PHOTOCONTROL OF STEM ELONGATION IN PLANT NEIGHBORHOODS - EFFECTS OF PHOTON FLUENCE RATE UNDER NATURAL CONDITIONS OF RADIATION

We investigated the effects of photon fluence rate on internode elongation in fully de-etiolated plants growing under sunlight. Our goal was to find out whether perception by the stems of fluence rate changes related to canopy density may be involved in the modulation of internode growth in canopies formed by plants of similar stature (e.g. crop stands). Using Datura ferox L. and Sinapis alba L. seedlings growing under natural radiation, we found that internode elongation is promoted by localized shading. This effect was observed with internodes receiving light with a high (> 0.9) or a low (0.3) red (R) to far-red (FR) photon ratio. Selective removal of the different wavebands from the light impinging on the internodes showed that part of the response to fluence rate is due to photons in the R + FR range. The blue (B) component, most likely acting through a specific photoreceptor, also inhibited elongation. However, changes in the fluence rate of B light did not have detectable effects on the response of the internodes to R:FR ratio. Fibre-optic studies and measurements with integrating-cylinder sensors in even-aged populations of seedlings showed that both the quality and quantity of radiation received by the stems are profoundly influenced by changes in canopy density. When density is very low (leaf area index = LAI less-than-or-equal-to 1) only the R:FR ratio is reduced, due to FR reflected from nearby leaves. In the LAI range of 1 to 2, though a large proportion of the leaf area is still receiving full sunlight, the photon fluence rate at the stem level drops dramatically. These results suggest that in even-aged populations of LAI > 1 elongation growth is promoted by the low R:FR ratio and the reduced fluence rate. Perception of these two factors at the stem level may elicit morphological adaptations in the canopy before the onset of severe competition among neighbours for the resource of light.