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Carry-over of enhanced ultraviolet-B exposure effects to successive generations of a desert annual: interaction with atmospheric CO2 and nutrient supply


C.F. Musil, fax + 21/ 7976903, e-mail


The performance of fifth generation offspring of a desert annual (Dimorphotheca sinuata DC.) were compared in the absence of UV-B, under variable atmospheric CO2 and nutrient supply, after four consecutive generations of concurrent exposure of their progenitors to UV-B at ambient (seasonal range: 2.55–8.85 kJ m–2 d–1) and enhanced (seasonal range: 4.70–11.41 kJ m–2 d–1) levels. Offspring of progenitors grown under elevated UV-B exhibited a diminished photosynthetic rate, a consequence of a reduced leaf density, and diminished foliar levels of carotenoids, polyphenolics and anthocyanins. Conversely, nonstructural carbohydrate and chlorophyll b levels were increased. Altered physiology was accompanied by reduced apical dominance and earlier flowering, features generally considered under photomorphogenic control, increased branching and inflorescence production and greater partitioning of biomass to reproductive structures, but diminished seed production. Many of these changes were magnified under nutrient limitation and intensified under atmospheric CO2 enriched conditions. The latter disagrees with current opinion that elevated CO2 may reduce detrimental UV-B effects, at least over the long-term. Observed correlations between seed production and polyphenolic, especially anthocyanin, levels in offspring, and indications of diminished lignification (thinner leaves, less robust stems and fewer lignified seeds set) all pointed to the involvement of the phenylpropanoid pathway in seed formation and plant structural development and its disruption during long-term UV-B exposure. Comparisons with earlier generations revealed trends with cumulative generations of enhanced UV-B exposure of increasing chlorophyll b and nonstructural carbohydrates, decreasing polyphenolics and biomass allocation to vegetative structures, and diminishing seed production despite increasing biomass allocation to reproductive structures. Notwithstanding some physiological compensation (increased chlorophyll b), the accumulation and persistence of these ostensibly inherited changes in physiological and reproductive performance suggest a greater impact of elevated UV-B on vegetation, primary production and regeneration over the long-term than presently envisaged.