To study the relationship between copper sensitivity and membrane
permeability in Arabidopsis, the seedling
growth rates (measured as root extension and f. wt increase) of 10
ecotypes with differing copper sensitivities were
compared with their patterns of potassium efflux and copper uptake.
At 36 h, inhibition of root extension in 40 μM
Cu correlated well with K+ efflux (r=0·96)
and a decrease in osmolality (r=0·96). By contrast, neither tissue
osmolality nor K+ efflux correlated significantly with growth
inhibition after 4 h Cu treatment. In detailed time-course studies of two
of the less sensitive ecotypes (Shadhara and Ws) and two of the more
sensitive ecotypes
(Berkeley and Limeport) in 20 μM Cu, growth, K+
retention and osmolality decreased rapidly during the first 4 h.
Only the two less sensitive ecotypes recovered during the following 32 h.
A similar pattern was observed with
40 μM Cu, except that growth inhibition was more marked.
Interestingly, after 4 h the less sensitive ecotypes
exhibited greater inhibition than the more sensitive. In time-course
measurements, the Cu contents of the four
ecotypes were not significantly different when normalised on a f. wt basis.
We conclude that variations in the metal
sensitivities of Arabidopsis ecotypes are not due to constitutive
differences in membrane permeability but, rather,
to differences in the ability to reverse K+
efflux. The possibility that short-term Cu-induced K+-leakage is
regulated by channels rather than lipid oxidation is discussed.