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.