Effects of boron deficiency on photosynthesis and carbohydrate status in leaves of mung bean (Phaseolus aureus Roxb)

Effects of boron (B) deficiency on leaf functions of mung bean, photosynthesis and carbohydrate contents, were investigated to establish whether the inhibited growth of boron deficient plants was caused by diminished photosynthesis. Mung bean (Phaseolus aureus Roxb) seedlings were grown in solution culture at two B levels, 0.2 mol/L B (low level) and 50 mol/L B (high level) two days after the appearence of the second trifoliate leaf of mung bean. Changes in chlorophyll content, gas exchange and carbohydrate content were determined during the growth of the second trifoliate leaf. The results demonstrated that the boron deficiency has less effects on chlorophyll content and internal CO2 concentrations (Ci), decreased the CO2 assimilation and stomatal conductance (Gs). The calculated relative stomatal limitation (Ls) of boron deficient plants is enhanced. The boron deficiency increases soluble sugar, particularly glucose, and starch contents in contract to the reduce of photosynthetic rates. The results suggest that the inhibited growth of boron deficient plants is not brought about by shortage of carbohydrate, but the low photosynthetic rate due to the low sink activity.