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Distribution of Single Fiber Tensile Properties of Four Cotton Genotypes JIHUA LIU, HONGBO YANG,

AND

YOU-LO HSIEH1

Fiber and Polymer Science, University of California, Davis, California 95616, U.S.A. ABSTRACT The distributions of breaking force and elongation of single fibers from five cultivated cotton varieties, African-51 (G. herbaceum), Asian-163 (G. arboreum), Maxxa and Texas Marker-1 (G. hirsutum), and Pima-S7 (G. barbadense), are studied. The range and distribution patterns of single fiber breaking force and elongation are significantly different for these cultivars and appear to be highly dependent on genotypes. Single fiber breaking force ranges from 16, 17, 20, 24, and 28 g for Pima-S7, TM-1, Maxxa, African-51, and Asian-163, respectively, and single fiber breaking elongation ranges from 16, 17, 18, 18, and 24% for these varieties. The single fiber breaking forces of Pima-S7 and TM-1 are symmetrically distributed, whereas those of the other three are asymmetrically distributed. African-51 and Maxxa have longer right tails, whereas Asian-163 has a longer left tail. Distributions of single fiber breaking elongation for all five varieties are asymmetrical and positively skewed, with the African and Asian cultivars having longer right tails. Within each cultivar, fibers of varying lengths have similar distributions in their breaking forces and elongation. This lack of relationship with fiber length suggests that these fibers’ tensile properties may be independent of length development, i.e., during elongation of the primary cell wall through the early stage of secondary cell wall synthesis. Single fiber breaking force and elongation are positively correlated (r ⫽ 0.259 to 0.443) for all five varieties, with Pima having the highest correlation coefficient.

Single fiber tensile properties are critical to the processing efficiency of cotton fibers into products and the quality of these products. The mean single fiber tensile properties and their variations have been reported to have significant effects on fiber bundle and yarn strengths (2, 6, 12–14). Suh et al. [13, 14] reported that efficiency loss of tensile properties in a fiber bundle was largely (46%) due to variations in the single fiber breaking elongation and, to a lesser degree (7%), to the slack in the fiber bundles. Sasser et al. [12] reported that yarn strength was better correlated with mean single fiber strength than with bundle strength. The coefficients of variance of single fiber strength within any individual Upland cotton varieties were around 35%, much higher than those (⬃12%) among thirty-five varieties. These findings suggest that yarn strength depends not only on single fiber strength, but also on the uniformity of single fiber breaking elongation. The higher the single fiber strength and the lower the variations of single fiber breaking elongation, the closer the bundle and yarn tensile strength would be to the sum of single fiber strength. Ideally, fiber bundle tenacity would equal the total single fiber break-

1

Corresponding author: [email protected]; (530) 752-0843

Textile Res. J. 75(2), 117–122 (2005)

ing tenacity had all fibers within the bundle equal breaking elongation and no slack. The sources of variations in single fiber tensile properties are many. It is generally agreed that the tensile properties of cotton fibers are dependent on genotype, environmental or growth conditions, and competition for nutritional resources [1, 5, 6, 9, 10]. Variations in single fiber tensile properties have mainly been reported for fibers of varying unknown sources, since bale mixing is a common practice in textile processing. In mixed cotton fiber populations, single fiber elongation has been reported to fit a normal distribution, whereas single fiber breaking force fits the Weibull distributions [13, 14]. Our studies of several calibration cotton standards have shown the distributions of single fiber breaking elongation and toughness to be positively skewed with a long right tail [3, 4]. Since these findings were based on mixed populations, the reported variations were attributed to a combination of all potential sources, i.e., genotype, growth conditions, and nutrition. Even within a cultivated variety and under optimal growing condition, fiber tensile properties vary by plants, boll positions within a single plant, seed locations within a boll, and regions on the same seed [5–7]. The overall variations of single fiber tensile properties within an 0040-5175/$15.00

