FIGURE 10 The regulatory model and function of root cortical senescence (RCS) in the root growth and development of different cotton varieties. Comparative analysis of the regulatory mechanism of RCS on drought tolerance of the different cotton under drought stress. In drought-tolerance varieties, endogenous hormones (showing increased indole-3-acetic acid and reduced abscisic acid contents) are involved in the regulation of RCS. Increased lacunae/cortex ratio, reduced cortical cell files and cortex/stele ratio, further reduced root respiration and metabolic enzyme activity (phosphofructokinase, malate dehydrogenase and glucose-6-phosphate dehydrogenase). This might be due to the redistribution of more resources to root growth (reflected by increased root dry weight, total root length, and convex hull area), ultimately increasing the aboveground drought tolerance (reflected by increased leaf water potential, relative leaf water content, and stomatal opening under drought stress). The regulatory mechanism of RCS varies based on plant species, drought severity and specific growth stages. ”Ji 228” exhibited a lower incidence of RCS than ”Guoxin 02”. Pink arrows denote positive regulation, while green arrows indicate negative regulation. Long and short arrows represent higher and lower metabolic costs, respectively.