LASER-INDUCED CHLOROPHYLL FLUORESCENCE RATIO IN CERTAIN PLANTS EXHIBITING LEAF HELIOTROPISM

Laser-induced F-685/F-720 chlorophyll fluorescence of intact leaves in solar tracking plants from Euphorbiaceae (Ricinus communis L.), Amara nthaceae (Amaranthus gangeticus L., Amaranthus viridis L.), Malvaceae (Hibiscus cannabinus L., Althaea rosea L.), Fabaceae (Dolichos lablab L.) Capparadaceae (Cleome gynandra L.) exhibited remarkable diurnal co nstancy in contrast to those of compass plants from Poaceae (Oryza sat iva L.) and Amaranthaceae (Telanthera ficoides Meg.) with fixed leaf o rientation. The F-685/F-720 ratio remained constant in both the diahel iotropic and paraheliotropic plants, but it decreased during midday in compass plants. The acquired spectra were analysed in terms of Gaussi an parameters to determine the relative intensity of contribution of v arious bands; the data suggested a decrease in both the F-685 and the F-720 bands at 1230 h in compass plants while they remained unaltered in reorienting leaves. The typical midday depression in F-685/F-720 ra tio observed in compass plants may be due to down-regulation of both p hotosystems, I and II. This unique behaviour of diurnal constancy in l aser-induced F-685/F-720 suggests that the leaves of diaheliotropic pl ants maximise light-use efficiency throughout the day and avoid the ha zard of midday depression of photosynthesis.