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The nitrate (NO3-) reduction process is complex and has a high energy cost for plants when N is provided exclusively as NO3-. For this reason, the addition of part of N in the form of ammonium (NH4+) may improve the development of many species. There are no studies showing how proline spray may affect plants under NH4+/NO3- treatment in varying proportions. The objective was to evaluate the role of exogenously applied proline on N assimilation, photosynthesis, chlorophyll content, antioxidant activity, mineral uptake, and biomass production of tomato plants in initial growth treated with NO3-/NH4+ in varying proportions. The experiment was carried out with NO3-/NH4+ proportions (100/0, 70/30 e 50/50) and exogenous proline doses rate (0, 10 e 20 mmol L-1). The parameters of production, nutrition and enzyme activity of the antioxidant system of tomato plants were evaluated. The NO3-/NH4+ proportions and proline spray did not affect the plant biomass, but the SPAD value, CO2 assimilation, and stomatal conductance were lower in plants supplied with NO3- exclusively. Proline concentration in the shoots was increased with the proline rate applied exogenously. The nitrate reductase activity was enhanced by proline rates only in plants under NO3- exclusively, whereas glutamine synthase was not affected by proline spray. However, proline spray decreased NH4+ concentration in shoots of the plants grown under 100/0 and 70/30 NO3-/NH4+, but not in the roots. The NO3- concentration and accumulation in roots were raised by proline spray only in plants grown under NO3- exclusively, indicating that proline is an excellent osmoregulator, which explains the maintenance of NO3- in the roots. Proline spray increased N assimilation and mineral uptake. Ascorbate peroxidase, catalase, and guaiacol peroxidase activity in shoots was higher in plants treated with a proline rate of 20 mmol L-1. Producers using proline spray for tomato plants should consider the NO3-/NH4+ proportions, because the proline supply affects NO3- and NH4+ uptake, nitrate reductase activity, and antioxidant activity in tomato plants under treatment from N ionic forms.

Exogenous proline affects nitrogen assimilation, mineral uptake, and antioxidant activity in tomato plants under NO3–/NH4+ proportions