The impact of nitrogen application on antioxidant characteristics of dryland potato leaves and yield in the mountainous region of southern Ningxia, China

Nitrogen deficiency in the middle and late stages of dryland potato fertility is prone to cause early leaf failure, and it is important to study the effects of different nitrogen applications on the antioxidant characteristics, dry matter accumulation characteristics and yield of dryland potato, and to explore the appropriate nitrogen application rate suitable for potato production in the mountainous areas of southern Ningxia, in order to provide technological support for green and efficient cultivation of potato in this region. The experiment was carried out in Haiyuan County, Ningxia, in 2019-2020, with 'Qingshu No. 9' as the test material, adopting a randomized block group experimental design, A total of four nitrogen treatments, 0 kg·hm⁻² (N0), 75 kg·hm⁻² (N1), 150 kg·hm⁻² (N2) and 225 kg·hm⁻² (N3), were set. The parameters related to antioxidant enzyme activities, non-enzymatic protective substances and dry matter mass of the leaves were determined. The results of the 2-year study showed that the appropriate N application rate could significantly increase potato yield, improve the dry matter accumulation characteristics and delay leaf senescence, with the best performance in the N2 treatment (N application rate of 150 kg·hm⁻²). Compared with no N fertilization, all N treatments increased leaf area index (LAI) and leaf relative chlorophyll content (SPAD) significantly at the late reproductive stage of potato (90-105 d after seedling), and at the same time, had significant effects on the enhancement of leaf antioxidant enzyme activities, and the sustained optimization of the dry matter accumulation characteristics, among which, the advantage was obvious with the N2 treatment. In addition, N2 significantly increased the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in the late vegetative stage of potato, effectively reduced the accumulation of malondialdehyde (MDA) and proline (Pro) compared with other treatments; significantly increased the rate of dry matter accumulation (Gmax), the amount of growth at the time of the maximum rate of accumulation (Wmax), and prolonged the active period of accumulation (D), which improved the dry matter accumulation characteristics, increased the dry matter quality of tubers, and then enhanced the yield; Meanwhile, the yield regression equations showed that the optimal N application range for maximum theoretical yield was 150-196 kg·hm⁻² (2019) and 150-169 kg·hm⁻² (2020), which could provide a reliable N application program for high potato yields in the region. Correlation analysis showed that potato yield was significantly and positively correlated with its leaf SOD, POD and CAT activities, maximum dry matter accumulation rate (Gmax) and active accumulation period (D); both were significantly and negatively correlated with leaf MDA and Pro accumulation. Therefore, a N application rate of 150 kg·hm⁻² (N2) can effectively optimize the dry matter accumulation characteristics of potato, improve the activity of leaf protective enzymes in the middle and late stages of reproduction, and reduce the accumulation of membrane lipid peroxidation products MDA, which can be used as the recommended N application rate for green, high-yield and high-efficiency cultivation of potato in the dry zone of Ningxia.