Dov Koller

THE VECTORIAL PHOTO‐EXCITATION IN SOLAR‐TRACKING LEAVES OF Lavatera cretica (MALVACEAE)*,†

Abstract— The diaphototropic responses of the solar‐tracking leaves of Lavatera cretica were studied under constant levels of vectorial photo‐excitation (negligible variations in angle of incidence and fluence rates). The results showed the following:

Action dichroism in perception of vectorial photo-excitation in the solar-tracking leaf ofLavatera cretica L.

The leaf lamina ofLavatera cretica L. exhibits a diaphototropic response that discriminates between two opposite, constant vectorial excitations by white light beams whose fluence rates differ by as little as 10% (50 versus 45 μmol·m−2·S−1). The relationship between the response (angular velocity of laminar reorientation) and the fluence-rate ratio is linear. The lamina similarly discriminates between two such excitations by polarized light, one with the electrical vector transverse to the plane of the two beams (θ) and the opposite one with the vector parallel to that plane (⪙). When two such beams were of equal fluence rate, the lamina reoriented towards the ⪙ beam. When the fluence rate of the θ beam was maintained at 50 μmol·m−2·s−1 and that of the ⪙ beam was reduced, the response to the latter (angular velocity of laminar reorientation) was reduced progressively. Further reduction in the fluence rate of the ⪙ polarized beam eventually resulted in reorientation in the opposite direction (towards the θ beam) and the response to the latter increased progressively with the reduction in fluence rate. The equilibrium was at a ⪙/θ ratio of 0.62. Measurements of reflectance of oblique beams of ⪙ and θ polarized light from the upper laminar surface, and of transmittance of such light ghrough the lamina, eliminated the possibility that optical dichroism of the lamina contributed significantly to these results. The implications of this action dichroism to the postulated mechanism of perception of vectorial excitation by these leaves is discussed.

Phototropic Responses of the Pulvinules and Associated Laminar Reorientation in the Trifoliate Leaf of Bean Phaseolus vulgaris (Fabaceae)

Summary Movements of the terminal and lateral leaflets in the trifoliate leaf of pinto bean (Phaseolus vulgaris L.) cv. Bulgarian, in response to directional light, were studied by kinetic analysis. The pulvinule responds to unilateral light by contraction of the sector that intercepts the light and concomitant expansion of the opposite sector. The rate of the resulting laminar reorientation depends on the excitation of the photo-receptors in the pulvinule, which changes with angle of the incident light and as pulvinar topology changes in response to light. This dependence provides feed-back control of the eventual steady-state of the response. The pulvinule perceives directional light from all sides, but only as unilateral signals, and integrates these signals in its response. Phototropic responses of the pulvinule are modified by its asymmetry, which differs in the terminal and lateral leaflets. The asymmetry is expressed by axial torsion, superimposed on the positive phototropic curvature. The resulting laminar reorientations reflect this asymmetry. Laminar reorientation in directional light is a passive result of the pulvinar response and may combine changes in azimuth and axial rotation with changes in elevation.

Light-driven movements of the trifoliate leaves of bean (Phaseolus vulgaris L.). Spectral and functional analysis

Summary The light-driven responses of the terminal leaflet of bean were analyzed spectrally and functionally. Laminar elevation increases rapidly in response to continuous overhead exposure of its pulvinus to blue light. This response is enhanced in its early stages by simultaneous exposure to red light. The pulvinus responds similarly to continuous overhead unmixed red, or far-red light, albeit at much lower rates. The response to overhead red, alone, or during enhancement of the response to blue, was not affected by simultaneous far-red. However, the response to blue alone, or enhanced by mixture with red, was partially inhibited by simultaneous exposure to far-red. The results suggest that the response to blue resulted mostly from a blue-absorbing pigment system, but may involve some absorption by phytochrome, while responses to red or far-red, with and without blue, may be mediated by high-irradiance responses of phytochrome. Functional differences between the responses to red and blue become apparent when the abaxial (lower), or lateral sectors of the pulvinus are exposed to them, separately and in combination. These differences suggest that red controls the photonastic unfolding of the pulvinus, whereas blue controls its phototropic responses. These responses co-exist in the same tissue, but are separate and additive.

