(Na⁺-K⁺)-stimulated ATPase in leaves of C₄ plants: possible involvement in active transport of C₄ acids

Abstract

Leaves of three C₄ plants, Setaria italica, Pennisetum typhoides, and Amaranthus paniculatus possessed five- to ten-fold higher activities of a (Na⁺-K⁺)-dependent ATPase than those of two C₃ plants, Oryza sativa and Rumex vesicarius. Na⁺-K⁺ ATPase from leaves of Amarathus exhibited an optimal pH of 7.5 and an optimal temperature of 35°C. It required 40 mM K⁺ and 80 mM Na⁺ for maximal activity. Ouabain partially inhibited (Na⁺-K⁺)-dependent ATPase activity in leaves of C₄ plants. Ouabain also blocked the movement of label from initially formed C₄ acids into end products in leaves of only C₄ plants, Setaria and Amaranthus but not in a C₃ plant, Rumex. We propose that Na⁺-K⁺ ATPase may mediate transfer of energy during active transport of C₄ acids from mesophyll into the bundle sheath.