Salinity-Induced Potassium Deficiency Causes Loss of Functional Photosystem II in Leaves of the Grey Mangrove, Avicennia marina, Through Depletion of the Atrazine-Binding Polypeptide

Abstract

Photosynthetic properties were studied in relation to the ionic composition of leaves of Avicennia marina grown in low and high salinity (i.e. 50 and 500 mol m^-3 NaCl) nutrient solution containing either 0.01 or 10 mol m^-3 K⁺. Leaves accumulated high concentrations of NaCl, but changes in photosynthesis were associated with changes in leaf K⁺ concentrations. The effects occurred at two levels. (1) With decrease in leaf K⁺ from 379 to 167 mol m^-3, a 21% decline in light and CO₂ saturated rates of oxygen evolution per leaf area was consistent with a 24% decrease in chlorophyll content. (2) Leaves containing only 103 mol m^-3 K+ showed drastic loss of light and CO₂ saturated photosynthetic capacity (42%) and photochemical dysfunctioning under limiting light conditions as manifest in a 38% decrease in quantum yield. Thylakoids isolated from these low K⁺ leaves showed no decrease in per chlorophyll concen- trations of photosystem I, cytochrome f/b complex and ATPase, but had 37% fewer atrazine-binding sites (corresponding to photosystem II reaction centres) than those from leaves with higher K⁺ concentrations. The decline in atrazine-binding sites in isolated thylakoids was sufficient to account for the loss of quantum yield in intact leaves. These results identify the atrazine-binding polypeptide of photosystem II as one site of sensitivity to salinity-induced K⁺ deficiency.