Optimum Plant Density Improved Cotton (Gossypium hirsutum L.) Root Production Capacity and Photosynthesis for High Cotton Yield under Plastic Film Mulching

Cotton photosynthetic efficiency and the root&ndash:shoot relationship are two important physiological indexes affecting the final yield, but the interactive effects of plastic film mulching and planting density on the cotton photosynthetic efficiency and root&ndash:shoot relationship have rarely been reported. We aimed to investigate the optimal plant density with or without plastic film mulching for improved seed cotton yield in southern Xinjiang. Therefore, a two-year field experiment was conducted to investigate the effects of plastic film mulching (with or without plastic film mulching) and planting density (D1: 9 &times: 104 plants ha&minus:1: D2: 18 &times: 104 plants ha&minus:1: D3, 22 &times: 104 plants ha&minus:1, local conventional planting density: D4, 27 &times: 104 plants ha&minus:1) on the cotton root&ndash:shoot relationship, photosynthetic parameters, and seed cotton yield. Our results showed that the seed cotton yield was improved under plastic film mulching at all planting densities, but economic income was significantly lower in comparison to without plastic film mulching in 2023. Compared with D3, seed cotton yield and economic income at D2 increased by 6.9% and 12.2%, either with or without plastic film mulching, respectively. The highest increase in the seed cotton yield in D2 under plastic film mulching was due to the greatest improvements in the root production capacity and photosynthesis. The boll capacity of the root system (BCR) and boll loading of the root system (BLR) in D2 were the highest among all treatments with film mulching, being 9.0% and 16.9% higher than that in D3 in 2022 and 2023. However, the root&ndash:shoot ratio (R/S) was 7.1% and 6.9% lower in D2 than D3, under film mulching, in 2022 and 2023. Moreover, moderate plant density (D2) improved the SPAD value, chlorophyll fluorescence (Fv/Fm and PIabs), and photosynthetic parameter (Pn, Tr, and Gs) and decreased Ci compared with other planting density treatments in both years. Further analyses with correlation analysis showed that the seed cotton yield was highly positive correlated with BLR, BCR, and the photosynthetic parameter. In summary, suitable planting density (18 &times: 104 plants ha&minus:1) combined with plastic film mulching has the potential to obtain high yields by enhancing the efficiency of photosynthetic assimilates, improving the capacity of cotton root production, providing a reference for suitable planting density under plastic film mulching.