Topical shading substantially inhibits vegetative branching by altering leaf photosynthesis and hormone contents of cotton plants

Cotton plants have complex branching patterns formed by distal fruiting branches (FB) and basal vegetative branches (VB). Our prior study showed that cotton’s vegetative branching was considerably inhibited under high plant density via altered hormone contents in VB tips and photosynthetic production in VB leaves. Here, our current objective was to determine whether or not shading of VB reduced vegetative branching, and whether this effect was elicited via a similar mechanism(s) as that under high plant density. To this end, cotton was planted on 76-cm row widths at a moderate density (5 plants m⁻²). Vegetative branches of cotton plants were shaded to approximate 60% of full solar exposure by using polyethylene nets during squaring; those without shading served as the control. The growth and development of VB, as well as underlying agronomic, physiological and molecular events of both shaded and control plants were examined. Shading VB reduced its numbers by 56.2%; its length by 88.8% and the ratio of VB biomass to total biomass by 91.3%, but it increased FB biomass by 30.8%, compared with the control at 69 days after seeding. Compared with control, shading did not reduce seedcotton yield, suggesting a yield stability similar to that found previously under high plant density. The net photosynthetic rate, chlorophyll content and ribulose-1,5-bisphosphate carboxylase activity in VB leaves were considerably decreased by shading. The sucrose biosynthesis gene GhCYFBP in leaves and GhphyB gene in tips of VB were down-regulated under its shading, followed by lower soluble sugar and starch contents. Suppressed photosynthetic production in leaves of VB was a key reason for poor vegetative branching under shaded conditions. Shading diminished both the auxin (IAA) content and auxin transport from VB tips by decreasing the expression of corresponding genes for auxin biosynthesis (GhYUC5) and transport (GhPIN1and GhPIN5). This treatment also reduced the content of cytokinins (CTKs) in the VB tips by decreasing the expression of the CTKs biosynthesis gene, GhIPT3. By contrast, the SL receptor gene, GhD14 was upregulated in the VB tips, followed by a greater content of strigolactones (SLs). We proposed that lower IAA, CTKs and brassinosteroid contents plus higher SLs content due to differential expression of hormone-related genes in the VB tips suppressed vegetative branching when shaded. Our results suggest this mechanism is not unlike how high plant density inhibits vegetative branching in cotton.