Physiological indexes of six plant species from the tibetan plateau under drought stress

During adaptation to the environment,plants can gradually develop drought resistance via a process of natural selection. Plants are often under water stress in the cold and dry environmental conditions on the Tibetan Plateau,and these conditions can directly affect various physiological activities. The aim of this study was to analyze and evaluate the drought resistance of selected plants from the Tibetan Plateau,and to provide a reference for selecting drought-tolerant plants. We collected six species of plants from the Tibetan Plateau; Hedysarum multijugum Maxim,Comarum salesovianum( Steph.)Asch. et Graebn. ,Berberis diaphana Maxim. ,Sibiraea laevigata( Linn.) Maxim. ,Caragana arborescens( Amm.) Lam.and Sophora moorcroftiana Benth Baker. To protect themselves against drought and low-temperature stress,plants have evolved efficient antioxidant systems to scavenge reactive oxygen species. We evaluated changes in the amounts of malondialdehyde( MDA),proline,soluble protein,chlorophyll,soluble sugars,and starch in leaves of these species subjected to increasingly severe drought conditions in a pot experiment. The soil moisture was greater than 80% of field capacity for the normal level,50% —70% for mild drought,30% —50% for moderate drought,and less than 30% for severe drought. In all six plant species,the MDA content increased slowly from day 0 to day 12 of the drought treatment,and then increased significantly after day 16 to reach a peak when the soil water content was approximately 12%. Caragana arborescens showed the smallest increase in MDA content under drought,but had the highest MDA content( 4. 72 mmol /g)among the six plant species. The proline content increased in all six species under drought,and there were significant differences among species in the amount of proline accumulated. The largest increase in proline content was in S.moorcroftiana( peak value,9981. 67 mg /g; 37. 43 times that in CK),and the smallest was in S. laevigata( peak value was 0. 49 times that in CK). The leaf soluble protein content increased under drought conditions in all six species,but began to decrease when the drought conditions became more severe. The largest decrease in protein content under severe drought conditions was in S. moorcroftiana( a decrease of 39. 22%,compared with the peak protein concentration). The chlorophyll concentrations in the leaves of the six species first increased and then decreased as the drought conditions became more severe. In the moderate drought period,the chlorophyll content of seedling leaves began to recover,probably because of a decrease in leaf water content. In all six species,the chlorophyll content on day 24 of the drought was lower than that on day 0. The largest decrease in chlorophyll concentration was in C. salesovianum( 15. 43 mg /g). The soluble starch contents in all six species decreased under drought because of a reduction in photosynthesis; however,all species showed an increase in soluble sugars content,which improved the osmotic adjustment capability of cells. The largest increases in soluble sugars contents and the largest reductions in starch contents were in S. moorcroftiana and C.arborescens. These species were able to retain higher cell turgor pressure at lower water potential,compared with the other species. After day 16 of the drought treatment,the decreases in starch and soluble sugar contents in the leaves indicated that prolonged water stress caused decomposition of biomass,reduced biosynthesis,and a weaker ability for osmotic adjustment. The drought resistance of plants is the sum of many physiological factors,and represents complex interactions among these factors. Thus,it is a complicated physiological adjustment mechanism. The six species were ranked differently in terms of each of the physiological indexes measured. Therefore,we used principal component analysis to select the most important physiological indexes of drought resistance: chlorophyll,MDA,and starch contents. The results of the principal component analysis indicated that species could be ranked,in terms of most to least drought resistant,as follows: C.arborescens H. multijugum S. moorcroftiana B. diaphana S. laevigata C. salesovianum.