Photosynthetic gas exchange, chlorophyll fluorescence and some associated metabolic changes in cowpea ( Vigna unguiculata) during water stress and recovery

Abstract

The responses of photosynthetic gas exchange and chlorophyll fluorescence along with changes in carbohydrate and proline levels were studied in cowpea ( Vigna unguiculata) during water stress and recovery. Three experiments were conducted under greenhouse and laboratory conditions. Decreased CO 2 assimilation rates during water stress were largely dependent on stomatal closure, which reduced available internal CO 2 and restricted water loss through transpiration. During the initial phase of stress, photochemical activity was not affected, as revealed by lack of alterations in fluorescence parameters associated with photosystem II (PSII) activity. Development of non-radiative energy dissipation mechanisms was evidenced during stress by increases in non-photochemical quenching and decreases in efficiency of excitation capture by open centers. At an advanced phase of stress, a down-regulation of PSII activity was observed along with some impairment of photochemical activity, as revealed by decreases in the maximum quantum yield of PSII (Fv/Fm). However, this impairment did not limit the overall photosynthetic process, since assimilation rates recovered, upon rewatering, independent of the still present decreased Fv/Fm values. Complete recovery of all gas exchange and fluorescence parameters occurred 3 days after rewatering. However, on the first day after water stress relief, assimilation rates only partially recovered in spite of the availability of internal CO 2, suggesting some non-stomatal limitation of photosynthesis. Accordingly, the downregulation of PSII activity observed during stress persisted at this time. Our results on carbohydrate metabolic changes revealed an accumulation of soluble sugars in water-stressed leaves, which also persisted for 1 day after rewatering. This finding suggest a transient end-product inhibition of photosynthesis, contributing to a minor non-stomatal limitation during stress and initial phase of recovery. Increases in proline level were small and their onset was delayed after stress imposition, so that it may rather be a consequence and not a stress-induced beneficial response.