Ni2+ toxicity in rice: Effect on membrane functionality and plant water content

The heavy metal nickel is an essential mineral trace nutrient found at low concentrations in most natural soils. However, it may reach toxic levels in certain areas and affect a number of biochemical and physiological processes in plants. Wilting and leaf necrosis have been described as typical visible symptoms of Ni2+ toxicity. The plasma membrane (PM) of root cells constitutes the first barrier for the entry of heavy metals but also a target of their toxic action. This work studies the relationship between disturbances of membrane functionality and the development of the typical symptoms of Ni2+ toxicity. Rice plants (Oryza sativa L. cv. Bahia) grown in nutrient medium containing 0.5 mM Ni2+ showed a significant decrease in water content as a consequence of the stress. Addition of Ni2+ to the solution bathing the roots induced a concentration-dependent PM depolarization but the activity of the PM-H+-ATPase was not inhibited by the presence of Ni2+ and the initial resting potential recovered in less than 1 h. In the short term (hours), membrane permeability of root cells was not significantly affected by Ni2+ treatments. However, in the long term (days) a drastic loss of K+ was measured in roots and shoots, which should be responsible for the changes in the water content measured, since stomatal conductance and the transpiration rate remained unaffected by Ni2+ treatment. The effects induced by Ni2+ were not permanent and could be reverted, at least in part, by transferring the plants to a medium without Ni2+.