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individual boll are associated mainly with seed locations in the boll and less with fiber length [7]. Fibers from medial sections of a cotton seed have the highest single fiber tenacities, even if all fibers from a single seed are of the same genotype and under the same growing conditions [5]. Even under culture conditions where the competition for nutrition resources is minimized, cell walls are thicker on fibers located near the micropylar ends of the seeds [1]. These findings are mainly derived from selected Gossipium hirsutum cultivars. Very little has been reported on the variation and distribution of tensile properties of fibers from other cotton species, let alone under optimal growing conditions. In recent years, much effort has been made by molecular biologists to improve cotton fiber quality by genetic means. A better understanding of these fiber variations and their origins is critical to developing effective strategies to improve fiber quality. Obtaining information on the variation, distribution, and correlation of individual fiber tensile properties within the same genotype, specifically the same position on seeds and same seed location in bolls, is vital. In this paper, we investigate the distributions of single fiber tensile properties of greenhouse-grown fibers from the same source, i.e., within an individual boll and from a specific section of the seed. The main focus is on the analysis of single fiber tensile, properties within the most similar and well-defined population within a genotype. We include two diploid species, i.e., Gossipium (G.) herbeceum, G. arboreum, and two tetraploid species, i.e., G. hirsutum and G. Barbadense. These samples represent all four species of cotton genotypes and allow comparisons among the full range of genotypes.

Materials and Methods Five cultivars of four cotton species, i.e., African-51 (G. herbeceum), Asian-163 (G. arboreum), Maxxa and Texas Marker-1 (G. hirsutum), and Pima-S7 (G. barbadense) were grown under standard greenhouse conditions, six plants of each cultivar. Each flower was tagged on the day of anthesis (flowering) as well as when the boll dehisced (opened). For each cultivar, first-position plant-mature bolls from between the fourth to the tenth fruiting branches were harvested 5 to 7 days after boll opening. Twenty to thirty plant-mature bolls were collected and weighed. Three mature bolls with boll weights closest to the average boll weight of those collected were selected. Within each boll, all seeds (five to eight total) from one locule were used. All fibers were manually removed from each seed. One hundred fibers from the middle section of each cotton seed were randomly selected for single fiber tensile measurements. The remain-

ing fibers from the middle sections on the same seeds were grouped by their lengths in 5-mm intervals. Fifty fibers randomly selected from each fiber length group were selected for single fiber tensile measurements. All fiber samples were conditioned at 70°F and 65% relative humidity at least 48 hours prior to measurement. A Mantis single fiber tensile instrument (Zellweger Usters, Inc.) was used in this study to accumulate a large amount of data as described in our previous reports [4 –7]. All single fiber tensile properties were measured using the middle segments of the fibers. All randomly selected fibers from the three bolls for each cultivar were considered as one population for analysis of their distributions. The histograms of single fiber breaking force and elongation were constructed using a 1-g breaking force and 1% elongation range as an interval. The correlation between single fiber breaking force and elongation was analyzed using the data of all randomly selected individual fibers from the three bolls.

Results and Discussion The histograms of fiber frequencies versus single fiber breaking force and elongation for the five varieties are presented in Figures 1 and 2, respectively. The distribution patterns of these tensile properties are highly dependent on genotypes. Single fiber breaking force ranges are the widest for the two diploids, i.e., up to 24 and 28 g for African-51 and Asian-153 cottons, respectively, followed by Maxxa of the tetraploid, i.e., up to 22 g. The upper ranges of breaking forces for the other tetraploids are 16 and 17 g for Pima-S7 and TM-1, respectively. The breaking elongation of Asian cottons has the widest range, up to 24%, whereas those of African and the tetraploid cultivars (Maxxa, TM-1, and Pima-S7) are close to each other in the 16 to 18% range. For breaking force and elongation distributions, the percentage of fibers in the peak category, the coefficients of variance (CV) of the peak, and the percentages of fibers to the left and right sides of the peaks are summarized in Tables I and II, respectively. The breaking force of Maxxa, one of the two tetraploid cultivars studied, is most varied with a CV of 45%, followed by the two diploid (CV ⫽ 40%) and the other tetraploid species (37% for TM-1 and 39% for Pima-S7). The breaking elongation of Maxxa, on the other hand, is the least varied (CV ⫽ 30%), followed by the other tetraploids (CV ⫽ 36% for both TM-1 and Pima), then the diploids (44% for both African and Asian cottons). The single fiber breaking force of Pima-S7 is the most symmetrically distributed, with close to 50% of fibers at each side. The single fiber breaking force of the TM-1 cultivar is nearly symmetrically distributed with a slight

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FIGURE 1. Distribution of single fiber breaking force of five cotton cultivars (n denotes the number of fibers).

