Effects of leaf scorch on photosynthetic characteristics, fruit yield, and quality of walnuts

The consequences of walnut (Juglans regia L.) leaf scorch (WLS) were studied using the cultivated varieties, Wen185 (Juglans regia 'Wen 185') and Xinxin2 (Juglans regia 'Xinxin2') in the Aksu region, Xinjiang, China. Photosynthetic parameters and indoor chemical analysis were used to determine the variations in photosynthetic characteristics, osmotic regulatory substances, antioxidant enzyme activities, and changes in fruit yield and quality between diseased and healthy leaves. Net photosynthetic rate (P _n) and stomatal conductance (G _s) of Xinxin2 diseased leaves were lower and intercellular CO₂ concentration (C _i) was higher than in healthy leaves. P _n, G _s, and C _i of Wen185 leaves were lower than those of healthy leaves initially. During the peak stage of disease, P _n and G _s of Wen185 were lower, whereas C _i was higher than in healthy leaves. The initial fluorescence (F ₀) of diseased leaves was higher and the maximum photochemical efficiency of photosystem II (PSII, F _v/F _m) was lower. The decrease in F _v/F _m of diseased Wen185 leaves was smaller than in Xinxin2. Malondialdehyde (MDA) content in Wen185 and Xinxin2 diseased leaves was higher than in healthy leaves. From late June to mid-July, the superoxide dismutase (SOD) activity and soluble protein (SP) content in the diseased leaves were higher than in healthy leaves, becoming lower in late August. Plant yield, single fruit dry weight, fruit longitudinal diameter, fruit shape index, kernel extraction rate, fat content, and protein content of the diseased plants were lower. Single fruit fresh weight, fruit transverse diameter, and fruit lateral diameter in Wen185 plants were similar but differed in diseased Xinxin2 plants. WLS reduces carbon assimilation and PSII reaction center activity leading to intensified membrane lipid peroxidation, gradual imbalance of osmotic regulation homeostasis, and decreased antioxidant capacity.