Photosynthesis Research 16:211-218 (1988) © Kluwer Academic Publishers, Dordrecht - Printed in the Netherlands

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Leaf photosynthetic characteristics of seedlings of actinorhizal Alnus spp. and Elaeagnus spp.

BENO[T C(~)T]~, l ROGER WILLIAM CARLSON: & JEFFREY OWEN DAWSON 1 Department ~?f Forestry, 110 Mumford Hall, 1301 West Gregory Drive, University c~["

Illinois, Urbana, IL 61801, USA; 2Department of Environmental Studies, 408 South Goodwin A re., University ~?[ Illinois, Urbana IL 61801, USA

Received 4 August 1987; accepted in revised form 8 December 1987

Key words: CO2 fixation, early successional plant, leaf N, plant architecture

Abstract. Single leaf photosynthetic characteristics of Alnus glutinosa, A. incana, A. rubra, Elaeagnus angustifolia, and E. umbellata seedlings conditioned to ambient sunlight in a glasshouse were assessed. Light saturation occurred between 930 and 1400/~mol m 2 s 1 PAR for all species. Maximum rates of net photosynthesis (Pn) measured at 25 °C ranged from 12.8 to 17.3#mol COrm 2s ~ and rates of dark respiration ranged from 0.74 to 0.95/~mol COrm 2s ~. These values of leaf photosynthetic variables are typical of early to mid- successional species. The rate of Pn measured at optimal temperature (20°C) and 530 llmol m 2s ~ PAR was significantly (p < 0.01) correlated with leaf nitrogen concentra- tion (r = 0.69) and negatively correlated with the mean area of a leaf (r = -0.64). We suggest that the high leaf nitrogen concentration and rate of Pn observed for Elaeagnus umbellata and to a lesser degree for E. angust(folia are genetic adaptations related to their crown architecture.

Abbreviations: Pn net photosynthesis

Introduction

Early successional habitats are usually open sites characteristically exposed to high light intensity and a wide variation in temperature (Bazzaz 1979). Certain plant species have adapted to these habitats. Early successional species, for instance, can have higher light saturated rates of photosynthesis than late successional species (Boardman 1977, Bazzaz 1979, Bazzaz and Carlson 1982). However, leaf photosynthetic capacity of woody plants often exhibits strong correlation with leaf N content (Field et al. 1983, Mooney et al. 1983, Lincoln and Mooney 1984). Soil nitrogen (N) availability is frequently low in early successional habitats (Miller 1983), possibly limiting photosynthesis. Plant species capable of fixing atmospheric N 2 can over-

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come this limitation and get a competitive edge over plant species incapable of Nz-fixation in these habitats (Dawson 1983). The characteristically high leaf N concentrations of actinorhizal (Frankia-nodulated) plants (Ro- driguez-Barrueco et al. 1984) should give them a high photosynthetic capac- ity on a leaf area basis compared with species incapable of N2-fixation.

Actinorhizal species in the genus Alnus and Elaeagnus are used extensively for land reclamation in the U.S.A. Autumn olive (Elaeagnus umbellata Thunb.) and Russian olive (Elaeagnus angustifolia L.) are planted and volunteer on reclaimed minespoils in the eastern U.S.A. Both species are shrubby and have lanceolate to oblong leaves up to 6cm in length. In contrast, alders, including Alnus glutinosa (L.) Gaertn., Alnus incana (L.) Moench and Alnus rubra Bong., more frequently attain tree stature and have oval to elliptic leaves up to 10cm in length. Because features of plant architecture, including crown size, branching patterns, the arrangement of the foliage, and the form and size of individual leaves interact to determine the capacity of a plant to fix carbon and grow in a particular light environ- ment (Bazzaz 1979, Chazdon 1985, Garbutt 1986), we hypothesize that differences in leaf photosynthetic characteristics among these species of Alnus and Elaeagnus exist and that the differences are associated with differences in crown architecture of the species.

