Effects of low nitrogen supply on relationships between photosynthesis and nitrogen status at different leaf position in wheat seedlings

Hydroponic experiments were conducted to investigate the effects of low nitrogen (N) nutrition on photosynthesis and its relationships with N status in wheat (Triticum aestivum L.). Two wheat cultivars, Zaoyangmai and Yangmai158, differing in low N nutrition tolerances, were used. The results show that under low N nutrition the area of the first top leaf was significantly reduced, while there was no significant difference in the top second and third leaf areas compared with the control for either cultivar. The net photosynthetic rate and chlorophyll content were significantly reduced in the top three leaves of Zaoyangmai, while no significant difference in these factors was observed in the top first and second leaves of Yangmai158 compared with control under N-limited conditions. The effective quantum yield of photosystem II (PSII) photochemistry and the maximal quantum yield of PSII photochemistry were only slightly altered in both cultivars, indicating that PSII was not damaged by low N nutrition. In addition, the non-photochemical quenching coefficient increased significantly in the top three leaves of Zaoyangmai, and only in the top third leaf of Yangmai158 under N-limited conditions. Furthermore, the ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and soluble protein contents decreased significantly in the top second and third leaves of Zaoyangmai, while no significant difference was observed in the top first and second leaves of Yangmai158 between low N nutrition and control. We concluded that in Yangmai158, N status changed less, and it maintained almost normal photosynthesis in young leaves, thus Yangmai158 could be more tolerance to low N nutrition.