Water-water cycle involved in dissipation of excess photon energy in phosphorus deficient rice leaves

The water-water cycle which may be helpful for dissipating the excitation pressure over electron transport chain and minimizing the risk of photoinhibition and photodamage was investigated in rice after 10-d P-deficient treatment. Net photosynthetic rate decreased under P-deficiency, thus the absorption of photon energy exceeded the energy required for CO2 assimilation. A more sensitive response of effective quantum yield of photosystem 2 (Phi(PS2)) to O-2 concentration was observed in plants that suffered P starvation, indicating that more electrons were transported to O-2 in the P-deficient leaves. The electron transport rate through photosystem 2 (PS 2) (J(f)) was stable, and the fraction of electron transport rate required to sustain CO2 assimilation and photorespiration (J(g)/J(f)) was significantly decreased accompanied by an increase in the alternative electron transport (J(a)/J(f)), indicating that a considerable electron amount had been transported to O-2 during the water-water cycle in the P-deficient leaves. However, the fraction of electron transport to photorespiration (J(o)/J(f)) was also increased in the P-deficient leaves and it was less sensitive than that of water-water cycle. Therefore, water-water cycle could serve as an efficient electron sink. The higher non-photochemical fluorescence quenching (q(N)) in the P-deficient leaves depended on O-2 concentration, suggesting that the water-water cycle might also contribute to non-radiative energy dissipation. Hence, the enhanced activity of the water-water cycle is important for protecting photosynthetic apparatus under P-deficiency in rice.