Effect of potassium fertilization on the shrinkage of ‘Short branch’ winter jujube

Abstract:【Objective】Winter jujube is the most characteristic fruit in China.‘Short branch’winter ju- jube, also known as second generation of‘Zhanhua’winter jujube, is a new excellent bud mutation cul- tivar selected from the natural variation of winter jujube in Zhanhua district, Binzhou city, Shandong province. The shrinkage of‘Short branch’winter jujube initiates from the mature stage is a new dis- ease occurring in Shandong, Hebei, Shanxi, Shaanxi and other major cultivation areas in recent years. Once the disease occurs, the fruit loses its commercial value. In 2019, the incidence in Zhanhua district and Wudi counties was 35.2%-41.0%. No pathogenic bacteria have been isolated from the shrunk part, and without symptoms such as flesh browning and bitterness. It is thus different from the traditional fruit shrink disease. The shrinkage has become an important factor restricting the development of winter jujube industry in China. Studies have shown that potassium is closely related to the metabolic process of fruit trees, and it is an activator of many enzymes, participating in the synthesis, transportation and transformation of sugars and starch. Potassium is effective to enhance the hydrophilicity of protoplast and cell osmotic pressure, increase the sugar reserve in the tree, increase the cellulose content in the epi- dermis of branches and fruits, and reduce the wrinkled fruit of citrus. The application of potassium fertil- izer can also improve the shelf life, fruit firmness and coloration in peach fruit. Therefore, potassium is likely to be related to the shrinkage of‘Short branch’winter jujube. But the association between them is not clear. In order to verify their relationship and physiological mechanism, and then development an effective control method, this study was carried out.【Methods】The experiment was conducted in a win- ter jujube orchard in Zhanhua district and Wudi county, Binzhou city, Shandong province，from June to October 2018 and June to October 2019. According to the phenology of winter jujube, June 20 to July 20 was the young fruit stage, July 20 to August 22 the expansion stage, and August 22 the maturation stage of fruit. There were 4 treatments, the control, K2SO4 spray at young fruit stage, K2SO4 spray at expan- sion stage and K2SO4 spray at the early maturation stage. The concentration of the sprays was 0.05 g·mL- 1. For each stage, spray was conducted 3 times at an interval of 7 days around 18:00 pm until dripoff. Spraying water was used as the control. A single factor randomized experiment design was used. Each treatment had 3 repetitions each with 3 trees. There were 36 trees in total. Then protopectin and cellu- lose, activities of wall-degrading enzymes, cell wall component and microstructure and fruit hardness were analyzed.【Results】Spraying potassium at different stages reduced the rate of fruit shrinkage, in- creased the contents of pectin and cellulose in different degrees, decreased the activities of pectinase and cellulase, increased fruit hardness, and improved the microstructure of fruit. Among the treatments, spraying potassium during the fruit expansion stage was most effective in preventing fruit shrinkage and increasing pectin content, which were 60.65% lower and 95.4% higher than the control, respective- ly. Spraying potassium at the expanding and early mature stages helped to maintain the integrity of cuti- cle and the density and tidy arrangement of the epidermal cells, and reduce the number of cavities in pulp. The effect was the best when spray was made the in expanding stage. There were significant corre- lations between fruit shrinkage rate and pectin content as well as its catabolic enzyme activity, with a correlation coefficient of 0.997 6 and 0.994 6, respectively. Spraying potassium had little effect on cellulose content and there was no significant correlation between shrinkage rate and cellulose content as well as cellulase activity.【Conclusion】Spraying K2SO4 on the leaves during fruit expansion stage is the best way to reduce the occurrence of shrinkage in‘Short branch’winter jujube during maturation. The treatment might effectively promote photosynthesis, increase the protein content, promote the sugar translocation, and thus reduce fruit shrinkage incidence. Based on our study, spraying 0.05 g · mL- 1K2SO4 every 7 days from July 20 for 3 times can significantly increase the pectin content, enhance the fruit hardness and improve the fiber structure of the fruit, and thus is an effective measure to control fruit shrinkage in‘Short branch’winter jujube.