Plant Physiol. (1986 80, 778-7810032-0889/86/80/0778/04/$O 1.00/0

Diurnal Phototropism in Solar Tracking Leaves ofLavatera cretica

Received for publication May 23, 1985 and in revised form October 31, 1985

AMNON SCHWARTZ* AND DoV KOLLERThe Hebrew University ofJerusalem, Department ofAgricultural Botany, Rehovot 76100, Israel

ABSTRACT

On a clear day, leaf laminas of Lavatera cretica tracked the solarposition throughout the day. The laminar azimuth did not diverge fromthe solar azimuth by more than 120 from sunrise to sunset. Tracking ofthe solar elevation started 1 to 2 hours after sunrise and ceased 1 to 2hours before sunset. On an overcast day, the laminas reoriented horizon-tally. After sunset, following a clear day, the laminas performed anocturnal reorientation, with three well defined phases. During the initialphase the laminas relaxed their strained sunset-facing orientation to oneperpendicular to their petioles. This equilibrium configuration was main-tained throughout the following phase, which was apparently concernedwith time-measuring. During the final phase, the laminas reoriented,before sunrise, to a position facing the direction of the anticipated sunrise.This directional information is phototropic and was retained for 3 to 4diurnal cycles, probably in the pulvinus itself, which is the site of theresponse. Laminas of plants transferred from sunlight either to darkness,or to a simulated natural photoperiod under overhead illumination, werefacing the originally anticipated direction of sunrise at the time of eachof the three to four subsequent sunrises (after which they reverted to thedark orientation in darkness, or to the horizontal one with overheadillumination). Cotyledonary laminas required directional information forthe nocturnal reorientation during 3 or 4 cycles of simulated sunrise tosunset transitions.

Expanded leaves of Lavatera cretica track the sun each daythroughout their mature existence. The mechanisms involveperception of light as a vectorial excitation in laminar tissuesassociated with the veins, and response of motor cells arrangedin a cylindrical sheath in the pulvinus that is situated in theuppermost 3 to 4 mm of the petiole (10). The pulvinus bendswhen motor cells in one sector of the sheath expand, while thesein the opposite sector contract (11, 18). The remainder of thepetiole, below the pulvinus, does not change its radial orientationon the stem, which is determined by its orthostichy, but itsvertical elevation decreases progressively during leaf ontogenyfrom about 90° to about 30° at full maturity (10). Consequently,the petiole's contribution to solar-tracking is virtually restrictedto the pulvinus. The diurnal time-course of solar-tracking wasstudied by Yin (18) in leaves of Malva neglecta. His resultsshowed that on sunny days, leaves started tracking the sun within2 to 3 h of sunrise and thereafter maintained their laminas nearlynormal to the sun until sunset. After sunset the laminas reversedtheir direction of movement, until they faced the anticipateddirection ofsunrise, several hours before sunrise. Yin (18) showedthat the nocturnal reorientation could not be ascribed to 'sleepingmovements' nor could it be related to the geographical orienta-tion of the leaf. The nocturnal movement that brought thelaminas to face the anticipated direction of the original sunrise

persisted for several days after the plants had been rotated 1800,until the leaves adapted to the new position and faced theanticipated direction of the prevailing sunrise.While the solar-tracking movement of the lamina throughout

the day evidently maximizes the light-harvesting capacity of theleaf (1, 2, 7, 14), the nocturnal movement maximizes the dura-tion of this light-harvesting activity, by reorienting the leaf to afavorable position to begin its next light-harvesting cycle.The purpose of this study was to characterize and analyze the

nature of the diurnal reorientation of the leaf laminas under thedirectional control of the sun.

