Studies On The Pesponse Of Sand Pear To High-temperature And Heat-tolerance Mechanism

Pear is an important commercial fruit crop widely cultivated in China except Hainan province. As one of the four main originating cultispecies in China, Chinese sand pear is widely cultivated in South China as it originates in Yangtze River and its south region and could adapt to the high temperature and high humidity conditions. However, climatic conditions remain the limiting factors for pear yield and quality, especially the unusual temperature changes during the critical periods of fruit development, for example, freeze injuries or poor pollination and fertilization caused by extreme temperature during blooming period, fruit dropping and sunburn near mature period, early defoliation and re-blooming after fruit maturity, and these phenomena probably are related to high-temperature conditions. With growing global warming, extremely high temperature conditions appear frequently in summer, which leads to obvious problems in pear industry. Therefore, the study on pear adaptability to high temperature condition will serve to improve cultivating measures and fruit quality.Firstly, one-year-old pot-grown pear trees (Pyrus pyrifolia Nakai cv. Cuiguan and Wonhwang) were exposed to two heat stress regimes, namely constant short-time (40℃,48h) or chronic heat stress (11:00-17:00,40℃,6d), to study the changes of mesophyll cellultrastructure and photosynthesis. The results showed that: under constant short-term heat stress, the numbers of osmiophilic globule and lipid droplet increased; starch grains in chloroplasts were not plump; chloroplasts and mitochondria were swollen; chromatin was agglutinated. Relative chlorophyll content (SPAD), net photosynthesis rate (Pn) and maximum photochemical efficiency of photosystem Ⅱ (Fv/Fnd were significantly decreased, which indicated that the leaf photosynthetic capability declined under heat stress. Moreover, the adaptation of pear leaves to heat stress was species-specific, because the damage ratio of chloroplast in ’Cuiguan’was55%, which was lower than that in’Wonhwang’(57%). Under chronic heat stress, mesophyll cell ultrastructure was not obviously damaged, but leaf photosynthetic capability was still restrained.To study the different thermotolerance in pear cultivars, the thermotolerance mechanism of ’Cuiguan’ and ’Wonhwang’ was analyzed under chronic heat stress. The results showed that:both MDA and H2O2were accumulated in pear leaves after heat stress, which indicated the biomembrane system was injuried by ROS. Ascorbate peroxidase activity in’Cuiguan’was increased, but not in’Wonhwang’. Six APX gene members were identified from’Cuiguan’leaves, which encoded three cytosolic APXs (PpcAPX1、PpcAPX2and PpcAPX3), one peroxisomal APX (PppAPX) and two chloroplastic APX (PpsAPX and PptAPX). The isoenzymes cAPX1and sAPX were confirmed to be localized in the cytoplasm and chloroplasts, respectively. Totally,62exacting elements related to stress response were identified in APX gene promoters, and the number, location and appear frequency were different among APX gene members. Inducible expression of PpAPX genes in the cytoplasm, chloroplasts and peroxisomes was consistent with increased APX activity in’Cuiguan’, whereas only weak induction of PpAPX genes was observed in’Wonhwang’, which was one of the reasons for different thermotolerance in pear cultivars.Proteomic analysis showed that the pre-treatment by sub-high temperature or H2O2could induce acquired thermotolerance in’Cuiguan’leaves under following heat stress. Large majority of down-regulated proteins were related to photosynthesis and stress response, which indicated the inhibition of photosynthesis and initiation of stress response under heat stress. Heat shock proteins including Hsp17.5, Hsp17.8and Hsp70were up-regulated under heat stress. And the up-regulation of HSP was mainly related to the induction of HsfA2.Based on the different thermotolerance, heat-sensitive cultivar’Wonhwang’was chosed to study the influence of heat stress on fruit quality. The data showed that: sorbitol accumulation in leaves was decrease as a result of photosynthesis inhibition, but sucrose content was increased. The decrease of sorbitol in leaves was a result of down-regulation of S6PDH activity and up-regulation of NAD+-SDH activity, while only NAD+-SDH activity was inhibited in fruit flesh. One S6PDH (PpS6PDH), three NAD+-SDH (PpSDHl-3) and two SOT genes (PpSOTl-2) were indentified from ’Wonhuang’. Moreover, the transcriptional response of these genes to heat stress was tissue-specific. PpS6PDH was down-regulated in leaves while no apparent changes in fruit. Up-regulation of PpSDHl and PpSDH3resulted in enhanced enzyme activity in leaves, while down-regulation of PpSDHl and PpSDH2in fruit resulted in inhibited enzyme activity in flesh. The unloaded of sorbitol in leaves was inhibited as expression of PpSOT2was down-regulated under heat stress. Sucrose metabolism both in leaves and flesh was also changed under heat stress. With the weakened sorbitol synthesis in leaves sucrose anabolism was enhanced, and sucrose in flesh was also accumulated, in addition, the sucrose accumulation both in leaves and flesh was catalyzed by SUS and SPS.