Ecophysiology of the hypotonic response in the salt-tolerant charophyte alga Lamprothamnium papulosum

The ecophysiology of the hypotonic response was studied in the charophyte a lga, Lamprothamnium papulosum, which was grown in a marine (SW; 1072 mosmol kg(-1)) and a brackish (1/2 SW; 536 mosmol kg(-1)) environment. The cells produced an extracellular mucilage identified by histochemical staining as a mixture of sulphated and carboxylated polysaccharides. The thickness and chemical composition of the mucilage layer was a function of environmental salinity and cell age. Mucilage progressively increased in thickness from t he apex (9 SW cells: 12.6 +/- 1.8 mu m; 15 1/2 SW cells: 4.8 +/- 0.7 mu m) to the base of the plants (15 SW cells: 44.8 +/- 3.3 mu m; nine 1/2 SW cell s: 23.8 +/- 2.5 mu m); with a corresponding increase in the sulphated propo rtion. The mucilage was significantly thicker in SW plants. Hydraulic condu ctivity (Lp) at the apex of SW plants, measured by transcellular osmosis, w as 8.3 x 10(-13) m s(-1) Pa-1. This was close to Lp of freshwater Chara (8. 5 x 10(-13) m s(-1) Pa-1) which lacked mucilage. Basal SW cells with thicke r mucilage had a smaller apparent Lp of 3.5 x 10(-13) m s(-1) Pa-1. The ele ctrophysiology of the resting state and hypotonic response was compared in cells from the two environments based on current/voltage (I/V) analysis. Th e resting potential difference (PD) and conductance differed (11 SW cells: - 102.4 +/- 10.1 mV, eight SW cells: 18.6 +/- 2.4 S m(-2); 19 1/2 SW cells: -125.7 +/- 5.9 mV, 8.3 +/- 0.8 S m(-2)). The type of cellular response to a hypotonic shock (decrease of 268 mosmol kg(-1)) also differed. In 1/2 SW plants, only the apical cells with thin mucilage responded classically with depolarization, conductance increase, Ca2+ influx, cessation of cytoplasmi c streaming, and K+ and Cl- effluxes. Older cells making up the bulk of the plants responded with depolarization, but continued cytoplasmic streaming, and had only a small increase in conductance; or depolarized transiently w ithout altering the I/V profile, conductance or streaming speed. Most cells remained depolarized and in the K+ state 1 h post-shock, Cells treated wit h the K+ channel blocker tetraethylammonium chloride also depolarized and r emained depolarized. The SW cells depolarized but otherwise responded minim ally to a 268 mosmol kg(-1) drop in osmolarity and required a further 268 m osmol kg(-1) down-step to elicit a change in the conductance. A spectrum of responses was measured in successively older and more mucilaginous cells f rom the same marine plant. We discuss the ecophysiological significance of the mucilage layer which modulates the cellular response to osmotic shock a nd which can be secreted to different degrees by plants inhabiting environm ents of different salinity.