MATERIALS AND METHODS

The diurnal movement of the leaf lamina was measured inplants of Lavatera cretica L. (Malvaceae). Potted plants wereinitially grown for about 4 weeks in a phytotron (4) under a 16-h photoperiod (04:00-20:00), and a diurnal temperature altera-tion of 27°C (08:00-16:00) and 22C (16:00-08:00), and werethen transferred to the field at least 4 weeks prior to the meas-urements.Laminar reorientation consists of tilting in the direction to

which the pulvinus bends, and was specified by following changeswith reference to an imaginary line normal to the laminar surface,by the 'laminar angle' of this line with the horizontal plane andby its 'azimuth angle' with the magnetic north. A hand-heldinclinometer was used to determine laminar angle (in the planeof symmetry of the leaf: horizontal = 00), while a combinedcompass-inclinometer was used to define azimuth angle as wellas laminar angle.Measurements at night, or during the dark period in the growth

cabinets, were made by weak green light from a flashlight.The growth cabinets were set at 17° and supplied with overhead

illumination from a bank of 'daylight' Power-Groove fluorescenttubes (General Electric), supplemented by equal wattage ofkryp-ton incandescent lamps (radiant flux between 400 and 700 nmwas 200 ± 20 ,umol m-2 s-1).The light sources used in the experiments with seedlings were

500 W Quartzline lamps (General Electric Q500T3/CL) in areflector housing equipped with a 'hot mirror' (reflectance max-imal and transmittance minimal in the long-wave range, and thereverse in the short-wave range) in front, and a linear, paraboli-cally curved 'cold mirror' (reflectance minimal and transmittancemaximal in the long-wave range, and the reverse in the short-wave) at the back. This allowed most of the radiant heat toescape backwards and directed most of the photosyntheticallyactive radiation forward.

RESULTS

Diurnal Course of Laminar Reorientation. The spatial laminarorientation of three mature leaves (22-30 d after expansion) oneach of eight field-grown plants was followed at hourly intervals,by means of the compass-inclinometer, between sunrise andsunset on a clear midsummer day (June 21) and on a totally

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DIURNAL SOLAR TRACKING OF LA VATERA CRETICA L. LEAVES

overcast winter day (February 15). The tracking efficiency of thelamina for the solar azimuth is presented in Figure 1, while thatfor the solar elevation is shown in Figure 2.On the clear day the laminar azimuth (Fig. 1) was about I10

to the West of the solar azimuth at sunrise and maintained thislead over the first 3 to 4 h. It then started to lag (5-6°) behindthe solar azimuth, and this lag was maintained for about 4 h.During the final 5 h of daylight the two angles did not differ bymore than 20. The tracking of the solar elevation (Fig. 2) did notstart at sunrise but only when the sunlight became approximatelynormal to the laminar surface. Thereafter, laminar tracking ofthe solar elevation continued throughout most of the day. Lam-inar angle was at first somewhat in advance ofthe solar elevation,but later it consistently lagged behind, until it gradually stabilizedat about 700, some time before sunset.On the overcast day, the laminar angle, which started out at

about 550 at sunrise, changed to about 00 within 1 to 2 h andretained this orientation for the remainder of the day. Thelaminar azimuth remained unchanged (data not shown).Laminar response to withdrawal of information from the sun

was tested by transferring the plants, during the daylight hours,to a growth cabinet, in darkness or in light. Upon transfer todarkness, the laminas reoriented so as to become approximatelynormal to their petioles. Upon transfer of an illuminated growth-cabinet, the laminas reoriented so as to stabilize horizontally, i.e.normal to the prevailing light.The nocturnal reorientation of the lamina was followed in 12

field-grown plants during a midsummer night (June 22-23), withthree types of fully expanded leaves, differing in their petiolarorientation: toward the west, toward the east, and somewhat

20. l l1r l 1 1

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16 20

FIG. 1. Tracking efficiency of the leaf lamina of L. cretica for thesolar azimuth. Azimuth of the normal to the laminar surface 0, withreference to solar azimuth of June 21 (straight line ESW). Positive andnegative values indicate advance and lag in tracking, respectively.

05 07 09 11 13 15LOCAL TIME (h)

17 19

0

(o,e)

20 'CD

40 2w

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FIG. 2. Tracking efficiency of the leaf lamina of L. cretica for thesolar elevation. Laminar angle (between laminar plane and horizontalplane) on a clear day (June 21, *) and on an overcast day (Feb. 15, 0),with reference to the solar elevation (June 21, ---- ; Feb. 15, - --).

