Mechanism Of Fruit Cracking And Molecular Marker In Watermelon(Citrulls.lanatus)

The fruits of watermelon are often found cracking duiring the whole Fruit development. Fruit-cracking results in lower yield and quality, easier infection to the deseases and more difficulties for sales. So it is a great significance to study mechanism of fruit-cracking in watermelon. However, fruit-cracking indicators and evaluation criteria have not yet established, and some good crack resistance merterials have not been well used. Therefore, a group of tests, such as morphology, physical and chemical mechanisms, genetic mechanism, and molecular marker and so on, was carried out for understanding watermelon fruit cracking. In this study, we proposed evaluation criteria and reliable identification technology on fruit cracking-risistance. On these bases, we combine traditional methods with molecular technology for in-depth exploration fruit cracking theory and application. It is very important for further breeding on crack-resistant and germplasm improvement in watermelon. The major results are as follows:1. The correlation between fruit agronomic traits and fruit-cracking of watermelon, grades and symptoms of fruit-cracking in the field, and an accurate, stable, scientific identification method of fruit cracking were studied.Resistance to fruit cracking of different gene-types watermelon was evaluated under opening and protected conditions. Grading standards and evaluation criteria of fruit-cracking in watermelon are drawn up. According to the comprehensive evaluation, the variance of the cracking indices were significant in opening field (F=5.783**, F0.01=1.90, df=13) and protected field (F=10.7218**, F0.01= 1.90, df=13). The correlation coefficients of cracking indices in both conditions were significant (r=0.937**, r0.01=0.6614, df=13).The correlation and path analysis on 14 agronomic traits of watermelon were studied. The thickness of pericarp and green epicarps were significantly related to the rate of fruit cracking and can be taken as a main reference trait for selecting anti-cracking varieties.By taking fruit-cracking rate of inbred line in the field as reference, we compare the stability, sensitivity and practicality of different identification methods, such as morphological index (pericarp thickness, green pericarp) and artificially inducing methods (pressure method, extract decompression, cut cracking), by correlation analysis and cluster analysis. The results show that the fruit cut cracking method, which is easy and convenient to determinate and is practicable to reflect the true fruit resistance of watermelon varieties, can be used as the identification method of fruit cracking in watermelon.2. The relationship between water, KC1 and rate of fruit cracking in watermelonThe impact of water on watermelon fruit cracking is discussed. In the condition of the Water-control and water irrigation, we determinate water content of the various parts of cracking fruit and non-crack fruit. The results shows, watermelon fruit cracking rate, in water-controlled treatment, was significantly reduced; The relationship between rate of fruit cracking and water content in the rind of fruit cracking was significantly negatively correlated (r=-0.975), and the ratio of water content of fruit pulp and peel between cracking and non-cracking fruit was significantly correlated (r= 0.9074).The relation among KCl, physiological Emezy and fruit cracking resistance of watermelon were studied. The results show, correlation coefficient between superoxide dismutase (SOD), peroxidase (POD), malondialdehyde (MDA) and fruit cracking rate in the field was -0.52843,-0.46383, and 0.15047, which have no significant variance. When we spray KCl on the leaves during the development of watermelon, the cracking rate is decreasing, SOD, POD are increasing and MDA decreasing with the KCl concentration increased.3. Genetic Analysis of Fruit cut cracking and Green epicarpin WatermelonIn this paper, inheritance of fruit cut cracking and green epicarpwere analyzed by applying the joint segregation analysis of a mixed genetic model of major gene plus polygene in six generations(P 1, P2, F1, B1, B2, and F2)derived from two crosses of L-103 (fruit-cracking susceptible)×X-101 (fruit-cracking resistance). The cut fruit cracking was dominated by two major genes with additive-dominance-epistatic effects plus polygenes with additive-dominance effects(E-1 model)in Cross X-101×L-103, The hereditability of major genes(hmg2)was 40.34%,25.08%,33.10% in B1,B2,F2 population, and that of polygenes(hpg2) detected in B1,B2,F2population was 32.65%,0,50.20%. It also indicated that the environmental variance and the genetic variance accounted for 39.55% and 60.45%, on everage, of the phenotypic variance in each population respectively, indicating that the environmental factors have a great effect on fruit cracking resistance. The green epicarpwas dominated by two major genes with additive-dominance-epistatic effects plus polygenes with additive-dominance effects (E-1 model) in the Cross. Hereditability of major genes were 94.63%,93.88% in F2 and Blpopulation, while was zero in B2 population. Hereditability of polygenes only detected in B1 population was 0.544%. It indicated that the green epicarpin is dominated by major genes. In addition, the environmental variance was little in F2 population, so hereditability of major genes can be favorable to an efficient selection in early generation.4. The SRAP reaction system of Watermelon and screen of SRAP molecular markers linked with fruit cracking resistanceIn this experiment, the reaction system of watemelon SRAP was optimized, in terms of Mg2+, dNTPs, primers, Taq E and template DNA 5 factors and their 4 levels with the primer Em3-Me3, by an orthogonal design.The watermelon appropriate SRAP-PCR reaction system in a 10μL reaction system was:10xPCRbuffer2uL, Mg2+3.0 mmol/L, dNTPs-0.2mmol/L, primer 0.5umol/L, template DNA40ng, Taq polymerase 0.5U.In this paper, bulked segregant analysis was used to screen SRAP molecular markers linked to fruit anti-cracking of watermelon using two P1, P2 and extreme types of DNA pools. By the watermelon appropriate SRAP-PCR reaction system, 1(me8-eml) primer was found with polymorphic bands from 300 pairs of SRAP primer combinations. These were further amplified in 163 F2 single DNA. The results shows 125 single plant DNA have the same maker band as the anti-cracking parent in 138 resistant single plant, Only 5 single plant DNA has the maker in 25 susceptible single plant. This marker was testified with individual DNA of the F2 population and the band could only be amplified in the high cracking resistant plants. Linkage analysis using the software of MAPMAKER/Exp3.0 indicated its genetic distance to the fruit cracking resistance is 9.9cM.