| Cotton plants are one of the mainstays of the global economy which produce the most prevalent natural fiber used in the textile industry. Yield and quality in upland cotton are composed of cotton bolls whose biomass accumulation and distribution are basal for their yield and quality formation. Nitrogen takes part in all process of cotton boll development, and the nitrogen accumulation and distribution all most determines the balance growth of every part of cotton boll (cotton seed, cotton shell and fiber). Field experiments were carried out in Xuzhou (117°11’E,34°15’N, Yellow River Valley) and Anyang (114°13’E, 36°04’N, Yellow River Valley) using two cotton cultivars Kemian 1 (average fiber strength is 35 c·-tex-1) and NuCOTN (average fiber strength is 32 cN·tex-1) with two sowing dates (25-Apr and 25-May) and three nitrogen levels (0,240 and 480 kgN·ha-1). The main aim is to study the effects of sowing dates, nitrogen and plant physiological age on biomass and nitrogen accumulation in cotton bolls and their relationship with boll quality (fiber quality and cotton seed quality). And the changes of carbon and nitrogen in the subtending leaf of cotton boll and its relationship to biomass of cotton boll (cotton seed and fiber) were also studied. Meanwhile, the synergistic effects of plant physiological age and nitrogen on the accumulation and distribution of biomass and nitrogen in cotton boll and their relationship with boll quality were investigated in detail.The main results were as follows:1. Effects of sowing date on the accumulation and distribution of biomass and nitrogen of cotton bollsBy setting sowing date experiment, the response of the accumulation and distribution of biomass and nitrogen of cotton bolls to temperature and its relationship with fiber and seed quality characteristics were investigated. The results showed that the effects of late sowing date on biomass and nitrogen accumulation/distribution was significant which result in lower cotton weight and poor cotton boll quality. Compared with the normal sowing date (25-Apr), in late sowing date (25-May) (1) the mean daily temperature during boll development stage in the middle fruiting branch decreased from 24.8℃and 24.1℃to 20.8℃and 19.4℃in 2005 and 2007 respectively. (2) The cotton shell biomass increased while its nitrogen accumulation amount decreased. In fiber and cotton seed, the start time of rapid biomass and nitrogen accumulation was later and the maximal rate of biomass and nitrogen accumulation became smaller and its appearance turned later which resulted in the decrease of their biomass and nitrogen accumulation amount. The maximal rate of fiber nitrogen accumulation appeared earlier than that of biomass accumulation which showed opposite in cotton seed. (3) Biomass and nitrogen distributive indices increased in cotton shell while decreased in cotton seed, with little change of nitrogen distributive indices in fiber. (4) Boll weight, fiber strength and seed protein/oil content were also decreased when exposed to low temperature. These results indicated that low mean daily temperature (below 20.8℃) during boll development stage affected the accumulation and distribution of photosynthesis production and nitrogen in cotton bolls (shell, seed and fiber), which decreased the accumulation amount of biomass and nitrogen reducing to poor qualities of fiber and seed.2. Effects of nitrogen on the accumulation and distribution of biomass and nitrogen of cotton bollsTo investigate the response of the accumulation and distribution of biomass and nitrogen of cotton bolls to nitrogen and its relationship with fiber and seed quality characteristics, three nitrogen levels (0,240 and 480 kgN·ha-1) were applied in field experiment. Data showed that nitrogen changed the biomass and nitrogen accumulation characteristics of cotton boll (shell, fiber and seed), and then affected its weight and quality. In every part of a single boll, at 240 kgN·ha-1, its biomass was highest except cotton shell, its nitrogen content was in the middle and its nitrogen accumulation amount was highest. Thus the boll weight was highest and the cotton boll quality (fiber and cottonseed quality) was the best. At 0 kgN·ha-1, the start time of rapid biomass and nitrogen accumulation was earlier and the rate of biomass and nitrogen accumulation became slower, the biomass and nitrogen accumulation amount were lower, which resulted in the lowest boll weight and poorest cotton boll quality. At 480 kgN·ha-1, nitrogen content and accumulation amount increased in cotton boll. In mature one, biomass distributive indices decreased in cotton fiber while nitrogen distributive indices increased in cotton seed. And seed protein content increased while boll weight, fiber strength and seed oil content decreased. These results indicated that low nitrogen rate affected the transportation of photosynthesis production and nitrogen to boll, but high nitrogen rate affected the distribution of cotton