Boll-leaf system gas exchange and its application in the analysis of cotton photosynthetic function

Estimating the boll development and boll yield from single-leaf photosynthesis is difficult as the source-sink relationship of cotton (Gossypium hirsutum L.) is complicated. As the boll-leaf system (BLS), which includes the main-stem leaf, sympodial leaf, and non-leaf organs, is the basic unit of the cotton source-sink relationship and yield formation, the concept of "BLS photosynthesis" is introduced in this study. We speculate that the characteristics of BLS gas exchange can more accurately reflect the photosynthetic function of the system, thus revealing the law of photosynthesis in the process of boll development. The results showed that the photosynthetic rate of single leaves measured by a BLS chamber was consistent with that measured by a standard single-leaf chamber. BLSs exhibited typical light response curves, and the shape of the curves was similar to those of single leaves. The light compensation point and respiration rate of BLSs were higher than those of single leaves, while the apparent quantum efficiency of BLSs was lower. Compared with single leaves, the duration of the photosynthetic function of BLSs was longer. Increasing plant density decreased the gas exchange rate per unit BLS more significantly under field conditions. There was a better linear correlation between the net CO2 assimilation rate, respiration rate of BLSs and boll biomass. Therefore, we think that the gas exchange of BLSs can better reveal the changes in photosynthetic function of BLSs and boll development. This provides a new basis for analyzing the mechanism and regulation of cotton yield formation.