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Copper (Cu) is an essential element that plays a very important role in plant growth and development. Its accumulation in soil is now considered a serious environmental problem endangering agricultural production and human health. In this study, the influence of exogenous nitric oxide (NO) on photosynthesis, Cu, magnesium (Mg), and iron (Fe) contents was studied in tomato seedlings under Cu stress grown in hydroponic culture. The results showed that the addition of sodium nitroprusside (SNP), a donor of exogenous NO, significantly alleviated photosynthetic inhibition in Cu-stressed tomatoes, including the recovery of photosynthetic pigment content and chlorophyll a fluorescence and the improvement of net photosynthetic rate, stomatal conductance, and transpiration rate. Application of exogenous NO by SNP reduced the excessive accumulation of Cu and recovered Mg and Fe contents in Cu-stressed tomatoes. This positive involvement of exogenous NO in tomato tolerance to Cu stress was further confirmed by addition of 2-(4-carboxyphenyl)-4,4,5,5- tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a specific NO scavenger, which resulted in further reduction of photosynthesis and more severe disturbance of ion homeostasis in Cu-stressed seedlings. These results indicate that rebalance of ion content was involved in alleviation of photosynthesis inhibition in Cu-stressed tomatoes by exogenous NO.

The effects of 2 nitrate levels, 14 (CK) and 140 mmol L-1 (T), on the leaf gas exchange variables of cucumber (Cucumis sativus L. cv. Xintaimici) seedlings grown in hydroponic culture were investigated. Photosynthetic light- and CO2-response curves from CK and T seedlings were determined and used for the analysis of photosynthetic capacity. The results showed that nitrate stress resulted in a significant reduction of net photosynthesis of T seedlings compared with CK. At the same time, the apparent quantum yield, light-saturated net photosynthesis, carboxylation efficiency, and CO2-saturated net photosynthesis in nitrate-stressed cucumbers also decreased significantly with the increase of treatment time. At 12 days, the apparent quantum yield, light-saturated net photosynthesis, carboxylation efficiency, and CO2-saturated net photosynthesis in nitrate-stressed cucumbers were 47%, 60%, 64%, and 54% lower than CK, respectively. The relative effect of stomatal resistance on photosynthesis (S%) in nitrate-stressed cucumber seedlings increased significantly during the first 8 days and returned to the level of CK thereafter. This evidence indicates that not only stomatal but also nonstomatal limitations might be involved in the reduction of net photosynthetic rate in nitrate-stressed seedlings.