Relationship between water and nitrogen uptake in nitrate- and ammonium-supplied Phaseolus vulgaris L. plants

Sole ammonium supply provokes negative effects on dry-mass formation, leaf growth, and water uptake of ammonium-sensitive plants. To study the effects of N form on nutrient and water uptake and aquaporin expression, French bean plants were grown in a split-root system. Five treatments were compared: homogeneous nitrate (NN) and ammonium (AA) supply; spatially separated supply of nitrate and ammonium (NA); and half of the root system supplied with N-free nutrient solution, the other half with either nitrate (NO) or ammonium (AO). Ten days after onset of treatments, root dry mass (DM) and water-uptake rate (WUR) were significantly reduced under ammonium compared to nitrate supply. WUR from nitrate-supplied vessels was 80% higher than that from N-free nutrient solution, while WUR from N-free nutrient solution was 130% higher than that from ammonium-supplied vessels. Potassium uptake was lower under ammonium supply and the ratio of N : K uptake of treatment AA was significantly higher compared to others. High K uptake from N-free nutrient solution of AO plants resulted in a ratio of N : K uptake comparable to nitrate-supplied plants, but shoot growth resembled that to plants under sole ammonium supply. Within 24 h after onset of treatments, expression of aquaporin was lower under ammonium compared to nitrate supply. From these data, it can be concluded that reduced root water transport under ammonium supply is directly related to aquaporin activity.