Regulation of stomatal movement and photosynthetic activity in guard cells of tomato abaxial epidermal peels by salicylic acid.

Salicylic acid (SA), a signalling molecule in plant-pathogen interactions induces stomatal closure in intact leaves and it has a direct control over stomatal movement by increasing the levels of reactive oxygen species (ROS) and nitric oxide (NO) in guard cells (GC). Stomatal closure on the abaxial epidermal peels of tomato leaves was induced at 10-7 and 10-3M SA but stomata remained open at 10-4M. At concentrations that reduced stomatal aperture, the ROS and NO levels were raised. The accumulation of ROS and NO could be prevented by specific scavengers, which were effective inhibitors of the SA-induced stomatal closure. In contrast with other plant species, the guard cells (GCs) of tomato did not show a long-lasting accumulation of ROS in the presence of 10-4M SA and their NO content decreased to below the control level, leading to stomatal opening. Increasing SA concentrations resulted in a significant decrease in the maximum and effective quantum yields of PSII photochemistry and in the photochemical quenching parameter of GCs. In the presence of 10-7 and 10-4M SA, the chloroplasts of GCs sustained a higher electron transport rate than in the presence of 10-3M, suggesting that the SA-induced inhibition of GC photosynthesis may affect stomatal closure at high SA concentrations.