Study On Fruit Quality And Seed Desiccation Sensitivity Of Main Longan Cultivars In Chongqing

Longan(Dimocarpus longan Lour.)is an evergreen subtropical fruit tree originated in southern China.It‘s long cultivation history can be traced back to the Han Dynasty.Longan fruits have high economic value because of their fruit good flavor such as sweet and refreshing,and the medicinal effect such as tonifying deficiency and replenishing intelligence.Longan fruit product is also an advantageous agricultural product in China because of its wide planting range and rich resources.Chongqing is the northern edge region of longan cultivation in China.The fruit maturity is about one month later than that in the southern main producing areas,which can extend the shelf life.However,the varieties of longan in Chongqing are complexity.The relative research on fruit quality and standardized seedling raising is less.As a typical recalcitrant seed,longan seed is not tolerant to dehydration,and will soon lose germination ability under natural dehydration.Based on the selection of high quality late maturing varieties and the basic characteristics of recalcitrant seeds,the analysis on morphological characteristics and physiological and biochemical indexes of longan fruits of six main cultivars of Chongqing longan was carried out in this study to screen out the longan varieties with high comprehensive quality.Then,the seeds were taken out from the ripe fresh fruit,and a single factor gradient dehydrated experiment was design to explore the effect of water stress on the vigor and germination of longan seeds.By the further analysis on physiological and biochemical characteristics and molecular biology,we hope to can prove the dehydration sensitive parts of longan seeds.The study results have important guiding significance for the selection of longan superior varieties,seed storage,germplasm conservation and propagation.Therefore,this paper designed some experiments and following research contents were carried out.The mature fresh fruits of six main varieties were collected from longan base in Yongchuan District of Chongqing City.The external quality and internal quality of fruits were determined.Correlation analysis and principal component analysis were used to compare the characteristic differences and correlation among varieties.the evaluation target indexes of high-quality varieties were selected from many measurement indexes.Finally,the high-quality late maturing varieties were selected by ranking according to the membership function value.Then,the seeds of Youtanben,a high-quality variety,were selected as the test materials for dehydration sensitivity analysis.The seeds were put into a sealed desiccator with quantitative color changing silica gel and dehydrated slowly at 25℃.Sample every 4 hours.The gradient dehydration range was 0-40 h.The dehydrated seeds were tested for physiological and biochemical indexes,germination tests,paraffin sections and gel electrophoresis.The main research results were as follows.1.There were significant differences in fruit quality among different longan varieties.In appearance quality,varieties could be identified according to fruit size,peel color,peel crack,pulp color,etc.There was no significant difference in internal quality,which showed that the stable genetic characteristics of longan.The correlation between single fruit quality and edible rate was not high,which indicated that large fruit and fruit weight did not mean high edible rate.While pulp flavor was significantly correlated with soluble solids.The soluble solids content of varieties was also high with the good flavor.The genetic variation among varieties is stable and abundant.Seven indexes with high contribution rate were selected as the key indexes in the evaluation.They were fruit shape,single fruit weight,fresh weight of pulp,edible rate,soluble solids,VC content and titratable acid.Based on the membership function analysis and comprehensive evaluation of the quality of each variety,it is considered that Chuliang and Youtanben are excellent late maturing varieties of longan in Chongqing.These two varieties are good materials for breeding and quality improvement.2.After gradient dehydration,the viability and germination rate of longan seeds decreased with the reduction of moisture content.Seeds almost lost viability after excessive water loss.The results showed that the moisture content of fresh seeds was as high as 41.51%,which could germinate rapidly under suitable conditions.The germination rate increased when seeds were dehydrated to 38.71%.It indicated that moderate dehydration could promote seed germination and 38.71% was the suitable moisture content for seed germination.When dehydrated to 28.28%,the seed viability decreased significantly,it‘s germination rate decreased by nearly 50%,which means that 28.28% moisture content was the critical water content of longan seeds.When dehydrated to about 23.12%,the seed viability decreased by nearly 74%,the germination energy and germination index approached to 0,and the seeds almost lost vitality,which indicates that 23.12% moisture content was the lethal water content of seeds.3.The changes of physio-biochemical characteristics of seeds with different water contents were evaluated by the contents of osmotic substances,inclusions and activities of antioxidant enzyme system.During the dehydration process,water loss caused serious damage to the seed biofilm system.The electrolyte extracted.The soluble sugar content increased significantly.The soluble protein content went upward during the dehydration period.Meanwhile,the antioxidant enzyme system responded rapidly to resistance to water-loss stress.With the increasing of water-loss,the content of soluble protein and the activity of antioxidant enzymes decreased significantly,indicating that the seeds were seriously damaged and the metabolism system was disordered.Seeds almost lost vitality.Under the light microscope,the cell structure changes of endosperm of longan seeds a were observed such as cell wall and cell membrane rupture,starch grains decomposition and cell vacuolation in the process of dehydration.These change indicated that water stress led to the disintegration of a large number of substances in cells and significantly affected the seed viability.The study results of physio-biochemical characteristics and microstructure analysis showed that the sensitive part of longan seed dehydration was biofilm system.4.Based on the fact that the content of soluble protein in dehydrated seeds,gel electrophoresis was used to analyze the changes of protein composition in the process of seed dehydration.The results showed that dehydration could induce seeds to obtain a certain tolerance to dehydration.The moderate dehydration could induce the expression and accumulation of specific proteins which resulted in the protein bands becaming clearer.With the continuous decrease of water content,the protein content of seeds decreased significantly,and the protein bands were fuzzy.There were almost no protein bands in the seeds with lethal water content,whch indicated that longan seeds were seriously damaged and protein denatured.The results of band analysis of protein gel electrophoresis further confirmed the synthesis,expression and denaturation of longan seeds with specific resistance proteins to water stress during dehydration.