Sink-source imbalance could cause accumulation of total non-structural carbohydrates (TNC) and down-regulation of photosynthesis. Sugiura et al. investigate how sink-source imbalance causes photosynthetic down-regulation in soybean (Glycine max), French bean (Phaseolus vulgaris), and azuki bean (Vigna angularis).
Among the three legume plants, maximum photosynthesis was down-regulated with increase in TNC only in French bean, whereas decrease in sink-source ratio caused anatomical changes and increase in cell wall content in source leaves, especially in soybean. The results suggest that down-regulation of photosynthesis is caused not only physiologically such as through a decrease in Rubisco, but also morphologically, such as through an increase in cell wall thickness that could reduce chloroplast CO2 concentration.