To test these hypotheses we assessed, under controlled environmental conditions, single leaf photosynthetic characteristics of actinorhizal Alnus glutinosa, A. incana, A. rubra, Elaeagnus angustifolia and E. umbellata. Seedlings rather than mature trees were studied because of the importance of seedlings of these species in colonizing disturbed sites.

Materials and methods

Seeds of Alnus glutinosa, A. incana, Elaeagnus angustifolia and E. umbellata were obtained from F.W. Schumacher Co., Sandwich Mass. U.S.A. Seeds of A. rubra Bong. were provided by Bruce Rottink of Crown Zellerbach Corp., Forestry Research Division, from a source in Columbia County, Oregon. Seeds were germinated in Jiffy-7 peat pellets in a glasshouse. The pellets were then placed in a mixture of Drummer silty loam soil (a fertile prairie-derived soil from Illinois, high in organic matter), sand, and sphag- num peat moss (2:1:1) in one-liter containers. Two weeks after germination, we inoculated each seedling with one cm 3 1% v/v of Frankia ArI3 (packed cells at 1000g for 5min) provided by Berry and Yorrey (1979). Three seedlings per species were grown for 14 weeks in a growth chamber. Light was supplied by a mixture of white fluorescent and incandescent lamps

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yielding from 150 to 300#molm--Xs 1 PAR depending on plant height. Seedlings were grown with a 16-h photoperiod, temperatures of 24/16°C and relative humidities of 60/75% for the day/night periods. Plants were transferred to a glasshouse adjacent to the experimental apparatus for measuring gas exchange. Temperatures in the glasshouse averaged 30°C during the day and 20 °C at night and the photoperiod was extended to 16 h using sodium vapor lamps. We conditioned the plants to ambient sun light in the glasshouse for at least a week before beginning photosynthetic meas- urements in February. Mean height of seedlings of each species was less than one meter and all seedlings were nodulated (Table 1).

One newly formed, fully expanded leaf on each seedling was placed in a plexiglass assimilation chamber (9 cm in diameter by 2 cm in height). Leaves were illuminated with light from quartz-iodide incandescent lamps directed through 10cm of water. Measurements were made at a maximum of 1400 #molm -~ s ~ PAR at leaf surface. Lower light intensities were obtained by shading with sheets of muslin. Gas exchange was measured using an open system (Bazzaz and Carlson 1982). We maintained the concentration of CO2 between 300 and 320 ppm by adding either pure CO2 or Nx containing 21% O~ from gas cylinders. We measured COx with an infrared gas analyzer (Horiba, Irvine, CA) and relative humidity with a Weather Measure (Sacra- mento, CA) humidity indicator. To assess plant water status, we calculated the stomatal conductance for H20 by dividing the transpiration rate by the saturation water vapor deficit (Tranquillini et al. 1986). Air turbulence was adjusted to maintain less than I°C difference between air and leaf tem- peratures.

Rates of Pn of one newly formed mature leaf per seedling were measured at 25°C, and 1400, 930, 530, 290, 125, 85, 45 and 0#molm Xs ~ PAR. To obviate the possibility of different temperature adaptations of the species, the three plants were assayed for Pn at 30, 25, 20, and 15°C at 530 #mol m- 2 s-~ PAR while relative humidity was maintained between 45 and 60%. We used a sub-optimal light intensity to diminish evaporation and therefore minimize water deficit for the largest plants. Rates of Pn measured

l'ahh' 1. Height and nodulat ion of the study, seedlings (Mean _+ SE; N = 3).

Species Height Nodules (cm) ( # plant ~) (rag DW plant ~)

Elaeagnus angust(/olia 97 4- 6 91 _+ 22 311 4- 7 E. umhellata 61 4- 6 57 _+ 12 337 4- 98 .41nusglutinosa 60 _+ 4 113 4- 9 589 4- 22 .4. incana 46 ± 8 84 _4_ 21 369 4- 135 4. ruhra 42 4- 5 47 + 23 257 _+ 69

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at different temperatures and 530 vmol m -2 s -1 PAR are reported only for the optimal temperature of 20 °C. The plants were allowed to equilibrate for 30 min at each light intensity or temperature before a measurement of Pn was taken. The apparent quantum yield and light compensation point were estimated as the slope and X-intercept, respectively, of the initial linear portion (0-125/~mol m -2 s -1 PAR) of the light response curve measured at 25 °C.

