Laser-induced Chlorophyll Fluorescence Ratio in Certain Plants Exhibiting Leaf Heliotropism

Abstract

Laser-induced F₆₈₅/F₇₂₀ chlorophyll fluorescence of intact leaves in solar tracking plants from Euphorbiaceae (Ricinus communis L.), Amaranthaceae (Amaranthus gangeticus L., Amaranthus viridis L.), Malvaceae (Hibiscus cannabinus L., Althaea rosea L.), Fabaceae (Dolichos lablab L.) Capparadaceae (Cleome gynandra L.) exhibited remarkable diurnal constancy in contrast to those of compass plants from Poaceae (Oryza sativa L.) and Amaranthaceae (Telanthera ficoides Moq.) with fixed leaf orientation. The F₆₈₅/F₇₂₀ ratio remained constant in both the diaheliotropic and paraheliotropic plants, but it decreased during midday in compass plants. The acquired spectra were analysed in terms of Gaussian parameters to determine the relative intensity of contribution of various bands; the data suggested a decrease in both the F₆₈₅ and the F₇₂₀ bands at 1230 h in compass plants while they remained unaltered in reorienting leaves. The typical midday depression in F₆₈₅/F₇₂₀ ratio observed in compass plants may be due to down-regulation of both photosystems, I and II. This unique behaviour of diurnal constancy in laser-induced F₆₈₅/F₇₂₀ suggests that the leaves of diaheliotropic plants maximise light-use efficiency throughout the day and avoid the hazard of midday depression of photosynthesis.