Cotton fiber maturity is an important fiber character that measures a relative degree of fiber primary cell wall and secondary cell wall thickness Thibodeaux Rajasekaran, 1999. Fiber thickness is a cellulose deposition process inside the primary cell wall; the secondary cell wall will be full of lumen when completed (Stewart, Oosterhuis, Heitholt, & Rauney, 2010). Cellulose, as the main component of the fiber wall, is in the form of Î²-1, 4 glucan chains (Stewart et al., 2010). Fibers with less cellulose depositions and thinner secondary cell walls are immature, less developed fibers; mature, well developed fiber cell walls are composed of about 95% cellulose (Meinert & Delmer, 1977; Benedict et al., 1992; Goynes Jr, 2003).
Maturity, relative to other fiber characteristics, has a greater impact on the appearance of the fabric; it will largely affect the fabric dyeing as well as the dyed appearance (Estur & Knappe, 2007). The increase of cell thickness and deposited cellulose improves the ability and mass of the fibers to absorb more water and bind more cellulosic dyes (Stewart et al., 2010). In most cases, maturity is expressed as maturity ratio which is the cell wall area divided by squared perimeter of the same circle (Lord & Heap, 1988). The higher the maturity ratio, the more mature fibers exist and the better dye uptakes (W. Stanley Anthony, 2004). Therefore, cross-sections and cotton-scope were carried out to measure cotton fiber maturity.
3.1.1 SEM of fiber cross-sections
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In this section, micrographs of developing cotton fibers from 30 dpas and 56 dpas are presented from Table 2 to Table 9 at 1000 magnifications. Tables 2 to 5 show the SEM images of cotton samples harvested in 2010 under well-irrigated (IC), semi-irrigated (SC), and dry-land (DC), respectively. Tables 6 to 9 are the SEM images of cotton fibers harvested in 2011 under IC, SC, and DC irrigation treatments, respectively.
The morphologies of fibers harvested at 30 dpa under different irrigation levels from both the year 2010 and 2011 are assessed. The micrographs from genotype IA of the year 2010 show that the fibers at 30 dpas are younger. The secondary cell walls are thin and poorly developed. The SEM images of the cross-sections at 30 dpa from other 2010 and 2011 genotypes present no clear differences. All the fiber images harvested at 30 dpas have less mature and ribbon-like cross-sections. At this developmental stage, they don't show the common "bean" shape of mature cross-sections because of the low amount of cellulose deposition and a thinner secondary cell wall.
The 2010 SEM images of the cross-sections at 56 dpas from IA cultivar under IC and SC irrigations (Table 2) exhibit a well-developed and thicker secondary cell wall. Fibers at this stage have reached a full maturity. Around half of cross-section images under DC treatment in Table 2 present thinner secondary cell walls, which are immature fibers. Therefore, we can conclude that the morphologies of 2010 cultivar IA are sensitive to irrigation levels. The secondary cell walls of IB cultivar under IC and SC irrigation levels are fully developed; most fibers are mature fibers. The fibers under DC treatment, compared to IA cultivar, are much better and well developed; however, there are still some immature fibers. The morphologies of 2010 IB cultivars are sensitive to irrigation treatments as well. The same results and conclusions are obtained for IC and ID cultivars from 2010.
For the 2011 samples, they present more immature fibers for IC, SC, and DC irrigation treatments, because of the dry weather and lower rain precipitations during the whole year (USDA-ARS weather data). The analysis of SEM images indicates that there are visual changes on the morphology of cotton fibers from 30 dpas to 56 dpas for all the fibers of different genotypes and irrigation treatments. There is an increase in the thickness of the cell wall which is caused by the increase in the amount of cellulose deposited from 30 dpas to 56 dpas.
In conclusion, the SEM is helpful to visualize the cotton fibers' morphology changes at different developmental stages and different treatments. The results show that the secondary cell wall developments of all the cotton fibers are sensitive to the irrigation treatments.
3.1.2 Maturity ratio from the cottonscope
Maturity ratio, as an express way of maturity, is the cell wall area divided by squared perimeter of the same circle (Lord & Heap, 1988). Table X presents the general descriptions for maturity of raw upland cotton (Uster Technologies, 2004).
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TableX. Maturity data (Uster Technologies, 2004).
Figure A to Figure F show the maturity ratio results from cottonscope. They give maturity ratio changes of four cultivars from well-irrigated, semi-irrigated, and dry-land treatments as a function of dpa. Figures A to C are the year 2010 results. With the development of dpa, there is an increasing tendency for the maturity ratio. Fibers from well-irrigated seem less mature than semi-irrigated and dry-land fibers at the same dpas. Well-irrigated fibers reach mature, around 0.85 maturity ratio, at 56 dpa. Semi-irrigated fibers attain 0.85 maturity ratio between 40 and 47 dpa; especially for IB cultivar, fibers has a high maturity ratio at 56 dpa. The fibers from dry-land have different performances: IA cultivar reaches a medium maturity ratio at 40 dpa; however, maturity ratio drops afterwards to 0.6 at 56 dpa which is immature fiber. ID cultivar goes to the medium ratio at 56 dpa. IB and IC cultivars mature at 47 dpa and keep the same level at 56 dpa. Figures D, E, F show the results of year 2011. Regarding to well-irrigated crops, cultivar IB, IC, and ID mature at 47 dpa while cultivar IA reach median mature at 56 dpa; by 56 dpa IA, ID cultivar have a high maturity ratio and IB, IC cultivars have very high maturity ratio. For semi-irrigated cotton fibers, all the four cultivars mature around 40 dpa; IA, IB, and ID cultivars have high maturity ratio at 56 dpa while IC cultivar has a very high maturity ratio. Cultivar IA, IB, and ID from dry-land attain a medium maturity ratio at 40 dpa and reach high maturity ratio at 56 dpa; cultivar IC mature at 35 dpa and also goes to a high maturity ratio at 56 dpa. As far as 2011 year concerned, well-irrigated fibers are more mature than semi-irrigated fibers; semi-irrigated fibers are more mature than dry-land fibers; additionally, results from 2011 are better than the results of 2010. This is because the cotton is planted in the field; the abundant rainfull in 2010 influences the irrigation treatment (USDA-ARS weather data). We don't have much precipitation in 2011, the water irrigated to the plants is exactly what they obtained. In conclusions, irrigation treatment impacts cotton fiber maturity.