Effects of water deficit in different growth stages coupling with nitrogen application rates on photosynthetic traits and quality of peanuts

Regulated deficit irrigation has been ever increasingly utilized to cope with the limited irrigation water resources for better crop production. This study aimed to explore the effects of water deficit in different growth stages with the nitrogen application rates on the photosynthetic characteristics, yield, and quality of peanuts. A split-plot experimental design was conducted in the lysimeters at the Experimental Farm of College of Water Conservancy of Shenyang Agricultural University, Shenyang, China, during the 2020 and 2021 peanut growing seasons (May to October). An investigation was made on the effects of nitrogen application rates (0 (N0), 50 (N50), and 100 kg/hm2 (N100)) on the photosynthetic rate, transpiration rate, yield, yield components, and quality of peanut under different levels of water deficit (full irrigation, IF, the lower limit for irrigation was 70%~75% field capacity; regulated deficit irrigation, IRD, the lower limit for irrigation at the flowering and pod filling stages was 55%~60% field capacity)). The soil moisture content was monitored at the 5-day interval during the peanut growing season. The photosynthetic rate and transpiration rate of peanut leaves were determined at the flowering and pod setting stages, respectively, using the Li-Cor 6400 portable photosynthesis measurement system. Three pieces of leaves were selected for determination each time. After harvest, the peanut yield and yield components were determined at the 14% moisture on a per-plot basis. The Near Infrared Quality Analyzer was also utilized to determine the peanut quality, including the protein, oil, oleic acid, linoleic acid content, and Oleic-Linoleic ratio (O/L). The clustering analysis was performed to identify the correlations between the different peanut quality traits under different irrigation and nitrogen treatments using the "pheatmap" package in R studio software. The results showed that the regulated deficit irrigation reduced the photosynthetic rate and transpiration rate of peanut leaves at the flowering stage, compared with the full irrigation. After rehydration at the pod setting stage, the photosynthetic rate and transpiration rate in the regulated deficit irrigation treatment was higher than those in the full irrigation due to the compensatory effect. The N100 significantly increased the photosynthetic rate and transpiration rate under regulated deficit irrigation at the flowering and pod setting stages, compared with the non-nitrogen control. Consequently, the regulated deficit irrigation significantly increased the peanut yield, 100 fruit weight, and 100 kernel weight, compared with the full irrigation. The highest peanut yield was achieved in the regulated deficit irrigation coupled with the 100 kg/hm2 nitrogen fertilizer treatment (IRDN100), which was 13.4% higher than that of the traditional water and nitrogen treatment (IFN100) (two-year average). The peanut produced in the IRDN100 treatment presented a relatively higher protein, oil, oleic acid, linoleic acid content, and O/L, indicating better oil output and storage quality. Therefore, the IRDN100 treatment can be expected to save water for the high peanut yield and quality. The finding can provide a theoretical reference for the water-saving, quality-improving, and efficiency-increasing in peanut production in arid and semi-arid areas.