Nitrogen metabolism in the roots of rubber tree (Hevea brasiliensis) plants supplied with nitrate or ammonium as nitrogen source during hypoxia

Abstract The plants may be exposed to excessive soil moisture condition, in which oxygen supply (O 2 ) to the root system becomes difficult due to the low diffusion rate gas in water. Given the fundamental importance of O 2 in plants aerobic metabolism, the flooding can to affect the plants development in ecosystems. O 2 lack in the cultivation substrates negatively influences the productivity of many economically important species such as rubber tree (Hevea brasiliensis Müll.Arg.). Amazonian species undergoes flooding periods at different lifecycle stages. The application of nitrate (NO 3 − ) has been observed to increase the plant tolerance subjected O 2 deficiency. The nitrogen role in plants under O 2 deficiency is very important, but is not well understood. In this study we evaluate nitrogen metabolism changes (roots enzymatic assimilation) and the nitrogen compounds transport to the shoot in rubber trees under O 2 deficiency treated with nitrate or ammonium. Rubber tree plants were grown in nutrient solutions of Bolle-Jones (1957) 1/2 strength. When plants were 12 months-old they were divided into six treatments consisting of the complete nutrient solution of Bolle-Jones (1957) free nitrogen, with 8 mM nitrogen (KNO 3 ) or with 8 mM nitrogen ((NH 4 ) 2 SO 4 ) and two oxygen availability conditions (normoxia-control plants: roots kept under nutrient solution and continuous aeration and hypoxia-flooded plants: roots kept under nutrient solution without aeration). Root and sap xylem material was harvested for biochemical analysis every 7 days (days 7, 14 and 21). The results showed that under hypoxia, nitrate treated plants showed higher nitrate reductase (NR) activity compared to the control. Glutamine synthetase (GS), glutamine-2-oxoglutarate aminotransferase (GOGAT) and glutamate dehydrogenase (GDH) activities were significantly decreased in stressed plants regardless of the nitrogen source. However, under normoxia or hypoxia, nitrate treated plants showed higher (GS, GOGAT and GDH) activity, in comparison with the ammonium treatment. Moreover, under hypoxia nitrate treated plants, presented higher alanine and Gaba concentrations in the xylem than plants treated with ammonium. Therefore nitrogen assimilation, amino acid biosynthesis and transport are in general less affected by hypoxia, however, nitrate addition attenuates stress effects