Short-term impacts of enhanced UV-B radiation on photo-assimilate allocation and metabolism: a possible interpretation for time-dependent inhibition of growth

To test the hypothesis that plant source-sink relations are important in de termining response to UV-B radiation, a short-term (45 d) field experiment was conducted at Abisko Scientific Research Station, Abisko, Sweden (68 deg rees N). Tillers of the grass Calamagrostis purpurea were grown outdoors at levels of UV-B radiation representing 25% ozone depletion. Growth, respira tion, photo-assimilate allocation and UV-B protective compounds were subseq uently measured. There were no significant effects of enhanced UV-B on total plant dry weigh t, leaf area, Shoot: Root ratio, leaf weight ratio, leaf area ratio, specif ic leaf area, tiller number per plant or blade thickness of this species. H owever, the amount of UV-B absorbing compounds and respiration rates were s ignificantly increased in young and mature leaves. Increases in leaf respir ation were accompanied by alterations in plant carbohydrate allocation at e nhanced UV-B. The amount of soluble root carbohydrates was reduced followin g UV-B exposure. Enhanced UV-B also caused increases in the soluble sugar: starch ratio of young leaves, the stem and total aboveground biomass. The i mportance of source-sink relations and constitutive versus induced defense are discussed in relation to UV-B response.