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Variability of Malate Dehydrogenase among Cotton Cultivars with Differing Fiber Traits

USDA-ARS, Cotton Physiology and Genetics Res. Unit, P.O. Box 345, Stoneville, MS 38776

^* Corresponding author.

Little is understood about the biochemical basis of differences in quantitative traits of cotton (Gossypium hirsutum L.) fiber. Malate dehydrogenase (MDH) is implicated as an enzyme that may have effect on extension growth during the development of cotton fiber. Our objectives were, first, to determine if variability among cultivars exists for MDH, and, second, to determine if variability in MDH can be correlated with quantitative traits of fiber. Twenty-four cultivars G. hirsutum were chosen. Polyacrylamide gel electrophoresis was used to search for MDH allozyme variation in etiolated cotyledons. Additionally, MDH activity in developing fiber was measured. All cultivars were grown in 1990 at two sites near Stoneville, MS. At 12 days post anthesis (DPA), immature bolls were harvested and the fiber separated from the ovules. An extract was prepared from the fiber and assayed for MDH activity and protein concentration. Other bolls were allowed to mature and open naturally, and from these bolls fiber was taken and tested for eight traits: elongation, maturity (ratio of wail thickness to fiber diameter), micronaire reading (fiber fineness), perimeter, 2.5 and 50% span length, strength, and wall thickness. Allozyme variation for MDH was not detected. The MDH specific activity ranged from 13.8 to 18.8 µmol NADH min^–1 mg^–1 protein, with a mean of 16.4 µmol NADH min^–1 mg^–1 protein in 12 DPA fiber. Variation among cultivars for MDH specific activity and all fiber traits was significant (P < 0.05). Though implicated in extension growth, MDH specific activity was not correlated with the length of mature cotton fiber. Only correlations between MDH specific activity and three traits of fiber walls (maturity, r = –0.476; micronaire, r –0.469; and wall thickness, r = –0.475 were significant. Cotton cultivars vary significantly for MDH activity in developing fibers and high MDH activity is indicative of low micronaire reading, thin fiber walls, and decreased maturity.

Received for publication July 3, 1991.