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FIGURE 2. Distribution of single fiber breaking elongation of five cotton cultivars (n denotes the number of fibers).

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TEXTILE RESEARCH JOURNAL TABLE I. Percentage of fibers in single fiber breaking force histograms of five cotton cultivars. % Fiber

Cultivar

African-51

Asian-163

Maxxa

TM-1

Pima-S7

Peak Left side Right side CV

10.2 40.3 59.7 40

10.6 55.0 45.0 40

12.4 39.5 60.5 45

14.2 48.3 51.7 37

17.2 49.5 50.5 39

TABLE II. Percentage of fibers in single fiber breaking elongation histograms of five cotton cultivars. % Fiber Cultivar

African-51

Asian-163

Maxxa

TM-1

Pima-S7

Peak Left side Right side CV

24.3 27.6 72.4 44

11.8 29.9 70.1 44

18.2 47.9 52.1 30

14.9 39.1 60.9 36

14.8 33.7 63.3 36

positive skew. The breaking forces of the other varieties are asymmetrically distributed. African-51 and Maxxa have longer right tails compared to the longer left tail of Asian-163. Note that Asian-163 cottons have the highest proportion of the lower breaking force fibers (55%), whereas African-51 and Maxxa have the lowest. Distributions of fiber breaking elongation of all cultivars are asymmetrical and positively skewed with variable lengths of right tails. The two diploid species have significantly longer right tails (70 –72%), whereas Maxxa has the lowest (52%) proportion of fibers with higherthan-mean breaking elongation. Pima-S7 and TM-1 have 61– 63% fibers in their right tails. Within each cultivar, we also examined the distributions of single fiber tensile properties of fibers grouped by lengths. Within each cultivar, we found similar distributions of single fiber tensile properties among all length groups. Therefore, we show only the distributions of single fiber breaking force and elongation of Pima-S7 fibers grouped by the fiber lengths (Figure 3) to exemplify the similarities among the length groups. The fact that fibers from different length groups have tensile property distributions identical to the randomly selected samples from the entire population indicates that fiber tensile properties are independent of fiber lengths within the same boll of an individual cultivar. Overall fiber length developes during the cell elongation stage, usually over a period of 20 to 25 days. Cotton fiber cells initiate over a period of 2 to 3 days around anthesis, and the cells that initiate earlier develop into longer fibers. Similar distributions of breaking force and extension among the different length groups suggest that variations in ten-

FIGURE 3. Distribution of single fiber tensile properties within each fiber length group for Pima-S7: (a) breaking force, (b) breaking elongation.

sile properties may not be associated with the initiation or elongation stages of fiber development. In other words, longer fibers that initiate earlier or elongated more have tensile properties similar to those shorter fibers that initiate late or elongate less. Our previous findings have shown that mean single fiber breaking force, cell wall thickness, and tenacity increase most significantly during the first two weeks of secondary cell wall development [5]. Such increased fiber tenacity is associated with increased crystallinity and crystal dimensions in the cellulose. Several questions about the sources of single tensile property variations remain to be addressed. One is if or how secondary cell wall thickness is associated with fiber lengths. Another question is whether variations in tensile properties develop during this first half or throughout the entire secondary cell wall development. Another angle to these data is if and how these distributions indicate the potential for improving the tensile properties of each species. In any distribution with a longer right tail, fewer fibers are in the left tails, meaning a proportionally smaller number of fibers possess lower single fiber tensile properties than mean values within the population. This could mean that the potential for

