BIOLOGIA PLANTARUM 53 (3): 588-592, 2009 588 BRIEF COMMUNICATION Leaf morphological and physiological responses to drought and shade in two Populus cathayana populations X. HUANG 1,2 , X. XIAO 1 , S. ZHANG 1 , H. KORPELAINEN 3 and C. LI 1 * Chengdu Institute of Biology, Chinese Academy of Sciences, P.O.Box 416, Chengdu 610041, P.R. China 1 Faculty of Landscape Architecture, P.O.Box 140, Southwest Forestry College, Kunming 650224, P.R. China 2 Department of Applied Biology, P.O. Box 27, FI-00014 University of Helsinki, Finland 3 Abstract 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 drought- tolerant 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, 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 whole- plant 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 September 2008, accepted 9 March 2009. Abbreviations: APX - ascorbate peroxidase; CAT - catalase; C mass - leaf carbon content per mass; GPX - guaiacol peroxidase; LCP - light compensation point; LS - leaf size; N mass - leaf nitrogen content per mass; P N - net photosynthetic rate; P Nmax - maximum photosynthetic rate; PNUE - photosynthetic nitrogen-use efficiency; R d - dark respiration rate; RWC - relative water content; SLA - specific leaf area (leaf mass/area ratio); SOD - superoxide dismutase; TLA - total leaf area. Acknowledgements: The research was supported by the Outstanding Young Scientist Program of the National Science Foundation of China (no. 30525036) and the Program of “Knowledge Innovation Engineering” of the Chinese Academy of Sciences (No. KSCX2-YW-N-064). * Corresponding author; fax: (+86) 28 85222753, e-mail: licy@cib.ac.cn