Protection effect of nitric oxide on photosynthesis in rice under heat stress

The effect of exogenous applied nitric oxide on photosynthesis under heat stress was investigated in rice seedlings. High temperature resulted in significant reductions of the net photosynthetic rate (PN) due to non-stomatal components. Application of nitric oxide donors, sodium nitroprusside (SNP) or S-nitrosoglutathione (GSNO), dramatically alleviated the decrease of PN induced by high temperature. Chlorophyll fluorescence measurement revealed that high temperature caused significant increase of the initial fluorescence (Fo) and non-photochemical quenching (NPQ) whereas remarkable decrease of the maximal fluorescence (Fm), the maximal efficiency of PSII photochemistry (Fv/Fm), the actual PSII efficiency (ΦPSII), and photochemical quenching (qp). In the presence of SNP or GSNO pretreatment, the increase of Fo and decrease of Fm, Fv/Fm, ΦPSII and qp were markedly mitigated, but NPQ was further elevated. Moreover, with SNP or GSNO pretreatment, H2O2 accumulation and electrolyte leakage induced by heat treatment were significantly reduced, whereas zeaxanthin content and carotenoid content relative to chlorophyll were elevated. The potassium salt of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a specific NO scavenger, arrested NO donors mediated effects. These results suggest that NO can effectively protect photosynthesis from damage induced by heat stress. The activation effect of NO on photosynthesis may be mediated by acting as ROS scavenging, or/and alleviating oxidative stress via maintaining higher carotenoid content relative to chlorophyll or/and enhancing thermal dissipation of excess energy through keeping higher level of zeaxanthin content under heat stress.