Photomodulation of Axis Extension in Sparse Canopies

Abstract

A fiber optic probe inserted into plant tissues was used to investigate the effects of canopy density on the light environment in different organs. The red:far-red ratio inside the stem of Datura ferox L. seedlings and the estimated phytochrome photoequilibrium were strongly reduced by the presence of neighbors forming canopies too sparse to cause any mutual shading at the level of the leaves. In such canopies, changes in plant density had limte effects on the light regime inside the leaves of the succulent Aeonium haworthfi (S.D.) Webb et Berth., particularly when the lamina was kept nearly normal to the direct rays of the sun. In field experiments using D. ferox and Sinapis alba L. seedlings, the elongation of the intemodes responded to various types of localized light-quality treatments that simulated different plant densities in sparse canopies. The responses were quantitatively similar to those elicited by changes in plant density. The evidence supports the hypothesis that, in stands formed by plants of similar size, the red:far-red ratio of the light that impinges laterally on the stems is among the earliest environmental cues that allow plants to detect local canopy density and adjust axis extension accordingly.Stem elongation is markedly affected by the presence of neighboring plants, and changes in light quality are currently viewed as an important factor in the perception of canopy density (2,5,14,20). It has recently been shown (1, 2) that in stands formed by individuals ofsimilar size, relatively small density increments in the LAI2 range of 0 to 1 stimulate stem elongation without involving a significant reduction in PAR interception per plant. In these conditions, elongation rate is well correlated with the R:FR ratio of the light parallel to the ground collected by an integrating-cylinder probe standing vertically in the canopy (1,2 3, 6, 16, 21, and stem; 4, 7, 8, 10, 13, 16, 21). In an open canopy, leaves and stems are, to some degree, exposed to direct sunlight and to the light scattered by neighbors but, because of the complicated optics of plant tissues (9, 30), the effects of canopy density on the light conditions within the organs are difficult to predict. Rich et al. (23) found that the amount of light inside internodes of intact seedlings increases markedly when the source of light is moved from above to the side. Thus, the light scattered by adjacent plants might make a greater contribution to the light regime inside the stem than that expected from its low relative contribution to global radiation. If this were the case, a shift in the spectral composition of the light impinging laterally on the axis might affect phytochrome status in the stem, which would be the major site of density perception in sparse canopies. The experiments reported here were carried out to test this hypothesis.In the first part of this paper, we address the influence of canopy density on the light environment of stems and leaves, as evaluated with a fiber optic probe inserted into the plant tissues. In...

Foraging for light: photosensory ecology and agricultural implications

“…This hypothesis was soon supported by a number of studies that demonstrated the following: (I) plants grown under full sunlight in the field respond to augmented FR radiation with increased elongation and apical dominance (Ballare et al 1987); (2) FR provides a reliable signal of the presence of neighbours well before plants experience a reduction in light availability (e.g. Ballare et al 1987); (3) the FR proximity signal is especially strong in the internodes (Ballare, Scopel & Sanchez 1989), which previous work had shown to be very sensitive to R:FR changes (Morgan, O'Brien & Smith 1980); (4) these changes in FR are quantitatively correlated with the density and proximity of the surrounding vegetation (Ballare et al 1989), and (5) filtering out FR from the light received by individual internodes in evenaged canopies can prevent the elongation response to crowding (Ballare et al 1990).…”

Phytochrome-Mediated Detection of Changes in Reflected Light

“…These results suggest that, first, plants may be capable of detecting and reacting not only to actual, but also potential shading; the early detection of nearby vegetation, before the occurrence of actual shading, might play a significant role in the process of adaptation to changing light conditions (1)(2)(3). Second, because relatively small amounts of reflected light are sufficient to elicit a significant response in plants in full sunlight, the detecting system is apparently capable of measuring a weak signal against a much stronger background one.…”

“…stems) are in the best position for the interception of horizontally propagated light, the component in which, according to measurements, the changes in light quality and quantity, resulting from the presence of nearby plants, are more pronounced than in direct light (1,22). Fourth, phytochrome is apparently involved in the detection of changes in reflected light (1)(2)(3)22).…”

“…Recent field studies have shown that relatively small amounts ofsunlight reflected by nearby plants can bring about a significant increase of stem elongation in seedlings in full sunlight, before the occurrence of any significant shading (1,2); the increase in stem elongation was closely correlated with the decrease in R:FR of the reflected light (1,2). These results suggest that, first, plants may be capable of detecting and reacting not only to actual, but also potential shading; the early detection of nearby vegetation, before the occurrence of actual shading, might play a significant role in the process of adaptation to changing light conditions (1)(2)(3).…”

“…Light-grown plants were used in the field studies (1, 2), and spectrophotometric measurements of phytochrome are not possible in green tissues (17). Projected values, calculated from internal light measurements (2), and spectrophotometric assays in phytochrome solutions exposed to the light reflected by a tobacco canopy (22) Table I.…”