younger leaves with vertical petioles. During the early hours ofthe night, the lamina exhibited epinasty, which resulted in its'negative' cupping, i.e. toward the petiole. Nocturnal reorienta-tion of the laminas followed a West to East arc in the verticalplane. Figure 3 shows that the laminas started to turn toward theEast at the beginning of the dark period. This movement lasted1 to 3 h, but whereas during that period the laminas on west-facing petioles had rotated only by about 150 and stabilized in awesterly direction, those on east-facing petioles had rotated byabout 1 10° and stabilized in an easterly direction. Laminas onvertical petioles rotated by about 75° and stabilized horizontally.In the stable orientation, the lamina was approximately normalto its petiole. The period of stable orientation came to an endseveral hours before sunrise, earlier in the leaves with west-facingpetioles and with vertical petioles than in those with east-facingpetioles. The first to do so were those on west-facing petioles,which now made the greatest angular displacement, while thelast to do so were those on the east-facing petioles, which nowmade the least angular displacement. Those on vertical petioleswere again intermediate between these two extremes. The ensu-ing reorientation brought the laminas to face the direction of theanticipated sunrise, at least 2 h before sunrise actually occurred.

Nocturnal Reorientation in Absence of Solar-Tracking. Agroup of potted plants that had been grown outdoors for about50 d were rotated by 1800 at sunset. At sunrise on the followingday all their laminas were facing in the direction ofthe previouslyanticipated sunrise, rather than the prevailing one. Yin's (18)results with M. neglecta and Wainwright's (16) results withLupinus arizonicus were thus confirmed with L. cretica. Anothergroup of potted plants were transferred to the growth cabinet atmidday, after their sunset orientation had been marked on thepot, and were subjected to overhead illumination till sunset.Nocturnal reorientation of the lamina was determined at inter-vals during the following night in 12 leaves with vertical petioles,concurrently with those of control plants left outdoors (Fig 3).The results show that the laminas, which started the dark periodin the horizontal orientation, established under the overheadillumination in the growth cabinet, reoriented during the darkperiod toward the previous East, or anticipated sunrise direction,as the equivalent leaves on the control plants. These leavesexhibited normal nocturnal reorientation oftheir laminas to facethe anticipated sunrise direction despite the fact that their solar-tracking was stopped at midday without allowing them to per-ceive the (westerly) sunset position.The conclusion that pre-sunrise orientation is determined by

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LOCAL NIGHT TIME (h)22 01

3 6 9TIME IN DARKNESS (h)

FIG. 3. Nocturnal reorientation in the field of leaf laminas of L.cretica on west-facing 0, east-facing 0, and vertical A A petioles duringthe night of June 22 to 23. A, plants exposed to overhead illuminationbetween 12:00 and 19:00 on June 22. Positive and negative laminarangles represent inclinations above and below horizontal plane, respec-tively, toward west (W) or east (E).

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SCHWARTZ AND KOLLER

the direction of the preceding sunrise may be questioned, how-ever, as appears from results ofanother experiment. Three groupsof potted plants, whose sunset orientation had been marked onthe pots, were moved at sunset to separate-growth cabinets, oneof which was maintained continuously in darkness, another wasilluminated for the duration of the natural daylight (05:00-19:00), and the third was illuminated continuously. Laminarangle was measured on eight leaves from each treatment at 30-min intervals between (simulated) sunrise (05:00) and 09:30 on4 consecutive days. At the time of sunrise on the 1st d, leaves inboth the dark and the natural photoperiod regimes were orientedin the direction oftheir originally anticipated sunrise, after whichthey gradually became horizontal. The post-'sunrise' reorienta-tion was more rapid in the plants exposed to the 'natural'photoperiod than in those maintained in continuous darkness.Laminas ofcontinuously illuminated plants remained horizontalthroughout. Figure 4 shows that nocturnal reorientation occurredrepeatedly, for as many as three cycles, despite the fact that theplants were no longer exposed to solar-tracking outdoors, but todarkness, or to a natural photoperiod (05:00-19:00) under ver-tical illumination. In the latter'case, the only direction fromwhich the leaves were illuminated at the beginning and the endof the diurnal photoperiod was from above. It is therefore clearthat the leaves had stored the directional information from theirprevious solar-tracking experience for as long as three cycleswithout a solar-tracking experience. The degree of initial orien-tation in the direction ofthe anticipated sunrise was progressivelydiminished each day, to the same extent in the dark grown groupand in the group exposed to the natural photoperiod (overheadartificial illumination 05:00-19:00).

