Microamperometric estimation of photosynthesis inhibition in a single algal protoplast

The single-cell level inhibition of photosynthesis by some chemicals was investigated from the light-irradiation induced response of intracellular oxygen reduction current at an ultramicroelectrode inserted into a single algal protoplast. The addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an electron-transport inhibitor, decreased a current peak appeared immediately after the light irradiation. The pI50 value (the negative decadic logarithm of the concentration needed for 50% inhibition) was determined from the change in response patterns and found to be 6.9 for DCMU. 2,4-Dichlorophenoxyacetic acid (2,4-D), an auxin herbicide, showed no effect on photosynthesis in the effective concentration range. 2,4-Dinitrophenol (DNP), a proton carrier, accelerated the oxygen production upon light irradiation.