Relationship between the suppression of photosynthesis and starch accumulation in the pod-removed bean

Effects of pod removal on the photosynthetic rates and their underlying biochemical properties were examined in leaves of bean (Phaseolus vulgaris L.). Pod removal led to decreases in the photosynthetic gas-exchange rate, stomatal conductance and intercellular CO₂ pressure. However, these decreases in photosynthesis were not associated with changes in the amounts of ribulose-1,5-bisphosphate carboxylase (Rubisco) and other several photosynthetic components, but were associated with an increase in starch content. In addition, such a decrease in the photosynthetic rate was not observed when measured with O₂ evolution under conditions of saturating CO₂. Similarly, although a decrease in CO₂-limited photosynthesis for a given Rubisco content was found in depodded plants, neither the specific activity nor activation state of Rubisco decreased. The decrease in the photosynthetic gas-exchange rate by pod removal was recovered by shading the plants. CO₂-limited photosynthesis per unit of Rubisco protein was also completely recovered at the same level in the plants with pods. At the same time, starch accumulation was also decreased by shading. From these results, we concluded that the suppression of photosynthesis observed in depodded beans was caused by the hindrance of CO₂ diffusion due to starch accumulated in the chloroplast.