| Fiber strength is an important criterion of cotton quality and the secondary wall thickening stage is a key period to its formation. Fiber strength was affected by genotype and environmental condition. The study was about the mechanism of cotton fiber strength formation in different cotton cultivars. Field experiments were carried out in Nanjing, Jiangsu Province (cotton belts in lower basin of the Yangtze River) in 2004-2007.1. In 2004, fourteen cotton cultivars were used to study the characters of cellulose accumulation and the relationships between cellulose accumulation and fiber strength. Based on the cotton cellulose accumulation velocity and fiber strength, the fourteen cultivars could be clustered into three groups, and DSC-1, KC-1 and AC-33B were their representational cultivars respectively. In 2004 and 2005, the two representational cultivars (KC-1,35 cN tex-1, DSC-1,26 cN tex-1) with great difference in fiber strength were as materials to detect the relationship enzyme activities in cotton fiber development with fiber specific strength.2. In 2006 and 2007, two cotton cultivars with different low temperature sensitivity during fiber strength formation (KC-1 is a low temperature insensitive cultivar and SC-15 is a low temperature sensitive cultivar) were used to investigate:(1) Effects of sowing date and boll position on fiber physiological parameters (2) Physiological mechanisms of cotton fiber strength formation in two cotton cultivars with different low-temperature-sensitivity. (3) Response of enzymes activity in two cotton cultivars fiber with different temperature sensitivity to low temperature and its relationship with fiber strength. (4) Effect of cotton physiological age on sucrose metabolism in fiber of two cotton cultivars under different temperature condition. The main results were as follows:1. Physiological and molecular mechanisms of cotton fiber development with different fiber strength(1) The relationship between cellulose accumulation of cotton fiber and fiber strength. Fourteen cotton cultivars with various fiber strength were used to study the characters of cellulose accumulation and the relationships among them. The dynamic curves of cotton cellulose accumulation were all in accordance with the model of logistic on bolls from different fruiting branches. Based on the logistic model, some eigenvalues were gained. Variability and relativity analyses were made on them. The results showed that these was a wide range of variation for Vmax (the maximal speed of cotton cellulose accumulation) and T (the duration of cotton cellulose speed accumulation) and fiber strength had positive correlation with T but negative correlation with Vmax. The cluster analysis results based on cotton fiber cellulose accumulating parameters (T and Vm) and fiber strength by the SPSS software showed that, the fourteen cultivars could be grouped into three types by two criteria. Based on the cotton cellulose accumulation velocity, they could be clustered into rapid, gentler and medium groups, and based on the fiber strength, they could be clustered into lower, high and medium groups, and DSC-1, KC-1 and AC-33B were their representational cultivars respectively. The results also indicated that, there were differences among genotypes in cellulose accumulation characters and cotton fiber strength, and tight relationship between cotton cellulose accumulation and cotton fiber strength, the gentler speed cellulose accumulation in secondary thickening of fiber cell wall is in favor of forming high fiber strength.(2) Activity changes for enzymes associated with fiber development two cotton cultivars with different fiber strength. To detect the relationship enzyme activities in cotton fiber development with different fiber strength, two cotton cultivars (KC-1 which is a cultivar with high strength fiber, average fiber strength is 35 cN-tex-1. DSC-1 which is a cultivar with low strength fiber, average fiber srength is 26 cN-tex-1.) were used to investigate the characteristics of sucrose synthase (SuSy) andβ-1,3-glucanase which play very important roles in developing cotton fiber, and dynamic changes of the related gene (sucrose synthase,β-1,3-glucanase,β-1,4-glucanase, Expansin) expression. The results showed that there were differences between the two cultivars in variations of the cellulose deposition and fiber specific strength which was related with the changes of key enzyme activities and the gene expression levels. The activity and gene expression of sucrose synthase andβ-1,3-glucanase were higher in KC-1 than in DSC-1. High concentration and high gene expression of SuSy andβ-1,3-glucanase in cotton-fiber resulted in a long period and tempered cellulose accumulation and high strength fiber formation. Dexiamian 1 had low activities of the key enzymes and low expression levels which led to a short and rapid cellulose accumulation duration and low strength fiber. From results above, we suggest that the different characteristics of key enzymes activity (sucrose synthase andβ-1,3-glucanase) in fiber of different cotton cultivars may cause the differences in the cellulose accumulation and fiber strength.2. Mechanism of fiber strength formation in two cotton cultivars with different low-temperature-sensitivity(1) Effects of sowing date and boll position on fiber physiological parameters in two cotton cultivars with different low-temperature-sensitivity. The effects of planting date and boll position on fiber development and fiber strength was studied by comparing the fiber physiological parameters (cellulose, callose and sucrose) in two cotton cultivars (KC-1 and SC-15) that developed at different planting dates or boll positions. The results showed that, fiber development and fiber strength were affected by sowing date and boll position. For the bolls in the same boll position, low temperature in late sowing caused a declined in sucrose andβ-1,3-glucose transformation, which retrained cellulose synthesis and fiber strength formation. Boll position affected fiber development, but the influence of boll position on cotton in the two sowing dates was different. In the normal sowing date, the sucrose andβ-1,3-glucose transformation in cotton fiber of middle fruiting branch were highest, it had the best fiber strength. In late sowing date, the temperature was lower with the fruiting higher, so matter metabolism in cotton fiber in upper fruiting branch was affected, and it had the lowest fiber strength. Of the cultivars tested KC-1 was better performer in late sowing date and the different boll position compared with SC-15. And the physiological parameters in cotton fiber were affected by planting date and boll position that was the important factor of the influence of sowing date and boll position on fiber strength.