Effects of water-fertiliser coupling on the photosynthesis and quality of Lycium barbarum based on predicted crop evapotranspiration (ETc)

Lycium barbarum is an important economic crop in the arid region of Northwest China, and the regulation of irrigation and fertilisation is an important way to improve the quality and yield of Lycium barbarum. To explore the effects of water-fertiliser coupling on photosynthesis, quality and yield of Lycium barbarum under irrigation methods based on predicted crop evapotranspiration (ET_c), ET_c was calculated via reference evapotranspiration (ET_o) predicted on the basis of public weather forecasts, and the irrigation water volume was determined as a proportion of this ET_c. A field experiment was conducted via a completely randomised experimental design with five irrigation water volumes (W0 (100% ET_c), W1 (90% ET_c), W2 (80% ET_c), W3 (70% ET_c) and W4 (65% ET_c)) and three fertiliser application rates (high fertiliser (FH), medium fertiliser (FM) and low fertiliser (FL)). The results revealed that the chlorophyll content, G_sw, C_i of Lycium barbarum leaves and 100-grain weight and yield of fresh fruit of Lycium barbarum increased with increasing irrigation, and the protein content, fat content, total sugar content and polysaccharide content in the dried fruits of Lycium barbarum first increased and then decreased with increasing irrigation under the same level of fertilisation, and the maximum value of these indexes reached 70% ET_c - 100% ET_c. At the same irrigation level, E, A, G_sw, C_i of Lycium barbarum leaves and protein content, fat content, total sugar content and polysaccharide content in the dried fruits of Lycium barbarum increased and then decreased with increasing of fertiliser application volume, and these indexes reached the maximum value at the fertiliser application rate of FM. A comprehensive evaluation based on principal component analysis (PCA) revealed that the optimum treatment in both years was W0FM (irrigation level of 100% ET_c, corresponding to irrigation water in the range of 254.2-309.4 mm, and fertiliser application of N-P₂O₅-K₂O of 315-82-135 kg ha^-1), which was significantly greater in 2021 (2022) than in the CK. E, A, G_sw and C_i of daily changes of Lycium barbarum leaves in the W0FM treatment in 2021 (2022) increased by 46.54% (31.53%), 7.08% (59.26%), 18.55% (10.74%) and 34.58% (29.81%), respectively. In 2021 (2022), W0FM treatment increased fat content, polysaccharide content, and betaine content of dried fruits of Lycium barbarum and the 100-grain weight and yield of fresh fruit by 2.88% (10.11%), 1.56% (10.02%), 8.37% (21.69%), 13.57% (24.81%) and 31.39% (71.50%), respectively. The results of this study may provide a theoretical basis for improving the quality and efficiency of Lycium barbarum in the field in the arid zone of Northwest China.