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

摘要

Plants have several mechanisms to weaken oxidative stresses brought about by the excess photon energy under drought. In addition to such morphological responses to drought as curling of leaves, plants have biochemical protection mechanisms against oxidative stresses under drought. To weaken the damages driven by the excess photon energy not utilized by the photosynthetic carbon reduction (PCR) cycle, photosystem II (PSII) dissipates the energy as heat (Deming-Adams and Adams 1992). This process isdriven by accumulation of protons in the thylakoid lumen and can be detected as nonphotochemical quenching (NPQ). The cyclic electron transport system within PSII and the xanthophyll cycle participate in the dissipation of the excess photons with and without the involvement of the reaction center of PSII, respectively (Miyake and Yokota, 2001). In this context, the biochemical events for the excess photon energy dissipation may be referred to as "photon sink". Exposed to drought, furthermore, plants increase the activities of superoxide dismutatse (SOD) and ascorbate peroxidase (APX) (Smirnoff 1993) and the amount of antioxidants, glutathione (GSH) and alpha- tocopherol. These increases in the antioxidative activities contribute to lower the steady-state concentration of O_2~- and H_2O_2, both of which are produced at PSI, and to prevent the PCR enzymes and thylakoidal lipids from oxidative inactivation and peroxidation, respectively (Asada 1999).