After determining the photosynthetic characteristics of a leaf, its area was measured using an area meter (Li-Cor model LI-3000). It was dried at 65 °C for 48 h and weighed to determine its specific weight (mg cm 2). Fully expanded leaves from the top of each plant were analyzed for N concentra- tion by the micro-Kjeldahl procedure (Bremner and Mulvaney 1982).

Data were analyzed using a one-way ANOVA. Means of species were compared using the LSD multiple range test, and the Pearson product- moment correlation was used to measure linear relationships between leaf characteristics (SAS Institute 1982). A probability level of 5% was used for all statistical analyses.

Results and discussion

Comparisons with non-actinorhizal plants The rates of Pn at 1400pmolm-2s ~ PAR ranged from 12.8 to 17.3 #mol CO2 m 2s 1; they were not significantly different from one another except for the two extreme values observed for A. incana and A. glutinosa, respec- tively (Table 2). Although herbaceous plant species often have higher photosynthetic rates than woody species (Larcher 1969, Mooney 1972), maximum rates of Pn measured in our study for woody N2-fixing plants are high compared with the results obtained by Singh et al. (1974) for her-

Table 2. L e a f p h o t o s y n t h e t i c c h a r a c t e r i s t i c s o f a c t i n o r h i z a l Elaeagnus s p p . a n d A l n u s s p p .

s e e d l i n g s c o n d i t i o n e d t o a m b i e n t s u n l i g h t ( M e a n + S E ; N = 3).

Species Dark resp. Quantum yield Net photosynthesis Net photosynthesis I

(pmol CO 2 (~mol CO 2 i~mol I) at 1400~molm 2s t at 530~umolm 2s-1

m 2s t) (,amolCO 2m 2s i) (pmolCO 2m 2s I)

Elaeagnus 0.95 + 0.16 a 2 41 + 2 a 15.8 + 1.5 ab 21.7 Z 0.8 ab angustijblia E. umbellata 45 + 1 a Alnus glutinosa 44 _+ 4 a A. incana 39 _+ 5 a A. rubra 41 _+ 6 a

0.93 + 0.10 a 14.7 _+ 0.5 ab 23.0 _+ 2.2 a 0.86 + 0.08 a 17.3 + 2.1 a 18.6 + 0.6 b 0.74 + 0.15a 12.8 + 1.7b 19.4 ± 0 .7ab 0.80 + 0.25 a 13.6 + 1.0 ab 19.3 + 1.6 ab

I Measured at 20°C

2 Means of species in a column followed by the same letter are not significantly different (p < 0.05).

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baceous C3 plants grown at similar light intensity. Overall, the rate of Pn of all species increased 1.8% (SE = 1.25%) when light intensity was increased from 930 to 1400#molm 2s l PAR. Therefore, light saturation likely occurred between 930 and 1400/~mol m -2 s--l PAR for all species. These high levels of light saturation are much higher than levels observed for late successional trees (Loach 1967, Wuenscher and Kozlowski 1970). Rates of dark respiration ranged from 0.74 to 0.95 Ftmol CO2 m -2 s-1 but differences among species were not significant (Table 2). These values are higher than the rates of dark respiration measured in late successional trees (Loach 1967). The values of light-saturated Pn, light saturation, and dark respira- tion of these N2-fixing seedlings showed very small interspecific variation and correspond to levels of early- to mid-successional plants (Bazzaz 1979, Bazzaz and Carlson 1982).

Apparent quantum yields ranged from 39 to 45 #mol CO2 #mo1-1, but differences among species were not significant (Table 2). Although high quantum yields have usually been associated with shade-adapted or late successional plants (Loach 1967, Bazzaz 1979, Oberbauer and Strain 1986), some recent studies have shown higher quantum yields in early successional plants than in late successional plants, and in sun leaves than in shade leaves (Bazzaz and Carlson 1982, Jurik 1986). Our results are consistent with the latter observations.

