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.