Concurrently with the experiment described above, a group ofpotted plants were transferred at the beginning ofthe dark periodto the growth cabinet with the natural photoperiod, and eightleaves with vertical petioles were debladed by excision of theentire lamina except for vestiges of the major veins radiatingfrom the pulvinus. At the time ofdeblading, the laminas oftheseleaves (and all other leaves) were facing in the direction ofsunset(which was marked on the pots). Nevertheless, at the time of thefollowing sunrise (05:00), the top ofthe pulvinus with its attachedvestiges of veins were also facing the direction of the anticipatedsunrise (Fig. 4), and this inclination was also gradually changedto the horizontal. Such debladed leaves did not repeat theirperformance on the following days. Leaves of field grown plantsthat had been similarly debladed exhibited the same nocturnalreorientation during the night following their deblading.

Establishment of Directionality for Nocturnal Reorientation.This was tested using 14-d-old seedlings that had been grown ina growth cabinet with overhead illumination (15-h photoperiod),and whose cotyledonary laminas (which also exhibit a diapho-totropic response 12]) were horizontal throughout each daily

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LOCAL TIME (h)FIG. 4. Laminar angle in leaves of potted 50 d old L. cretica plants

at time of sunrise (05:00) and subsequently, during four consecutive 24-h cycles after transfer from the field to continuous darkness 0, or to asimulated natural photoperiod under overhead illumination: 0, intactleaf; A, leaf with lamina excised, except for vein-convergence adjoiningpulvinus.

photoperiod. The seedlings were placed in a row with theircotyledons in parallel. Two quartz-halogen light sources werethen placed, one on each side of the row of seedlings, facing oneof the paired cotyledons of each seedling. The light sources wereprogrammed to illuminate the seedlings obliquely from the sideduring the first half of the 15-h light period (AM) after which theother lamp would do the same from the opposite side during thesecond half (PM). Laminar angle was measured at intervals duringthe daily dark period. At the beginning of each dark period thelaminas were all facing the second (PM) light source. The laminaproximal to that light source was at a negative angle (i.e. itsmidvein was directed below the horizontal plane), while thelamina of the opposite cotyledon, distal to the (PM) light source,was at a positive angle. Time-course of laminar reorientation ofthe opposite cotyledons during the dark periods of4 consecutivelight/dark cycles is shown in Figure 5. During the 1st and 2nddark periods, the laminas of both cotyledons started to reorientright away (angle decreasing on the AM side and increasing onthe PM side), and both stabilized at a slightly negative angle. (Atthe end of the 2nd dark period the angle was somewhat morenegative.) During the subsequent (3rd and 4th) dark periods, thelaminas reoriented so that the one that was proximal to the PMlight source and started the dark period with a negative angleended the dark period with a positive angle, while the oppositeone exhibited the reverse change. Consequently, both cotyledonsended the 3rd and 4th dark periods with their laminas inclinedto face the direction of the anticipted AM light source. Thekinetics of the reorientation during the 3rd and 4th dark periodsdiffered in the opposing cotyledons. The lamina that started thedark period at a positive angle completed most of its nocturnalreorientation during the first 3 h of the dark period, while theone that started the dark period at a negative angle made mostof its nocturnal reorientation during the last 3 to 4 h of the darkperiod (and its terminal angle increased markedly from the 3rdto the 4th dark period).

DISCUSSIONLeaves of plants growing outdoors tracked the solar position

throughout most of the daylight hours following the change insolar elevation (Fig. 2) and in solar azimuth (Fig. 1). The absenceof solar-tracking for a considerable period at the beginning andthe end of the daylight period (also previously observed [16,18]) may only be an apparent one, since at these times the sunis just above the horizon and its beams may be at least partiallyobstructed by local topographic features and by neighboringplants.

TIME IN DARKNESS (h)FIG. 5. Nocturnal reorientation of cotyledonary laminas (a, b) of L.

cretica seedlings after 10, 20, 30 and 4A cycles ofexposure to unilateralillumination, during the day, for 7.5 h from source proximal to cotyledona (AM), followed by 7.5 h from source proximal to cotyledon b (PM).Insert: Position of cotyledons a and b at the beginning of dark period;arrows indicate direction of nocturnal reorientation, depicted in panelsA and B, respectively.