Interrelationships between leaf characteristics Leaf N concentration differed significantly among species but no significant difference was detected among the specific leaf weight of the species (Table 3). Leaf N concentration was greatest in E. umbellata and lowest in A. rubra (Table 3). In another study, leaf litter of Elaeagnus umbellata gathered from a stand floor in winter has been found to be almost twice as high in total

Table 3. Leaf nitrogen concentrat ions and physical characteristics o f the leaves (Mean _+ SE;

N = 3).

Species Nitrogen Leaf area Specific leaf (mg g ~) (cm 2 leaf ~) weight

(mg DW cm 2)

Elaeagnus angustifolia 33.0 _+ 1.2 b I 11.9 _+ 1.1 c 7.2 _+ 0.3 a E. umbellata 43.3 _+ 0 . 8 a 11.1 _+ 1 .4c 7.8 + 0 . 6 a Alnus glutinosa 29.4 + 1.4 b 43.7 _+ 2.2 a 8.0 _+ 0.4 a A. incana 29.3 _+ 1.1 b 30.4 _+ 1.9 b 7.0 _+ 0.2 a A. rubra 24.1 _+ 1.9 c 28.1 + 2.2 b 7.9 _+ 0.4 a

Means of species in a column followed by the same letter are not significantly different (p < 0.05).

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Table 4. Coefficients of correlation between the rate of photosynthesis measured at 530/~mol m-2s ~ and 20°C, and leaf physico-chemical characteristics of the species.

Photosynthesis Nitrogen Leaf area Specific leaf weight

Photosynthesis 1.00 0.69 *j 0.64* - 0.39 Nitrogen 0.69* 1.00 - 0.63* - 0.09 Leaf area 0.64* - 0.63" 1.00 0.16 Specific - 0.39 - 0.09 0.16 1.00 leaf weight

Coefficients of correlation followed by an asterisk are significant at p < 0.05.

nitrogen concentrat ion as leaf litter of Alnus glutinosa (Carlson and Dawson 1984). In contrast, mean leaf area was greatest for A. glutinosa and lowest for the two species of Elaeagnus (Table 3). Leaf N concentrat ion was negatively correlated with the mean area of a leaf (Table 4). Leaf size is partly genetically-determined and partly under control o f the environment. When growth conditions are similar for all species, differences in leaf size among species can be at tr ibuted to differences in their genotype. In our study, interspecific variation in leaf size and leaf N concentrat ion occurred al though all seedlings were grown in the same environment and inoculated with the same Frankia endophyte. Perhaps the high leaf N concentrat ion in Elaeagnus umbellata and to a lesser degree in E. angustifolia is also a genetically-predetermined characteristic of these two species.

Rates of Pn measured at 530~molcm-2s -l and 20°C were positively

correlated with leaf N concentrat ion and negatively correlated with the mean area of a leaf (Table 4). Therefore, smaller leaves tended to have higher leaf N and higher rate of Pn than larger leaves. Small leaves and high leaf N concentrat ion were characteristic of Elaeagnus species which also exhibit a shrubby growth form that likely minimizes self-shading. Because crown architecture of plants affects the capacity of a plant to fix CO2 in a particular light environment (Bazzaz 1979, Chazdon 1985, Garbu t t 1986), and because leaf photosynthet ic capacity can adapt to the growth environment over evolutionary time (Mooney and Gulmon 1979, Jurik 1986), we suggest that the high leaf N concentrat ion and rate of Pn observed for Elaeagnus um- bellata and to a lesser degree for E. angust~/'olia are genetic adaptat ions related to their crown architecture.

Acknowledgements

This research was supported by the Natural Sciences and Engineering Research Council of Canada (Postgraduate studentship) and the USDA Hatch Project 1-6-52616 of the Illinois Agricultural Experimental Station.

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