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To address the issue of water eutrophication and to use water more effectively, we conducted experiments on rice (Oryza sativa L.) grown in floating culture. From 2009 to 2011, we compared the photosynthesis and root characteristics of the rice, hybrid line Zhuliangyou 02, grown under a conventional tillage and in a floating culture in Huaihua, the home of hybrid rice. Rice in the floating culture showed a higher net photosynthetic rate and stomatal conductance than that under the conventional tillage. The activities of phosphoenolpyruvate carboxylase and NADP-malic enzyme were 32 and 28% higher, respectively, in rice in the floating culture than under the conventional tillage. Rice in the floating culture also showed significantly greater number of roots, root activity, and antioxidant enzyme activity than that under the conventional tillage. Compared with rice under the conventional tillage, rice in the floating culture had 18 and 24% higher tiller number and effective panicle number, respectively. These results suggested that the floating culture system can promote rice production through enhancing root absorption, increasing effective panicle number, and improving the photosynthetic rate. In addition, rice cultivated in the floating culture could remove excess nutrients from water, which addresses the problems of a lack of arable land and water pollution.

Photosynthetic electron flux allocation, stomatal conductance, and the activities of key enzymes involved in photosynthesis were investigated in Rumex K-1 leaves to better understand the role of nitric oxide (NO) in photoprotection under osmotic stress caused by polyethylene glycol. Gas exchange and chlorophyll fluorescence were measured simultaneously with a portable photosynthesis system integrated with a pulse modulated fluorometer to calculate allocation of photosynthetic electron fluxes. Osmotic stress decreased stomatal conductance, photosynthetic carbon assimilation, and nitrate assimilation, increased Mehler reaction, and resulted in photoinhibition. Addition of external NO enhanced the stomatal conductance, photosynthetic rate, activities of glutamine synthetase and nitrate reductase, and reduced Mehler reaction and photoinhibition. These results demonstrated that osmotic stress reduced CO 2 assimilation, decreasing the use of excited energy via CO 2 assimilation which caused significant photoinhibition. Improving stomatal conductance by the addition of external NO enhanced the use of excited energy via CO 2 assimilation. As a result, less excited energy was allocated to Mehler reaction, which reduced production of reactive oxygen species via this pathway. We suppose that Mehler reaction is not promoted unless photosynthesis and nitrogen metabolism are prominently inhibited.

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