Analysis of the optimal application time of prohexadione calcium and its physiological effects on the growth regulation of new shoots of sweet cherry

【Objective】The origin of sweet cherry (Prunus avium L.) was believed to be traced back to a region south of the Caucasian mountains around the Caspian and Black Seas before they were disseminated across Europe and Russia by explorers. In China, its main production areas are in the northern regions. In recent years, sweet cherry has gradually become the main cultivated tree species in southern leisure picking orchards. However, due to the influence of factors such as climate, some varieties of sweet cherry trees are prone to vigorous growth, resulting in dense canopies. This results in insufficient light entering the inner chamber, leading to frequent occurrence of pests and diseases, difficulty in pruning, and even affecting flowering. Eventually, it will impact yield and fruit quality. Prohexadione calcium (Pro-Ca) is a new type of plant growth regulator, featuring low toxicity, no residue, easy absorption, and no pollution to the environmental air. However, the application effect of Pro-Ca is influenced by factors such as spray concentration and time. To achieve better control of plant growth, the timing of applying is more crucial than the concentration. This study aimed to determine the appropriate period for spraying Pro-Ca to control the growth of sweet cherry shoots in the southern China, and analyze the ef-fects of Pro-Ca on the physiological indicators related to the new shoots of sweet cherry.【Methods】The treatment was conducted during the two-year periods of 2022—2023. The 7-year-old Brooks/ (Gisela 6 rootstock) was selected as the test material. Using the same concentration of Pro-Ca (≈166.67 mg ·L-1 ) to spray at the initial stage of new shoot emergence and new shoot rapid growth respectively. The inhibitory effects on shoot growth were evaluated by comparing the growth and elongation of new shoots with those of controls. And for the year with better inhibitory effects, the relevant physiological indicators were measured, including carbohydrate content, photosynthetic pigments content, SPAD, endogenous hormones content, and leaf microstructure of the new shoots. Combined with cluster analysis and correlation analysis, the influence of Pro-Ca on the physiological indicators of the new shoots' leaves was determined.【Results】The results revealed that when Pro-Ca was applied at the initial stage of new shoot emergence, it exerted scarcely any inhibitory effect on the growth of vegetative shoots in the summer, while when the Pro-Ca was applied at the initial stage of rapid new shoot growth, the growth of vegetative shoots in the summer was significantly inhibited. From June to October, the total growth length of new shoots in the treatment group and the control group was 25.94 cm and 29.19 cm respectively, indicating that the treatment group had a 11.12% decrease compared to the control group. The total growth length of internodes in the treatment group and the control group was 4.55 mm and 5.17 mm respectively showing that the treatment group had a 12.00% decrease compared to the control group. After treatment with Pro-Ca, the contents of soluble protein and starch in the new shoot leaves were increased rapidly. By the beginning of August, these two indexes contents were higher than those of the control. The rate of increase in glucose, fructose and sucrose contents in the new shoot leaves of the treatment group was slower than that of the control group, and the values were significantly lower. From July to October, through a series of dynamic changes, by early November, the glucose, fructose and sucrose contents in the leaves of the treatment group were all higher than those of the control group. Among them, the contents of glucose and fructose had significant differences between the two groups. The contents of IAA, ZR, GA3 and GA4 in the new shoot leaves had a significant decrease within the first month of the treatment, while the content of ABA increased. From August to September, the contents of all five endogenous hormones showed the same trend among the groups. By November, the treatment group exhibited lower levels of IAA, ZR, GA3, and GA4 compared to the control group, but contained higher concentrations of ABA. Meanwhile, the SPAD value, chlorophyll a and chlorophyll b content were all increased in the treatment group. The analysis of the leaf anatomical structure indicated the thickness of the leaves also increased. Compared with the control group, the leaf tissue structure exhibited higher compactness, but the PT/ST ratio decreased. Cluster analysis indicated that the period from 0 days to 60 days was the optimal time for the Pro-Ca effect to be exerted. Cluster analysis revealed that the period from 0 to 60 days represented the optimal time window for the Pro-Ca effect to be exerted. The correlation analysis indicated that there was a highly significant or significant correlation between the growth elongation of new shoots and the indicators of carbohydrates, chlorophyll, and endogenous hormones. Among them, more than four related indicators belonged to endogenous hormone catogory. In particular, GA3 had a correlation with six other indicators, of which three of these indicators had highly significant correlations. Notably, Pro-Ca had a significantly impact on the hormones within the new shoot leaves than on carbohydrates and photosynthetic pigments.【Conclusion】The treatment with Pro-Ca caused a series of changes in physiological indicators such as carbohydrates, endogenous hormones, photosynthetic pigments content within the leaves of new shoots with the most pronounced effects observed on endogenous hormones. Pro-Ca primarily influences plant growth by modu-lating endogenous hormone levels, thereby achieving a growth-inhibiting effect. In southern China or similar habitats, a single spray of 125-250 mg · L- 1 of Pro-Ca at the initial stage of new shoot rapid growth can inhibit the summer vegetative shoots growth of Brooks sweet cherry. If a more effective shoot suppression effect is desired, a second spray can be applied during the period when the effect weakens. This study will provide a theoretical basis for establishing scientific and effective technical measures for regulating the nutritional growth of sweet cherry in south region and similar habitat areas.