Acclimation And Adaptation Of Three Leguminous Shrub Species To Drought Stress

Drought is often a key factor limiting plant establishment, growth and distribution in many regions of the world. The harsh environmental conditions and long-term anthropogenic disturbance had resulted in habitat destruction in the dry valley of Minjiang river, southwest China. Recently, it tended to be more severe on the vegetation degradation, soil erosion and water loss, natural disaster, as well as desertification, which impact on regional booming economy and harmonious development, and would be very dangerous to the environmental security in the middle and lower reaches of Yangzi River. Therefore, ecological restoration in the dry valley is one of the vital tasks in China. Water deficit is known to affect adversely vegetation restoration in this place. Moreover, in the context of climate change, an increased frequency of drought stress might occur at a regional scale in the dry valleys of Minjiang River. The selection of appropriate planting species for vegetation restoration in regard to regional conditions is an important issue at present and in further. The research on responses of indigenous species to drought stress could provide insights into the improvement of the vegetation restoration in the dry valleys of Minjiang River.In this paper, the responses of three indigenous leguminous shrubs, Sophora davidii, Bauhinia faberi var. microphylla and Campylotropics polyantha, to various soil water supplies were studied in order to assess drought tolerance of seedlings, and to compare interspecific differences in seedlings' responses to drought stress. The results were as follows: 1 Growth and reproduction of shrubs in response to drought stressSeedling growth reduced significantly while leaf senescence accelerated under drought stress, the cumulative responses to prolonged drought were found. The capacity for reproduction was limited by severe drought stress, and improved by moderate drought stress. Leaf responses were more sensitive than shoot to various water supplies.2 WUE, biomass production and its partitioning of shrubs in response to drought stressDrought stress reduced significantly the total dry mass and their components of seedlings, and altered more biomass allocation to root system, showing higher R/S ratio under drought. Water use (WU) and water-use efficiency (WUE) of both S. davidii and B. faberi var. microphylla declined strongly with drought stress. The WU C. polyantha of also declined with drought stress, but WUE improved under moderate drought stress.3 Anatomical characteristics and ultrastructures of leaves in response to drought stressThere were xeromorphic for S. davidii leaves and mesomorphic for B. faberi var. microphylla and C. polyantha at the all water supplies. The foundational changes in leaf structures were not found with drought stress. However, mesophyll thickness, the areas of stomatal and epidermis reduced slightly while the densities of stomatal and epidermis increased under severe drought stress. Variations in these parameters could mainly be duo to cell size. Other structures did not displayed significant changes with drought stress.4 Physiological responses of shrubs to drought stressThe gas exchange parameters and leaf relative water content (RWC) were affected by moderate stress, while chlorophyll fluorescence and chlorophyll content were only affected by severe stress. Drought stress decreased net photosynthesis rate (Pn), stomatal conductance, light-use efficiency and RWC, and increased leaf temperature. The respiration rates (Tr) were kept within a narrower range than Pn, resulting in a progressively increased instantaneous water use effiecency (WUEi) under drought stress. Moreover, drought stress also affected the response curve of Pn to RAR, there was a depression light saturation point (Lsat) and maximum Pn (Pnmax) for moderate and severe stressed seedling. However, diurnal changes of gas exchange parameters did not differ among water supplies although maximum daily Pn declined under severe stress. Severe stress reduced Fv/Fm, Yield and qP while increased NPQ and chlorophyll content. Photosynthetic activity decreased during drought stress period due to stomatal and non-stomatal limitations. The relative contribution of these limitations was associated with the severity of stress. The limitation to Pn was caused mainly by stomatal limitation under moderate drought stress, and by the predominance of non-stomatal limitation under severe stress. In this case, 40% FC water supply may be a non-stomatal limitation threshold.5 Interspecific differences in drought tolerance of shrubsThree shrubs exhibited good performance throughout the experiment process, even if at 20% FC treatment there were no any seedlings died, 80% FC water supply was suitable for their establishment and growth. S. davidii minimized their water loss by reducing total leaf area and growth rate, as well as maintained higher RWC and Pn compared to the other two species under drought stress, thus they might be more tolerant to the drought stress than the other two species. On the contrary, it was found that C. polyantha and B. faberi var. microphylla had higher water loss because of their stomatal conductance and higher leaf area ratios. They reduced water loss with shedding their leaves and changing leaf orientation under drought stress. Based on their responses, the studied species could be categorized into two: (1) S. davidii with a tolerance mechanism in response to drought stress; (2) C. polyantha and B. faberi var. microphylla with drought avoidance mechanism. These results indicated that slow-growing shrub species are better adapted to drought stress than intermediate or fast-growing species in present or predicted drought conditions. Therefore, selecting rapid-growing species might leave these seedlings relatively at a risk of extreme drought.