The production conditions and adaptation mechanism of sunburn in apple

Abstract: Apple production areas are mainly distributed in semi-arid areas, with the shortage of water resources and the extensive application of dwarfing rootstocks. The occurrence of apple sunburn is be- coming increasingly serious, reducing fruit quality and the economic benefits. Fruit bagging in apple is widely used in China where the reason causing apple sunburn might be different from that in other pro- duction areas, so it is urgent to study apple sunburn based on China’s climate condition and cultivation patterns. In this paper, the authors analyzed and summarized the types of apple sunburn, the develop- ment conditions and the adaptation mechanism of apple peel, so as to provide a theoretical basis for the prevention of apple sunburn in China. At present, apple sunburn is divided into three types: sunburn ne- crosis, sunburn browning and photooxidative sunburn. Excess radiant heating and/or exposure to excess sunlight are the direct causes of sunburn, other factors (relative humidity, wind speed, variety and culti- vation management) could interact with high temperature and strong light to influence the occurrence and development of sunburn although each of them alone might not induce the apple sunburn, but.The sunburn necrosis occurs when the temperature of fruit surface (FST) exceeds a threshold (52±1 °C), and lasts for 10 min, causing thermal cell death with complete inactivation of the photosynthetic, and black or brown necrotic spots appear on the sunny side of the fruit peel. The sunburn browning appears when the FST reaches 46-49 °C and lasts for more than 60 min, yellow, brown or dark brown spots will ap- pear on the sunny side of the fruits in most varieties and which is the most common sunburn type in non-bagging apples. The photooxidative sunburn usually occurs when the shaded fruits are suddenly ex- posed to the high temperature and strong light, and white spots will appear on the fruit surface. The pho- tooxidative damage occurs on the apple skins during the typical sunburn formation. When the apple is exposed to the strong light, the light energy absorbed by the skin tissue will gradually increase, when it  exceeds photosynthetic requirements, a large number of reactive oxygen species (ROS) will be pro- duced, leading to photooxidation. If the fruit continues to be exposed to the high temperature and strong light, sunburn damage will occur. In this process, the skin tissue will reduce the accumulation of the heat and the ROS, and the damage to photosynthetic organs occur due to the changes of its own physio- logical and biochemical processes, such as accumulating photoprotective pigments and promoting bio- synthesis of antioxidants and so on. However, when stresses exceed certain thresholds (duration and in- tensity), these mechanisms become inefficient, resulting in sunburn symptoms. In the course of sunburn development, the pigment content and composition of apple peel will change significantly. The content of chlorophyll is decreased and the content of carotene and flavonoids are increased, but the content of anthocyanin, the main pigment of red apple, also vary with different types of sunburn. Anthocyanin ac- cumulation was found around the photooxidative sunburn spots, while in the interior of sunburn spots, anthocyanin content was lower. For the Sunburn browning, the content of anthocyanin in the spots of fruits was lower than that of the normal apple skins. Chlorophyll and carotenoids are also the main pig- ments that determine fruit color. In all types of sunburn, the content of carotenoids and flavonoids are in- creased, indicating that these yellow pigments have a higher stability to the bright light and high temper- ature than other pigments, and had a more stable protective effect on the fruits which potentially would be damaged by sunburn. Under the excessive light conditions, the ROS is formed by direct photoreduc- tion of O2 in the PS I reaction center, at the same time the ROS is cleared by a comprehensive antioxi- dant system composed of enzymes and non-enzymatic antioxidants. The enzyme system includes super- oxide dismutase (SOD), polyphenol oxidase (PPO), peroxidase (POD) and catalase (CAT). Non-enzy- matic systems include substances such as ascorbic acid and glutathione circulation (AsA-GSH). In the process of scavenging ROS by the enzyme system, the SOD can transform the initial product ·-of pho- toreduction into H2O2, and then H2O2 is mainly reduced to H2O by CAT, POD, PPO and AsA- GSH, while AsA-GSH is thought to be a more effective photoprotective metabolic pathway.