Electron flux in photosystem II and photon/electron sinks in wild and domesticated watermelon plants during the progress of drought stress

Abstract

Responses of activities of the enzymes involved in electron sinks in photosynthesis and of electron flux in photosystem II (PS-II) to drought under illumination at 1000 µmol photons m-2 s-1 were analyzed with wild and domesticated watermelon plants. Although the different responses of both species to water deficit were revealed in the water status and morphological appearance of leaves, the rates of transpiration and net CO2 assimilation gradually decreased to almost zero in the first 3 days. While RuBisCO showed a gradual decrease in activity in the prolonged stress, the activation states of the enzyme and fructose bisphosphatase were kept high during the experimental period in both species. Wild and domesticated watermelon plants induced chloroplast ascorbate peroxidase, but not the cytosolic enzyme, during drought stress. High activities of monodehydroascorbate reductase and other enzymes involved in regeneration of ascorbate might cause the reduction levels of ascorbate and glutathione to be kept at high levels. Although the photochemical efficiency of photosystem II was not damaged in both species under drought for 5 days, the flux of electrons in PS II was regulated in different ways in both species. Wild watermelon leaves repressed the electron flux in PS II to the level, which was persisted after the cessation of the net CO2 assimilation, earlier than the domesticated species. The level of down-regulation of the flux was closely related to the strengths of non-photochemical quenching of both species.