Effects of calcium and IAA treatments on active oxygen metabolism and related antioxidant gene expressions in the peel of cracking sensitive fruit of Miguang tangerine

Abstract:【Objective】Miguang tangerine is world-famous for its golden color, juice, sweetness, sour and aroma. However, the fruits are prone to cracking in the middle and late stage of ripening stage and easy to abscise and rot, affecting the fruit quality and causeing great economic losses and restricting the further development of citrus industry in Jiangxi province, China. Imbalanced metabolism of reactive oxygen species (ROS) can lead to cell membrane damage, which increases the sensitivity of the peel to environmental changes, affecting fruit dehiscence. Antioxidant enzymes (SOD, CAT, APX, GPX, POD) can scavenge ROS and reduce or prevent the formation of hydroxyl radicals. The plant ROS scavenging  system is coordinated to keep ROS content in a relatively stable state and maintain the stability of bio- logical membrane system. Application of mineral elements such as calcium can promote the peel growth, enhance its protection ability, and reduce fruit cracking. The mechanism of calcium reducing fruit cracking of Miguang tangerine will provide theoretical basis for effective prevention and mitiga- tion of the fruit cracking in production.【Methods】Miguang tangerine fruit peels were used as the ex- perimental materials. In the late of May and June, the whole plants were sprayed with tap water (con- trol), 0.14 g·L-1 Ca (Ca), and 0.14 g·L-1 Ca+20×10-6 IAA (Ca+IAA) until the leaves and fruits wetted. Each treatment was repeated six times. Fruit samples were collected on 30th day of each month from June to November during fruit development period and taken back to the laboratory to determine the contents of , H2O2 and MDA, the expressions levels of related genes CsSOD, CsCAT, CsPOD, CsAPXand CsGPX, as well as the changes of relative enzyme activities. In order to analyze the effects of differ- ent calcium treatments (Ca, Ca+IAA) on fruit cracking rate, Excel was used to sort out the experimental data and was used to plot, IBM SPSS Statistics 25.0 and DPS was applied to analyze the significance differences.【Results】In this study, the contents of and H2O2 were at a high level in June, which acti- vated antioxidant enzymes to decompose them in time, resulting in a decrease the content of ROS in July. The oxidative stress increased after July, especially in August and September when fruit cracking broke out, which was the key period for ROS accumulation. However, the insufficient scavenging capacity of antioxidant enzymes would destroy the balance of antioxidant system, leading to the continu- ous membrane lipid peroxidation, resulting in fruit cracking. This cracking could damage peel cell mem- brane, leading to ROS accumulation and MDA generation, thus causing damage to peel cells and reducing fruit resistance. This might be the reason why the contents of , H₂O₂ and MDA in the peel of cracked fruits were always higher than those in the normal fruits. The calcium treatment could signifi-cantly reduce ROS and MDA accumulation in the peel, and alleviate the cellular toxicity. Meanwhile, calcium ion improved antioxidant capacity, significantly enhanced the expression levels of antioxidant enzymes related genes CsSOD, CsCAT, CsAPX and CsGPX, and reduced the expression levels of Cs- POD. Furthermore, the activity of antioxidant enzymes (SOD, CAT, APX, GPX) increased and POD ac-tivity decreased to improve the ductility of cell wall, which slowed down the occurrence of fruit crack- ing, and kept the whole antioxidant system in dynamic balance.【Conclusion】The above results showed that the fruit cracking of Miguang tangerine was a serious physiological disease. It is closely related to ROS and antioxidant enzyme metabolism. Dehiscent fruits often occure in August and September, which is a critical period of the highest content of ROS in Miguang peel. The calcium treatments could significantly enhance the peel antioxidant related gene expression, increasing the activities of antioxi- dant enzymes in order to scavenge ROS. Therefore, maintaining the balance of active oxygen metabo- lism would play a vital role in the peel cells for reducing the dehiscent fruit rate. This study would pro- vide theoretical basis for preventing fruit cracking in production.