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Inheritance of Seedling Root Length and Relative Root Weight in Cotton¹

A perennial problem in cotton, Gossypium hirsutum L., involves obtaining a vigorous, uniform stand when planted early. Strains that grow roots longer and higher in relative root weight (mg/mm length) than currently used cultivars when temperatures are cool may help alleviate this. Using the standard cotton, cool temperature (18°C) germination test we determined root lengths and relative root weights for 124 day-neutral composite F₃ strains. Variability among lines was significant for each seed and seedling parameter. Fixed model diallel analyses of five strains indicated that among the five strains one would have difficulty in selecting for long roots with high relative root weight. Generation mean analysis indicated from one of two crosses that large amounts of additive, dominance, and additive by additive epistatic effects were present for the two primary root traits. One should thus be more successful if selection is delayed to the F₃ to allow genetic recombination of additive and additive by additive epistatic genes to occur. Recurrent selection should be a useful method of breeding. The cross of strains 3822 x 3851, involving accessions T-214 and T-225, respectively, should be useful for developing lines with root length equal or longer than ST 213 and relative root weight greater than ST 213. We speculate that plants with long roots and a high relative root weight should possess increased levels of resistance to seedling disease through a mechanical type of resistance; i.e., the plants should have more root tissue to slough off as diseased tissue and yet maintain a viable root system and a healthy plant than would plants with short roots with a low relative root weight.

Key Words: Seedling disease • Cotton genetics • Diallel analysis • Generation mean analysis

² Research associate, Dep. of Agronomy, Mississippi Agric. and For. Exp. Stn.; research geneticist, USDA-ARS, Crop Sci. Res. Lab., P. O. Box 5367, Mississippi State, MS 39762; and agronomist, Dep. of Agronomy, Mississippi Agric. and For. Exp. Stn., Mississippi State, MS 39762.

Received for publication December 22, 1982.