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 is prone to cause early leaf failure. Therefore, it is important to study the effects of different nitrogen applications on the antioxidant characteristics, dry matter accumulation characteristics, and yield of dryland potatoes and to explore the appropriate nitrogen application rate suitable for potato production in the mountainous areas of southern Ningxia. This research will contribute to the provision of technological support for green and efficient potato cultivation 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, including 0 kg·hm⁻² (N0), 75 kg·hm⁻² (N1), 150 kg·hm⁻² (N2) and 225 kg·hm⁻² (N3), were applied. Parameters related to antioxidant enzyme activity, non-enzymatic protective substances and dry matter mass were determined. The results of the two year study showed that the appropriate N application rate could increase potato yield, improve the dry matter accumulation characteristics and delay leaf senescence, with the best performance under the N2 treatment (N application rate of 150 kg·hm⁻²). Compared with no N fertilization, all N treatments increased leaf area index and leaf relative chlorophyll content at the late reproductive stage of potatoes (90—105 d after seedling). Simultaneously, appropriate N application rate could increase antioxidant enzyme activity and optimize the characteristics of dry matter accumulation, and the advantage of N2 treatment was obvious. Compared with other treatments, N2 treatment can increase the activity of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in the late reproductive stage of potatoes, and effectively reduce the content of malondialdehyde (MAD) and proline (Pro). In addition, N2 treatment can increase the maximum dry matter accumulation rate (G_max) and growth at maximum rate of dry matter accumulation (W_max), and prolonged the number of days of active dry matter accumulation (D). This improved the dry matter accumulation characteristics, increased the dry matter quality of tubers, and enhanced the yield. Meanwhile, the yield regression equations showed that the optimal N application range for the 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 leaf SOD, POD, and CAT activities, G_max and D. Yield and dry matter accumulation were significantly and negatively correlated with the content of leaf MDA and Pro. Therefore, an N application rate of 150 kg·hm⁻² (N2) can effectively optimize the dry matter accumulation characteristics of potatoes, 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), and it can be used as the recommended N application rate for green, high-yield and high-efficiency cultivation of potatoes in the dry zone of Ningxia.