3.5 RCS is closely related to root and aboveground growth
To clarify the physiological role of RCS in regulating the development of the whole root system and aboveground parts, we measured several traits, such as root architecture and shoots, at different time points (Figures 7-8). There were genotypic differences in the response of cotton roots to drought stress (Figure 7A). Specifically, compared to ”Ji 228”, the total root length, root dry weight, specific root surface area, root volume, root tissue density, average length - lateral roots, average length - all roots, and convex hull area of the ”Guoxin 02” were increased by 45.92%, 90.00%, 98.70%, 37.56%, 84.00%, 28.04%, 30.63%, 16.97%, and 15.65%, respectively, under drought stress (Figures 7BCDEGIJM). However, the lateral root angle was significantly reduced by 23.34% (Figure 7K).
The impact of drought stress on aboveground traits of cotton plants was the highest at 15 days after treatment (Figure S2 and Figure S3). Under drought stress, the F v/F m, leaf area, leaf water potential, plant height, relative leaf water content, stomatal length, stomatal width, and stomatal opening of ”Guoxin 02” increased by 80.21%, 40.05%, 15.56%, 18.42 %, 29.91%, 15.09%, 23.68%, and 18.75%, respectively, (Figures 8ABCFGIJK) compared to ”Ji 228”. However, the leaf water saturation deficit and stomatal density showed a significant reduction of 9.79% and 5.91%, respectively (Figures 8EL).
Pearson correlation analysis showed that the cortical senescence percentage at the 6 cm, 9 cm and 12 cm segments from the root tip were positively correlated with root total length, root dry weight, root tissue density, leaf water potential, leaf relative water content, andF v/F m (p < 0.05, Figures 3ABCGHI).
3.6 Structural equation model of RCS and drought tolerance
To further investigate the direct and indirect effects of RCS on cotton drought tolerance, we generated a structural equation model to examine the relationship between RCS and cotton drought tolerance and determine the path coefficients between the two (Figure 9). The structural equation model showed a significant correlation between RCS and drought tolerance, with a P -value of 0.033*. This indicated that the pathway was valid, with an effect coefficient of 0.986. Overall, the model demonstrated a good fit (Table S2).