We investigated sex-related photosynthetic responses to drought in the dioecious species, Populus cathayana Rehd. Plants were subjected to two watering regimes (100% and 30% of field capacity) in a semi-controlled environment. Drought significantly decreased leaf area (LA), total number of leaves (TNL), specific leaf area (SLA), relative water content, net photosynthetic rate (P(n)), transpiration (E), stomatal conductance (g(s)), intercellular CO(2) concentration (C(i)), light saturation point (L(SP)), apparent quantum yield (Phi), carboxylation efficiency (CE), light-saturated photosynthetic rate (P(max)), maximum efficiency of PSII (F(v)/F(m)) and maximum effective quantum yield of PSII (Yield), and increased the total chlorophyll concentration (TC), CO(2) compensation point (Gamma), non-photochemical quenching coefficient, peroxidase (POD) activity and carbon isotope composition (delta(13)C). Moreover, differences between males and females were detected in many of these responses. In the drought treatment, males exhibited significantly higher LA, TNL, TC, concentration of carotenoids (Caro), P(n), E, g(s), C(i), L(SP), Phi, CE, P(max), F(v)/F(m), photochemical quenching coefficient, POD activity and delta(13)C, but a lower SLA, chlorophyll a/b ratio, carotenoids/total chlorophyll ratio and Gamma than females. However, Caro, L(SP), Gamma, Phi, CE and POD activity were apparently associated with sex-related resource demands, because significant differences in these traits were detected between the sexes under well-watered conditions. Our results indicate that drought stress limits photosynthetic capacity more in females than in males.

Quercus aquifolioides Rehder & E.H. Wilson, an evergreen alpine and subalpine species, occupies a wide range of habitats in the Wolong Nature Reserve, southwestern China. We measured age-related carbon (C) and nutrient (N, P, K, Mg and Ca) contents, C/N, carbon isotope composition (d 13 C) and specific leaf area (SLA) in the leaves and branches of Q. aquifolioides trees along an altitudinal gradient ranging from 2,000 to 3,600 m. The results showed that both age and altitude significantly affected the morphological and physiological properties of Q. aquifolioides. Young tissues possessed higher contents of N, P, K and Mg, lower Ca contents, both on a dry mass basis (subscript ''M'') and on a unit area basis (subscript ''A''), and lower C/N and d13 C values than did the old ones. The levels of N M and d 13 C increased with increasing altitude above 2,800 m, but decreased with increasing altitude below 2,800 m. In contrast, C/N and SLA showed opposite patterns, and other nutrient contents, including P M , K M , Ca M and Mg M , exhibited irregular changes with elevation. On the other hand, d13 C was positively correlated with N M in both leaves and branches, and negatively correlated with SLA in leaves along the altitudinal gradient. Our results also showed that both the Mg M level of leaves and the Ca M level of branches, besides the functional correlations between the N M level and the structure of leaves, are responsible for or accompanied by variation in d 13 C. In addition, d 13 C was negatively correlated with C/N in both leaves and branches along an altitudinal gradient. It follows that high-altitude plants achieve higher water use efficiency (WUE) at the expense of decreasing nitrogen use efficiency (NUE, derived from C/N), whereas plants at 2,800 m can maintain relatively higher NUE but lower WUE. These characteristics probably reflect the physiological potential of Q. aquifolioides for vigorous growth and metabolism at the optimum altitude (around 2,800 m). With increasing distance from the optimum altitude, NUE decreases. The observed intraspecific variation in the trade-off between WUE and NUE may partially explain the altitudinal distribution of Q. aquifolioides in relation to moisture and nutrient availability.

Detailed information on plants’ responses to varying temperature conditions will be useful when assessing the potential effects of climate change. We conducted reciprocal transplantations in Abies faxoniana Rehd. et Wils. to detect responses of seedlings to different winter (non-growing-season) temperatures in the Wanglang National Nature Reserve, China. Winter temperature variation might alter nitrogen allocation between 1-year-old leaves and branchlets. In leaves, coupling acclimation between photosynthesis potential (evident in pigment content and composition and carbon isotope composition (δ13C)) and adversity tolerance (detectable in peroxidase activity, malondialdehyde content, and nonstructural carbohydrate composition) to winter temperature variation was documented, whereas in branchlets, warming winter did not result in a δ13C-discriminative respiration process at tissue level. Although the experiment included only a short winter period, warming winter was found to pose a negative influence (decreased storage and increased leaf thickness) on A. faxoniana seedlings of subalpine forest understory. As both genetic adaptation and phenotypic plasticity could be responsible for such physiological variation, a detailed altitudinal investigation and a long-term experiment on A. faxoniana seedlings are needed to properly assess their responses to climate change.