SPECIES-SPECIFIC DIURNAL CHANGES IN CHLOROPHYLL FLUORESCENCE IN TROPICAL DECIDUOUS AND EVERGREEN PLANTS GROWING IN THE FIELD DURING SUMMER

Diurnal changes in chlorophyll (Chi) a fluorescence characteristics of sun-exposed and shade leaves of some tropical and temperate tree spec ies growing under field conditions on sunny and cloudy days during sum mer were recorded. Significant reduction in photochemical efficiency o f photosystem (PS) 2 measured as F-v/F-m ratio during periods of maxim um irradiance was observed in sun-exposed leaves of some plants. The e xtent of this reduction was probably a function of the plant species. Among the tropical species tested, Boehmeria rugulosa, Celtis australi s, Dalbergia sissoo, Eugenia jambolana and Grewia optiva showed maximu m reduction (15-22 %), whereas species like Ficus racemosa and Sapium sebiferum showed least decline (< 7 %) in the F-v/F-m ratio. On the ot her hand, no significant reduction in this ratio was observed in Bauhi nia purpurea. Temperate species, viz. Acer oblongum and Aesculus indic a, were most susceptible to high irradiances, Prolonged exposure of th ese plants to high irradiances resulted in lesion development in their leaves characterized by reduction in pigment contents and F-v/F-m rat io. Plant species with considerable midday decline in F-v/F-m ratio sh owed (1) slight but significant rise in initial fluorescence (F-0), (2 ) pronounced decrease in F-m value, and (3) significant reduction in a rea over the curve between F-0 and F-m indicating the pool size of ele ctron accepters on the reducing side of PS2. The depression in F-v/F-m ratio though reversible could not be alleviated even after watering t he plants at hourly intervals. No reduction in F-v/F-m was observed in shade leaves of the same plants and/or when measurements were taken o n cloudy days. Hence high solar radiation and high atmospheric evapora tive demand existing during summer period could probably be the major contributory factors for this reduction.