Boll/leaf ratio improves the source–sink relationship and lint yield during the boll setting stage of cotton

Abstract

ContextCoordination of the cotton source–sink relationship is important. Although some indicators have been used to reflect the source-sink relationship in cotton, few indicators can directly demonstrate the source–sink relationship from the perspective of the cotton population. ObjectiveThis study aimed to determine one or several indicators that can be used to directly reflect the source–sink relationship of the cotton population. MethodsThree field experiments were conducted with a conventional cotton cultivar (‘Sikang1’) in 2018 and 2019. Experiment I consisted of six planting densities: 3.0 × 104, 3.3 × 104, 3.6 × 104, 3.9 × 104, 4.2 × 104, 4.5 × 104 plants·ha−1. Experiment Ⅱ was composed of two types of studies. One was made up of one control and two square removed treatments, including 10 and 20% of squares removed from the whole plant, with no square removed as the control. The other study consisted of one control and two leaf-cutting treatments, in which 25 and 50% of the leaf area were cut off from the whole plant. ResultsLint yield was enhanced by 11.2–66.1% in 2018, and the boll number per leaf increased from 0.225 to 0.282 boll·leaf–1. The boll number per square meter of leaf area increased from 22.7 to 27.2 bolls·m–2 leaf, and the boll dry weight per square meter of leaf area increased from 119.3 to 149.7 g·m–2 leaf in 2018. The results for 2019 were similar to those for 2018. There was significant linear positive correlations between lint yield and the above three measurements of the boll/leaf ratio. Moreover, these three measurements of the boll/leaf ratio were significantly and linearly positively correlated with each other, indicating that the three boll/leaf ratio measurements were similar and represented the coordination of the source–sink relationship of cotton population.The increased proportion of square removal led to a decrease in the boll number per leaf area. The canopy apparent photosynthesis rate, sucrose phosphate synthase and soluble acid invertase activities of subtending leaves decreased, but the sucrose and glucose contents increased and decreased, respectively. The proportion of 14C assimilates exported from the subtending leaf to the opposite boll also decreased, and the dry weight of the reproductive organs decreased.When the leaf area index was adjusted to the optimum level by leaf cutting at the maximum leaf area index above the optimum range, the boll number per square meter of leaf area increased from 10.9 to 26.2 and 21.9 bolls·m–2 leaf, and the boll dry weight per square meter leaf area increased from 55.6 to 155.1 and 141.9 g·m–2. The lint yield was also increased by 24.9% in 2018. ConclusionThe boll/leaf ratio can be used as an index to reflect the source–sink relationship of the cotton population. A higher boll/leaf ratio is beneficial for the population’s photosynthetic production, carbohydrate inversion, and transportation of photosynthetic products to the boll, which is conducive to increasing the lint yield in cotton. ImplicationsThe results suggest that the boll/leaf ratio can be used as a diagnostic index for the coordination of the source–sink relationship.