Responses of Root Physiology and Yield of Apple Tree to Water and Nitrogen Amounts under Drip Fertigation

In order to reveal the responses of yield, growth, root activity, antioxidant status, and nitrogen metabolism of apple tree to water and nitrogen amounts, a 2-year field experiment was conducted in 2018/2019 and 2019/2020, which comprised four nitrogen doses, i.e., 0 (N 1 ), 120 (N 2 ), 240 (N 3 ), and 360 (N 4 ) kg N ha −1 and two levels of upper irrigation limit, i.e., 85% (W 1 ) and 100% (W 2 ) of field capacity. Physiological parameters of roots were measured at fruit-inflating and post-harvesting phases. New shoot length, yield attributes, N content of roots, and leaves were determined at harvest stage. The results showed that single fruit weight, number of fruits, and yield increased with N application amount, and the maximum value reached at N 3 in both years. W 2 increased yield by 11.55% compared with W 1 due to the increase of single fruit weight in 2019. Antioxidant enzymes and malondialdehyde (MDA) content were enhanced under N starvation (N 1 ) and excess (N 4 ), and were not affected by irrigation level regardless of growth phases. Root activity, nitrate reductase activity, glutamine synthetase activity, and glutamate synthetase activity were affected by N dose and they have different thresholds for reaching the maximum. The content of free amino acids in roots increased with the N dose, and reached peak level at N 3 . W 1 has neither caused water stress on roots nor affected nitrogen absorption of plants compared with W 2 . Taken together, W 1 N 3 could save agricultural water, ensure the yield, improve root N absorption, and avoid oxidative stress for apple planting in semi-humid and drought-prone regions.