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

Abstract

Effects of pod removal on the photosynthetic rates and their under lying 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. H owever, these decreases in photosynthesis were not asso-ciated with changes in the amounts of ribulose-1,5-bisphosphate carb oxylase (Rubisco) and other several photo-synthetic 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 speci fic activity nor activation state of Rubisco decreased . The decrease in the photosynthetic gas-exchange rate by pod removal was rec overed by shading the plants. CO₂-limited photosynthesis per unit of Rubisco protein was also completely recovered at the same l evel in the plants with pods. At the same time, starch accumu-lation 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.