| Tomato(Lycopersicon esculentum Mill.)is an important vegetable worldwide.With abundant nutrition,it becomes one of the most popular vegetables.Cracking of the tomato fruit epidermis may often occurs in the Yangtze River in summer due to the rainy anh high temperature.Cracking of the epidermis of harvested fruit degrades the appearance and increases the susceptibility of the fruit to infections by opportunistic pathogens,resulting in huge economic losses of toato production.Therefore,screening of cracking-resistant tomato and interpreting the mechanism of tomato cracking have specific practical and theoretical significance.In order to screen the agronomic characters related to tomato fruit cracking,make a clear relationship between pericarp and tomato fruit cracking,rich tomato fruit cracking mechanism,the following studies have been conducted:1.In order to screen the agronomic characters related to tomato fruit cracking,and provide academic evidence for germplasm evaluation of cracking-resistant tomato,we adopted 44 tomato genotypes as plant materials.And 16 agronomic characters were investigated and measured in this study.Then,the correlation analysis,path analysis,multiple linear regression analysis and principal component analysis were conducted to explore the relationship between different agronomic characters and tomato fruit cracking.The results showed that the variation of different agronomic characters was between 1.37%and 102.22%.When the Euclidean distance was 12.5,the tomatoes could be divided into three groups by cluster analysis.The correlation analysis and path analysis found that four agronomic characters were the most closely related to tomato fruit cracking.Dry weight and number of locules showed positive correlation with the cracking rate,while firmness and pericarp thickness/transverse diameter showed negative correlation with the crack rate.Furthermore,with the four agronomic characters as independent variable and the fruit cracking rate as dependent variable,the regression equation was built by multiple linear regression analysis,which was Y=0.717+0.08X1-0.674X2+0.007X3+0.0004X4.The four agronomic characters can reduce to two components including fruit morphology and skin mechanical properties by principal component analysis.2.In order to investigate the influences of anatomical structure and cell wall composition in pericarp on the fruit cracking of tomato,a cracking-resistant genotype'LA1698' and a cracking-susceptible genotype 'LA2683' were used as tomato materials in this study.At the stage of mature green,turning stage and red ripe,the anatomical structure of tomato pericarp was observed and the cell wall composition,such as pectin and hemicellulose were determined,to compare the differences of the two genotypes and analysis on the relationship between these indexes and tomato fruit cracking.The results illustrated that the fruit cracking of the two genotypes appeared at the stage from turning stage to red ripe,and the fruit cracking rate of 'LA2683' was higher than 'LA1698'.There were significant differences on the anatomical structure between two tomato genotypes:the cuticle and thickness of subepidermal cell layer of 'LA1698' were thicker.The cuticle and epidermis of 'LA1698' linked more closely,and cells were smaller and more neatly arranged,but the characteristics of 'LA2683' were opposite and its subepidermal cells were obviously shrinked.The water soluble pectin(WSP)content in cell wall of the susceptible genotype increased during the maturation of tomato,while the WSP content of cracking-resistant genotype increased followed by a decrease.The other components in cell wall of the two genotypes were all decreased.The chelator-soluble pectin(CSP)?Sodium carbonate-soluble pectin(SSP)?Total pectin and hemicellulose(combined with cell wall tightly and loosely)content of 'LA1698' were significantly higher than that of 'LA2683'.The WSP/(CSP+SSP)ratio of 'LA1698' was from 0.35 at the mature green period to 0.72 at the red ripe period,and the ratio of 'LA2683' was from 0.58 to 1.18.In conclusion,fruit cuticle and subepidermal thick,subepidermal thickness cells smaller and regular arrangement,high content of covalently and ionically bound pectin and hemicellulose content,and smaller ratio of WSP/(CSP+SSP)might be the key reason for fruit cracking-resistant.3.In order to investigate the impact of mineral content and cell wall hydrolase activity of pericarp on the pericarp characteristics and fruit cracking of tomato,and enrich the mechanism of tomato fruit cracking,a cracking-resistant genotype 'LA1698' and a susceptible genotype 'LA2683' were used as materials in this study.Our aim is to compare the mineral content and the main hydrolase activity in cell wall of pericarp of the cracking-resistant and cracking-susceptible genotypes,and analyze the relationship between the physiological indexes and tomato fruit cracking.At the stage of tomato mature green,breaker stage,turning stage,pink stage and red ripe,the hardness and consistency of pericarp,the Ca2+,Mg2+,K+ and B elements content in pericarp,the activity of PG.PME??-Gal and CX in pericarp were determined.The results showed that the content of Ca2+ and B was higher,the Mg2+ content was lower after breaker stage,the K+ content of the cracking-resistant genotype was lower than the susceptible genotype except for pink stage in pericarp of the cracking-resistant tomato fruit as compared with the cracking-susceptible genotype.The activity of four hydrolases of the susceptible genotype was higher than the cracking-resistant genotype,and the variation trends of hydrolases activity of the two genotypes were the same during tomato mature process.The activity of PG was very low at mature green and breaker stage and increased rapidly after breaker stage;(3-Gal had a higher activity at the pre-mature of tomato fruit,while the activity of PME and CX were relatively stable.In summary,a lower content of Ca2+ and B and a higher activity of cell wall hydrolase could promote the degradation of cell wall and result in the decline of the hardness and extensibility of pericarp,which might be the main reason for fruit cracking. |
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