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.
Research Award Fund for Outstanding Middle-Aged and Young Scientists of Shandong Province [BS2011NY010]
; Research Fund for the Doctoral Program of Higher Education of China 
; China Agriculture Research System [CARS-25-D]