Leaf nitrate accumulation influences the photorespiration of rice (Oryza sativa L.) seedlings

Aims
The form of nitrogen (N) supply influences photorespiration in C3 plants, but whether nitrate (NO3−) regulates photorespiration and, if so, the underlying mechanisms for such regulation are still unclear.

Methods
Three hydroponic experiments were conducted in a greenhouse to investigate the relationships between leaf NO3− concentrations and photorespiration rates in rice (Oryza sativa L.) genotypes cv. ‘Shanyou 63’ hybrid indica and ‘Zhendao 11’ hybrid japonica or using mutants that overexpress NRT2.1 (in cv. ‘Nipponbare’ inbred japonica). We estimated photorespiratory rate from the CO2 compensation point in the absence of daytime respiration (Γ*) using the biochemical model of photosynthesis.

Results
Higher Γ* values under high N level or NO3− were significantly and positively correlated with leaf NO3− concentrations. Further elevating leaf NO3− concentrations by either resuming NO3− nutrition supply after N depletion (in cv. ‘Shanyou 63’ hybrid indica and ‘Zhendao 11’ hybrid japonica) or using mutants that overexpress NRT2.1 (in cv. ‘Nipponbare’ inbred japonica) increased Γ* values. Additionally, the activities of leaf nitrate reductase (Nr) and concentrations of organic acids involving in the tricarboxylic acid (TCA) cycle synchronously changed as environmental conditions were varied.

Conclusions
Photorespiration rate is related to the leaf NO3− concentration, and the correlation may links to the photorespiration-TCA derived reductants required for NO3− assimilation.