Changes in mineral nutrients and photosynthetic capacity and their correlations during leaf development and senescence in Shine Muscat grape

Abstract: 【Objective】In recent years, grape production has expanded rapidly in Guangdong province, where climate allows over 9 months of growth season for grape. Leaf senescence and abscission occur in Guangdong much later than in traditional production regions further north. In some cultivars like Shine Muscat and Oriental Star, there is no complete leaf shed throughout winter. During the active growth season, leaf senescence frequently occurs with the ripening of the berry. Mineral nutrients play important roles in performing photosynthetic functions. It is worth to examine mineral nutrient changes in leaves during development and senescence. Also, there has been no study on comparing patterns of mineral changes in senescing leaves during growth season and during short-day induced systematic leaf senescence in late fall. The present study was conducted to understand the change patterns of photosyn-thetic capacity and mineral nutrients in leaves of Shine Muscat grape during leaf development and se-nescence and analyze their correlations so as to find out the key elements for photosynthetic capacity de-velopment and maintenance during leaf maturation and senescence as well as the reutilization pattern of various nutrients during leaf senescence. These findings will provide important guidance for nutrient management in grape.【Methods】The experiment was carried out in the experimental grape orchard, South China Agricultural University, Guangzhou, China, in 2019 and 2020. Four- to five-year-old Shine Muscat vines on 5BB rootstocks in pergola trellis were used as the experimental materials. One month after berry clusters were harvested, leaves at all nodes of five canes from different vines were sampled during the postharvest tree recovery period in mid September 2019 after they were individually mea- sured for net photosynthetic rate (Pn) with a CIRAS-3 photosynthesis system (PP systems, Amesbury,MA, USA) and chlorophyll index (SPAD value) with an a SPAD-502 portable chlorophyll meter (Mi-nolta, Osaka, Japan). All the measurements were made from 9:00 to 11:00 in the morning in a clear day.The sampled leaves were then taken into the laboratory and individually washed with clean water and oven-dried at 65 ℃ for 3 days before used for mineral nutrient analysis. In late September 2020, a func-tional mature leaf at the 8th node from the tip on 3 canes with each of five vines was tagged for tracing changes in SPAD and photosynthetic parameters from late September to late December at an interval of one month. Leaves at the same node of neighboring canes were sampled for nutrient analysis in late September and late December. Based on differences in mineral nutrients between the functional leaves and the senesced leaves, the resorption rate, which reflects the remobilization ability of nutrients in se-nescing leaves, was calculated.【Results】(1) Leaf Pn and SPAD values increased as the node position in-creased. At node 8, Pn value reached a level similar to its maximum value obtained in leaf at node 16,while SPAD increased to a maximal value at node 10 and maintained relatively stable in older leaves,suggesting leaves at the 10 th node were fully mature. The senesced and yellowed leaves on the samecanes showed significantly reduced Pn and SPAD values compared with the leaves at the 16th node. (2)Concentrations of nitrogen, phosphorous, potassium and copper decreased initially with leaf expansion but maintained stabilized from node 8 up; calcium and manganese accumulated with leaf age and were the highest in the old senescing leaves; iron content decreased during leaf expansion but accumulated later; the remaining elements (magnesium, zinc and boron) showed no defined pattern. (3) During the growth season in mid-September, the yellowed and senescing leaves had lower contents of nitrogen,phosphorus, potassium, zinc, boron and copper compared with the functional mature leaf at the 16th node with the highest Pn value. The resorption rates of nitrogen, phosphorus, potassium, zinc, boron and copper in senescent leaves were 44.6%, 44.4%, 71.1%, 25.8%, 24.6% and 10.5%, respectively, indicat-ing these mineral nutrients in senescing leaves can be remobilized at different degrees; the resorption rates of magnesium, manganese, calcium and iron were close to zero or negative, indicating that these mineral nutrients can hardly be reutilized. (4) Correlation analysis between Pn and mineral nutrients con-tents in functional leaves at different nodes showed that only manganese had a significant and positive correlation with Pn. Manganese content and Pn could be perfectly regressed into a quadratic equation(R2=0.652 5, p=0.003), with a maximal Pn value occurring when manganese content was 66.96 mg·kg^-1.(5) During winter, systemic leaf senescence occurred in the vines, and only nitrogen, zinc and potassi-um had a positive resorption rate (24.3%, 18.4% and 0.6%, respectively), while those of phosphorus,calcium, magnesium, copper, iron, manganese and boron were all negative, indicating that these ele-ments accumulated with leaf senescence in winter and that these nutrients may loss as the leaves drop in winter. (6) During the senescence of leaves in winter, nitrogen was the only nutrient with a significant and positive linear correlation (R2=0.457, p=0.032) to Pn.【Conclusion】Based on the above results, the following conclusions could be drawn: (1) Leaf at the 8th node from the cane apex becomes fully mature and photosynthetic function maintains its maximal level until leaf senescence. (2) During the growth season, nitrogen, phosphorous, potassium, zinc and boron are remobilized in senescing leaves of Shine Muscat grape, but calcium, iron and manganese are not remobilized and accumulated with leaf aging. In winter, systematic leaf senescence occurs, and the mobilizability of all mineral nutrients from senescing leaves loses or becomes weaker in Guangzhou. (3) During the development of young leaves, manganese may be a limiting nutrient element for the formation of photosynthetic capacity in Shine Muscat leaves,while in old leaves, nitrogen serves as the key nutrient element for maintaining photosynthetic capacity.