Drought stress reshapes the genetic architecture of fiber quality and developmental traits in Egyptian cotton (Gossypium barbadense L.): a line × tester analysis under contrasting irrigation regimes.

Water scarcity is an increasing threat to Egyptian cotton production, yet the mechanisms by which drought stress modifies the genetic control of fiber development and plant architecture remain poorly characterized. This study investigated how water deficit alters the genetic architecture of eight fiber quality and agronomic traits in Egyptian cotton, using a line × tester mating design comprising seven lines crossed with three testers to produce 21 F1 hybrids, which were evaluated under normal irrigation and drought stress at the Sakha Agricultural Experimental Research Station during the 2025 growing season. Analysis of variance partitioned genetic variation into general combining ability (GCA) and specific combining ability (SCA) components, and estimated gene action, heterosis, and heritability under both environments. Under normal irrigation, fiber quality traits micronaire, fiber length (UHML), uniformity index, and fiber strength were predominantly governed by additive genetic effects, with GCA/SCA variance ratios ranging from 13.0 to 81.6. Under drought stress, all four fiber traits showed lower GCA/SCA ratios (0.13-0.32), consistent with a relatively greater contribution of non-additive (dominance and epistatic) variance in this dataset. Narrow-sense heritability for fiber length declined from 0.980 under normal conditions to 0.206 under drought, while broad-sense heritability remained high (0.979), a pattern consistent with a relative reduction in the additive genetic component under stress. Agronomic traits were predominantly non-additive under both environments, with the notable exception of the first fruiting node under drought, which exhibited an additive-dominant ratio of 1.11, suggesting that the developmental timing of reproductive branching becomes more additively heritable under stress. Among parental genotypes, Giza 85 and Giza 92 emerged as superior donors for fiber quality under drought, while Giza 70 and Giza 87 were the most effective combiners under normal conditions. The cross Giza 85 × Giza 86 showed the highest positive mid-parent heterosis for fiber length (8.48%), uniformity index (2.01%), and fiber strength (6.57%) under drought simultaneously, suggesting complementary genetic interactions that may buffer fiber development against water deficit. These findings suggest that drought is associated with a shift in the relative contribution of additive and non-additive genetic control over fiber quality and developmental traits in this population and environment, with potential implications for climate-resilient cotton breeding strategy. Hybridization-based approaches exploiting non-additive variance may be worth considering for drought-targeted programs, whereas pedigree selection appears more appropriate under favorable conditions; confirmation across additional environments and germplasm would strengthen these recommendations.