Banana ( Musa sp.) leaf gas exchange and chlorophyll fluorescence in response to soil drought, shading and lamina folding

Abstract

Reductions in net photosynthesis of banana ( Musa sp.) leaves may be due to both stomatal and non-stomatal limitations. To evaluate the significance of photochemical damage, we measured chlorophyll fluorescence in conjunction with net photosynthesis. Photochemical damage was reflected in either an increase in original fluorescence ( F O), or decreases in maximum fluorescence ( F M) or in the ratio of variable ( F V= F M− F O) to maximal fluorescence ( F V/ F M). The leaf surface receiving direct sunlight (adaxial) was more photochemically damaged, as measured by declines in chlorophyll fluorescence parameters than the abaxial leaf surface that received transmitted and indirect light. Damage increased over a diurnal period and was greater in droughted compared with irrigated plants. F V/ F M ratio declined significantly as plant water status, measured as relative leaf water content or leaf water potential, declined. The decrease in the F V/ F M ratio was due more to a decline in F M than an increase in F O. Shading of plants decreased photochemical damage in leaves but net photosynthesis ( P n) was lower than in sunlit plants because of the limitation of P n by low photosynthetic photon flux density. A combination of stomatal and non-stomatal factors reduced P n in droughted or shaded plants. Excessive sunlight can cause photochemical damage to banana leaves but this damage does not appear to be a limitation of P n in well-irrigated plants. The natural folding of banana laminae (not caused by wilting) reduces photochemical damage by reducing the flux density of radiation intercepted by the lamina surface.