flow and did not exhibit non-linearity (Fig. 3B, C). Thus, these data, and the very high ratio of Ar to Ax found in young internode tissue (Fig. 1A), indicate that flow in growing internodes may be limited by xylem development imposing substantial frictional resistances.

Our data do not support the concept that the root represents the rate-limiting resistance when water is moved mainly for transpiration (Boyer 1985). In no case did the A~ between the soil (soil gt assumed to be 0.02 MPa at field capacity) and the basal shoot internode, which represents a conservative estimate of A~ across the root (Schultz and Matthews 1988a), exceed the A~ in the shoot itself.

Irrespective of the experimental method, the Kh and k s of petioles were severalfold lower than the Kh and ks of internodes, indicating that petioles represent a substantial hydraulic barrier for water movement to the leaves (Figs. 4, 8). Leaf laminae also represented a large hydraulic resistance for water flowing in the soil-plant- atmosphere continuum for both sun- and shade-grown plants, although their ks is not directly comparable to that of the other organs, since it is based on total cross- sectional-area rather than xylem cross-sectional area (Fig. 11). The leaf lamina k s measured here reflected the ability of the vascular system to conduct water, not the protoplast or cell-to-cell pathway for growth (Boyer 1974). Although the cause of the different ks for sun and shade leaves was not investigated, shade leaves may have had lower vein density as shown in early work by Schus- ter (1908) and Wylie (1951).

Leaf specific conductivity, Huber value, and ecological implications. If the capacity to conduct water becomes decoupled from the transpiring surface, exposure of shade plants or parts thereof to high evaporative demand or drought will inevitably turn the advantage of having a large surface area into a liability. Tyree and Sperry (1988) recently developed a model in which this decoupling is indicated by changes in LSC. The model predicts that plant segments with low LSC values should be more prone to "catastrophic xylem dysfunction" than segments with high LSC values. Variations in LSC within canopies of woody species occur (Oren et al. 1986; Gart- ner 1991), but are usually masked by relating the total size of the canopy to the average stem conductance.

A decoupling was specifically shown in the present study for shade shoots of grape as a three- to fourfold reduction in LSC and HV of internodes and petioles (Fig. 12). In grapevine canopies, both sun and shade shoots develop on the same plant. Because of the higher resistance, A~r in shade stems and petioles would have to increase more than in sun stems and petioles to meet increased demand for transpiration by the atmosphere or to overcome soil drought. The impairment in the water- conducting system of shade-grown plants resulted in larger A~ in the stem compared with the sun-grown plants at similar flow rates. This may cause growth inhibition (Schultz and Matthews 1988a), cavitation, and ultimately abscission of these shoots (Zimmermann and Milburn 1982). The increased sensitivity of photosyn-

thesis to water stress of some plants from shade habitats (Gauhl 1979) may also be related to the reduced capacity for water transport to transpiring surfaces.

The results support the so-called plant segmentation hypothesis developed by Zimmermann (1983), but basically already suggested by Huber (1928). This states that under periods of stress, unfavorable low LSC, HV or nodal resistances would help to confine cavitation and embolism to expendable minor branches or (applied to the present study) to less productive shaded branches or stems. Since petioles had lower LSCs than internodes in both sun and shade plants (Fig. 12), leaves will be at a disadvantage as compared with stems during stress. Al- though the few large vessels in stems of sun-grown plants could make them more prone to losses in Kh (Zimmer- mann and Milburn 1982), sun plants maintained safety by developing a much larger number of vessels and tracheids in any of the smaller size classes compared to the shade-grown plants. In addition, shade can reduce lignification of tracheary elements (Zimmermann and Brown 1971), but the implications for water transport and cavitation are not clear.

Hans R. Schultz was supported in part by the Deutsche Forschungs- gemeinschaft (grant Ki-114/8-1). We wish to thank Dr. Thomas Geier, Institut ffir Biologie, Forschungsanstalt D-6222 Geisenheim, Germany for his advice on sample preparation and microscopy, and two anonomous reviewers for their helpful comments.

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Xylem development and hydraulic conductance in sun and shade shoots of grapevine (Vitis vinifera L.): evidence that low light uncouples water transport capacity from leaf area