Dual turgor regulation response to hypotonic stress in Lamprothamnium papulosum

Cells of the salt-tolerant charophyte Lamprothamnium respond differently to hypotonic challenge according to their position on the plant (i.e. cell ag e). Differences in electrophysiological response are coupled with differenc es in cell fine structure, and the presence or absence of extracellular muc ilage. (1) Young, apical (fast-regulating, FR) cells respond with sudden ce ssation of cyclosis, depolarization to -50 mV (in some cells by more than 1 00 mV) and increase in membrane conductance by up to an order of magnitude. Intracellular [K+](v), [Na+](v) and [Cl-](v) decrease 1 h after hypotonic challenge. Patch-clamping cytoplasmic droplets reveals two types of K+ chan nel, 150 pS and 35 pS, and a small conductance Cl- channel, 35 pS (conducta nces at estimated tonoplast resting potential between zero and 20 mV). Extr acellular mucilage is thin (<5 mu m thick) or lacking, similar to freshwate r Chara. Unlike freshwater charophytes these cells have a canalicular vacuo lar system of large surface area and compartment the fluorochrome 6 carboxy fluorescein in the cytoplasm rather than the vacuolar system. (2) Older bas al (slow-regulating, SR) cells do not cease streaming on hypotonic challeng e and depolarize only slightly (by approximately 20 mV) with small or no ch ange in membrane conductance. After 1 h the intracellular [K+](v), [Na+](v) and [Cl-](v) scarcely change. Patch-clamping cytoplasmic droplets reveals two types of KC channel, medium conductance 90 pS and low conductance (as i n FR cells). The large conductance K+ channel was not observed. The CT chan nel was more active in SR cells. The cells were coated with extracellular m ucilage more than 10 mu m thick. In a similar manner to freshwater Chara, t hese cells compartment 6 carboxyfluorescein in a large central vacuole. In the older cells, making up the bulk of any given plant, the simultaneous de velopment of extracellular mucilage and a large central vacuole which compa rtments 6 carboxyfluorescein is associated with a minimal electrophysiologi cal response to hypotonic challenge. The significance of these findings for salt-tolerance is discussed.