Effects of cadmium on photosynthetic oxygen evolution from single stomata in Brassica juncea (L.) Czern

Scanning electrochemical microscopy (SECM) was utilized to investigate photosynthetic oxygen evolution from single stomata in leaves of live Brassica juncea (L.) Czern cultured in nutrient solution to which 0.2 or 0.01 mM CdC12 had been added. The bulk leaf surface serves as an insulator normally; therefore, a typical negative feedback was observed on the probe approach curves (PACs) when the probe approached epidermal cells. When the probe tip approached an open stoma, a higher tip current was detected due to the O2 release from this stoma. Thus, SECM can be used to map the O2 concentration profile near the leaf surface and study stomatal complex structure size and density. The oxygen release from single stomata was also analyzed by comparison of experimental PACs with those simulated by COMSOL multiphysics software (version 3.4). In addition to an increase in the stomatal complex size and a decrease in the complex density, the Cd accumulation caused up to a 26% decrease in photosynthetic rate determined at the level of a single stoma. The O2 evolution was also monitored by recording the tip current vs time when a tip sat above the center of a stoma. Periodic peaks in O2 release-time curves were observed, varying from 400 to 1600 s. The opening and closing activities of single stomata were also imaged by SECM.