Structural Aspects of Photosystem I from Dunaliella salina

Abstract

The Na⁺ and K⁺ transport characteristics of Spergularia marina (L.) Griseb. were considered in order to compare the systems by which these two physiologically different cations are managed during initial acquisition and subsequent partitioning in midvegetative plants. Uptake of ²²Na⁺ and ⁴²K⁺ and redistribution of labels in pulse-chase studies were compared under steady state growth conditions or with the concentration of one of the ions elevated. At high external concentrations, the initial ⁴²K⁺ accumulation and transport to the shoot was associated with a small, rapidly exchanging, cellular compartment similar to that previously indicated for Na⁺ (D Lazof, JM Cheeseman 1986 Plant Physiol 81: 742-747). At 1 mol m⁻³, K⁺ was conducted to the shoot through a root compartment, the specific activity of which rose much more slowly than the rapidly exchanging compartment. After a lag of approximately 5 minutes, ⁴²K⁺ translocation approached a constant rate with a half-time of 14 minutes compared to 5 minutes for ²²Na⁺ or for ⁴²K⁺ at higher external levels. At all external levels, prolonged translocation of ⁴²K⁺ was measured when a 10 minute pulse was followed by an unlabeled chase, again suggesting a conducting compartment distinct from that for Na⁺. It is suggested that the K⁺ conducting compartment, possibly the `bulk cytoplasm,' is associated with the active K⁺ transport system generally found in higher plants.