Quantitation of Fiber Quality and the Cotton Production-Processing Interface: A Physiologist's Perspective

Traditionally, ideal cotton (Gossypium ssp.) fibers are said to be as white as snow, as strong as steel, as fine as silk and as long as wool. It is difficult to incorporate these specifications favored by cotton processors into a breeding program or to set them as quantitative goals for cotton producers. Since the 36 JOURNAL OF COTTON SCIENCE, Volume 4, Issue 1, 2000 early 1980s in the USA, the USDA-AMS cotton classing offices have become the primary connection for fiber quality between cotton producers and processors. The high volumes of cotton passing through the classing offices every year have forced workers there to make compromises for the sake of speed and productivity, and to develop rapid, semiautomatic classing techniques that have blurred some fiber-quality definitions in ways that may favor one industry segment over another. The vertical integration of the U.S. cotton industry from field to fabric depends on efficient use and cooperative refinement of the existing line of communication. Fiber-classing technologies now in use and under development and evaluation allow quantitation of fiber properties, application of improved standards for end-product quality, and, most importantly, creation of a fiber-quality language and a system of fiber-quality measurements that can be meaningful and useful to producers and processors alike. A cotton physiologist working in production research examines the interface between cotton production and processing in terms of the fiber properties currently quantified by the USDA-AMS cotton-classing offices, describes the measurement protocols available, and investigates possible environmental sources of the significant variations in fiber quality that reduce grower and processor profits. The interaction of growth environment, genetic potential, and fiber properties quantified at harvest are discussed where appropriate data or references exist. From the physiologist’s perspective, the fiber quality of a specific cotton genotype is a composite of fiber shape and maturity properties that depend on complex interactions among the genetics and physiology of the plants producing the fibers and the growth environment prevailing during the cotton production season. Fiber shape properties, particularly length and diameter, are largely dependent on genetics. Fiber maturity properties, which are dependent on deposition of photosynthate in the fiber cell wall, are more sensitive to changes in the growth environment. The effects of the growth environment on the genetic potential of a genotype modulate both shape and maturity properties to varying degrees. Anatomically, a cotton fiber is a seed hair, a single hyperelongated cell arising from the protodermal cells of the outer integument layer of the seed coat. Like all living plant cells, developing cotton fibers respond individually to fluctuations in the macroand microenvironments. Thus, the fibers on a single seed constitute continua of fiber length, shape, cell-wall thickness, and physical maturity (Bradow et al., 1996b,c, 1997a). Environmental variations within the plant canopy, among the individual plants, and within and among fields ensure that the fiber population in each boll, indeed on each seed, encompasses a broad range of fiber properties and that every bale of cotton contains a highly variable population of fibers. Successful processing of cotton lint depends on appropriate management during and after harvest of those highly variable fiber properties that have been shown to affect finished-product quality and manufacturing efficiency (Bradow et al., 1996b). If fiber-blending strategies and subsequent spinning and dyeing processes are to be optimized for specific end-uses and profitability, production managers in textile mills need accurate and effective descriptive and predictive quantitative measures of both the means and the ranges of these highly variable fiber properties (Moore, 1996). In the USA, the components of cotton fiber quality are usually defined as those properties reported for every bale by the classing offices of the USDA-AMS, which currently include length, length uniformity index, strength, micronaire, color as reflectance (Rd) and yellowness (+b), and trash content, all quantified by the High Volume Instrument (HVI) line. The classing offices also provide each bale with the more qualitative classers’ color and leaf grades and with estimates of preparation (degree of roughness of ginned lint) and content of extraneous matter. The naturally wide variations in fiber quality, in combination with differences in end-use requirements, result in significant variability in the value of the cotton lint to the processor. Therefore, a system of premiums and discounts has been established to denote a specified base quality. In general, cotton fiber value increases as the bulkaveraged fibers increase in whiteness (+Rd), length, strength, and micronaire; and discounts are made for both low mike (micronaire less than 3.5) and high mike (micronaire more than 4.9). Ideal fiber-quality specifications favored by processors traditionally have been summarized thusly: “as white as snow, as long as wool, as strong as steel, as fine as silk, and as cheap as hell.” These 37 BRADOW AND DAVIDONIS: FIBER QUALITY, COTTON PRODUCTION, AND PROCESSING specifications are extremely difficult to incorporate into a breeding program or to set as goals for cotton producers. Fiber-classing technologies in use and being tested allow quantitation of fiber properties, improvement of standards for end-product quality, and, perhaps most importantly, creation of a fiberquality language and system of fiber-quality measurements that can be meaningful and useful to producers and processors alike. GENETIC POTENTIAL, GENETIC CONTROL, AND ENVIRONMENTAL VARIABILITY Improvements in textile processing, particularly advances in spinning technology, have led to increased emphasis on breeding cotton for both improved yield and improved fiber properties (Meredith and Bridge, 1972; Green and Culp, 1990; Patil and Singh, 1995). Studies of gene action suggest that, within upland cotton genotypes there is little non-additive gene action in fiber length, strength, and fineness (Meredith and Bridge, 1972); that is, genes determine those fiber properties. However, large interactions between combined annual environmental factors (primarily weather) and fiber strength suggest that environmental variability can prevent full realization of the fiberquality potential of a cotton genotype (Green and Culp, 1990). More recently, statistical comparisons of the relative genetic and environmental influences upon fiber strength suggest that fiber strength is determined by a few major genes, rather than by