VA Shepherd, MJ Beilby and D Heslop
Plant, Cell and Environment, 1999, 22: 333-346
Abstract
The ecophysiology of the hypotonic response was studied in the charophyte
alga Lamprothamnium papulosum, growing in a marine (SW;
1072 mOsmol kg-1 ) and a brackish (1/2 SW; 536 mOsmol
kg-1 ) environment. The cells produce an extracellular
mucilage identified by histochemical staining as a mixture of sulphated
and carboxylated polysaccharides. Mucilage layer
thickness and chemical composition was a function of environmental
salinity and cell age. Mucilage was significantly thicker in SW plants,
and in cells from both environments it progressively increased in thickness
from the apex (9 SW cells: 12.6 microns, SE (standard error): 1.8 microns;
15 1/2 SW cells: 4.8 microns, SE: 0.7 microns) to the the base of
the plants (15 SW cells:
44.8 microns, SE: 3.3 microns; 9 1/2 SW cells: 23.8 microns,
SE: 2.5 microns), with a corresponding increase in the sulphated proportion.
At the apex of SW plants, hydraulic conductivity (Lp), measured by transcellular
osmosis, was 1 x 10-12 m/s/Pa, approaching that of freshwater
Chara, which lacks mucilage. Basal cells with thicker mucilage had
a smaller Lp of 5 x 10-13 m/s/Pa. The electrophysiology of the
resting state and hypotonic response was compared in cells from the two
environments using current/voltage (I/V) analysis. The resting potential
difference (PD) and conductance differed (11 SW cells: -102.4 mV, SE: 10.1
mV, 8 SW cells: 18.6 Sm-2; SE: 2.4 Sm-2;
19 1/2 SW cells: -125.7 mV, SE: 5.9 mV;: 8.3 Sm-2,
SE: 0.8 Sm-2). The type of cellular response to a hypotonic
shock (decrease of 268 mOsmol kg-1) also duffered. In
1/2 SW plants only the apical cells with thin mucilage responded classically
with depolarisation, conductance increase, Ca2+ influx (as indicated
by cessation of cytoplasmic streaming) and large K+ and
Cl- effluxes (fast-regulating cells). Older cells, making
up the bulk of the plants, responded with depolarisation, but continued
cytoplasmic streaming, and had only a small increase in conductance associated
with a K+ state I/V profile. Some basal cells depolarised transiently
without significantly altering the I/V profile or streaming speed. This
latter behaviour was very similar to that exhibited by cells exposed to
the K+ channel blocker TEA. SW cells responded minimally to
a 268 mOsmal kg-1 drop in osmolarity and requiried a further
268 mOsmol kg-1 down-step to elicit a response.
The full spectrum of responses was
measured in successively older and more mucilagenous cells from the same
marine plant. We discuss the ecophysiological significance of the mucilage
layer which modulates the cellular response to osmotic shock and which
can be secreted to different degrees by plants inhabiting environments
of different salinity.
Keywords : Lamprothamnium, Characeae, hypotonic effect, marine algae, mucilage, sulphated polysaccharide, salinity tolerance, electrophysiology.
