Short-term water stress affecting NO3− absorption by almond plants.

Identifying the rates of nitrate absorption by almond plants under water stress is important for improving nitrogen use efficiency in almond orchards. This study aimed to determine the rates of nitrate absorption by almond plants subjected to short-term water stress and no water stress, and the kinetic NO 3 − absorption parameters in well-watered three-year-old almond plants. Two assays were performed. In the first, fourteen plants arranged in completely randomized design received nutrient solutions with water potentials (Ψ w ) of 0.0, −0.2, −0.4 and −0.7 MPa and containing 1.2 mmol l −1 KNO 3 . We accessed the NO 3 − depletion in these solutions over an 8-h interval and estimated regression curves of the quantity of nitrate in the nutrient solution ( Q ) as a function of time ( t ). Subsequently nitrate uptake rates were calculated. Water loss; nitrate reductase activity; nitrate and total N of the sap; nitrate concentration of leaf dry matter; and nitrate and nitrite concentration of root dry matter were, either, evaluated. The second assay was performed the same way of the first one, with four well-watered plants submitted to 0.3 mmol l −1 KNO 3 as initial concentration, and allowed estimating NO 3 − absorption parameters V max , K m and C min . We observed a linear decrease in the nitrate content of the containers receiving 1.2 mmol l −1 of NO 3 − over a period of 8 h. In this concentration, the rates of NO 3 − uptake ranged from 1.11 to 3.43 μmol g −1 h −1 and decreased in low water potentials at the root medium. Root nitrate reductase activity followed the same trend of nitrate absorption. In the low NO 3 − concentration range, the absorption showed a Michaelis–Menten pattern, being the kinetic parameters of NO 3 − absorption 1.15 ± 0.27 μmol g −1 h −1 , 28.81 ± 4.12 μmol l −1 and 17.93 ± 1.1 μmol l −1 for V max , K m , and C min , respectively. We concluded that NO 3 − uptake of almonds is affected by short periods of water stress, with harmful effects occurring at Ψ w below −0.18 MPa. [ABSTRACT FROM AUTHOR]