FEBRUARY 2005 improvement is lower. Based on that, G. arboreum and G. herbeceum would have the lower improvement potential for breaking force than the G. hirsutum and G. barbadense cultivars. It seems that a more viable and likely approach for improving the breaking force of any species is to increase its peak value and uniformity. The fact that all varieties have long right tails in their breaking elongation histograms suggests that the improvement potentialities of breaking elongation are much lower than those of breaking force. We plotted the single fiber breaking force and elongation of all randomly selected mid-ovule fibers together to analyze their co-variation properties (Figure 4). We observed positive correlations between individual fiber breaking force and elongation for all five cultivars. This means that within the same genetic background, fibers that break at a lower extension also break with less force, and those that elongate more at break also require higher forces. Although positively correlated for all five varieties, the correlation coefficient r values of these linear regressions between breaking force and elongation vary by their genotypes. The r values for African-51 and Asian-163 are the lowest and are lower than those of all tetraploid cultivars. The single fiber breaking forces of Pima have the highest correlation with their breaking elongations. Because yarn and bundle tenacity is dependent upon single fiber breaking forces, one question is how the distribution of single fiber tensile properties contributes to yarn and bundle properties. It is thought that the distribution of breaking elongation may impact bundle tenacity more significantly because fibers with lower extension break first and do not contribute to the strength of remaining fibers in the bundle. African-51 and Asian163 fibers have much longer right tails in their breaking elongations, meaning that larger percentages of fibers have higher breaking elongations. This may translate to higher bundle and yarn strengths because of a higher proportion of higher extension fibers. On the other hand, these fibers are also shorter, which is less favorable for yarn strength. During bundle tensile processing, fibers with lower extension contribute little to bundle tenacity because they break before others in the bundle. The correlations between the breaking force and elongation, however, may be more useful indicators of bundle strength. The higher correlation, i.e., higher slope, and/or the closer correlation, i.e., higher coefficient r value, among fibers may mean a greater contribution to bundle breaking force. The r values of the linear regressions are in the descending order of G. barbadense, G. hirsutum, G. herbeceum, and G. arboreum.

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FIGURE 4. Correlation between breaking force and elongation of five cotton cultivars (linear regression equation with r denoting the correlation coefficient).

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Conclusions In this study, we have examined single fiber tensile properties of plant-mature fibers from five cultivated cottons. There are significant variations in single fiber breaking force and elongation within each cultivar, even though each fiber population is from the same genotype and is grown under the same condition. Single fiber breaking force ranges from 1, 16, 17, 20, 24 or 28 g for Pima-S7, TM-1, Maxxa, African-51, and Asian-163, respectively, whereas single fiber breaking elongation ranges from 1, 16, 17, 18, 18 or 24% for Maxxa, Pima-S7, TM-1, African-51 and Asian-163, respectively. The distribution patterns of single fiber tensile properties are highly dependent on genotypes, but independent of fiber lengths. The single fiber breaking forces of Pima-S7 and TM-1 have similar and nearly symmetrical distributions, whereas those of the other three varieties are clearly asymmetrically distributed with longer right tails for African-51 and Maxxa and a longer left tail for Asian163. Distributions of single fiber breaking elongation for all cultivars are asymmetrical and positively skewed with variable lengths of right tails. The fact that all varieties have a long right tail in the histogram indicates that the improvement potential of breaking elongation is much lower than that of breaking force. The more viable approach for improving the breaking force of any species is to increase its peak value and uniformity. In terms of relating single fiber to bundle tensile properties, the variations and distribution patterns of breaking elongation may have a more significant impact than breaking force. The longest right tail distribution properties and highest variations of breaking elongation for African-51 and Asian-163 imply that fibers from these species (G. herbeceum and G. arboreum) contribute less to their bundle and yarn tensile properties. During testing of bundle tensile strength, fibers with a lower extension at break contribute little to bundle tenacity because they are broken first, before the majority of fibers in the bundle. The correlation coefficients between breaking force and elongation are in the descending order of G. barbadense, G. hirsutum, G. herbeceum, and G. arboreum, suggesting the order of potential for increasing single fiber properties toward improved bundle tenacity. Future work with information on cell wall thickness, fiber size and structure, and linking with bundle properties will further enhance our understanding on the genetic link in cotton fiber tensile properties.

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