The Characteristics Of Sugar Accumulation And Its Relationship With Fruit Size During The Fruit Development In ’Hosui’ And ’Yali’ Pear

Sugar is one of the key indicators of fruit quality. A lot of research has been carried out in fruit sugar accumulation, but this study in pear is limited. Pear is cultivated in China with great area and rich varieties, and the growth and bearing habit among cultivars are quite different. However, the differences of sugar accumulation and its regulation in different species need to be studied systemly. On the other hand, high quality and large fruit pear cultivation has become an urgent requirement for industrial development in pear; however, the relationship between sugar accumulation and fruit size is still unknown. Therefore, in this study, we used the high-sugar, large fruit shape pear cultivar’Hosui’and the low-sugar, small shape pear cultivar ’Yali’as materials, to study the two varieties photoassimilate distribution, fruit sugar accumulation, changes in fruit sugar metabolism enzymes, gene expression differences and the effect of fruit thinning and different canopy structure on fruit size and sugar accumulation, to explore the sugar accumulation mechanism on pear fruit. The main purpose of this study was to provide the theoretical basis for the production of the large size and high quality pear fruit. Main results as following:1. Comparing the net photosynthetic rate of leaves in’Yali’pear and’Hosui’pear, the rusults indicated that which in’Yali’pear is higher than which in’Hosui’pear.14CO2tracer results indicated that photoassimilate tranport from the leaves to the fruit core and seeds firstly, then transport to flesh of the fruit. Transport of photoassimilate in’Hosui’pear was significantly faster than which in’Yali’pear.2.’Hosui’pear fruit total soluble sugar was significantly higher than which in’Yali’ pear. The difference was due to the different content of sucrose in the two cultivars.’Hosui’ pear sucrose accumulation rate rapidly increased from112days after anthesis (DAA) to fruit ripening, but glucose and sorbitol contents were decreased during this period. The sucrose content in ripening’Hosui’ fruit was up to43.08%. The’Yali’pear fruit sugar accumulation was increased smoothly during its development, although a small amount of sucrose increased before harvest, the proportion of sucrose was less than10%.3. Fruit weight and soluble solids content (sugar) accumulation was significantly correlated both of the two cultivars, and correlation coefficient r=0.6306**,0.8028**. In ’Hosui’pear, sucrose was straight line correlation with the fruit weight; glucose, fructose, sorbitol and total soluble sugars was in a curve line correlation with the fruit weight, and correlation coefficient r=0.9896**,0.9689**,0.9575**,0.9955**,0.9852**, respectively. In’Yali’pear fruit, sucrose, glucose, fructose, sorbitol and total soluble sugars was in a straight line correlation with the fruit weight, and correlation coefficient r=0.9681**,0.9789**,0.9414**,0.9620**,0.9661**, respectively, all of these data reach the significantly difference. In’Hosui’pear fruit, soluble sugar was increased with the increment of fruit weight, but increase rate slowed down after300g. The increased sugar was sucrose, but fructose and glucose was decreased slightly. In’Yali’ pear fruit, soluble sugars was increased with the increment of fruit weight, showed a significant linear relationship between the two factors, and no upper limit.4.Both of’Hosui’ and’Yali’ pear, the small fruit of the early stage showed bad enlargements in the later stage, less sugar accumulation, which was mainly due to the difference of sucrose content difference between large and small fruit in’Hosui’pear, and the former is3.25times larger than the latter. The small fruit of’Hosui’pear was mainly accumulation fructose and glucose, which development curve smoothly, accumulation pattern was consistent with the’Yali’pear fruit. In’Yali’pear, large fruit accumulated more sucrose, glucose, fructose, sorbitol than small fruit.5. Sugar accumulation in fruit was regulated by endogenous hormone both in’Hosui’ and’Yali’pear. The content of ABA, GA, IAA and ZT in early stage of’Hosui’pear fruit was higher than which in’Yali’ pear. These endogenous hormones in large fruit was higher than small fruit in both of the two cultivars, indicating high