Baoli Duan

Sex-related adaptive responses of Populus cathayana to photoperiod transitions.

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 O(2)(*-) in females, whereas reduced by higher O(2)(*-) 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.

Intra- and inter-sexual competition of Populus cathayana under different watering regimes

1Previous 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. We employed Populus cathayana males and females as a model to investigate intra- and intersexual competition under well-watered condition and drought stress. 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. 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.

Water deficit affects mesophyll limitation of leaves more strongly in sun than in shade in two contrasting Picea asperata populations

The aim of this study was to examine the response of internal conductance to CO(2) (g(i)) to soil water deficit and contrasting light conditions, and their consequences on photosynthetic physiology in two Picea asperata Mast. populations originating from wet and dry climate regions of China. Four-year-old trees were subjected to two light treatments (30% and 100% of full sunlight) and two watering regimes (well watered, drought) for 2 years. In both tested populations, drought significantly decreased g(i) and the net photosynthesis rate (A) and increased carbon isotope composition (delta(13)C) values in both light treatments, in particular in the sun. Moreover, drought resulted in a significantly higher relative limitation due to stomatal conductance (L(s)) in both light treatments and higher relative limitation due to internal conductance (L(i)) and abscisic acid (ABA) in the sun plants. The results also showed that L(i) (0.26-0.47) was always greater than L(s) (0.12-0.28). On the other hand, drought significantly decreased the ratio of chloroplastic to internal CO(2) concentration (C(c)/C(i)), photosynthetic nitrogen utilization efficiency (PNUE) and total biomass in the sun plants of the wet climate population, whereas there were no significant changes in these parameters in the dry climate population. Our results also showed that the dry climate population possessed higher delta(13)C values with higher ratio of internal conductance to stomatal conductance (g(i)/g(s)), suggesting that increasing the g(i)/g(s) ratio enhances water-use efficiency (WUE) in plants evolved in arid environments. Thus, we propose that the use of the g(i)/g(s) parameter to screen P. asperata plants with higher water deficit tolerance is certainly worthy of consideration. Furthermore, g(i) is an important variable, which reflects the population differences in PNUE, and it should thus be included in plant physiological investigations related to leaf economics.

Interactions between drought, ABA application and supplemental UV‐B in Populus yunnanensis

To test whether drought and ABA application alter the effects of enhanced UV-B on the growth and biomass allocation of Populus yunnanensis Dode, cuttings were grown in pots at two ABA levels, two watering regimes and two UV-B levels for one growth season. Exposure to enhanced UV-B radiation significantly decreased plant growth and photosynthesis under well-watered conditions, but these effects were obscured by drought, which alone caused growth reduction. Drought may contribute to masking the effects of UV-B radiation. The accumulation of UV-B absorbing compounds and the increase of the ABA content induced by drought could reduce the effectiveness of UV-B radiation. ABA application did not have large direct effects on biomass accumulation and allocation. Evidence for interactions between UV-B and ABA was detected for only a few measured traits. Therefore, there was little evidence to support a pivotal role for ABA in regulating a centralized whole plant response to enhanced UV-B. Yet, we recorded an ABA-induced decrease in stomatal conductance (g(s)) and increase in UV-B absorbing compounds and carbon isotope composition (delta(13)C) in response to enhanced UV-B. The allometric analysis revealed that regression models between root and shoot biomass in response to enhanced UV-B are different for plants under well-watered and drought conditions. Enhanced UV-B led to a significant displacement of the allometric regression line under well-watered condition, while allometric trajectories for both UV-B regimes did not differ significantly under drought condition.

Intraspecific variation in drought response of Populus cathayana grown under ambient and enhanced UV-B radiation

