T. Nakamura et al., DIFFERENCES IN MECHANISMS OF SALT TOLERANCE BETWEEN RICE AND BARLEY PLANTS, Soil Science and Plant Nutrition, 42(2), 1996, pp. 303-314
Mechanisms of salt tolerance were estimated in terms of the ATPase act
ivity in roots, the Na excluding ability of roots, and the regulation
of Na translocation from roots to shoot by comparing Na tolerant plant
s (barley) and Na sensitive plants (rice) grown at high NaCl concentra
tions. The results obtained were as follows. 1) As the relative dry we
ight (dry weight at high NaCl concentration in the medium to the dry w
eight at 2 mM NaCl concentration in medium) of rice decreased remarkab
ly with the increase of NaCl concentration in the medium compared to t
hat of barley, salt tolerance of barley was higher than that of rice.
High salt tolerance of barley was ascribed to the fact that Na translo
cation from roots to shoot was prevented at high NaCl concentration in
the growth medium. 2) ATPase activity in roots of rice decreased rema
rkably with the increase of NaCl concentration (50 to 200 mM NaCl), wh
ile that of barley grown at a high NaCl concentration remained high. S
ince the effect of the NaCl concentration in the reaction solution on
the ATPase activity in vitro was negligible, high NaCl concentration i
n medium may not inhibit directly ATPase but may inhibit indirectly AT
Pase synthesis or denature membranes, etc. 3) ATPase activities in the
plasma membrane and tonoplast of barley roots were higher than those
of rice when roots were exposed to NaCl concentrations in the range fr
om 0 to 25 mM in the medium. 4) When Na-22 tracer experiment was condu
cted over a short period of time, both Na exclusion from roots and Na
retranslocation from shoot to roots were higher in rice than in barley
, regardless of Na concentration (2 to 150 mM Na) in medium because AT
Pase activity of the plasma membrane and tonoplast in rice roots remai
ned high under short Na treatment. In rice other factors were involved
, such as Na sensitivity of leaf tissues after prolonged exposure. Thu
s, immediately after the exposure of roots to NaCl, roots of rice plan
ts actively excluded Na from roots to medium, and a large amount of Na
was retranslocated from shoots to roots in rice plant compared to bar
ley. However when rice plants were exposed to high NaCl concentrations
for a long period of time, the functions of Na exclusion and Na retra
nslocation from shoots to roots could not be maintained because the AT
Pase activity of roots of rice plants decreased due to the high Na con
centration in the medium, and the metabolism of leaf tissues was more
sensitive to Na in rice than in barley.