Comparative analysis revealed that a xantha rice mutant (cv. Huangyu B) had higher ratios of chlorophyll (Chl) a/b and carotenoids/Chl, and higher photosynthetic efficiency than its wild type parent (cv. II32 B). Unexpectedly, the mutant had higher net photosynthetic rate (P N) than II32 B. This might have resulted from its lower non-photochemical quenching (qN) but higher maximal photochemical efficiency (FV/FM), higher excitation energy capture efficiency of photosystem 2 (PS2) reaction centres (FV′/FM′), higher photochemical quenching (qP), higher effective PS2 quantum yield (ΦPS2), and higher non-cyclic electron transport rate (ETR). This is the first report of a chlorophyll mutant that has higher photosynthetic efficiency and main Chl fluorescence parameters than its wild type. This mutant could become a unique material both for the basic research on photosynthesis and for the development of high yielding rice cultivars.

The effects of water deficit on photochemical parameters and activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) were investigated in two cultivars of rice. The soil water content fell to about 40% of field water capacity after withholding irrigation for 14 d. As a result, the net photosynthetic rate (Pn) was significantly depressed in both cultivars. The maximum photochemical efficiency (F_v/F_m) remained unchanged. The photochemical quenching coefficient (qP) and the actual quantum yield of electron transport rate of PSⅡ(Φ_(PSⅡ)) in cultivar IR 1552 were not different between plants under water deficient condition and the irrigated control, but their SOD, CAT and POD activities of the drought-stressed plants were significantly higher. . However, those photochemical parameters in cultivar Azucena were significantly lower after withholding irrigation and all antioxidant enzyme activities decreased. The results indicated that there was different response to water stress between two rice cultivars.

After rice plants (japonica rice Azucena) were treated with various concentrations of polyethylene glycol (PEG) for 16 days, characteristics of gas exchange, chlorophyll fluorescence, MDA content and activities of antioxidant enzymes (SOD, POD and CAT) in the leaves were measured. The reasons for photosynthetic declination caused by different PEG concentrations were remarkably different, varying in three phases. In the first phase, under 10% PEG treatment, net photosynthetic rate (P_(n)) significantly decreased, but stomatal conductance (G_(s)) and intercellular CO_(2) concentration (C_(i)) increased slightly, while chlorophyll fluorescence parameters and photosynthetic pigment content showed little change, suggesting that the decrease in P_(n) was not due to the stomatal limitation. In the second phase, under 15% PEG treatment, the pigment content and fluorescence parameters began to change but P_(n) decreased with a significant decrease in G_(s) and C_(i), indicating G_(s) became a main factor limiting P_(n). In the last phase, under 20% PEG treatment, P_(n), G_(s), fluorescence parameters and pigment content fell signifiantly further, whereas C_(i) exhibited no further decrease, demonstrating that the decline in P_(n) was largely attributed to the damage of photosynthetic membrane. Although severe water stress caused the reduction of the abilities for harvesting and transferring light energy, the excess excitation energy still occurred and might lead to the production of reactive oxygen species triggering the antioxidant system. However, it could still damage photosynthetic apparatus and result in the reduction of photochemical activity of PS II.

Transgenic rice plants with an antisense gene inserted via Agrobacterium tumefaciens were used to explore the impact of the reduction of Rubisco activase (RCA) on Rubisco and photosynthesis. In this study, transformants containing 15% to 35% wild type Rubisco activase were selected, which could survive in ambient CO2 concentration but grew slowly compared with wild type controls. Gas exchange measurements indicated that the rate of photosynthesis decreased significantly, while stomatal conductance and transpiration rate did not change; and that the intercellular CO2 concentration even increased. Rubisco determination showed that these plants had approximately twice as much Rubisco as the wild types, although they showed 70% lower rate of photosynthesis, which was likely an acclimation response to the reduction in Rubsico activase and/or the reduction in carbamylation.