High plant density inhibits vegetative branching in cotton by altering hormone contents and photosynthetic production

Manual removal of vegetative branches (VB) is a traditional cultural practice widely adopted in Chinese cotton production, but the increasing labor cost in the country has reduced its cost-effectiveness. It is well known that high plant density inhibits the VB growth of cotton and can be a potential alternative to the traditional practice, but the underlying mechanisms are poorly understood. This study mainly aimed to determine how the VB growth of cotton could be inhibited by increasing plant density. Cotton was planted on 76-cm row widths at low (3 plants m(-2)), moderate (6 plants m-2) and high (9 plants m(-2)) plant densities. The VB growth and development, as well as the underlying agronomic, physiological and molecular events were examined. The high plant density decreased the number of VB by 67.3% and the ratio of VB biomass to total biomass by 95.0% at 125 days after seeding (DAS) compared with the low plant density. The net photosynthetic (Pn) rate, chlorophyll content and ribulose-1, 5-bisphosphate (RuBP) carboxylase activity in VB leaves were considerably decreased under high plant density, followed by reduced soluble sugar and starch contents, suggesting a suppressed photosynthetic production in the leaves. This decreased photosynthesis resulting from reduced RuBP carboxylase activity and chlorophyll content was one of the reasons for the poor VB growth under high plant density. High plant density increased the auxin (IAA) content and promoted auxin polar transport by increasing the expression of the auxin biosynthesis gene, GhYUCS and auxin polar transport gene, GhPIN1. It also increased the content of cytokinins (CKs) by increasing the expression of the CKs biosynthesis gene, GhIPT3 in the main-stem tips, and decreasing the IAA and CKs contents as well as auxin polar transport by reducing the expression of GhYUCS, GhPIN1, GhPIN5 and GhIPT3 in the VB tips. The GhPhyB gene was down-regulated by high plant density, but the strigolactones (SLs) content was increased by up-regulating the SL receptor gene, GhD14 in the VB tips. The decreased IAA, CKs, GA and BR contents plus the increased content of Sts due to differential expression of hormone -related genes in the VB tips was another reason for the inhibition of vegetative branching. Although high plant density did not increase seedcotton yield, it improved the fiber quality by decreasing the number of VBsourced bolls. Our results suggested that high plant density can be a substitute for the traditional plant pruning.