Hormonal Regulation of Growth form in the Arctic-Alpine Cushion Plant, Silene acaulis

Whether Silene acaulis L. grows as a cushion or a loose mat of vegetation appears to be primarily regulated by soil temperature. Under growth chamber conditions, in soils at 13?C or less in the root zone, this species assumes a cushion form due to almost total suppression of internodal elongation, while in warmer soils substantial internodal growth leads to a mat form. Roots of this species exhibit relatively high respiration rates and low activation energies similar to those reported for cold-adapted tissues. This and other observations indicate that low soil-temperature stunting is probably not entirely the result of reduced root activity. Abscisic acid applied to the shoots of plants in warmer soils suppressed growth, and it is therefore likely that stunting in this and possibly other species is due to increased ABA levels produced in cold-stressed roots. These results are compatible with the proposition that progressive stunting at higher elevations and perhaps timberline formation result in large part from reduced average rhizosphere temperatures.