Effect of temperature on photosynthetic activities of senescing detached wheat leaves

Abstract

Changes in various components of photosynthetic activity during the dark induced senescence of detached wheat leaves, maintained at 25°C (control) and 35°C (mildly elevated temperature treatment), were examined. Senescence-associated decline measured up to 96 h, in photosynthetic activity was appreciably hastened at 35°C, than at 25°C as evident by the relative higher losses of chlorophyll, photosystem (PS) II and PS I catalyzed photochemical activities and ribulose-1,5-bisphosphate (RuBP) carboxylase activity. In addition, a comparatively higher rise in light scattering profile of isolated chloroplasts was noted at 35°C than at 25°C. Senescence-induced degradation of chlorophyll was faster at 35°C than at 25°C; on the other hand, the degradation of carotenoids was faster at 25°C than at 35°C. Furthermore, the ratio of carotenoids to chlorophyll increased with senescence up to 96 hours, higher ratio being obtained at 35°C than at 25°C. Both PS II and PS I activities showed a transient rise in the beginning phase of dark incubation, whereas loss in chlorophyll was continuous throughout the period of senescence. The initial rise observed in photochemical activities was attributable to the uncoupling of electron transport from photophosphorylation. Elevated temperature treatment resulted in greater inactivation of RuBP carboxylase than control. It appears that during senescence the loss in chlorophyll and RuBP carboxylase activity are triggered simultaneously.