DIFFERENTIAL STIMULATION OF AN ARID-ENVIRONMENT WINTER EPHEMERAL DIMORPHOTHECA-PLUVIALIS (L) MOENCH BY ULTRAVIOLET-B RADIATION UNDER NUTRIENT LIMITATION

A South African winter ephemeral D. pluvialis was exposed, under low and high nutrient conditions, to four different daily doses of biologically effective UV-B radiation. These simulated different depletions (range 0-30%) in the ozone layer at the southerly distribution limit (33 degrees 56'S) of this species, and included daily UV-B doses received at the northerly distribution limit (26 degrees 38'S) without ozone depletion. Growth inhibition by increased UV-B radiation was observed during early vegetative stages, but only under low nutrient conditions. Thereafter, net CO, assimilation rate, growth and reproduction were stimulated by an increase in UV-B radiation, though doses above those approximating a 20% ozone depletion appeared to be inhibitory. Differential stimulation occurred in the two nutrient treatments. Under high nutrient conditions, photosynthesis (specifically carboxylation efficiency), and numbers of leaves, inflorescences and diaspores per plant, and leaf areas increased, but leaf thickness decreased with increased UV-B radiation. Under low nutrient conditions, dry masses of leaves, stems, inflorescences and diaspores, and total aboveground dry masses increased with increased UV-B radiation. Foliar organic carbon and nitrogen concentrations and foliar concentrations of UV-B absorbing compounds were unaffected by increased UV-B radiation, but foliar P concentrations declined. Diaspore viability declined with increased UV-B radiation. The net effect was a 35 to 43% reduction in viable diaspore production under high nutrient conditions at UV-B doses equivalent to those currently received at the northerly distribution limit during the reproductive phase. It is concluded that anticipated increases in W-B radiation could reduce regeneration success, and seedling survival in areas of low soil fertility, particularly at lower latitudes, and consequently increase the risk of localized population extinctions from stochastic causes.