NICKEL DEFICIENCY SYMPTOMS ARE INFLUENCED BY FOLIAR ZN:NI OR CU:NI CONCENTRATION RATIO

Abstract

The occurrence of nickel (Ni) deficiency of pecan [Carya illinoinensis (Wangenh.) K. Koch] in orchards is an increasingly common problem. There is uncertainly regarding the primary cause of the problem, as orchard soils have plenty of Ni. The influence of essential micronutrients on the endogenous bioavailability of Ni is unknown and is a possible factor triggering Ni deficiency. This study examines the linkage between Ni deficiency and endogenous foliar concentration of Ni, zinc (Zn), and copper (Cu). This relationship was tested in a greenhouse study using ‘Desirable’ seedlings trees growing in an orchard soil known to cause Ni deficiency in potted trees. Amendment of the potting soil with various amounts of either Zn-sulfate or Cu-sulfate produced seedling trees possessing a variety of Zn:Ni and Cu:Ni concentration ratios in developing foliage, and growth/morphological symptoms expressing various degrees of Ni deficiency. Severity of Ni deficiency was unrelated to foliar Ni concentration, but strongly linked to foliar Zn:Ni or Cu:Ni ratios. Deficiency symptoms increased sigmoidally with increasing Zn:Ni or Cu:Ni ratio, and were correctable, regardless of the Zn:Ni or Cu:Ni ratio, in seedling trees by foliar applications of Ni-malate extracted from Alyssum biomass. Thus, Zn and Cu in foliage can function antagonistically to reduce the bioavailability of Ni and therefore trigger expression of Ni deficiency. Soil Zn or Cu supplements did not detectably affect foliar Ni concentration; thus, root uptake does not appear to be inhibited by Zn or Cu. It is concluded that Ni deficiency in pecan orchards is likely to be partially due to either Zn or Cu fertilization-induced reductions in the physiological availability of Ni, perhaps via either competitive inhibition or sequestration. It is also concluded that long-term or excessive Zn and/or Cu fertilization of crops potentially triggers Ni deficiency through endogenous antagonisms; thus, potentially threatening the sustainability of commercial pecan enterprises.