Root gravitropism is affected by many environmental stresses,including salinity,drought,and nutrient deficiency.One significant environmental stress,excess ammonium (NH4+),is well-documented to inhibit root elongation and lateral root formation,yet little is known about its effects on the direction of root growth.We show here that inhibition of root elongation upon elevation of external NH4+ is accompanied by a loss in root gravitropism (agravitropism) in Arabidopsis.Addition of potassium (K+) to the treatment medium partially rescued the inhibition of root elongation by high NH4+ but did not improve gravitropic root curvature.