Influence of calcium on fruit cracking and its physiological mechanism in white flesh loquats

Abstract: 【Objective】White flesh loquat (Eriobotrya japonica Lindl.) has good quality and sweet taste, which is deeply loved by consumers. However, the peel of loquat is thin, and easy to crack in rainy weather from fruit expanding to mature stage. Fruit cracking can result in quality reduction and production decline, and lead to serious economic losses. Therefore, this study aimed to clarify the effect of calcium and related physiological changes on fruit cracking of white flesh loquats, and reveal the mechanism of calcium regulating physiological process.【Methods】The experiment was carried out at a loquat orchard of Citrus Research Institute of Zhejiang province, Taizhou (E121.25°, N28.65°), from2019 to 2020. 8-year white-fleshed loquat trees(Ruantiaobaisha) were taken as testing materials. 40 loquat trees with same growth vigor were randomly selected to investigate the fruit quantity and cracking fruit number of each tree, and calculate the cracking fruit rate(Cracking fruit rate/% = Cracking fruit number/total fruit number ×100). Then normal fruit samples were collected from trees with light cracking(cracking rate2+ content in fruits at different development stages. According to the results of fruit cracking investigation, the fruit trees with severe cracking degree were selected for spraying 0.05% CaCl2 on November 25, 2019(30 days after flowering), December 25, 2019(60 days after flowering) and January 25, 2020(90 days after flowering). Distilled water was sprayed as a control. The cracking rate was investigated in May 2020, and normal fruits were picked from calcium-spraying trees and cracked fruits were picked from control trees on May 15,with the same sampling method as above. The peel and pulp were separated, some of them were frozen in liquid nitrogen and then stored in a-80 ℃ ultra-low temperature refrigerator for determination of Ca2 +concentration, superoxide dismutase(SOD), catalase(CAT), peroxidase(POD), polyphenol oxidase(PPO), pectinase(PG) and cellulase(CL) activities, as well as GA3, IAA and ABA contents. The other part was green-killed at 105 ℃ for 20 minutes and then dried and grounded into powder at 60 ℃for the determination the contents of total nitrogen(N), phosphorus(P), potassium(K), calcium(Ca)and magnesium(Mg). Various analysis and correlation analysis were applied by using Excel 2010 and SAS 9.0. The data of each experimental treatment and the control were analyzed for significant difference. Excel 2010 was used to construct the graph.【Results】Loquat fruit cracking began at later fruit expansion stage, and reached the peak from April 28 to May 6 in southeast Zhejiang province. The content of P in normal fruit peel was 7.06 g·kg^-1, which was significantly higher than that in cracked ones(4.54 g·kg^-1), but there was no significant difference in pulp. The calcium content in cracked fruits was lower than that in normal fruits at the same part, and there was a significant difference in calcium content between normal and cracked fruits. The calcium content in peel was 19.72% and 36.41% higher than that of cracked ones, respectively. Fruit cracking rate was negatively correlated to Ca concentration in peel evidently, and the correlation coefficient was 0.8665. SOD and CAT activity of normal fruit peel was significantly higher than that of cracked ones, while the contrary result was detected in POD and PPO activity. SOD activity of normal fruit peel was 17.81% higher than that of cracked fruits, and SOD activity of normal fruit pulp was 40.58% higher than that of cracked ones. CAT activity of peel and pulp of cracked fruits was 17.78% and 7.87% higher than that of cracked fruits at the same part, respectively.A significant negative correlation was found between fruit cracking rate and SOD and CAT activity in peel. On the contrary, both the POD and PPO activities were lower in peel of normal fruits than those of cracked ones. POD activity of normal fruit peel was 21.20% lower than that of cracked ones, and there was no significant difference in peel between two treatments. PPO activity in peel of normal fruit was significantly lower than that of cracked fruits by 26.01%, and it was found positively correlated with fruit cracking rate. The activities PG and CL of cracked fruits peel were much higher than those of normal ones(28.82% and 54.85%, respectively), and the same results were found in pulp. Fruit cracking rate was significantly and negatively correlated with PG and CL activity in peel. GA3level in pulp and ABA content in peel were found to evidently increase in cracked fruits. Ca content in fruit pulp was positively correlated with SOD activity, but negatively correlated with PPO, PG and CL activities as well as ABA content. Ca2 +concentration was the highest at young fruit stage, decreased significantly at expansion stage, and reached the lowest value at maturity stage. Exogenous calcium treatment significantly increased fruit calcium content and reduced fruit cracking rate. SOD and CAT activity significantly increased in fruits with calcium treatment than that in non-treated ones, while POD, PPO, PG and CX activities were found to evidently decrease. ABA content in peel and GA3content in pulp decreased by19.41% and 28.18% in calcium treated fruits than that in control ones, respectively.【Conclusion】The absorption of calcium nutrient by loquat fruit mainly occurred from young fruit stage to expansion stage. The deficiency of calcium nutrient in fruit was one of the important reasons for loquat fruit cracking. Spraying calcium at flowering and young fruit stage could significantly reduce fruit cracking rate by regulating the antioxidant enzyme system, cell wall hydrolase system and hormone synthesis pathways.