| Cotton is the most important natural fiber crop, and also an important plant oil and protein crop in the world. The genus Gossypium comprises of approximate5tetraploid species and45diploid, including4cultivated species, G. hirsutum L., G. barbadense L., G. arboretum L. and G. herbaceum L.. Of the cultivated species, upland cotton (G. hirsutum L.) supplies over95%of the world's total fiber production.It's an important goal for cotton breeding to attain high yield, and lint yield are consist of plant per unit area, boll number per plant, boll weight and lint percentage. Lint percentage is closely associated with lint yield, and it is an important factor to improve cotton production. Compared to other yield traits of cotton, lint percentage is slightly influenced by environment and is provided with high genetic force. Photosynthesis is physiological basis to attain high yield, and is important to improve photosynthesis for increasing yield potential of a crop. Chlorophyll is the important component of chloroplast and the material basis of leaf photosynthesis, as well as the important indicator of leaf function duration. Chlorophyll content reflects the number of photosynthetic units of per unit leaf area and the plant nitrogen level, and represents the photosynthetic ability and the aging degree of a plant. Varieties with high level of chlorophyll are not premature senility and their leaf function duration is longer than other varieties, which varieties can make greater use of solar energy to synthesize dry matter and to form more economic output. The previous studies showed that the characters associated with yield, photosynthetic function, and fiber quality traits are mostly quantitative traits in cotton, and it is slow to improve quantitative traits with conventional breeding methods. Recent progress in DNA molecular markers provides plant breeders with a fast and precise way to modify quantitative traits in crop breeding. Using the markers closely-linked with QTLs for quantitaive traits to select traits will be effective in crop improvement. However, there are very few reports about QTL affecting physiological traits associated with light utilization in cotton. In the present study, a recombinant inbred line population derived from the cross between two upland cotton cultivars, Yumian1and T586, was used to increase marker density of genetic linkage map with microsatellite marker, and to detect QTLs affecting lint percentage and photosynthetic traits.1. Polymorphism of markers and genotypingIn total,6955SSR primer pairs were used to screen the polymorphism among Yumian1and T586, and305polymorphic primer pairs were obtained, acconting for4.37%of the tatal primer pairs. The polymorphic primer pairs produced334loci when they were used to genotype the270recombinant inbred line families of (Yumian1×T586). The1546loci (including the previous1212loci of our lab) were tested by Chi-square, the result showed that501loci distorted from the expected segregation ratio (P<0.05), accounting for32.4%of the total loci.2. Increasing density of cotton genetic mapThe1546loci were used to construct the linkage groups by Joinmap4.0, a map with1458loci and71groups was obtained. The map spanned4357.1cM with an average distance of3.0cM between two markers. Seventy-one linkage groups were assigned to26chromosomes of tetraploid cotton, which including one to five linkage groups. Thirty-eight linkage groups were assigned to A-subgenome, containing701loci, and spanning2150.3cM with an average distance of3.1cM between two markers. Thirty-three linkage groups were assigned to D-subgenome, containing757loci, and spanning2206.8cM with an average distance of2.9cM between two markers.3. Phenotypes of mapping parents and F2:7familiesIn year2007-2010, the lint percentage of Yumian1were41.61%,42.03%,42.56%and41.27%, respectively, which was higher than that of T586(6.51%,7.94%,6.62%and6.35%). In year2007-2008, the photosynthetic rate of Yumian1were21.98 μmolCO2/m2·s and26.51μmolCO2/m2·s, which was higher than that of T586(18.00μmolCO2/m2·s