Comparative efficiency of citrus rootstocks in the absorption of 15N and 32P labelled nutrients

Abstract

Comparative efficiency of some commercially important citrus rootstocks and the scion cultivar Coorg mandarin in the absorption and utilization of N and P was studied through field and pot culture experiments and also kinetics of P absorption in solution culture using their isotopes 15N and 32P. Trifoliate oranges had significantly higher N concentration in the shoot but lower rate of N utilization. The nitrogen derived from fertilizer (Ndff) in the shoot was significantly more in Citrumelo, Troyer and Carrizo citranges, the two Trifoliate oranges and the scion cv. Coorg mandarin indicating their greater efficiency in the absorption nitrogen than the other rootstocks Rough lemon, Sour orange, Cleopatra and Kodakithuli mandarins. Trifoliate oranges, Citrumelo, Troyer citrange also had finer roots and higher anion exchange capacity. In studies on the comparative efficiency of citrus rootstocks in the absorption of P in pot culture, it was found that Trifoliate oranges, Rough lemon, Troyer citrange Kodakithuli mandarin and the scion cv. Coorg mandarin derived more P from the fertilizer (Pdff) than Cleopatra mandarin and Sour orange. Field studies on the comparative efficiency of P absorption by scion cultivars Italian lemon and seedless lime budded on seven rootstocks revealed that Italian lemon on two hybrid rootstocks Carrizo citrange and Citrumelo derived significantly more fertilizer P than the other, where as in Seedless lime, Rough lemon and Trifoliate oranges resulted in higher Pdff. Mean effects of the two scion cultivars showed that the dwarfing rootstock Trifoliate oranges resulted in higher Pdff. Of the two scion cultivars, Italian lemon derived more P from the fertilizer than Seedless lime. Rootstocks Troyer citrange, Citrumelo, and scion cv. Coorg mandarin had higher inflow (ln) and maximum uptake (lmax) rates than the others, indicating that they had greater number of absorption sites. In Rough lemon and Citrumelo the (lmax) increased with increase in the concentration of P in the external solution indicating that these two rootstocks respond to increase in the concentration of P in the soil solution brought about by fertilizer application. Rootstocks Rough lemon and Trifoliate oranges had lower Km values (external P concentration corresponding to half the maximum rate of uptake) implying greater affinity of their roots for P and that they may be more efficient on unfertilized soils of low fertility.