| Drought and salinity are not only global problems, but also the major environmental factors limiting the growth of biological desert crust algae. With the growing shortage of fresh water resources, drought and soil salinity have been threatening the development of crust succession and the environment in the arid and semi-arid areas of northwest China particularly.In this study, drought resistance of desert algal crusts in the eastern margin desert region of Tibetan Plateau was researched by field studies, sampling and laboratory cultivation, so as to filter out the algae species which have strong drought resistance. The main results were showed as follows:1. The changes of osmotic adjustment, proline and soluble protein content. With the increased intensity of PEG osmotic stress,free proline content and soluble protein content of the four algae species increased and then emerged in a downward trend.Free proline content and soluble protein content of the first and the second algal species is the highest in 20% PEG and 30% PEG osmotic stress respective, the third algal species has the highest content in30% PEG osmotic stress and the forth algal species in 10% PEG osmotic stress. The rate of increase is greater, the osmotic adjustment is stronger, so the drought resistance is stronger also. By comparing the four algal species, it found that the increase of free proline content and soluble protein content of the third algae species is the highest, the osmotic adjustment is the strongest, so the drought resistance is also the strongest, osmotic adjustment ability of the forth algae is the weakest, so its drought resistance is the weakest.2. The change of membrane permeability and MDA content. the MDA content of the first algae species is the higest in 10% PEG osmotic stress,while the second, third and forth species have the highest MDA content in 40% PEG osmotic stress, the membrane permeability and the MDA molar mass concentration increased obviously during the treatment of water stress,indicating that the membrane systems of the first algal species have maximal damage in 10% PEG, while the second, the third and the forth algal species have maximal damage in 30% PEG osmotic conditions, the lipid peroxidation is the highest respectively, so the drought resistance is the weakest.In 20% PEG and 30% PEG osmotic stress conditions, the second, third and forth algal species have lower MDA molar mass concentration and membrane permeability, so they have higher drought resistance. With the increasing intensity of PEG osmotic stress,theMDA content of the first algae species showed a downward trend. By comparing the four algal species, it found that the membrane permeability and the lipid peroxidation of the first algal species is the weakest, so it has the highest drought resistance.3. The change of protective enzyme activities .The first algae species has the highest CAT and SOD activities in 30% PEG and 20% PEGrespectively, the second and the third algae species have the highest SOD and CAT activities in 20% PEG,while the forth algae species in 10% PEG, indicating that the first, the second and the third algae species have higher ability to eliminate oxygen free radicals than the forth algae species, it can reduce the damage to the membrane system of algae species,so that the three algae species have stronger drought resistance, the forth algae species has lower drought resistance. With the increase of PEG stress intensity,SOD and CAT enzyme activities of the four algae species aredecreased, lipid peroxidation is further increased,resulting in O2-, H2O2 excessive accumulation, and the protective enzyme systems of the four algae species are not able to fully play their role inmaintaining membrane stability and removing oxygen free radicals actively,which result in weaker drought resistance of the four algae species. By comparing SOD and CAT enzyme activities of the four algal species, it found the first algae species has the highest drought resistance.
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