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DIURNAL SOLAR TRACKING OF LAVATERA CRETICA L. LEAVES

Solar tracking in L. cretica is a response to vectorial excitation,i.e. by light beams striking the laminar surface obliquely (10).This explains why the laminar surface is almost never completelynormal to the sunbeams, but diverges from the normal withinnarrow limits throughout the day. Laminar movement appar-ently leads that of the sun during the first part of the morningand follows it thereafter. A similar phenomenon has been ob-served in young leaves of M. neglecta (18) and L. arizonicus(16). The reasons for this are not known.The nocturnal reorientation of solar-tracking leaves and inflo-

rescenses has been attributed to a relaxation from strained con-figuration at sunset to a nonstrained one prior to sunrise (6, 18).However, the present study shows that the nocturnal reorienta-tion (Fig. 3) exhibits three distinct phases. The first starts im-mediately after sunset, when all laminas face west, and thepulvinus is abaxially (dorsally) concave in west-facing petiolesand adaxially (ventrally) concave in east-facing ones. During thisphase, laminas on west-facing petioles make a small angulardisplacement, those on east-facing petioles make a large one,while those on vertical petioles make an intermediate one. Atthe end of this phase the laminas stabilize with their surfacesmore or less normal to their petioles. This suggests that duringthis phase the unequal strain that exists in the different sectorsof the pulvinar motor tissue at sunset, as a result of theirdifferential turgor ( 18), may be relaxed by equalization of turgorthroughout the pulvinar motor tissue. This leads to the normalorientation of the lamina to its petiole.The next phase starts when the laminas have reached this

equilibrium orientation and lasts for several hours, during whichtheir orientation remains virtually unchanged. This 'stable' phaseends when the laminas start to reorient again. The observationssummarized in Figure 3 are not sufficiently detailed to indicatethe precise duration ofthe second phase, but it seems that laminasthat start this phase earliest (on west-facing petioles) also startthe last phase earliest, while those that are the latest to start phasetwo (on east-facing petioles) are also the latest to start the lastphase. It is thus likely that the duration of 'stable' phase isapproximately the same in all leaves. It is therefore suggestedthat phase two, which is initiated when equilibrium is achieved,is concerned with time measuring, i.e. with the ubiquitous bio-logical clock, by which the circadian movements of nyctinasticleaves are phased (9).

In nyctinastic leaves, time measuring, as well as light percep-tion are located in the pulvinus itself (5, 13, 17). Pulvinaranatomy of these leaves restricts their diurnal reorientation tothe plane passing through their dorsal and ventral motor tissue('extensor' and 'flexor'), whether this takes place in response tolight/dark transitions, or under endogenous control by the bio-logical clock (8, 9). In the diaphototropic leaves of M. neglecta(18) and L. cretica (11), the motor tissue in the pulvinus isorganized as a sheath around the vascular core, which allowstheir omnidirectional reorientation. In these leaves, the site oflight perception is not located in the pulvinus (18) but in thelamina itself (1 1). Nevertheless, results summarized in Figure 4indicate that nocturnal reorientation of L. cretica may not re-quire participation by the lamina and thus suggest that thismechanism might be located in the pulvinus, as in nyctinasticleaves.

Phase three is initiated when the laminas start reorientatingagain, toward the east, namely in the direction of the anticipatedsunrise. While the direction of laminar reorientation duringphase one is dictated by the morphological orientation of thepetiole and is thius under endogenous control, the direction oflaminar reorientation during phase three is clearly dictated bythe previous solar-tracking performance of the leaf and is there-fore under exogenous control. Since the solar-tracking during

daytime is phototropic, the final phase of the nocturnal reorien-tation is in essence its extension and is therefore also phototropic,despite the fact that it is expressed only towards the end of thesubsequent dark period. In view of its unique features, we suggestthe term 'dark-phototropic response' (or 'nycti-heliotropism') todescribe the final phase of the nocturnal reorientation of theseleaves, despite the apparent internal contradiction.