Movements of the trifoliate leaf of bean (Phaseolus vulgaris L.) during a simulated day, and their consequences for solar-tracking fidelity and interception of solar radiation

Summary Movements of the terminal and lateral leaflets in the trifoliate leaf of pinto bean (Phaseolus vulgaris L.) cv. Bulgarian were studied under simulated daytime conditions (12-h photoperiod), during which a light source directed on the leaflet lamina was moved at the angular velocity of the earth (15 deg h-1) in a vertical arc centered on the pulvinule, either along the plane of the mid-vein, or transversely, at right angles to it. The time-course of change in laminar elevation, azimuth and rotation was recorded. These data were used to evaluate the fidelity of solar-tracking and calculate the changes of laminar orientation to the sun (angle of light incidence, AI) throughout the day. Interception of incident light throughout the day was estimated by the ratio between the PPF incident on the lamina and on a perpendicular surface, and was calculated as cosine of the angle of light incidence on the lamina. Leaflet movements contributed to the interception of solar radiation, but did not exhibit significant, or consistent solar-tracking. Calculated integrated interception by all three leaflets was similar in all directions of solar traverse over the trifoliate leaf.

Enhanced diaphototropic response to vectorial excitation in solar-tracking leaves of Lavatera cretica by an immediately preceding opposite vectorial excitation

Summary The rate of the diaphototropic response (angular velocity of laminar reorientation) is constant under a constant level of vectorial excitation by an oblique beam (angle of incidence and fluence rate of light intercepted by the lamina). Laminar elevation decreases when the oblique beam is oriented towards the base of the lamina (BO) and increases when the beam is oriented towards the tip (TO). The responses to TO and BO excitations were enhanced considerably when such excitations were immediately preceded by exposure to equivalent excitations in the opposite directions (BO and TO, respectively). Plants exposed to several cycles of «solar» tracking under simulated conditions were more responsive to vectorial excitation, whether or not it was preceded by an opposite excitation, but the enhancement by a preceding opposite excitation remained the same. Exposure to vectorial excitation enhanced the response to a subsequent opposite excitation even when the lamina was mechanically restrained from responding throughout the first excitation, although the effect was somewhat reduced. Exposure of the leaf to non-vectorial light (normal to the lamina) for the same time and at an equivalent fluence rate as vectorial excitation did not enhance the response to a subsequent vectorial excitation. The results suggest that in addition to providing on-line directional information, vectorial excitation may produce longer-lasting effects on membrane transport.

After-Effects of Vectorial Photo-Excitation in Solar-Tracking Leaves of Lavatera cretica L. — A Kinetic Analysis

Summary Leaves of Lavatera cretica L. were exposed to different constant levels of TO (at Al = +15° +30° or + 60°C), or BO excitation (at AI = −15°, −30°, or −60°), along their midvein. After 90, or 150min, they were transferred to light regimes that were not vectorially effective (white, red or darkness) and the time-course of the subsequent changes in LE was recorded. These changes are the expression of solute (and water) fluxes in the sector of pulvinar motor tissue subtending the midvein, whose associated photoreceptors had been vectorially excited. The diaphototropic response was non-inductive and required continuous VE. When such VE was terminated by darkness, the immediate reorientation was inertial, i.e. its direction was dictated by that of the preceding VE. This short-lived phase was replaced by the primary post-inertial phase, during which LE fell, irrespective of whether the preceding VE had been TO, or BO. However, the subsequent reorientation differed qualitatively when preceded by TO, or BO. Kinetic analysis showed that the post-inertial phases in laminar reorientation were quantitatively modified by the level (AI) and duration of the preceding VE. Thus, the laminar reorientation in darkness was not nyctinastic, but induced by the preceding VE. Non vectorial white or red light, had marked and different effects on the kinetics of both post-inertial phases, suggesting the involvement of different photoreceptors.

