Figure legends
FIGURE 1 The transverse cross-sections of the taproot of ”Guoxin 02” and ”Ji 228” varieties sampled at different areas on the taproot 15 days after the treatment and stained with acridine orange fluorescent nuclei. Note: acridine orange fluorescent nuclei are not visible in these images because they are located deeper within the tissue, as indicated by a laser scanning confocal microscope. The red arrow indicates the lacunae area.
FIGURE 2 Characteristics and occurrence mechanisms of RCS in different cotton varieties after 15 days of drought stress. Statistical analysis of (A) the cortical senescence percentage, (B) cortical cell files and (C) the cortical lacunae area. DT type, Drought-tolerant type ”Guoxin 02”; DS type, Drought-sensitive type ”Ji 228”; NS, no-stress; DS, drought stress; C, varieties; D, drought. Error bars represent standard errors of the means. Different lowercase letters in the figure indicate significant differences between the same root segments from different cotton varieties under drought stress treatments (p ≤ 0.05). P >0.05, no significance (ns);P <0.01 significant difference (**).
FIGURE 3 Effects of the drought treatment on the root metabolic costs of cotton. Statistical analysis of (A) phosphofructokinase, (B) malate dehydrogenase, (C) glucose-6-phosphate dehydrogenase, and (D) root respiration activities at 3 cm, 6 cm, 9 cm, and 12 cm segments from the taproot tips of the ”Guoxin 02” and ”Ji 228”. (E) Comparative image showing the differences in root metabolic costs between ”Guoxin 02” and ”Ji 228” under drought stress. Long and short arrows represent higher and lower metabolic costs, respectively. DT type, Drought-tolerant type ”Guoxin 02”; DS type, Drought-sensitive type ”Ji 228”; NS, no-stress; DS, drought stress; C, varieties; D, drought. Error bars represent standard errors of the means. P >0.05, P <0.05 andP <0.01 indicate no significance (ns), significant difference (*) and highly significant difference (**), respectively. Different lowercase letters in the figure indicate significant differences between different root segments under the same treatment (P ≤ 0.05).
FIGURE 4 Linear regression analysis showing the relationship between RCS and root growth, root metabolic costs, and aboveground growth under drought stress. (A) Total root length, (B) root dry weight, (C) root tissue density, (D) root respiration at 6 cm from the root tip, (E) root respiration at 9 cm from the root tip, (F) root respiration at 12 cm from the root tip, (G) leaf water potential, (H) leaf relative water content, and (I) the ratio of variable to maximum fluorescence (F v/F m ).
FIGURE 5 Effects of drought stress on the ratio and size of root cross-section in the two cotton varieties. (A) Representative image of the root tissue size of ”Guoxin 02” under drought stress. Statistical analysis of (B) stele/whole ratio, (C) cortex/stele ratio, and (D) lacunae/cortex ratio at the 3 cm, 6 cm, 9 cm, and 12 segments cm from the root tips of ”Guoxin 02” and ”Ji 228”. NS, no-stress; DS, drought stress; C, varieties; D, drought. Error bars represent standard errors of the means. P >0.05, no significance (ns); P <0.01 significant difference (**). Different lowercase letters in the figure indicate significant differences between different root segments under the same treatment (P ≤ 0.05).
FIGURE 6 Effects of the drought treatment on phytohormone levels in the two cotton varieties. Statistical analysis of (A) gibberellin, (B) zeatin riboside, (C) indole-3-acetic acid, (D) brassinolide, and (E) abscisic acid contents at the 3 cm, 6 cm, 9 cm, and 12 cm segments from the taproot tips of the ”Guoxin 02” and ”Ji 228”. DT type, Drought-tolerant type ”Guoxin 02”; DS type, Drought-sensitive type ”Ji 228”; NS, no-stress; DS, drought stress. Error bars represent standard errors of the means. P >0.05, no significance (ns); P <0.05, significant difference (*);P <0.01, highly significant difference (**). Different lowercase letters in the figure indicate significant differences between different root segments under the same treatment (P ≤ 0.05).
FIGURE 7 Effects of drought stress on the root traits of ”Guoxin 02” and ”Ji 228”. (A) Representative images of root traits of the two varieties at 5 d, 10 d and 15 d after drought stress. Statistical analysis of the (B) total root length, (C) root dry weight, (D) specific root surface area, (E) root volume, (F) average diameter, (G) root tissue density, (H) taproot length, (I) average length-lateral roots, (J) Average length- all root, (K) lateral root angle, (L) lateral root count, and (M) convex hull area. DT type, Drought-tolerant type ”Guoxin 02”; DS type, Drought-sensitive type ”Ji 228”. NS, no-stress; DS, drought stress; C, varieties; D, drought. Error bars represent standard errors of the means. P <0.05, significant difference (*); P <0.01, highly significant difference (**).
FIGURE 8 Effects of drought stress on the aboveground traits of ”Guoxin 02” and ”Ji 228”. (A) Representative images of the growth of ”Guoxin 02” and ”Ji 228” at 0, 5, 10 and 15 days after drought stress. Statistical analysis of the (B) ratio of variable to maximum fluorescence (F v/F m), (C) leaf area, (D) leaf water potential, (E) steam diameter, (F) leaf water saturation deficit, (G) plant height, (H) relative water content, (I) SPAD value, (J) stomatal length, (K) stomatal width, (L) stomatal opening, and (M) stomatal density. DT type, drought-tolerant type ”Guoxin 02”; DS type, drought-sensitive type ”Ji 228”. CK, no-stress; DS, drought stress; C, varieties; D, drought. Error bars represent standard errors of the means. P >0.05, no significance (ns); P <0.01, highly significant difference (**). Different lowercase letters in the figure indicate significant differences among different varieties and treatments at the same time point (P ≤ 0.05).
FIGURE 9 Structural equation modeling showing the effect of RCS on drought tolerance of cotton under drought stress. The standardized loading coefficients are indicated by the numbers. Positive loading coefficients signify positive correlations between the measured variables and the factors, while negative loading coefficients indicate negative correlations. The red solid and dashed lines represent positive and negative correlations, respectively.
FIGURE 10 The regulatory model and function of 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.