Involvement of the biological clock is supported by resultssummarized in Figure 4, which show that leaves could storedirectional information from their previous solar-tracking expe-rience. Furthermore, they could repeat the nocturnal reorienta-tion for a number of cycles in which directional information wasabsent (darkness, or overhead illumination for the duration ofthe natural photoperiod). This information was apparently pro-gressively dissipated. An analogous situation is known in nycti-nastic leaves, which continue to oscillate in circadian periodicityin absence of the diurnal light/dark transitions, but with aprogressively diminished amplitude (3). In the solar-trackingleaves of L. arizonicus the nocturnal reorientation failed after asingle photoperiod in a growth cabinet with overhead illumina-tion (16). However, the mechanism of solar-tracking in Lupinusis apparently different from that of M. neglecta and L. cretica,because light perception is located in the pulvinules, not in thelaminar parts of the leaflets ( 15).The directional information provided by (simulated) solar-

tracking apparently has to be repeated for 3 or 4 diurnal cyclesbefore it is expressed in the nocturnal reorientation, at least incotyledons (Fig. 5). The same number ofthe cycles without solar-tracking experience are required to dissipate that information inmature plants.

LITERATURE CITED1. BONHOMME R, C VARLET GRANCHER, P ARTIS 1974 Utilization de 1'energie

solaire par une culture de Vigna sinensis. II. Assimiliation nette et accroisse-ment de matiere seche, influence du phototropisme sur la photosynthese despremieres feuilles. Ann Agron 25: 49-60

2. EHLERINGER JR, I FORSETH 1980 Solar-tracking by plants. Science 210: 1092-1098

3. HOSHIZAKI T, KC HAMNER 1964 Circadian leaf movements: persistence inbean plants grown in continuous high-intensity light. Science 144: 1240-1241

4. KOLLER D, J KIGEL, S OVADIA 1977 A kinetic analysis of the facultativephotoperiodic response in Amaranthus retroflexus. Planta 136: 13-19

5. KOUKKARI WL, WS HILLMAN 1968 Pulvini as the photoreceptors in thephytochrome effect on nyctinasty in Albizzia julibrissin. Plant Physiol 43:698-704

6. LESHEM YY 1977 Sunflower a misnomer? Nature 269: 1027. MOONEY HA, JR EHLERINGER 1978 The carbon gain benefits of solar tracking

in a desert annual. Plant Cell Environ 1: 307-31 18. SATTER RL 1979 Leaf movement and tendril curling. In W Haupt, ME

Fienleib, eds, Encyclopedia of Plant Physiology (NS) Vol VII. Physiology ofMovements. Springer-Verlag, New York, pp 442-484

9. SATTER RL, AW GALSTON 1973 Leaf movement: Rosetta stone of plantbehaviour? BioScience 23: 407-416

10. SCHWARTZA 1980 Physiology ofdiaphototropism in leaves ofLavatera cretica,PhD thesis. Hebrew Univeristy of Jerusalem (Hebrew, with English sum-mary)

1 1. SCHWARTZ A, D KOLLER 1978 Phototropic response to vectorial light in leavesof Lavatera cretica. Plant Physiol 61: 924-928

12. SCHWARTZ A, D KOLLER 1980 Role of the cotyledons in the phototropicresponse of Lavatera cretica. Plant Physiol 66: 82-87

13. SIMON E, RL SATTER, AW GALSTON 1976 Circadian rhythmicity in excisedSamanea pulvini. Plant Physiol 58: 421-425

14. VARLET GRANCHER C, R BONHOMME 1972 Utilization de l'energie solaire parune culture de Vigna sinensis. I. Etude theorique de l'influence de l'heliotrop-isme des feuilles sur l'interception des rayonnements. Ann Agron 23: 407-417

15. VOGELMANN TC 1984 Site of light perception and motor cells in a sun-trackinglupine (Lupinus succulentus). Physiol Plant 62: 335-340

16. WAINWRIGHT CM 1977 Sun-tracking and related leaf movements in a desertlupine (Lupinus arizonicus). Am J Bot 64: 1032-1041

17. WATANABE S, T SIBAOKA 1973 Site of photo-reception to opening response inMimosa leaflets. Plant Cell Physiol 14: 1221-1224

18. YIN HC 1938 Diaphototropic movement of the leaves of Malva neglecta. AmJ Bot 25: 1-6

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