The Critical Concentration of Nickel Sufficient for Growth and Nutrient Accumulation of Newhall Navel Orange

In citrus production, there is an absence of established standards of critical Nickel (Ni) content for deficiency, sufficiency, and excess, which could be used to determine the nutritional status of plant Ni. In this study, to explore the critical Ni concentrations for deficiency and excess, we conducted a hydroponic pot culture experiment and investigated the effects of Ni levels on flower and fruit development, dry weight, and nutrient accumulation of Newhall navel orange. We found that 0.8 and 6.4 mg L^-1 of solution Ni were the turning point concentrations of Ni deficiency and excess for plants, respectively. Solution Ni deficiency (0 to 0.8 mg L^-1 of Ni) tended to promote vegetative growth and increase the dry weight of new leaves, but suppress flower bud number and fruit development. It also significantly promoted the accumulation of N, P, K, Ca, and Mg in old leaves and N and K in roots, but significantly reduced that of Fe, Mn, and Zn in roots. Excess solution Ni (6.4 to 12.8 mg L^-1 of Ni) reduced the water content of fruit peel and was accompanied by fruit cracking during the fruit expansion period, inhibited new leaf growth and whole plant biomass or dry weight, and significantly decreased nutrient accumulation in roots. Equations of dry weight and solution Ni levels for each plant organ were established, showing that 3.93 to 4.72 mg L^-1 of Ni was the sufficient concentration of solution Ni for the growth and development of Newhall navel orange, with the corresponding range of Ni contents in new and old leaves being 17.87 to 20.42 and 10.24 to 11.64 mg kg^-1, respectively. These findings provide reference for the recommended range of Ni sufficient for citrus growth.