Chemical and structural properties of cotton fiber base and associated seed coat

Cotton is the most abundant natural fiber which accounts for more than 26% of the world’s fiber consumption. Trash content present in a bale of cotton fiber has a significant impact on cotton fiber quality. Seed coat fragments (SCFs) are among the most difficult-to-remove trash particles due to their strong attachment to fibers. According to the International Textile Manufacturers Federation, about 32% of all cotton evaluated in 2016 was contaminated with SCFs. SCFs cause multiple problems in the yarn and during fabric manufacturing process, such as yarn breakage and differences in dye uptake. Many environmental and management practices contribute to produce SCFs and several studies reported that the generation of SCFs is cultivar dependent. The cotton fiber is nearly pure form of cellulose with very little amount of non-cellulosic components such as wax, pectins, inorganics, etc. The cotton seed coats are composed of cellulose, hemicellulose, lignin, pectin, and wax substances. We hypothesized that understanding the amount and distribution of these macromolecules in the fiber base and the associated seed coat will provide a new insight into their contributions to the generation of SCFs. A large population of F3 cotton lines were grown and four cotton cultivars were selected with differences in the SCFs count and size. Two among these cultivars generated the highest and the lowest amount of SCFs and the other two cultivars generated the largest and the smallest size of SCFs. Then, an integrated approach was developed with Fourier transformed infrared (FTIR) spectroscopy and microspectroscopy imaging, microscopy techniques, thermogravimetric analysis, and histochemical analysis to characterize cotton fiber base and associated seed coat. These techniques altogether provide information regarding morphological, compositional, and biomolecules distribution in fibers and seed coats. In particular, FTIR microspectroscopy is widely used to identify biochemical composition and their relative distribution in highly complex biological tissues. FTIR images of 8 µm thick seed coat sections were analyzed using functional group distribution images and multivariate data analysis methods. K-mean clustering technique produced representative spectra from each layer of the seed coat sections, which were further compared to identify biochemical differences between cultivars. The findings indicate that cultivars generating the largest and the smallest size of the SCFs do not seem to exhibit differences in the chemical composition and biomolecules distribution in fibers and seed coats. However, the other two cultivars generating different amounts of SCFs show major differences in distribution of wax (vibration ~2850 cm-1), hemicellulose (vibration ~1734 cm-1), pectin (vibration ~1624 cm-1), and lignin (vibration ~1504 cm-1) in the seed coats. This study demonstrates that FTIR imaging could be used as a powerful non-destructive technique to investigate the biochemical composition and distribution of biomolecules in the seed coat sections.