The cuttings of Populus przewalskii Maximowicz were exposed to three different watering regimes (100, 50, and 25% of the field capacity) in a greenhouse to characterize the morphological, physiological, and biochemical basis of drought tolerance in woody plants. Two contrasting populations of P. przewalskii were used in our study, which were from the wet and dry climate regions in western China, respectively. The results showed that there were significant differences in responses to three different watering regimes in both populations tested; drought not only significantly affected dry mass accumulation and partitioning but also significantly decreased chlorophyll pigment contents and accumulated free proline and total amino acids. On the other hand, drought also significantly increased the levels of abscisic acid, hydrogen peroxide, and superoxide radical as secondary messengers to induce the entire set of antioxidative systems including the increase of reduced ascorbic acid (ASA) content and the activities of superoxide dismutase, guaiacol peroxidase, ascorbate peroxidase, and glutathione reductase (GR). Moreover, there were different responses to drought stress between the two contrasting populations of P. przewalskii. Compared to the wet climate population, the dry climate population showed lower dry matter accumulation and partitioned more biomass to root systems, and accumulated more free proline and total amino acids for osmotic adjustment. The dry climate population also showed more efficient antioxidant systems with higher content of ASA and higher activities of ascorbate peroxidase and GR than the wet climate population. IntroductionPlants can avoid drought stress by maximizing water uptake (e.g. tapping ground water by deep roots) or minimizing water loss (e.g. stomatal closure, small leaves, etc.) (Kozlowski and Pallardy 2002). Apart from these morphological changes, plants have evolved to employ a variety of physiological and biochemical processes ranging from photosynthesis to antioxidant defenses and solute accumulation as components of drought tolerance . One of the early responses of plants to drought stress may be the increase in abscisic acid (ABA) content, which may induce stomata closure to reduce leaf transpiration and prevent the development of excessive water deficit in their (Asada 1999). There are many cases that plants growing in hostile environments exhibit increased oxy-stress enzyme activities to combat the deleterious effect of AOS (Duan et al. 2005, Jebara et al. 2005. On the other hand, chlorophyll contents change has been used to account for the stress effects on plants, and earlier study proved that chlorophyll contents usually decreased under drought stress due to their slow synthesis or fast breakdown (Ashraf 2003). Moreover, the other potentially important mechanism of drought tolerance is osmotic adjustment which can be achieved from the accumulation of compatible solutes in protoplasm (Misra and Gupta 2005). This osmotic adjustment allows cell enlargement and plant growth...

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We measured dry matter accumulation and allocation, photosynthesis, lipid peroxidation, osmotic adjustment, antioxidative defences and ABA content of Populus kangdingensis C. Wang et Tung under three different watering regimes (100%, 50% and 25% of the field capacity) to characterize the morphological, physiological and biochemical basis of drought resistance in woody plants. The results showed that drought stress caused pronounced inhibition of the growth and photosynthesis rate, and that the stomatal limitation to photosynthesis was dominant. The decrease in stomatal conductance effectively controlled water loss and increased water use efficiency. Drought also affected many physiological and biochemical processes, including increases in free proline, malondialdehyde and ABA content, and superoxide dismutase activity. On the other hand, the ABA content of leaves was significantly higher than that of stem and roots under all watering regimes; the high level of ABA in the leaf may result from the large import of ABA to leaves from other organs. These results demonstrate that there are a large set of parallel changes in the morphological, physiological and biochemical responses when plants are exposed to drought stress; these changes may enhance the capability of plants to survive and grow during drought periods.

The responses of photosynthetic gas exchange, chlorophyll fluorescence, activities of antioxidant enzymes and lipid membrane peroxidation of two contrasting Picea asperata Mast. populations to 30% of full sunlight (shade) and full sunlight (sun) were investigated under well-watered and drought conditions. Two contrasting populations were from the wet and dry climate regions in China, respectively. For both populations tested, drought resulted in lower needle relative water content (RWC), CO 2 assimilation rate (A), stomatal conductance (gs) and effective PSII quantum yield (Y), and higher non-photochemical quenching (qN), superoxide dismutase (SOD), ascorbate peroxidase (APX) activities as well as malondialdehyde (MDA) levels and electrolyte leakage in sun plants, whereas these changes were not significant in shade plants. For the wet climate population, shade plants showed higher chlorophyll contents (Chla, Chlb and Chla þ b) than sun plants under both well-watered and drought conditions. Our study results implied that shade, applied together with drought, ameliorated the detrimental effects of drought. On the other hand, compared with the wet climate population, the dry climate population was more tolerant to drought in the sun treatment, as indicated by less decreases in A and mass-based leaf nitrogen content (N mass ), more responsive stomata, greater capacity for non-radiative dissipation of excitation energy as heat (analysed by qN), and higher level of antioxidant enzyme activities as well as lower MDA content and electrolyte leakage. These results demonstrated that the different physiological strategies were employed by the P. asperata populations from contrasting climate regions when the plants were exposed to drought and shade.

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Aim This study aimed at predicting how sub-alpine coniferous ecosystems respond to global changes in the Eastern Tibetan Plateau by understanding soil microbial communities and activities, as well as variation in the quality and quantity of soil organic matter. Methods An experiment was conducted to examine soil microbial communities and their related soil processes in rhizospheric soil of two coniferous species that were exposed to two levels of temperature (unwarmed and infrared heater warming) and two levels of nitrogen (unfertilized and 25 g N m −2 a −1

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