Determination of the rate of photoreduction of O-2 in the water-water cycle in watermelon leaves and enhancement of the rate by limitation of photosynthesis

A study was performed to determine how the electron fluxes for the photosyn thetic carbon reduction (PCR) and the photorespiratory carbon oxidation (PC O) cycles affect the photoreduction of O-2 at PSP, which is the limiting st ep in the water-water cycle. Simultaneous measurements were made of CO2-gas exchange, transpiration and quantum yield of PSII [Phi(PSII)] using leaves of watermelon (Citrullus lanatus). The total electron flux in PSII [Je(PSI I)], as estimated from Phi(PSII), was always larger than the total electron flux required for the PCR and PCO cycles at various partial pressures of C O2 and O-2 and 1,100 mu mol photons m(-2) s(-1). This observation suggested the existence of an alternative electron flux (Ja). Ja was divided into O- 2-dependent [Ja(O-2-depend)] and O-2-independent [Ja(O-2-independ)] compone nts. The magnitude of half Ja(O-2-depend), 7.5 to 9.5 mu mol e(-) m(-2) s(- 1), and its apparent K-m for O-2, about 8.0 kPa, could be accounted for by the photoreduction of O-2 at PSI either mediated by ferredoxin or catalyzed by monodehydroascorbate reductase. The results indicated that Ja(O-2-depen d) was driven by the water-water cycle. A decrease in the intercellular par tial pressure of CO2 from 23 to 5.0 Pa at 21 kPa O-2 enhanced Ja(O-2-depend ) by a factor of 1.3. Saturation of the activities of both the PCR and PCO cycles by increasing the photon flux density induced Ja. These results indi cate the electron flux in PSII that exceeds the flux required for the PCR a nd PCO cycles induces the photoreduction of O-2 in the water-water cycle.