biomass and nitrogen in cotton boll, both of which decreased the boll weight and reduced to poor qualities of fiber and cotton seed.3. Changes of carbon and nitrogen in the subtending leaf of cotton boll and its relationship to biomass of cotton boll, cotton seed and fiberThe changes of total carbon and total nitrogen content in subtending leaf of cotton boll could be simulated with quadratic:Y=at2+bt+c (Y stands for the total carbon or total nitrogen content (%), t stands for boll age (d)). And the changes of C/N ratio could be simulated with Logistic equation. Correlation analysis indicated:the changes of total carbon affected the duration for cotton seed and fiber biomass speedy accumulation and the average rate of cotton boll, seed and fiber biomass during the speedy accumulation period. The changes of total nitrogen and C/N had effects on the duration of cotton boll, seed and fiber biomass speedy accumulation and then affected the biomass accumulation. Difference of nitrogen changed the C/N ratio in the subtending leaf of cotton boll which resulted in the changes of cotton boll, seed and fiber biomass accumulation. Appropriate nitrogen (240 kgN·ha-1) regulated the C/N ratio in the subtending leaf, increased the biomass accumulation of cotton boll, seed and fiber and then achived high yield and quality.4. Effect of plant physiological age on biomass and nitrogen accumulation in cotton bollTo study the effect of plant physiological age on biomass and nitrogen accumulation in cotton bolls and their relationship with boll quality, the biomass and nitrogen accumulation of cotton bolls were studied with different fruiting positions under the same temperature conditions by setting different sowing dates (25th April and 25th May). Date showed that the there was no difference of biomass and nitrogen accumulation of cotton shell, cotton seed, fiber and single boll between different plant physiological ages, but the dynamic accumulation of them changed significantly. Under younger plant physiological age conditions (bolls developed on lower positional fruiting-branches), the start time, the rate and the duration of the rapid biomass and nitrogen accumulation period was earlier, higher and longer than that under higher plant physiological age conditions (bolls developed on middle positional fruiting-branches) respectively, while the total biomass and nitrogen accumulation of the bolls with different plant physiological conditions were not different. Consequently, except that the fiber strength developed in the middle positions was higher than that in lower position, other fiber qualities (fiber length and micronaire) and cotton seed qualities (the gross protein content and gross oil content) were not different. These results suggested that the variation of plant physiological age seemed not to change the total biomass and nitrogen accumulation in cotton bolls, but may change the process of them. Fiber strength might be the most sensitive fiber character to the variation of plant physiological age. And higher fiber strength in the middle positions than that in lower positions might be mainly due to its more tempered biomass and nitrogen accumulation program.5. Difference study on the effects of N application rate on biomass and nitrogen accumulation and distribution under different plant physiological ageTo study the synergistic effects of plant physiological age and nitrogen on biomass and nitrogen accumulation in cotton bolls and their relationship with boll quality, we combine the sowing date and nitrogen experiments, settling the cotton bolls of different fruiting-branches under the same temperature and different nitrogen rates. The data showed that the synergistic effects of plant physiological age and nitrogen significantly affected the nitrogen and the biomass accumulation of boll shell, cotton seed, fiber and single boll and the formation of the cotton seed quality (cottonseed protein and oil content). Under younger plant physiological condition (bolls developed in the lower positional fruiting branch), the reduction of the cotton boll quality was not significant in low-nitrogen rate, while in high-nitrogen rate decreased significantly. And under elder plant physiological condition (bolls developed in the middle positional fruiting branch), the quality of fiber and cotton seed reduced not significantly in high-nitrogen rate while in low-nitrogen rate decreased significantly. Under the synergistic effects of plant physiological age and nitrogen, higher rate (mg d-1) of biomass and nitrogen accumulation would result in higher fiber strength. These results indicated that the quality of fiber and cotton seed was decided by the biomass and nitrogen accumulation progress and their distribution index in different cotton boll organs, while the nitrogen accumulation progress and distribution index were synergistically affected by the plant physiological age and nitrogen. |
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