(2) Physiological mechanism of cotton fiber strength formation between two cotton cultivars with different low-temperature-sensitivity. Two cotton cultivars with different low-temperature-sensitivity during fiber strength formation (KC-1 is a low-temperature-insensitive cultivar and SC-15 is a low-temperature-sensitive cultivar) were used to investigate the matter (sucrose,β-1,3-glucan and cellulose) content and the activity of sucrose synthase andβ-1,3-glucanase which play very important roles in developing cotton fiber. The effects of low temperature on cotton fiber development by setting different sowing date, thus the fiber development process can be arranged at different temperature conditions. Result showed that in normal sowing date, mean daily minimum temperature was 24.0℃and 25.4℃during fiber development period, cotton had high-strength fiber, sucrose synthase activity in cotton fiber was the highest,β-1,3-glucanase activity was the lowest, and sucrose transformation rate and cellulose accumulation were the highest. When mean daily minimum temperature was less than 21.1℃in late sowing dates, enzyme activity and matter content related to cotton fiber development was reduced, sucrose transformation rate,β-1,3-glucan and cellulose accumulation decreased, sucrose synthase activity decreased, andβ-1,3-glucanase activity increased, which resulted in lower fiber strength. Two cultivars had different sensitivity to low temperatures, the change range of SC-15 in sucrose transformation, cellulose accumulation and enzyme activity was lager than that of KC-1. Mean daily minimum temperature below 21.1℃which affected cotton fiber development. There were significant differences in the sensitivity of the related enzymes activities and matter content to low temperature between two cultivars, and that may be one of important factors which lead to the two cotton cultivars had different low temperature sensitivity during fiber strength formation.(3) Response of enzymes activities in cotton fiber with different temperature sensitivity to low temperature during fiber development. Two cotton cultivars with different low-temperature-sensitivity during fiber strength formation were used to investigate the activity of sucrose synthase, sucrose phosphate synthase, invertase andβ-1, 3-glucanase which were related to fiber development. The results of the influence of low temperature on cotton fiber development showed that the enzymes activity and the gene expression were influenced by low temperature conditions. In normal sowing date, mean daily minimum temperature was 24.0℃and 25.4℃during fiber development period, cotton had high-strength fiber and high cellulose content, sucrose synthase and sucrose phosphate synthase activity in cotton fiber was the highest, invertase andβ-1,3-glucanase activity was the lowest, the expression level of expansin,β-1,3-glucanase and sucrose synthase gene were higher. When mean daily minimum temperature was less than 21.1℃in late sowing dates, enzyme activity related to cotton fiber development was reduced, sucrose synthase and sucrose phosphate synthase activity decreased, invertase andβ-1,3-glucanase activity increased, and the expression level decreased, which resulted in lower fiber strength. There were significant difference in the sensitivity of the related enzymes activities to low temperature between two cultivars, the change range of SC-15 in enzyme activity (sucrose synthase, sucrose phosphate synthase andβ-1,3-glucanase) and expression level was lager than that of KC-1, and the three enzymes play important role in fiber strength formation of two cotton cultivars with different temperature sensitivity.(4) Effect of cotton physiological age on sucrose metabolism in fiber of two cotton cultivars under different temperature condition. Two cotton cultivars with different low-temperature-sensitivity during fiber strength formation were used to investigate sucrose metabolism which play very important roles in fiber development period. The effects of cotton physiological age on cotton fiber sucrose metabolism by setting different sowing date, thus the fiber development process can be arranged in same temperature condition but physiological age was different. Sucrose metabolism was affected by cotton physiological age, and under different temperature condition, the influence was different. When mean daily minimum temperature was 24.0℃during fiber development period, cotton physiological age had a highest influence on sucrose metabolism, cotton of middle fruiting branch had high-strength fiber, sucrose synthase activity in cotton fiber was the highest, and sucrose transformation rate and cellulose accumulation were the highest. When mean daily minimum temperatures were 22.6℃and 21.1℃, the difference between different fruiting branches was short. And the mean daily minimum temperature was only 18.1℃, there was no significant difference between different fruiting branches. Two cultivars had different sensitivity to low temperature, the change range of SC-15 in sucrose metabolism was lager than that of KC-1.
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