Physiological mechanisms underlying reduced photosynthesis in wheat leaves grown in the field under conditions of nitrogen and water deficiency

Reduced photosynthesis results directly from nitrogen or water deficiency in wheat plants, and leads to a decrease in grain yield. In this study, by measuring the effects of water and N deficiencies, both individually and combined, we characterized the responses of wheat (Triticum aestivum L. Yumai 49–198) plants to these two deficiencies using physiological measurements and comparative proteomics. Significant decreases in grain yield and leaf photosynthetic performance were observed in all deficiency conditions, and 106 photosynthetic proteins that showed responses were identified. Nitrogen deficiency induced the least change in photosynthetic proteins, and similar changes in most of these proteins were also observed for the combined nitrogen and water deficiencies. Water deficiency induced the largest change in photosynthetic proteins and resulted in the lowest 1000-kernel weight. Severe decreases in photosynthesis in both the water-deficiency and combined N and water deficiency groups were reflected mainly in an imbalanced ATP/NADPH ratio associated with the light reaction, which influences carbon metabolism in the Calvin cycle. Photorespiration was respectively stimulated or inhibited by N or water deficiency, while suppression of photorespiratory flux and activation of nitrogen recycling were observed in the combined N and water deficiency treatments. Comparison of photosynthetic proteins between experimental sites suggested that precipitation affected linear electron flow in the photoreaction, and thus photosynthetic efficiency. Our results provide a baseline for future studies of the roles of these photosynthetic proteins in the response to N or water deficiency and their effect on 1000-kernel weight.