| By using six generation of three summer squash crosses, we studied the heterosis performance, the correlation between characters and the genetic mechanism of heterosis of summer squash. The aim was to provide theoretical basis for heterosis utilization and parent selection.1. The result of heterosis analysis indicated that: there were all improvement in yield characters, fruit characters and plant characters, but the degree of improvement was different between each character of each cross. In yield composed characters, character of yield per plant showed the highest heterosis, yield of each hybrid F1 than that of their high parent was 34.2%, 10.6% and 77.7% respectively. Characters of fruit number and fruit weight of hybrid F1 were all higher than their high parents also. Character of female flower number and fruits setting ratio were improved, but not as high as that of fruit number and fruit weight. First female flower node and the days from tansplanting to flowering decreased, this made hybrid had the early maturity ability. In the fruit composing characters, fruit stalk length decreased ,fruit length, fruit width, fruit index, fruit stalk width and fruit flesh thickness all showed increase, but heterosis degree of each character of each cross was different, some characters were higher than that of high parent, some were higher than that of middle parent only. Plant characters were improved differently, plant height, stem diameter, leaf number and distance between each node were between the both parents; leaf length and leaf width increased, so leaf area increased and showed heterosis.2. Correlation analysis indicated that: yield and yield composing characters had intimate relation with fruit characters and plant characters. Yield per plant had significant relation with fruit number, fruit weight, fruit setting ratio, leaf width and leaf length, while fruit weight had significant relation with fruit length, fruit width, fruit flesh thickness, plant height, leaf length and leaf width; fruit number had significant relation with fruit setting ratio and leaf number. We considered fruit shape index, fruit uniformity and fruit flesh thickness as important characters evaluating fruit quality, correlation analysis indicated that fruit shape index had significant relation with fruit length, fruit width and fruit stalk width; fruit uniformity had significant relation with fruit stalk width, stem diameter, leaf length and leaf width. Fruit flesh thickness had significant relation with plant height, distance between nodes, leaf length and leaf width, these all indicated that good plant shape characters were important to the formation of good quality fruits. The result of analyzing plant characters indicated that there were significant relations between each other.3. genetic analysis indicated that: fruit color was a qualitative character and was controlled by a pair of genes, green was dominant to white; most other characters of summer squash were mainly controlled by major gene and polygene, and gene effect mainly included additive effect and dominant effect., though epastasis existed in the heredity of a few characters, its influence was small in relation to the effect of additive and dominant effect. In the heredity of offspring generation, polygene effect was important and occupied high percentage. In the heredity of most character, dominant effect was positive and higher than additive effect, so dominant effect was the genetic basis of heterosis. In some other characters, dominant effect was negative and lower than of additive effect, this made fruit stalk length decrease and first female flower low and days to flowering shorten, so these characters were improved also. In the heredity of other characters, dominant effect is lower than additive effect, this made some characters like fruit setting ratio, fruit stalk width, plant height, leaf number, stem diameter and distance between nodes lies between their parents, so these characters were improved, in the end all characters were improved. |
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