A kinetic analysis of the facultative photoperiodic response in Amaranthus retroflexus L.

The ontogenetic change taking place in the facultative photoperiodic response of A. retroflexus to inductive short-day (SD) conditions was studied by exposing plants to continuous induction after different initial exposures to long-days (LD), and comparing the kinetics of their developmental responses (cumulative number of plants with reproductive apices, flowering stage, and height of the apical dome). As the plants progressed from emergence to “autonomous” flowering (i.e., in non-inductive conditions), their response to continuous induction became progressively more rapid. Reproductive development was initiated following a progressively shorter lag-phase after the start of induction, but its subsequent rate remained unchanged. Until the onset of reproductive development, the undifferentiated upper part of the shoot apex (apical dome) elongated much more rapidly in SD than in LD. However, in both cases reproductive development was initiated when the apex had elongated to about the same extent, after which its elongation accelerated considerably, but to similar rates in both photoperiods. The data indicate that progress towards reproductive development takes place in inductive (SD), as well as in non-inductive (LD) photoperiods, but one cycle of the latter is as effective as 0.20–0.25 of a cycle of the former. —Plants induced at different stages in ontogeny started to change their subsequent branching pattern (ratio of leafy to leafless branches) as soon as induction was delayed beyond “autonomous” flowering.

Dynamic aspects of the response of the pulvinus in the leaf of bean plants(Phaseolus vulgaris L.) to photoexcitation

Summary The pulvinus of bean ( Phaseolus vulgaris L) responds to unilateral photo-excitation by phototropic curvature. Osmotically active solutes and water are transported from its exposed to the opposite sector of its motor tissue, resulting in differential changes in turgor pressure in these sectors and generation of a trans-pulvinar torque. A null-point approach was used to non-invasively study these dynamic changes in the terminal leaflet of bean. A variable torque was applied perpendicular to the midrib, to restrain laminar movement by precisely and continuously counteracting the generated torque. This equilibrium prevented curvature of the pulvinus and the associated opposite axial changes in volume in the opposite sectors of its motor tissue. The laminar torques measured were used to estimate stresses (changes in turgor pressure) generated within the motor tissue. These stress values were used to derive the corresponding changes in osmotic pressures and in solute concentration. Skotonastically downfolded leaflets were excited with white light to study their combined dynamic response to photonastic and phototropic excitation. Photonastically unfolded (horizontal) leaflets were excited with blue and red light, alone and in combination, to determine the spectral dependence of the dynamic pulvinar responses tophototropic excitation by itself.

Diaphototropic responses to polarized light in the solar-tracking leaf of Lupinus palaestinus Boiss. (Fabaceae)

Summary The diaphototropic response of the palmately compound leaf lamina of Lupinus palaestinus exhibits dichroic effects in plane polarized light. The lamina discriminates between two opposite oblique (non-polarized) beams, maintained at an equivalent angle of incidence (30°) transverse to its plane of symmetry, when their fluence rate ratio is 0.8 (50 and 40 μmol m -2 s -1 , respectively), by reorienting towards the stronger beam. When fluence rate of the two beams is equal (50 μmol m -1 s -1 ), but one is polarized parallel to their common plane (∥) while its opposite is polarized transversely to that plane (⊥), the lamina reorients towards the ∥ beam and away from the ⊥ beam. When the fluence rate of the ∥ beam is reduced, while that of the ⊥ beam is maintained constant, the angular velocity of laminar reorientation is smaller. Further reduction of the ∥ fluence rate eventually results in reversal of the direction of laminar reorientation, at a progressively increasing angular velocity. The relationship between the diaphototropic response and the ∥/⊥ fluence rate ratio is linear, with equilibrium between the two beams at a value of 0.586, suggesting that the ∥ beam may be ≈ 70 % more effective than the ⊥ beam.