variations in the growth environment (May, 1999). Indeed, spatial variations of single fertility factors in the edaphic environment were found to be unrelated to fiber strength and only weakly correlated with fiber length (Bradow et al., 1999b,c; Johnson et al., 1999). Genetic potential of a specific genotype is defined as the level of fiber yield or quality that could be attained under optimal growing conditions. The degree to which genetic potential is realized changes in response to environmental fluctuations such as application of water or fertilizer and the inevitable seasonal shifts such as temperature, day length, and insolation. Season-related shifts in cotton plant metabolism and fiber properties take the form of higher levels of fiber maturation in upland and pima bolls from July flowers, compared with the maturity levels of fibers in bolls from August flowers on the same plants (Sassenrath-Cole and Hedin, 1996; Bradow et al., 1996c; Bradow et al., 1997a). Similar effects of environment on genetic potential have been quantified in plant and field maps of micronaire and maturity (Bradow et al., 1996b, 1999b; Johnson et al., 1999). In addition to environment-related modulations of fiber quality at the crop and whole-plant levels, significant differences in fiber properties also can be traced to variations among the shapes and maturities of fibers on a single seed and, consequently, within a given boll. Comparisons of the fiber-length arrays from different regions on a single seed have revealed that markedly different patterns in fiber length can be found in the micropylar, middle, and chalazal regions of a cotton seed at either end and around the middle (Delanghe, 1986). Mean fiber lengths were shortest at the micropylar (upper, pointed end of the seed) in G. hirsutum, G. barbadense, and G. arboreum genotypes (Vincke et al., 1985). The most mature fibers and the fibers having the largest perimeters also were found in the micropylar region of the seed. After hand ginning, the percentage of short fibers less than 0.5 inch or 12.7 mm long on a cotton seed was extremely low. It has been reported that, in ginned and baled cotton, the short fibers with small perimeters did not originate in the micropylar region of the seed (Vincke et al., 1985; Delanghe, 1986). Further, AFIS-A2 (Advanced Fiber Information System, Model A-2, Zellweger, Knoxville, TN) measurements of fibers from micropylar and chalazal regions of seeds revealed that the location of a seed within the boll was related to the magnitude of the differences in the properties of fibers from the micropylar and chalazal regions (Davidonis and Hinojosa, 1994). Significant variations in fiber maturity also can be related to the seed position (apical, medial, or 1 Trade names are necessary for reporting factually on available data. The USDA neither guarantees nor warrants the standard of the product or the service. The use of the name USDA implies no approval of the product or service to the exclusion of others that may be suitable. 38 JOURNAL OF COTTON SCIENCE, Volume 4, Issue 1, 2000 basal) within the boll and locule. Degree of secondary wall thickening (quantified by the AFISA2 as the fiber cell-wall maturity measurement, degree of thickening, or æ) is lowest in seeds at the apex (Seed Location 1) of the locule and highest in seeds at the peduncle or basal end (Seed Location 7 or higher) of the locule (Table 1). Fiber length and maturity also exhibit both seed and locule location effects. Porter (1936) examined fiber length in relation to seed location in the locule and found that seeds near the apical or basal end of the boll produced the shortest fibers. In Table 1, the least mature fibers were found closest to the boll apex, regardless of fruiting site (boll position on the plant). Fibers from fruiting branches 9, 10, and 11 or higher were consistently longer and more mature than fibers from fruiting branch 7 (lower on the plant). Thus, the different microenvironments within the boll and within the plant canopy had significant effects on the properties of fibers produced within the same macroenvironment, that is, on the same plant in the same field in the same crop year. VARIATIONS IN FIBER QUALITY RELATED TO PLANT ARCHITECTURE AND SUBOPTIMAL GROWTH ENVIRONMENTS Canopy Architecture and Fiber Quality Cotton canopy architecture, particularly with respect to plant height and branch formation, is modified by such environmental factors as temperature (Hanson et al., 1956; Reddy et al., 1990; Hodges et al., 1993); growth-regulator application (Reddy et al., 1990; Cadena and Cothren, 1996; Legé et al., 1996); light intensity (Hanson et al., 1956; Sassenrath-Cole, 1995); and herbivory by insects and other animals (Terry 1992; Rosenthal and Kotanen, 1994; Sadras, 1996). Genotype canopy characteristics, such as solar tracking and leaf shape, and macroand microenvironmental factors interact to modulate canopy light distribution, which, in turn, alters photosynthetic activity within the canopy and the crop (Wells et al., 1986; Reddy et al., 1991; Sassenrath-Cole, 1995; Sassenrath-Cole and Heitholt, 1996). Thus, reduced photosynthetic rates and the modulation of other metabolic factors, in association with lower light intensities, may result in lower micronaire, fiber strength, and yield (Pettigrew, 1996). Boll Retention Patterns and Fiber Quality Another obvious architectural linkage among environment, yield, and fiber quality is seen in boll retention patterns. Environmental conditions that induce boll drop significantly alter the fiber quality in the remaining bolls by modifying partitioning of assimilates and metabolic resources within the reduced boll population. A clear connection between boll retention and micronaire distribution patterns can be seen in Figs. 1 and 2. Irrigation method was the macroenvironment treatment in a study of Pee Dee 3 (PD3) grown in Florence, SC, in 1992 (Bradow et al., 1997a,b). The irrigation treatments were natural rainfall or rainfall plus water added through micro-irrigation tubing laid in the root zone under each row (in-row) or between alternate rows (alternate-row). Both the in-row and alternate-row irrigation treatments delivered a season total of 90 mm of additional water in nine irrigation events. In comparison to both the rainfed and alternaterow treatments, the in-row irrigation treatment Table 1. Effect of seed location within the locule on upland Deltapine 51 cotton fiber properties quantified by AFIS-A2. Seed nearest the peduncle (basal) end of the locule is designated as location 7. Each value is an average of three bolls containing no motes (underdeveloped, low weight seeds). All data are from first branch position bolls. Data from nodes 9, 10, and 11 were pooled to obtain statistically valid populations. Cotyledonary node = 0. (Davidonis, unpublished.)