levels of endogenous hormones was associated with photoassimilate transport and accumulation to the fruit. Using GA3+GA4+7to treat the small fruit and using PP333to multiple treat surfaces of large fruit, finding GA3+GA4+7treatment increase in41.5%of weight,21.6%and32.5%increment of soluble sugar and sucrose in small fruit, respectively. In PP333treatment, the large fruit weight decreased35.6%, and the soluble sugar and sucrose decreased18.5%and27.8%, respectively. These results indicated that there was "positive synergy effect" between fruit size and sugar accumulation.6. The sorbitol and sucrose metabolism enzymes were measured during the development both of the’Hosui’and’Yali’pear. The results showed soluble acid invertase activity and cell wall acid invertase were higher in the early stage of fruit development, and decline to lower level in the later stage of fruit enlargement in’Hosui’and’Yali’pear fruits. Sucrose phosphate synthase showed higher in early stage of fruit development, lower in the fruit enlargement and increase in the ripening stage, and the sucrose phosphate synthase activity in’Hosui’pear was higher than which in’Yali’pear. NAD+-dependent and NADP+-dependent sorbitol dehydrogenase increased gradually with fruit development, both of these enzymes activity in’Hosui’pear was higher than which in’Yali’pear, suggesting the two enzymes may be beneficial to enhance the sink strength.7. By using three software packages of geNorm, NormFinder and BestKeeper, we identified that EF1α, TUB-b2and GAPDH were the most reliable reference genes across all samples together and across the developmental stage samples. EF1α, TUB-b2and ACT were most stable across the different genotypes samples. GAPDH and EF1α were the most stable reference genes across the different tissue samples. For phytohormone-treated samples, TUB-b2and GAPDH were the most stable reference genes. Using real-time quantitative PCR (RT-qPCR) technique studied the major metabolic gene expression including vacuole acid invertase (VAVI), sucrose synthase (SS), NAD+-dependent sorbitol dehydrogenase (NAD+-SDH), sorbitol-6-phosphate dehydrogenase (S6PDH) of different development periods and different size fruits. The results showed that VA VI expression was higher in the early stage and lower in the late stage of pear development; expression of SS and NAD+-SDH was higher in the early stage of large fruit than which in small fruit of ’Hosui’pear, but the NAD+-SDH expression in small fruit was higher than the large one in the later development stage of’Hosui’pear. In the later development stage, NAD+-SDH expression in’Yali’was higher than which in’Hosui’and which expression in large fruit was higher than in small fruit in’Yali’. These results further demonstrate that the sucrose accumulation-type for’Hosui’, hexose accumulation-type for’Yali’and sucrose accumulation in large fruit was higher than which in small fruit.8. Using’Hosui’pear as material to study the effect of fruit size and sugar accumulation on fruit thinning and different canopy structure. The results showed:(1) changing the sink and source relation by thinning fruit can change the fruit size and affecte fruit sugar accumulation. In’Hosui’pear fruit, the soluble sugar decreased19.3%, and the sucrose with the highest decrease,40.2%decrease compared with fruit heavy thinning. In the high fruit loading, the leaf photosynthetic rate increased at first and then declined. Osmiophilic granules in the leaves significantly increased and chloroplast lamellae fuzzy under high fruit load conditions.(2) Using the WinsCanopy2005a canopy analyzer, light meter, Li-6400photosynthesis system and SPAD-502chlorophyll meter to study different tree systems. The results showed that leaf area index and numbers of young bearing branch were significantly lower in horizontal trellis system (HTS) than that in delayed-open central leader system (DLS), canopy openness, direct PPFDs, indirect PPFDs and total PPFDs under tree were significantly or very significantly higher in HTS than that in DLS. Single fruit weight, total contents of sugar of HTS were higher than which of DLS. The net photosynthetic rates and SPAD values of the leaf from fruits assistant branch were affected by tree systems, but the SPAD values of leaf from lateral shoot of bearing part has no significant correlation with the fruit sugar accumulation.