Foliar and seed application of plant growth regulators affects cotton yield by altering leaf physiology and floral bud carbohydrate accumulation

The development of cotton (Gossypiwn hirsutum L) floral bud prior to flowering is fundamental for yield formation since it is the initial step in the fruiting cycle. However, little information is available on the early reproductive stage of cotton subjected to plant growth regulators (PGRs). A 2-year experiment was conducted to investigate the effects of gibberellic acid (GA(3)), N6-benzyladenine (6-BA) and N, N-dimethyl piperidinium chloride (DPC) with two application methods [(A) seed soaking and (B) foliar spraying at squaring stage] on cotton floral bud development, yield and yield components compared with the control (water application). The results showed that seedcotton yield increased by 6.394-7.5% and 12.1%43.3% respectively in GA(3) and 6-BA seed soaking treatments, and by 7.7%-8.5% and 8.2%-11.0% after foliar application compared to control due to increased boll numbers, which were associated with higher floral bud numbers. However, there was no response to yield for DPC treatment. Additionally, GA(3) and 6-BA significantly increased photosynthesis rate (Pn) accordingly affecting the subtending leaf carbohydrate content and translocation, resulting in higher sucrose and starch contents in floral buds, which substantially enhanced dry matter of floral buds and reduced abscission rate. Those responses in floral buds are conductive to favorable boll development, which was evidenced by higher boll retention and boll opening rate for GA(3) and 6-BA application relative to control. Specially, 6-BA showed equal effects to GA(3) on yield and yield components with application method B, whereas induced a greater promotion than GA(3) with application method A in the following aspects: (1) square numbers, boll numbers and seedcotton yield increased more obviously under 6-BA than GA(3) ; (2) the increases in boll retention and boll opening rate were larger under 6-BA than GA(3) ; (3) 6-BA induced higher carbohydrate content in floral buds than GA(3).