Antioxidant Characterization of Nitrogen Application for Mitigating Potato Senescence Post-Flowering Under High Temperature Stress

In order to explore the mechanism of nitrogen application under high temperature on the antioxidant characteristics of dryland potato in the mountainous area of southern Ningxia, and to elucidate the mechanism of nitrogen regulation, so as to provide the reference for the local development of nitrogen application measures favorable to alleviate high temperature stress.

A 2-year field in situ experiment was conducted in Dazui Village, Haiyuan County, Ningxia, from 2020 to 2021, using a split-zone experimental design with four N application levels as the main zones, namely 0 (N0), 75 kg·hm^-2 (N1), 150 kg·hm^-2 (N2), and 225 kg·hm^-2 (N3), and two temperature gradients as the sub-zones, namely (35±2) ℃ (HT) and (30±2) ℃ (CK). The effects of post-flowering high temperature stress on potato leaf area index (LAI), relative chlorophyll content (SPAD), antioxidant properties, membrane lipid peroxidation products and non-enzymatic protective substances were analyzed.

The 2-year results showed that potato LAI, SPAD, superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) showed a decreasing trend from 30d-35d after flowering under the nitrogen fertilizer×temperature intercropping condition; at the same time, the cell membrane permeability, malondialdehyde content (MDA) and proline content (Pro) of potato leaves showed an increasing trend. Taking 35d after flowering as an example, after high temperature stress, potato LAI decreased by 11.22%-21.20%, SPAD decreased by 23.29%-26.05%, SOD decreased by 12.27%-16.87%, POD decreased by 13.69%-17.71% and CAT decreased by 13.80%-18.39% compared with room temperature; while cell membrane permeability, MDA content and Pro content increased significantly. Meanwhile, after high temperature stress, LAI and SPAD were significantly increased, while SOD, POD, CAT as well as yield reached the highest under N2 treatment (150 kg·hm^-2) compared with other treatments and N2 could also reduce cell membrane permeability and MDA content to alleviate high temperature hazards and increase potato yield. To further explore the correlation between potato yield and N application, a quadratic relationship between yield and N application was found, which led to the derivation of the corresponding parameter values for the economically optimal N application rates of 132-142 kg·hm^-2 (HT) and 185-210 kg·hm^-2 (CK). Pearson's correlation analysis showed that under high temperature stress, yield was only related to leaf LAI, POD, and CAT. LAI, POD and CAT reached significant positive correlation and significant negative correlation with MDA and Pro, while it did not reach significant level with SOD, SPAD and cell membrane permeability. Meanwhile, through the principal component analysis, it was found that after 2 years of high temperature stress, the composite scores of different nitrogen application levels were N2>N3>N1>N0.

The application of nitrogen at 150 kg·hm^-2 could continue to improve the physiological and antioxidant characteristics of potato leaves and to optimize its yield effectively, and it was also consistent with the theoretical estimation of 2 years of post-flowering high temperature. It was found that the N application rate of 150 kg·hm^-2 could continuously improve the physiological characteristics of potato leaves and effectively optimize the yield, and the difference was very small with the theoretically estimated 2-year economic optimum N application rate (132-142 kg·hm^-2). Therefore, the present experiment could also take 150 kg·hm^-2 as the recommended N application rate for safe potato production in Ningnan mountainous area to cope with the increasingly serious local high temperature hazard.