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Summary1. Previous studies have demonstrated that poplar is sexually dimorphic in its responses to environmental stresses and provided evidence of sex-related differences in protein composition, gene transcription and morphological and physiological processes. However, sexual competition of poplar has not yet been studied. 2. We employed Populus cathayana males and females as a model to investigate intra-and intersexual competition under well-watered condition and drought stress. 3. Our results indicated that competition significantly affected sexual dimorphism of P. cathayana under different watering regimes. Sexual competition was environment-dependent and modified by drought. In females, drought intensified the negative effect of intrasexual competition. Similar resource utilization patterns of females may aggravate pressure for resources under drought stress. Under intersexual competition, females showed a greater competitive ability than males under well-watered condition, while males showed a slight competitive advantage under drought stress. Intersexual competition seems to alleviate the sexual dimorphism of P. cathayana in response to drought stress, as females increase fitness and males decrease fitness compared with intrasexual competition. 4. Sexual dimorphism in resource utilization patterns and niche segregation may contribute to sexual competition in a given environment. Sexual competition was found to affect competitive ability and fitness of both sexes, which may cause spatial segregation of sexes in P. cathyana populations.

Populus cathayana Rehd., a dioecious tree species, occupies a wide range of habitats in southwest China. Both males and females were grown under two regimes of photoperiod, from mid-length to short-day photoperiod (SD shift), or to long-day photoperiod (LD shift). SD shift triggered leaf senescence in both males and females by decreasing net photosynthesis rate (A), transpiration (E), and chlorophyll pigment (Chl), non-structural carbohydrate (NSC) and indoleacetic acid (IAA) contents, while increasing abscisic acid (ABA), malonaldehyde (MDA) and free proline (Pro) contents. The antioxidant enzyme (e.g. POD, CAT and SOD) activities and capability to maintain ultrastructural integrity also decreased under SD shift. Males exhibited faster leaf senescence than did females, as shown by greater decreases in A, E, Chl and IAA. However, males maintained a less senescent stage than did females, as indicated by higher values of A, Chl, NSC, IAA and antioxidant enzyme activities. Conversely, A, E, NSC and IAA contents and antioxidant enzyme activities were enhanced by lower O2 •-in females, whereas reduced by higher O2 •-in males under LD shift. Such sex-dependent responses of P. cathayana to photoperiod transitions showed that males and females possess different adaptabilities, which may relate to sex-specific leaf senescence speed under changing environments.

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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The effects of exogenous abscisic acid (ABA) on the acclimation of Picea asperata to water deficit were investigated in two populations originating from wet and dry climate regions of China. Exogenous ABA was sprayed onto the leaves, and changes in plant growth and structure, gas exchange, water use efficiency (WUE), endogenous ABA content, and antioxidant enzyme levels were monitored. The results demonstrated that ABA application affected the two P. asperata populations in different ways during the water deficit. ABA application resulted in significantly lower CO(2) assimilation rates (A) under water deficit in plants from the wet climate population, whereas there were no significant changes in this parameter in the dry climate population. On the other hand, ABA application significantly decreased the dry shoot biomass, stomatal conductance (g(s)), transpiration rate (E), and malondialdehyde (MDA) content, and it significantly increased the leaf mass per area (LMA), root/shoot ratio (Rs), fine root/total root ratio (Ft), WUE, ABA content, and the superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT) activities under water-deficit conditions in the dry climate population, whereas ABA application did not significantly affect these parameters in the wet climate population. The results clearly demonstrated that sensitivity to an exogenous ABA application is population-dependent in P. asperata. Direct evidence is presented that variation in physiological mechanisms rather than different rates of ABA absorption explain the population differentiation in the sensitivity to exogenous ABA, and that the physiological basis for the amplified response to water deficit caused by exogenous ABA, present mainly in the dry climate population, is related to internal ABA accumulation. These results provide evidence for adaptive differentiation between populations of P. asperata, and they support the expected relationship between environmental heterogeneity and the magnitude of plastic responses in plant populations.