Phosphorus Use Kinetics of Citrus Grafts Versus Rootstocks Alone: Morphological, Physiological, and Biochemical Responses

The efficiency of phosphorus (P) uptake in rootstocks and varieties of citrus can be estimated by kinetic parameters. The kinetic parameters of nutrient uptake are composed of the maximum rate of uptake (Vmax), Michaelis-Menten constant (Km), and minimum concentration (Cmin). This set of variables determines the rate of influx (I), which refers to the net rate of uptake of the ion in a solution concentration. Therefore, the main objective of the study was to evaluate whether there is a difference in P absorption efficiency between citrus rootstocks and grafts and correlate kinetic parameters with morphological, physiological, and biochemical variables of citrus rootstocks and grafts. The rootstocks 'Trifoliata' (T) and 'Citrumelo' (C), and grafts with the orange tree 'Valencia' (TV; CV) and mandarin 'Montenegrina' (TM; CM), were acclimated in Hoagland solution at 50% and evaluated for 102 h of kinetic march. Kinetic parameters, P uptake efficiency, and root morphological, biochemical, and physiological parameters were determined. The P use kinetics remains unaltered in grafted varieties when compared to rootstocks alone, with no significant difference in kinetic parameters (Vmax, Km, and Cmin). The rootstock 'T' showed higher values of P influx, P concentrations in tissue, gas exchange, photosynthetic pigments, a tendency for higher photosynthesis, and less stress or need for activation of the antioxidant system, indicating that it is more efficient in the uptake of P. The results can serve as a basis for selecting the best rootstock-scion combination, contributing to the rational use of phosphate fertilizers and, at the same time, to sustainable citrus production.