Ammonium under solution culture alleviates aluminum toxicity in rice and reduces aluminum accumulation in roots compared with nitrate

Abstract

Al stress and ammonium–nitrogen nutrition often coexist in acidic soils due to their low pH and weak nitrification ability. Rice is the most Al-resistant species among small grain cereal crops and prefers NH4+ as its major inorganic nitrogen source. This study investigates the effects of NH4+ and NO3− on Al toxicity and Al accumulation in rice, and thereby associates rice Al resistance with its NH4+ preference. Two rice subspecies, indica cv. Yangdao6 and japonica cv. Wuyunjing7, were used in this study. After treatment with or without Al under conditions of varying NH4+ and NO3− supply, rice seedlings were harvested for the determination of root elongation, callose content, biomass, Al concentration and medium pH. The results indicated that Wuyunjing7 was more Al-resistant and NH4+-preferring than Yangdao6. NH4+ alleviated Al toxicity in two cultivars compared with NO3−. Both NH4+-Al supply and pretreatment with NH4+ reduced Al accumulation in roots and root tips compared with NO3−. NH4+ decreased but NO3− increased the medium pH, and root tips accumulated more Al with a pH increase from 3.5 to 5.5. Increasing the NO3− concentration enhanced Al accumulation in root tips but increasing the NH4+ concentration had the opposite effect. These results show NH4+ alleviates Al toxicity for rice and reduces Al accumulation in roots compared with NO3−, possibly through medium pH changes and ionic competitive effects. Making use of the protective effect of NH4+, in which the Al resistance increases, is advised for acidic soils, and the hypothesis that rice Al resistance is associated with the preferred utilization of NH4+ is suggested.