Increasing root-lower improves drought tolerance in cotton cultivars at the seedling stage

Drought is an important abiotic stress factor in cotton production. The root system architecture (RSA) of cotton shows high plasticity in alleviating drought-related stress under drought stress (DS); however, this alleviation is cultivar dependent. Therefore, this study aimed to estimate the genetic variability of RSA in cotton under DS. Using the paper-based growth system, we assessed the RSA variability in 80 cotton cultivars at the seedling stage, with 0 and 10% polyethylene glycol 6000 (PEG6000) as the control and the DS treatment, respectively. Analysis of 23 above-ground and root traits in the 80 cotton cultivars revealed different responses to DS. On the 10th day after DS treatment, the variation degree of RSA traits under DS (5-55%) was greater than that of the control (5-49%). The 80 cultivars were divided into drought-tolerant cultivars (group 1), intermediate drought-tolerant cultivars (group 2), and drought-sensitive cultivars (group 3) based on the comprehensive evaluation value of drought resistance. Under DS, the root length-lower, root area-lower, root volume-lower, and root length density-lower were significantly reduced by 63, 71, 76, and 4% in the drought-sensitive cultivars compared to the control. Notably, the drought-tolerant cultivars maintained their root length-lower, root area-lower, root volume-lower, and root length density–lower. Compared to the control, the root diameter (0-2 mm)-lower increased by 21% in group 1 but decreased by 3 and 64% in groups 2 and 3, respectively, under DS. Additionally, the drought-tolerant cultivars displayed a plastic response under DS, characterized by an increase in the root-lower. Drought resistance was positively correlated with the root area-lower and root length density-lower. Overall, the RSA of the different cotton cultivars greatly varied under DS. Therefore, important root traits, such as the root-lower, provide great insights for exploring whether drought-resistant cotton cultivars can effectively withstand adverse environments.