Influência de diferentes fontes de nitrogênio na tolerância de plantas de girassol ao estresse salino

Abstract

Adequate nitrogen fertilization can reduce the deleterious effects of saline stress and promote plant growth, being this effect dependent on the nitrogen (N) source used. In this experiment, we aimed to study the influence of the N source on the tolerance of sunflower (Helianthus annuus L.) plants to salinity. In a greenhouse, sunflower seeds were sown in vermiculite, and after eight days, the seedlings were transferred to a hydroponic medium with either NO3− or NH4+ as the only N source, or a NO3-/NH4+ mixture (3:1, v/v). After six days, NaCl was added in the nutrient solution until a concentration of 75 mM was reached. The plants were harvested at 7 and 14 days after the imposition of saline stress. The growth of sunflower plants was reduced by salinity; however the plants supplied with the NO3-/NH4+ mixture showed the higher shoot growth under salt conditions. The stomatal conductance (gs), transpiration (E), net photosynthetic rate (A), internal CO2 concentration (Ci), instantaneous carboxylation efficiency (A/Ci) and water use efficiency (WUE) were little influenced by salinity. The NH4+ plants showed the lowest values of gs, A, A/Ci and WUE, in comparison to the other N sources. Similarly, plants supplied with NH4+ showed the lowest contents of chlorophyll (a, b and total), carotenoids and anthocyanins, which were little influenced by salinity. Electrolyte leakage (LV) and lipid peroxidation were not influenced by N sources, and salinity increased LV. Salt stress increased Na+ content and reduced K+ content, especially in the roots, but these contents were lower in the plants supplied with NH4+ compared to the other N treatments. In saline conditions, the highest increase in Cl- content was observed in plants supplied with NO3- and NO3-/NH4+, however plants supplied with NH4+ were the more accumulated Cl in their tissues. The NO3- content in the different parts of the plants and their translocation to the shoot were reduced by the salinity, being higher in the plants supplied with NO3-, followed by those supplied with NO3-/NH4+. The content of NH4+ and their translocation to the shoot were higher in the plants supplied with this ion, which also accumulated more N-amino compounds and free amino acids, but this was not able to reduce the NH4+ content in their tissues, justifying the toxicity observed in these plants. Plants supplied with NO3-/NH4+, as compared to those supplied with NO3-, accumulated more N-amino compounds and glutamine, asparagine, serine and proline under salt conditions. In general, the contents of N-total, soluble carbohydrates and starch were higher in plants supplied with NH4+, and no salt effects were observed in the latter two variables. Enzymes of N metabolism were affected by salinity, regardless of the N source used. In the leaves, nitrite reductase and nitrite reductase activities were higher in plants supplied with NO3- and NO3-/NH4+. There was a low correlation between the activity of glutamine synthetase and glutamate synthase and the content of glutamine and glutamate. These results suggest that sunflower plants are sensitive to NH4+, showing toxicity symptoms under this nutrition, even under control conditions. The combined nutrition of N (NO3-/NH4+) was the most efficient in reducing the deleterious effects of salinity in sunflower plants, as it promoted higher growth and a better ionic balance in stress conditions. Besides the plants under this nutrition were the more efficient in the use of N under salt stress.