Determination of the Rate of Photoreduction of O2 in the Water-Water Cycle in Watermelon Leaves and Enhancement of the Rate by Limitation of Photosynthesis

Abstract

A study was performed to determine how the electron fluxes for the photosynthetic carbon reduction (PCR) and the photorespiratory carbon oxidation (PCO) cycles affect the photoreduction of O2 at PSI, which is the limiting step 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(PSII)], 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 CO2 and O2 and 1,100 micromol photons m(-2)s(-1). This observation suggested the existence of an alternative electron flux (Ja). Ja was divided into O2-dependent [Ja(O2-depend)] and O2-independent [Ja(O2-independ)] components. The magnitude of half Ja(O2-depend), 7.5 to 9.5 micromol e- m(-2)s(-1), and its apparent Km for O2, about 8.0 kPa, could be accounted for by the photoreduction of O2 at PSI either mediated by ferredoxin or catalyzed by monodehydroascorbate reductase. The results indicated that Ja(O2-depend) was driven by the water-water cycle. A decrease in the intercellular partial pressure of CO2 from 23 to 5.0 Pa at 21 kPa O2 enhanced Ja(O2-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 indicate the electron flux in PSII that exceeds the flux required for the PCR and PCO cycles induces the photoreduction of O2 in the water-water cycle.

The relationship between CO2 assimilation, photosynthetic electron transport and water–water cycle in chill‐exposed cucumber leaves under low light and subsequent recovery

“…Furthermore, oxygenase has a relatively low affinity for O 2 compared with Mehler reactions, thus allowing these to proceed more effectively at low O 2 (Badger et al 2000). It has been argued that the water-water cycle is largely suppressed at 2% O 2 pressure (Wu, Neimanis & Heber 1991;Brestic et al 1995;Lovelock & Winter 1996;Kingston-Smith, Thomas & Foyer 1997;Miyake & Yokota 2000). Accordingly, cyclic flow of electron within photosystems might be involved in the increase in F PSII /F CO2 after the chill.…”

“…1998). Leaf absorption ( a ) was measured as described by Miyake & Yokota 2000). A/C i curves were also measured (von Caemmerer & Farquhar 1981) between 2000 and 0 mmol mol -1 CO 2 while leaf temperature and PPFD were maintained at 25 ∞C and 1200 mmol m -2 s -1 , respectively.…”

“…The rate of oxygenation by Rubisco (V o ) was estimated following von Caemmerer & Farquhar (1981) and the rate of carboxylation by Rubisco (V c ) was estimated as described by Miyake & Yokota (2000). Under atmospheric conditions, the electron fluxes in the two cycles can be expressed as Je(PCR) = 4 ¥ V c and Je(PCO) = 4 ¥ V o , respectively (Krall & Edwards 1992).…”

“…Under atmospheric conditions, the electron fluxes in the two cycles can be expressed as Je(PCR) = 4 ¥ V c and Je(PCO) = 4 ¥ V o , respectively (Krall & Edwards 1992). An alternative flux, Ja, caused by electrons that are not used by the PCR and/or PCO cycles in the total electron flux driven by PSII, can be estimated from Je(PSII) -Je(PCR + PCO) (Miyake & Yokota 2000). Ja(O 2 -dependent) was estimated from the differences between Ja (20% O 2 ) and Ja (2% O 2 ) (Miyake & Yokota 2000).…”

“…An alternative flux, Ja, caused by electrons that are not used by the PCR and/or PCO cycles in the total electron flux driven by PSII, can be estimated from Je(PSII) -Je(PCR + PCO) (Miyake & Yokota 2000). Ja(O 2 -dependent) was estimated from the differences between Ja (20% O 2 ) and Ja (2% O 2 ) (Miyake & Yokota 2000). Then, Ja(O 2 -independent) was estimated from the differences between Ja and Ja(O 2 -dependent).…”