Mepiquat chloride application combined with high plant population density promotes carbon remobilization in the roots of upland cotton.

Carbon reserves in cotton roots can be remobilized to support reproductive growth, thus potentially affecting cotton yield. However, the regulation of carbon remobilization in cotton roots and its relationship with cotton yield are still poorly understood. Plant population density (PPD) and mepiquat chloride (MC) have been hypothesized to affect the dynamics of nonstructural carbohydrate content and the resulting carbon remobilization in cotton roots through the regulation of carbohydrate metabolism enzyme activities. A mid-maturation cotton line 4003-6 was field-grown in 2019 and 2020. Three different levels of PPD (D1: 2.25 plants m, D2: 4.5 plants m, and D3: 6.75 plants m) and two levels of MC dosage (M0: 0 g hm, M1: 82.5 g hm) were combined to create six populations differing in terms of the source-sink relationship. The changes in the hexose, sucrose, and starch contents and the key carbon metabolic enzyme activities in cotton roots were examined during peak squaring (PS) to late boll opening (LB). The combination of the PPD of 6.75 plants m and MC application (M1D3) exhibited the greatest cotton yield and reproductive biomass-to-leaf area ratio from peak flowering (PF) onwards. M1D3 presented the greatest total nonstructural carbohydrate (TNC) remobilization amount of 2.96 and 3.80 g m, the highest efficiency of 39.11% and 48.39%, and the largest gross contribution to seed cotton yield of 0.66% and 0.79% in 2019 and 2020, respectively. The three parameters were positively correlated with the seed cotton yield except for the remobilization rate in 2019. Unlike the other treatments, the greater carbohydrate content per unit ground area in M1D3 prior to the PF stage was attributed to the higher sucrose phosphate synthase (SPS) and ADP-glucose pyrophosphorylase (AGPase) activities during the PS to first flowering (FF) stages. Conversely, the greater α-amylase and β-amylase activities in M1D3 at the PF stage accounted for the lower starch content at the EB stage, and the smaller vacuolar invertase (VIN) and cell wall invertase (CWIN) activities at the EB stage could be responsible for the lower hexose concentration at that time. The TNC remobilization amount had a positive association with the AGPase and SPS activities at the FF stage and with β-amylase activity at the PF stage in cotton tap roots in 2019 and 2020. This study provides a cotton yield-improving alternative through the promotion of carbon remobilization in roots using certain agronomic practices.