Ecophysiological Responses Of Populus Yunnanensis To Supplemental Ultraviolet-B, Drought And Exogenous Abscisic Acid

Currently, drought is one of the most serious environmental stresses. In arid and semi-arid regions, drought is a major constraint imposed on tree survival and growth. The decrease of ozone layer leads to a significant increase in ultraviolet-B (UV-B, 280-320 nm) radiation reaching the earth surface. In some places, plants suffer both UV-B and water stress simultaneously. Their combination will increase or decrease the sensitivity of plants to UV-B stress which lies on the species. On the other hand, abscisic acid (ABA), as a plant homone and growth regulator, is better for plants resistant to drought stress, but it is uncleared about the relationship between exogenous ABA and supplemental UV-B. In the present study, we employed Populus yunnanensis Dode as a model species to characterize the growth and ecophysiological responses of woody plants to supplemental UV-B, drought and exogenous ABA. The results are as follows:1. Both supplemental UV-B and drought affected the morphological, physiological and biochemical responses of P. yunnanensis. They decreased the plant height, basal diameter, total leaf area, average leaf area, biomass and photosynthesis, and increased specific leaf mass, the activity of guaiacol peroxidase (GPX), the content of proline, anthocyanins and malondialdehyde (MDA). However, drought decreased the leaf number and increased root/shoot ratio, fine root/total ratio, the activity of superoxide dimutase (SOD) and the contents of ABA, carbon isotope composition (δ¹³C), UV-absorbing compounds. While supplemental UV-B had no effects on them. The combination of drought and UV-B augmented the growth inhibtion acting as further lower plant height and smaller basal diameter, leaf area, biomass and higher MDA content. And compared with drought stress, root/shoot ratio and fine root/total root ratio decreased under the combination stresses. The photosynthesis, proline content and Catalase (CAT) activity became lower under combination stresses than that of either stress lonely. According to these results, we suggested that, compared with the effect of stress lonely, the combination of supplemental UV-B and drought did not mitigate the harmful effect, but augmented it.2. Under drought conditions, exogenous ABA increased root/shoot ratio, fine root/total root ratio and the specific leaf mass. That was to say exogenous ABA increased plant plasticity under drought conditions. Also ABA content, proline content, activity of GPX andδ¹³C were enhanced further. In addition the enhancement of MDA was restrained. So the results suggested that exogenous ABA increased the seedling capacity of resistance to drought.3. Under supplemental UV-B conditions, exogenous ABA augmented the growth restrain of UV-B to seedlings, which acted as further decreased leaf area, specific leaf mass and biomass. Compared with UV-B stress alone, proline content and photosynthesis were decreased and MDA content was increased under the combination of UV-B and ABA. These results suggested that although exogenous ABA increased the seedling capacity of resistance to drought, it augmented the growth restrain of supplemental UV-B to P. yunnanensis.