Adjusting leaf nitrogen allocation could promote photosynthetic capacity, and nitrogen accumulation in Cucumis sativus L.

Nitrogen (N) is essential for crop growth and development under stress. The photosynthetic capacity of crops depends on the leaf N content (Na), and leaf N allocation is an important factor affecting net photosynthetic rate (Pn). However, the relationship between N supply and leaf N allocation and the effects of different N forms on cucumber growth at low-temperature (LT) stress are still unclear. In this study, the effects of different N concentrations (NC) and ammonium (NH4+)-nitrate (NO3−) ratios (A-NR) supplied to grown LT-stressed cucumber at seedling and flowering to early fruiting stages on growth, photosynthesis, N accumulation, and leaf N allocation were assessed. NC, A-NR, and their interactions significantly affected leaf area, Pn, chlorophyll content, and N accumulation at seedling and flowering to early fruiting stages. When the cucumber plants were grown in LT environment, the maximum photochemical efficiency, leaf area, Pn, chlorophyll content, Na, water-soluble proteins N (Nw), and N accumulations in various organs were the greatest. The content of sodium dodecyl sulfate (a detergent) soluble protein N (Ns) was minimal when the NCs at the seedling and flowering to early fruiting stages were 10.5 and 28 mmol·L−1, respectively, and the A-NRs were both 1:1. Correlation analysis showed that leaf area, Pn, chlorophyll content, and N accumulation could be improved by increasing Na and Nw or reducing Ns. This finding suggested that increasing NC and NH4+ proportion in the nutrient solution could increase Na and Nw in leaves and reduce the content of Ns, thereby improving photosynthetic capacity to promote leaf growth and increasing cucumber LT resistance.