Interactive effects of nitrate-ammonium ratios and temperatures on growth, photosynthesis, and nitrogen metabolism of tomato seedlings

•We examined growth, photosynthetic rate, and nitrogen metabolism in tomato seedlings.•The ratio of nitrate and ammonium and temperatures interact to influence growth.•Proper supply of ammonium and nitrate can improving chilling tolerance.

Growth of many species of plants is optimal when the two major forms of N that are assimilated by plants are supplied at a particular ratio. This ratio is affected by both species and the environment. We assessed the effects of different ratios of nitrate to ammonium (N-A ratio) supplied to hydroponically grown chilling stressed and non-stressed tomato seedlings on several parameters. When the plants were grown in normal temperature (14-30 °C), growth parameters, photosynthetic rate, chlorophyll concentration, soluble protein in roots, and leaf nitrates were greatest when the N-A ratio was 75:25. The activities of glutamine synthetase (GS) and NADH-dependent glutamate synthetase (NADH-GOGAT) in leaves were maximal when the N-A ratio was 50:50, while NADH-GOGAT in roots was maximal when the ratio was 25:75. Soluble protein in leaves and NO3− content in roots and nitrate reductase activity were positively correlated with N-A ratio, while free amino acids, total N, and NH4+ content were negatively with this ratio. Under chilling temperature (5-18 °C), growth parameters, photosynthetic rate, soluble protein in roots, leaf nitrate, and GS activity in roots also had an optimal N-A ratio of 50:50, while GS activity in leaves and NADH-GOGAT activity were the highest when the N-A ratio was 25:75. Increased NH4+, resulting in an N-A ratio of 75:25, improved NR activity, and NO3− in roots in CT. The leaf chlorophyll content was not affected by the N-A ratio. Plant growth parameters, N content in roots, NO3− and NH4+ concentrations, photosynthetic rate, leaf chlorophyll content and key enzymes activities of nitrogen metabolism were influenced by the N-A ratio, while the concentrations of soluble protein, free amino acids, and total leaf N did not appear to be influenced by the interaction of temperature and N supply. The optimal N-A ratio of tomato seedling appears to be 75:25 for unstressed plants and 50:50 for plants grown in chilling temperatures.