Nitrate addition improves photosynthesis and flooding tolerance of rubber tree plants

The role of nitrogen in plants under oxygen deficiency is not well understood. Some studies indicate that nitrate reduction can act as an alternative electron sink (on proton consumption and NAD(P)+ regeneration) reducing the cytoplasmic acidosis induced by anaerobiosis. In this study, we evaluate the role of nitrogen (applied in nutrient solution as either 8mM NO3-or 8mM NH4+) in the metabolism and physiology of rubber trees (Hevea Brasiliensis) subjected to oxygen deficiency. The experiment lasted 21 days, with measurements of gas exchange, biochemical and anatomical analyses taken every 7 days (day 7, 14 and 21). Net photosynthesis, stomatal conductance and transpiration rate in stressed plants significantly decreased (around 50%) at 14 days of the treatment compared to control plants, regardless the nitrogen source. However, in plants treated with nitrate even under stress photosynthesis remained relatively high until the end of the experiment. When exposed to stress, plants treated with nitrate showed a higher efficiency of the antioxidant system enzymes (superoxide dismutase and ascorbate peroxidase), so the H2O2 content was higher in leaves of plants treated with ammonium and exposed to hypoxia. Moreover, plants grown in nitrate in waterlogged conditions were characterized by earlier aerenchyma formation than plants grown with ammonium. Therefore, our results indicate that NO3- is more beneficial than NH4+ for rubber tree plants metabolism under oxygen deficiency conditions.