| Better understanding of the mechanisms of fiber development in high-quality cotton may help improving fiber quality through angonomic practices. In the present study, two high-quality upland cotton (Gossypum histutum L) cultivars Luyuan 324 (from Shandong agricultural institute, fiber strength 26.5 CN/tex, ICC) and YM-1(from Southwest agricultural university, 25.7 CN/tex, ICC) and one common quality cultivar Lumianyan 18 (fiber strength 23.2 CN/tex, ICC) were grown in the research farm of Shandong Agricultural University from 2006 to 2007. The physiological characteristics of leaves, and cotton boll and fiber development in the two types of cotton were studied to reveal the mechanism of fiber quality formation. The main results are as follows:1. Leaf physiological characteristics of high-quality upland cottonThe photosynthetic rate of main-stem leaf and fruiting branch leaf decreased with leaf age both in high-quality cotton and common cotton. The photosynthetic rate of L18 were higher than that of Luyuan 324 andYM-1 in both upper and lower main-stem leaves. The photosynthesis rate of upper fruiting branch leaf of 324 and YM-1 were higher than L18 at 15 d of leaf age, and lower than L18 at other leaf ages. The photosynthetic rate of lower fruiting branch leaf of 324 and YM-1 were both lower than L18.The ETR andφPSⅡof high-quality cotton was higher than those of L18,which means that high-quality cotton non-radiant energy of leaf was lower and the energy was used sufficiently in photosynthesis. TheφPSⅡof lower leaf showed YM-1>L18>324 at 15 d leaf age, and there was a higherφPSⅡin high-quality cotton since 30 d leaf age. The high-quality cotton had higherφPSⅡsince 15 d of leaf age of upper leaves. The content of chla, chlb and chl(a+b) in L18 were obviously higher than 324 and YM-1, which consistents with higher photosynthetic rate in L18.The content of soluble sugar and sucrose in main-stem leaves and fruiting-branch leaves showed 324>YM-1>L18. There was a period in which soluble sugar content and sucrose content in main-stem leaves and fruiting-branch leaves of high-quality cotton increased rapidly, meanwhile the cellulose also accumulated rapidly in this period. Thus sufficient initial substrate was supplied in leaves of high-quality cotton. The sucrose contents were higher because of higher SPS activities in high-quality cotton leaves.In the process of cotton boll development, the SOD activities in main-stem leaves and fruiting-branch leaves of high-quality cotton increased at beginning, and then fell down, while the POD activities of high-quality cotton were 324>YM-1>L18. The MDA content of main-stem leaves increased with leaf age with 324>L18>YM-1 both in upper and lower main-stem leaves. The MDA content was L18>324>YM-1 both in upper and lower fruiting-branch leaves。2. Changes of single boll weight and its component factorsThe single boll weight, seed weight, cellulose dry weight per boll and cellulose weight per seed in high-quality cotton were higher than in common cotton, which showed YM-1>324>L18. The weight of single cellulose was L18 >YM-1>324.3. The matters content and enzymes activities related to fiber development in high-quality upland cottonThe soluble sugar and sucrose content in boll shell increased first and then decreased and then increased in the process of cotton boll, the peak appeared in 14 d of boll age, the second peak appeared in 35 d and 42 d of lower and upper boll respectively. The soluble sugar content in boll shell of 324 was highest, L18 was higher, YM-1 was lowest. The key period for fiber secondary wall development is 35 d of boll age. Lower soluble sugar and sucrose content of upper boll in the key period of cellulose accumulation maybe was one of the reasons for formation of low fiber quality.The soluble sugar content in fiber of 324 and YM-1 were higher than that of L18 at 14 d of boll age, but lower than that of L18 after 21 d. The sucrose content in cotton fiber reached the peak value at 14 d of boll age, and decreased rapidly because of sucrose rapid consumption. In the earlier period of fiber developement, the sucrose content of high-quality cotton was higher than that in common cotton. It is indicated that there was sufficient initial substrate for the cellulose illuviation on secondary wall in high-quality cotton.The simulation with Logistic equation of cotton cellulose accumulation showed that the accumulation duration was longer, the accumulation rate was moderate and the final cellulose content was higher. The SS activities of high-quality cotton were higher and consistent with cellulose accumulation duration in high-quality cotton. The fiber quality of L18 was lower because of lower SS activities. The study showed that, the SPS activities fell down before 21 d of boll age, which maybe affected by higher sucrose content. It indicated that the SPS activities difference was one of the reasons of fiber quality difference. The balance of sucrose synthesis and degeneration was completed by both SS and SPS, however, further research should be made in formation mechanism. The SPS activities increased from 21 d of boll age, sucrose was synthesized for fiber synthesis. The SPS activities of high-quality cotton were higher than L18 before 35 d of boll age and then lower thanL18, which maybe was the reasons for difference of fiber quality.The IAA content was higher in 324 and YM-1 fiber, which was beneficial to fibrogenesis and elongation, and laid a foundation for higher weight and length of fiber. The first peak of ABA content in L18 was earlier than that in high-quality cotton, because of earlier cellulose illuviation on secondary wall in L18. The higher ABA content in 324 and YM-1 was beneficial to change from first wall development to secondary wall development. The GA3 content of 324 and YM-1 was higher than L18 in 7-14 d of boll age and lower in 21-35 d. So the boll volume maybe affected by GA3 content in early age of boll development. The ZR content of three cultivars changed in single peak curve, the peak appeared in 35 d of boll age. The high-quality cotton had higher ZR content than common cotton except 35 d of boll age. The ZR content was 324>YM-1 in lower boll fiber, while YM-1>324 in upper boll fiber. It is indicated that, the hormone content affected the biological activity in cotton fiber elongation and thickens, thus affected the fiber quality.4. The formation characteristics of fiber of high-quality upland cottonThe formation characteristic of cotton fiber length, fiber strength, micronaire values and the fiber maturity were difference between high-quality cotton and common cotton. The fiber length of 324 and YM-1 was longer and the fiber rapid elongating stage was obviously longer than common cotton. The 3.2mm gauge length strength of 324 and YM- 1 was larger than common cotton,and the final fiber micronaire values and maturity were significantly lower than common cotton.5. Composition of cotton seed kernel of high-quality upland cottonThe soluble sugar and sucrose content of cotton seed kernel decreased in 28-42 d of boll age, and increased in 49 d and then decreased. The maturity of cotton seed of high-quality cotton was better than common quality cotton, because the soluble sugar and sucrose content were lower than common quality cotton. The cotton seed kernel fat content of 324 and YM-1 were lower than L18, there is no difference between 324 and YM-1. The protein content showed YM-1>L18>324 at 28 d of boll age, there was higher protein content in high-quality cotton after 28 d of boll age. The protein content of 324 was lower than YM-1 before 35d of boll age, while higher than YM-1 after 35d of boll age in lower boll of cotton. The protein content of 324 was lower than YM-1 of upper boll.
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