Changes in growth and physiology of tobacco and cotton under Ag exposure and recovery: Are they of direct or indirect nature?

Responses of tobacco and cotton seedlings to two chemical farms of Ag at va rious concentrations were studied. Cotton exhibited higher resistance than tobacco to Ag. The application of 1 mu g L-1 Ag, as AgI or AgNO3, induced n o toxicity symptoms in the growth and photosynthesis of cotton. However, th e corresponding concentration negatively affected the growth and physiology of tobacco. On exposure to higher Ag concentrations (10(3) and 5 X 10(3) m u g L-1 as AgNO3), root growth and concentrations of Ca, Mg, K, and Fe were significantly depressed, whereas concomitant Ag accumulation, especially i n roots, was increased in both species. The loss of nutrients and chlorophy ll was associated with disturbances in photosynthetic function. Because the re was no apparent change in Fv/Fm, there is a possibility that the faster rise time was due to an increase in LHC size to the PSII reaction centers. The decreased Fv/Fo reflects the inactivation of chlorophyll associated wit h the PSII reaction center. Stomatal conductance was decreased less than th e CO2 assimilation, possibly due to the direct and/or indirect effects of A g ions on stomatal guard cells. After the plants were transferred from the highest Ag concentration to a nutrient solution, recovery of tobacco, cotto n morphology, and physiology were observed.