Plant pruning affects photosynthesis and photoassimilate partitioning in relation to the yield formation of field-grown cotton

Fine pruning, or the artificial removal of vegetative branches (VB) and main stem tips (plant topping), is a traditional cotton (Gossypium hirsutum L.) cultivation practice. Pruning can be simplified without reducing yield by retaining the vegetative branches and adopting chemical topping; however, cotton yield formation under different plant pruning modes remains unclear. We aimed to determine if and how simplified pruning results in comparable yields to fine pruning in terms of canopy photosynthesis and photoassimilate partitioning. A two-year field experiment was thus conducted to determine the effects and interactions of vegetative branch management (retaining and removing of vegetative branches) and plant topping pattern (non-topping, manual topping, and chemical topping) on yield, canopy photosynthesis, and photoassimilate partitioning. Seed cotton yield, canopy photosynthesis, and photoassimilate partitioning were significantly affected by VB removal or retention and plant topping modes, but not by their interaction. Boll weight and harvest index under VB retention were reduced compared to VB removal, while boll density and biological yield increased. Compared with non-topping, the biological and seed cotton yield increased and the harvest index decreased under chemical topping, whereas these all increased under manual topping. Seed cotton yield was comparable between chemical and manual topping. The leaf area index (LAI) under VB retention was higher than under VB removal at peak squaring, peak flowering, and peak boll-setting, and comparable at boll-opening. The carbon assimilation rate (CAR) under VB retention increased compared with that under VB removal at peak flowering, peak boll-setting, and boll-opening. Meanwhile, VB retention partitioned more photoassimilates to the vegetative organs, and less to the reproductive organs, than VB removal at peak flowering and peak boll-setting. Compared with no topping, LAI and CAR under chemical and manual topping increased at peak-boll setting and boll-opening. Furthermore, the partitioning of photoassimilates to the reproductive organs under chemical topping was similar to that of non-topping at the peak boll-setting and boll opening stages, whereas this increased under manual topping. Cotton yield did not vary between VB managements due to the coordination between canopy carbon assimilation and assimilate partitioning. Manual topping improved both CAR and photoassimilate partitioning to the bolls, and thus increased the seed cotton yield compared with non-topping. By contrast, chemical topping reduced the photoassimilate partitioning to the reproductive organs, and the increased yield was attributed to the improved carbon assimilation rate. In summary, chemical topping achieved the same yield as manual topping, but the mechanism of yield increase differed from the perspective of photosynthetic production and assimilate partitioning.