Effect of post-harvest foliar fertilizer sprays on the photosynthetic capacity of walnut

【Objective】The study aimed to reduce the high branch drying rate and increase the continuous fruiting capacity of Wen 185 walnuts (Juglans regia L.) in Xinjiang through foliar surface spray of potassium dihydrogen phosphate (KH2PO4).【Methods】Based on field experiments and laboratory measurements, four foliar application concentration gradients were set: water (CK), 0.8% KH2PO4, 1.2% KH2PO4, and 1.5% KH2PO4. The spray was carried out three times after harvest with 10 days interval. The photosynthetic physiological parameters, leaf anatomical structure, and sugar metabolite content of Wen 185 walnut leaves after harvest were determined.【Results】The spray of KH2PO4 effectively increased photosynthetic parameters, sugar content, and improved leaf structure of Wen 185 walnut, and 1.5% KH2PO4 treatment had the most significant and optimal effect. On September 25, the 1.5% KH2PO4 treatment increased the net photosynthetic rate by 28.83% compared with the control. The trend of leaf stomatal conductance was basically consistent with that of net photosynthetic rate. At the late stage of fertilizer spraying, the stomatal conductance of the 1.5% KH2PO4 treatment group was 27.29% higher than that of the control. The transpiration rate of the 1.5% KH2PO4 treatment increased by 27.39% compared with the control. A total of 12 short-chain sugars were detected in walnut leaves, dominated by sucrose. The levels of sucrose, glucose and fructose peaked after fertilizer spraying and were significantly different from those of the other groups. The sucrose content in the leaves increased by 10.61% , glucose content by 13.23% and fructose content by a significant 35.63% after fertilizer spraying compared with the control. The increase of the total sugar content of 1.5% KH2PO4 treatment was the most significant (P＜0.05), with an increase of 13.50% compared with the control; the difference of the chlorophyll content of the 1.5% KH2PO4 treatment group and the control at the late stage of fertilizer spraying reached the maximum value. The spray of KH2PO4 significantly affected the stomata of Wen 185 walnut leaves. The effect of the 1.5% KH2PO4 treatment was most significant. The stomatal opening rate of the 1.5% KH2PO4 treatment increased by 18.41% compared with the control, whereas there was no significant difference between the stomatal opening rate of 0.8% and 1.2% treatment groups and the control. The thickness of upper and lower epidermal cells under the three concentration treatments was lower than that of the control during the fertilizer spraying period. After fertilizer spraying, the thickness of upper and lower epidermal cells decreased in all treatment groups, except for the 1.5% KH2PO4 treatment, which showed an increase in the thickness of upper and lower epidermal cells. The most significant increase in the thickness of fenestrated tissue was observed in the 1.5% KH2PO4 treatment during the fertilizer spraying period. The fenestrated tissue thickness increased in 1.2% KH2PO4 and 1.5% KH2PO4 treatments after fertilizer spraying, with 1.5% KH2PO4 treatment showing the most significant effect, increasing by 22.98% compared with the control. The fenestrated sea ratio (the ratio of fenestrated tissue thickness to spongy tissue thickness) of the 1.5% KH2PO4 treatment during the fertilizer spraying period was significantly different from that of the control, whereas none of the other treatment groups were significantly different from that of the control. After fertilizer spraying, the fenestrated sea ratio of 1.5% KH2PO4 treatment showed an increasing trend. The leaf thickness was significantly increased in 1.5% KH2PO4 treatments during the fertilizer spraying period, with the most significant effect in 1.5% KH2PO4 treatments, increasing by 20.73% compared with that of the control. The leaf pulp cell density showed significant enhancement in 1.5% KH2PO4 treatment both at mid- and post-fertilization sprays, with an increase of 13.08% compared with that of the control. Significant correlations exist among various physiological indicators in walnut leaves following KH2PO4 treatment. The net photosynthetic rate (Pn) and chlorophyll content showed consistent correlation patterns with other indicators. Specifically, they exhibited extremely significant positive correlations with total sugar content, transpiration rate (Tᵣ), and stomatal conductance (Gs), while displaying an extremely significant negative correlation with intercellular CO2 concentration (Cᵢ). Additionally, the palisade- spongy ratio was extremely significantly and positively correlated with palisade tissue thickness. The moisture content of the 1.5% KH2PO4 treatment group was 31.35%, which was 31.92% lower than that of the control group (CK) (P＜0.05). The germination rate of walnut buds treated with 1.5% KH2PO4 reached the maximum value of 72.95%, the 0.8% and 1.2% KH2PO4 treatments were basically similar, being 72.22% and 69.08%, respectively. The shoot drying rate of the 1.5% KH2PO4 treatment was 37.37%, which was significantly lower than that of the control group (CK) by 31.92% (P＜0.05).【Conclusion】Spraying KH2PO4 1-2 weeks after harvest had a significant promoting effect on photosynthetic and physiological indexes of Wen 185 walnut leaves, which could enhance the resistance of walnut trees to winter cold.