Leaf Age Dependence of Chlorophill Fluorescence Parameters in Water-stressed Leaves of Phaseolus vulgaris L.

Abstract

To investigate leaf age dependence in the responses of photosynthetic functions to water stress, the changes in fluorescence parameters of bean (Phaseolus vulgaris L.) leaves were examined while the leaf water potential (Ψ_LW) was gradually decreased by withholding water supply to the soil; leaves of three different ages were used from each plant. No significant leaf age dependence was detected in any fluorescence parameters when the Ψ_LW was higher than -0.9MPa, the point at which the net CO₂ uptake rate approached zero. However, at Ψ_LW lower than -0.9MPa, both the maximum PS II photochemical efficiency (F_V/F_M) and the quantum yield of non-cyclic electron transport in PS II (φ_e) decreased with decreasing Ψ_LW. The decreases in both F_V/F_M and φ_e were larger in younger leaves. More conspicuous leaf age dependence was detected in the response of the steady-state nonphotochemical quenching coefficient (q_N). Water stress-induced changes in all these fluorescence parameters recovered to pre-stress levels within a day after rewatering. The results showed that the changes in the fluorescence parameters did not result from damage to the photosynthetic systems, but from changes in physiological conditions caused by the cessation of CO₂ supply to the stroma or changes in stromal water status. It was concluded that leaf age is an important factor in determining the responses of fluorescence parameters to water stress.