Effect of nitrogen source and oxygen deficiency on carbon metabolism and antioxidant system of rubber tree plants (Hevea spp.)

Abstract

Oxygen deficiency in roots affects key functions such as nutrient and water uptake; furthermore, liquid-CO2 assimilation, stomatal conductance/ transpiration and carbohydrate translocation are also affected. Under oxygen depletion, there is also an uncontrolled increase of free radicals in cells. The nitrogen application has been observed to increase the plant tolerance to oxygen deficiency. Indeed, NO3− and NH4+ (nitrogen forms) may induce distinct metabolic responses under hypoxia. Nevertheless, the beneficial effects of N during hypoxia it is not fully explained. In this study, the nitrogen (N) role in the physiology and metabolism of rubber plants (Hevea brasiliensis) subjected to oxygen deficiency was evaluated. The experiment was conducted with plants supplied with KNO3 (8 mM N) or (NH4)2SO4 (8 mM N) in nutrient solution for 3 days. The experiments were arranged in a completely randomized design (CRD) in a factorial arrangement (2x2x4). The experiment had four treatments and four time points (12, 24, 48 and 72 hours) for stress measurements with four replicates, totaling 64 plants. Data were analyzed using analysis of variance (ANOVA), and the means were compared using the Scott-Knott test (p ≤ 0.05). Gas exchange measurements and biochemical analyzes were performed at 12, 24, 48 and 72 hours after treatment induction. The results showed that, net photosynthesis, stomatal conductance and transpiration rate in stressed plants decreased significantly when compared to the control plants, regardless of nitrogen source. However, under hypoxia, plants treated with NO3- showed significantly higher CO2 assimilation, overcoming the treatment with NH4+. Therefore, when comparing plants treated with NO3- and those treated with NH4+ under flooding conditions (for 3 days), the first presented higher sucrose production, consequently higher invertase isoform activity. Under hypoxia, NO3- treated plants also showed higher antioxidant system efficiency. Therefore, H2O2 content was higher in plants treated with NH4+. Thus, we concluded that carbon metabolism such as carbohydrate anabolism, catabolism and transport were negatively affected by hypoxia. However, NO3- addition in contrast to NH4+ treatment reduced damage caused by oxygen deficiency increasing the level of tolerance of the plant to stress.