Accumulation of cesium in leaves of Lepidium sativum and its influence on photosynthesis and transpiration

The study examines the transfer factor (TF) for cesium in a soil-plant system and cesium accumulation in cress Lepidium sativum L. plants grown in hydroponic culture and subjected to root and foliar application of 0.3 mM CsCl. The experiments showed a high TF for radiocesium: 2.97 (kBq/kg plant DW)/(kBq/kg soil DW). High accumulation of cesium was observed in leaves after both root and foliar treatments. A higher concentration of cesium (3 mM) caused significant disturbance in water uptake, tissue hydration (FW/DW) and production of biomass (DW). Accumulation of cesium in leaves affected gas exchange parameters. Stomatal conductance (C) and transpiration rate (E) were strongly inhibited but photosynthetic CO2 assimilation (P) was disturbed to a lesser extent. As a result, photosynthetic water utilization efficiency (P/E) was unaffected by 3 mM cesium at photosynthetically active radiation (PAR) of 220 mumol x m(-2) x s(-1). Increasing PAR from 220 to 450 mumol x m(-2) x s(-1) stimulated the photosynthetic rate after 3 days, but no stimulation was observed after 5 days of cesium treatment, in comparison with potassium-grown plants. Changes in chlorophyll fluorescence, indicating maximal quantum yield of photosystem II (PSII) photochemistry, were observed only as a late stress effect. Decreased stomatal opening was an early effect of cesium stress in Lepidium sativum, which resulted in limitation of transpiration and water uptake. It is suggested that the decrease in tissue hydration is what limits photosynthetic CO2 assimilation, synthesis of organic matter and light reactions of photosynthesis.