Eficiência na absorção e proposição de níveis críticos de nutrientes em citros

Abstract

Citrus plants adapt well in tropical, temperate, and subtropical countries, which turns them of great importance to the agriculture sector. Usually, kinetics parameters related to nutrient uptake are not evaluated, that can establish which rootstocks or grafts are more or less efficient to the absorption of nutrients. At the same time, in citrus and other fruit production in general, the need and nutrient doses are established considering the results of soils analysis, however, as citrus present internal reserves of nutrients, especially in perennial organs, and the roots occupies deeper soil layers than the diagnosticated layer, the foliar analysis can be adequate to contribute in the diagnosis of nutritional state. This research aimed to establish values of kinetic variables related to P absorption and propose nutritional reference values in citrus leaves. Two studies were conducted. Study 1 approached the selection of citrus rootstocks and grafts, in terms of P absorption efficiency. One experiment with two citrus rootstock (‘Trifoliata’ and ‘Citrumelo Swingle’) and grafts (‘Citrumelo + Montenegrina’; ‘Citrumelo + Valência’ and ‘Trifoliata + Montenegrina’; ‘Trifoliata + Valência’) was conducted. The rootstocks and grafts de citrus were cultivated for 10 days, in pots containing solution with CaSO4 at 0,1 mol L -1 . After this period, the plants were exposed to the Hoagland solution, where were conducted periodical collections of solution for 102 hours of evaluation. Afterwards, biochemical, physiological and morphological parameters of aerial part and roots, dry matter, P concentration were evaluated. Kinetical parameters or P absorption (Vmax, Km, Cmin and influx - I) were calculated using Influx software. Study 2 established and compared nutritional standards obtained through CND method for citrus in subtropical climate obtaining CL and SB in leaves. A database was used with the production of fruits and nutrient content in leaves, obtained in the four harvests, in commercial orchards not irrigated, where the fruit are destinated to the commercialization in natura. The method of multivariate calculation CND was utilized. In the Study 1, the rootstock ‘Trifoliata’ presented higher values of P influx, concentration of P in tissue, gas exchanges, pigments and tendency of greater photosynthetic rate, and stress or need of activation of the antioxidant system. In the Study 2, the method CND was effective in the diagnosis of nutritional state of citrus, in relation to the nutrient concentration in leaves of N, P, K, Ca, Mg, S, Cu, Fe, Mn, Zn and B in poor, adequate and excessive concentration. The SB established from CND index are different from the ones obtained in other countries and regions. The results of studies 1 and 2 can allow the rational use of fertilizers, decrease problems with environmental contamination and unnecessary expenses by the citrus farmers, besides to provide greater yields of citrus production in the fruit farms.