Cuttings from two contrasting Populus cathayana Rehder populations originating from Hanyuan (wet climate) and Ledu (dry climate) in western China were grown in a greenhouse to determine the effects of drought, shade and their interaction on the morphological and physiological traits of leaves. The dry climate population was more droughttolerant than the wet climate population, as indicated by smaller decreases in the leaf relative water content (RWC) and net photosynthetic rate (P N ), as well as by greater increases in antioxidative enzyme activities and free proline content under drought. On the other hand, the negative effects of shade on leaf traits were more pronounced in the dry climate population, which suggested that the dry climate population was more light-demanding. In addition, moderate shade alleviated the drought stress of P. cathayana not only by improving the leaf RWC but also by maintaining the positive carbon balance. In contrast, severe shade aggravated drought stress as indicated by a pronounced decrease in leaf size, carbon and nitrogen contents, maximum P N , free proline content and antioxidative enzyme activities.Additional key words: antioxidative enzymes, leaf area and thickness, net photosynthetic rate, nitrogen-use efficiency, proline. ⎯⎯⎯⎯In a natural environment, the availability of water and light are the two most important resources for plant survival and growth. The plasticity of seedlings to adjust leaf morphological and physiological performance in response to water deficit and light limitation plays an increasingly important role in the forest ecosystem (Aranda et al. 2005(Aranda et al. , Čaňová et al. 2008, as drought is becoming more severe and common worldwide. According to the trade-off mechanism (Smith and Huston 1989), a shaded plant has a higher leaf area than sun plants and invests more biomass in leaves but less in roots for efficient light capture (Niinemets and Valladares 2006), which may decrease its drought tolerance. On the other hand, there is increasing evidence that shade lessens the impact of drought by indirect effects, such as reduced leaf and air temperature, vapour pressure deficit and oxidative stress (Holmgren 2000). However, Sack and Grubb (2002) have proposed that the impacts of shade and drought on seedlings are independent. At the wholeplant level, drought stress usually leads to a decrease in photosynthesis and growth, which is associated with alterations in carbon and nitrogen metabolism (Raven et al. 2004). Moreover, to prevent the accumulation of reactive oxygen species under environmental stress, antioxidative enzymes, such as SOD, CAT, APX and GPX, are activated (Sivritepe et al. 2008). However, plants possess lower amounts of antioxidants due to a lower photosynthetic capacity under low irradiance as compared with high irradiance (Lermontova and Grimm 2006). Thus the antioxidant enzymes activities are important indexes for drought or shade tolerance of a plant. Additionally, free proline accumulates under stress, ⎯⎯⎯⎯Received 20 Sept...

We compared two populations of Populus cathayana Rehder, originating from altitudes 2 840 m and 1 450 m, to determine whether trees from different altitudes exhibit different tolerance to alkalinity. The tree cuttings were exposed to nutrient solutions with pH 7.9, 8.8, 9.8 and 10.4 and the salt concentration 200 mM. Na + and K + contents, and Na + /K + ratios in leaves and roots were greatly affected by pH values. At pH 10.4, the Na + /K + ratios in both leaves and roots sharply dropped in the higher altitude population but were maintained at higher levels in the lower altitude population. The patterns of pH-induced changes in contents of malondialdehyde and free proline, and antioxidative enzyme activities indicated that the higher altitude population exhibits greater tolerance to alkalinity stress than does the lower altitude population.Additional key words: free proline, malondialdehyde, poplar, salt-alkali mixed stress.