Ecophysiological Responses Of The Hippophae Plants To Enhanced UV-B Radiation At Both Population And Species Level

The continuous increase of greenhouse gases over the world has reducing the ozone layer, which further have intensified ultraviolet-B (UV-B) radiation (280-320nm) and will lead to a series of ecological effect on plants. Under such a condition, the plants occurring the arid and semi-arid regions of the northwest China may suffer more stress. It remains unknown how the different populations of the same species or different species with close relationship within the same genus respond to the enhanced UV-B radiation. In this thesis, the representative plants of sea buckthorn, Hippophae rhamnoides (four different populations from the same subspecies subsp. sinensis and one population from another subspecies subsp. yunnanensis) were selected to study intrspecific responses at the population level while three species, i.e. H. rhamnoides, H. goniocarp and H. neurocarpa, were used to investigate interspecific responses to the enhanced UV-B radiation. All species of Hippophae have been widely used as the pioneer species in ecological restoration. The experimental results will be important for further selection of seed resources. Photosynthetic parameters, the content of pigment, UV-absorbing compounds and antioxidant enzyme activity that characterize responsive processes of the photosynthetic ecophysiology were measured. The main results are as follows.(1) the enhanced UV-B radiation decreased the photosynthetic rates in all experimented populations and species;(2) The population from the arid habitat (Buerjin) exhibited higher tolerance to enhanced UV-B radiation than which from the semi-arid habitat (Baotou) although these populations from the similar altitude. Although the pigment contents of all populations had no obvious decrease under the enhanced UV-B radiation, populations from the high altitude (H. rhamnoides subsp. Yunnanensis, H. rhamnoides Pingliang and Qilian) enhanced the UV-absorbing compounds and antioxidant enzyme activity (such as SOD, APX and CAT) to respond to the UV-B radiation more obviously than those from the low altitude populations.(3) Both photosynthetic rate and pigment content of H. goniocarpa decreased faster that those of the H. rhamnoides and H.neurocarpa while the total amount of the UV-absorbing compounds and antioxidant enzyme activity increased less in the first species. H. neurocarpa exhibited highest tolerance to enhanced UV-B radiation.Based on these results, three main conclusions are drawn. First, Hippophae showed interspecific and inter-populational differences in ecophysiological responses to the enhanced UV-B radiation. The population or species from the high altitude exhibited higher tolerance to enhanced UV-B radiation. However, H. goniocarpa originated from hybrid between H. rhamnoides and H. neurocarpa showed no significant heterosis. Second, all populations and species adopted a similar strategy to respond to the enhanced UV-B radiations. The decrease of photosynthetic rate was mainly caused by the stomatal limitation. Because of the no obvious changes in the pigment contents, the species or populations from the high altitude showed high toleration of UV-B radiation mainly through enhancing the UV-absorbing compounds and intensifying antioxidant enzyme activity. Finally, the populations from the arid or low temperature habitat exhibited higher tolerance to the enhanced UV-B radiation when compared with those from the similar altitude.