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 NO3− 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 (Ψw) of 0.0, −0.2, −0.4 and −0.7 MPa and containing 1.2 mmol l−1 KNO3. We accessed the NO3− 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 KNO3 as initial concentration, and allowed estimating NO3− absorption parameters Vmax, Km and Cmin. We observed a linear decrease in the nitrate content of the containers receiving 1.2 mmol l−1 of NO3− over a period of 8 h. In this concentration, the rates of NO3− 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 NO3− concentration range, the absorption showed a Michaelis-Menten pattern, being the kinetic parameters of NO3− 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 Vmax, Km, and Cmin, respectively. We concluded that NO3− uptake of almonds is affected by short periods of water stress, with harmful effects occurring at Ψw below −0.18 MPa.