Effect of Drip Irrigation, N, K, and Zn Coupling on Pn of Densely Cultivated Apple on Dwarf Rootstock in Xinjiang, China

Abstract

This study aimed to determine the effect of irrigation amount (W), nitrogen (N), potassium (K), and zinc (Zn) on the net photosynthetic rate (Pn) of closely planted apple trees on dwarf rootstocks in arid areas of Xinjiang. Taking the “Royal Gala” apple as the experimental material, a mathematical model for Pn was established using the principle of four-factor five-level quadratic regression with a general rotation combination design. The results show that: (1) The regression equations reached significant levels (F = 37.06 > F0.01(11.11) = 4.54). (2) The effect of W, N, K, Zn on Pn is significant with relative importance W > N > Zn > K. (3) The results of single factor analysis showed that with an increase in W, N, K, and Zn, Pn exhibits an n-shaped parabolic response. (4) The positive coupling between W and N is significant, and the positive coupling between W and Zn is also significant. (5) Analysis of the interaction between sets of three factors revealed that W, N, and Zn could be combined to best effect, with the maximum value reaching 12.77 μmol·m−2·s−1. Compared with W×K×Zn and W×N×K, the combination of W×N×Zn reduces W by 9.2% and 6.3%, respectively, which indicates its suitability for use in the dry and water deficient planting environment in Xinjiang. (6) Within the 95% confidence level, when W is 258–294.75 mm, N is 33.44–39.51 kg/hm2, K is 53.82–69.39 kg/hm2, and Zn is 6.46–7.84 kg/hm2, the net photosynthetic rate reaches 11 μmol·m−2·s−1.