and13.75μmolCO2/m2·s); The stoma conductance of Yumian1were360.00mmol/m2·s and444.00mmol/m2·s, which was higher than that of T586(263.00mmol/m2·s and277.00mmol/m2·s); The intercellular CO2concentration of Yumian1were160.00μmolCO2/m2·s and249.00μmolCO2/m2·s, which was lower than that of T586(240.00μmolCO2/m2·s and269.00μmolCO2/m2·s); The transpiration rate of Yumian1were7.46mmol/m2·s and8.10mmol/m2·s, which was higher than that of T586(5.35mmol/m2·s and6.31mmol/m2·s); The chlorophyll content of two parents were close to each other.In year2007-2010, lint percentage ranged from0.94%to47.23%, from1.30%to42.43%, from0.64%to47.52%and from1.48%to43.27%, and averaged21.21%,22.96%,24.31%and21.45%, respectively; In year2007-2008, photosynthetic rate ranged from14.00μmolCO2/m2·s to25.00μmolCO2/m2·s and from10.10μmolCO2/m2·s to26.76μmolCO2/m2·s, and averaged19.84μmolCO2/m2·s and17.82μmolCO2/m2·s; Stoma conductance ranged from106.00mmol/m2·s to445.00mmol/m2·s and from140.00mmol2·s to496.00mmol/m2·s, and averaged288.00mmol/m2·s and398.26mmol/m2·s; Intercellular CO2concentration ranged from97.00μmolCO2/m2·s to263.00μmolCO2/m2·s and from160.00μmolCO2/m2·s to305.00μmolCO2/m2·s, and averaged205.00μmolCO2/m2·s and250.41μmolCO2/m2·s; Transpiration rate ranged from1.90mmol/m2·s to8.00mmol/m2·s and from1.54mmol/m2·s to10.22mmol/m2·s, and averaged5.38mmol/m2·s and6.47mmol/m2·s; Chlorophyll content ranged from30.51mg/g to43.67mg/g and from34.23mg/g to58.52mg/g, and averaged39.41mg/g and47.09mg/g.Singnificantly positive correlations were observered between photosynthetic rate and stoma conductance, photosynthetic rate and transpiration rate; Singnificantly negative correlations were observered between photosynthetic rate and intercellular CO2concentration; Singnificantly positive correlations were observered between stoma conductance and intercellular CO2concentration, stoma conductance and transpiration rate; Singnificantly positive correlations were observered between intercellular CO2concentration and transpiration rate; Positive correlations were observered between intercellular CO2concentration and chlorophyll content.Variance analysis indicated that lint percentage, photosynthetic rate, transpiration rate and chlorophyll content was significantly influenced by both genotype (P<0.01) and environment (P<0.01); Stoma conductance and intercellular CO2concentration was significantly influenced by environment. Among the Phenotypic value influenced by environment of photosynthetic traits, the largest was the chlorophyll content, followed by the intercellular CO2concentration, the stoma conductance, the transpiration rate and the photosynthetic rate.4. QTL mapping of lint percentage and photosynthetic traitsMultiple QTL mapping was conducted to identify QTL for lint percentage in four environments (in year2007-2010) and for photosynthetic traits (in year2007-2008) by using MapQTL6.0. In year2007-2010, a total of12QTL were indentified for lint percentage in four environments. These QTL explained phenotypic variation between3.5-21.2%. In year2007-2008, a totalof48QTL were indentified for photosynthetic traits in two environments. For photosynthetic rate, eight QTL were indentified, and explained phenotypic variation between3.5-14.8%. For stoma conductance, nine QTL were indentified, and explained phenotypic variation between3.6-6.1%. For intercellular CO2concentration, ten QTL were indentified, and explained phenotypic variation between3.5-7.2%. For transpiration rate, twelve QTL were indentified, and explained phenotypic variation between3.6-8.0%. For chlorophyll content, nine QTL were indentified, and explained phenotypic variation between3.3-19.0%.Among the12QTL detected for lint percentage, the favorable alleles of nine QTL were derived from Yumian1, the favorable alleles of three QTL were derived from T586, respectively. Among the48QTL detected for photosynthetic traits, the favorable alleles of23QTL were derived from Yumian1, the favorable alleles of25QTL were derived from T586, respectively.
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