Abstract• The effects of drought, enhanced UV-B radiation and their combination on plant growth and physiological traits were investigated in a greenhouse experiment in two populations of Populus cathayana Rehder originating from high and low altitude in south-west China.• In both populations, drought significantly decreased biomass accumulation and gas exchange parameters, including net CO2 assimilation rate (A), stomatal conductance (gs), transpiration rate (E) and photosynthetic nitrogen use efficiency (PNUE). However, instantaneous water use efficiency (WUEi), transpiration efficiency (WUET), carbon isotope composition (δ13C) and nitrogen (N) content, as well as the accumulation of soluble protein, UV-absorbing compounds and abscisic acid (ABA) significantly increased in response to drought. On the other hand, cuttings from both populations, when kept under enhanced UV-B radiation, showed very similar changes, as under drought, in all above-mentioned parameters.• Compared with the low altitude population, the high altitude population was more tolerant to drought and enhanced UV-B, as indicated by the higher level of biomass accumulation, gas exchange, water-use efficiency, ABA concentration and UV-absorbing compounds.• After one growing season of exposure to different UV-B levels and watering regimes, the decrease in biomass accumulation and gas exchange, induced by drought, was more pronounced under the combination of UV-B and drought. Significant interactions between drought and UV-B were observed in WUEi, WUET, δ13C, soluble protein, UV-absorbing compounds, ABA and in the leaf and stem N, as well as in the leaf and stem C:N ratio.• Our results showed that UV-B acts as an important signal allowing P. cathayana seedlings to respond to drought and that the combination of drought and UV-B may cause synergistically detrimental effects on plant growth.Résumé• Les effets d’une sécheresse, d’une augmentation du rayonnement UV-B et de leur combinaison ont été étudiés sur la croissance et les caractéristiques physiologiques de plants de deux populations de Populus cathayana Rehder originaires de haute et basse altitude dans le sud-ouest de la Chine.• Dans les deux populations, la sécheresse a réduit de manière significative l’accumulation de biomasse et les échanges gazeux foliaires, y compris l’assimilation nette (A) de CO2, la conductance stomatique (gs), la transpiration (E) et l’efficience photosynthétique d’utilisation de l’azote (PNUE). Toutefois, l’efficience instantanée d’utilisation de l’eau (WUEi), l’efficience de transpiration (WUET), la composition isotopique du carbone (δ{si13}C) et le contenu en azote (N), ainsi que l’accumulation de protéines solubles, des composés absorbant les UV et de l’acide abscissique (ABA) ont présenté une augmentation significative en réponse à la sécheresse. D’autre part, des boutures des deux populations, quand elles ont été conservées sous un rayonnement UV-B renforcé, ont montré des changements de tous les paramètres mentionnés ci-dessus, similaires à ceux induits par la sécheresse.• Par rapport à la population de faible altitude, la population de haute altitude, était plus tolérante à la sécheresse et au renforcement du rayonnement UV-B ; cela s’est traduit par le niveau plus élevé d’accumulation de biomasse, d’échanges gazeux, de l’efficience d’utilisation de l’eau, de la concentration d’ABA et des composés absorbant les UV.• Après une saison de croissance, la diminution de l’accumulation de biomasse et des échanges gazeux induite par la sécheresse, était plus prononcée dans le cadre de la combinaison du rayonnement UV-B et de la sécheresse. Des interactions significatives entre sécheresse et UV-B ont été observées pour WUEi, WUET, δ13C, les protéines solubles, les composés absorbant les UV, l’ABA et l’azote des feuilles et des tiges, ainsi que pour le rapport C/N des feuilles et des tiges.• Nos résultats ont montré que les UV-B agissent comme un signal important en permettant aux jeunes plants de P. cathayana de faire face à la sécheresse et que la combinaison de la sécheresse et des UV-B peut causer par synergie des effets néfastes pour la croissance des plants.

Sexual competition and N supply interactively affect the dimorphism and competiveness of opposite sexes in Populus cathayana

Several important dioecious species show sexual spatial segregation (SSS) along environmental gradients that have significant ecological effect on terrestrial ecosystem. However, little attention has been paid to understanding of how males and females respond to environmental gradients and sexual competition. We compared eco-physiological parameters of males and females of Populus cathayana under different sexual competition patterns and nitrogen (N) supply levels. We found that males and females interacting with the same or opposite sex showed significant differences in biomass partition, photosynthetic capacity, carbon (C) and N metabolism, and leaf ultrastructure, and that the sexual differences to competition were importantly driven by N supply. The intersexual competition was enhanced under high N, while the intrasexual competition among females was amplified under low N. Under high N, the intersexual competition stimulated the growth of the females and negatively affected the males. In contrast, under low N, the males exposed to intrasexual competition had the highest tolerance, whereas females exposed to intrasexual competition showed the lowest adaptation among all competition patterns. Sexual competition patterns and N supply levels significantly affected the sexual dimorphism and competitiveness, which may play an important role in spatial segregation of P. cathayana populations.

Differences in growth and physiological traits of two poplars originating from different altitudes as affected by UV-B radiation and nutrient availability

Cuttings of Populus kangdingensis and Populus cathayana originating from altitudes of 3500 and 1500 m in southwestern China, respectively, were grown for one growing season in the field under ambient or ambient plus supplemental ultraviolet-B (UV-B) radiation with two levels of nutrients. In both species, enhanced UV-B radiation significantly increased UV-B absorbing compounds and guaiacol peroxidase (GPX) activity, while no significant effects were observed in photosynthetic pigments and proline content. On the other hand, cuttings grown with high-nutrient availability had larger leaf area, higher total biomass and GPX activity as well as higher water use efficiency (WUE) (as measured by stable carbon isotope composition, delta(13)C) when compared with low-nutrient conditions, while UV-B absorbing compounds and ascorbic acid (AsA) content significantly decreased. Differences in responses to enhanced UV-B radiation and nutrient availability were observed between the two species. Nutrient-induced increases in chlorophyll a, chlorophyll b and total chlorophyll as well as in carotenoids were greater in P. kangdingensis than in P. cathayana. In P. cathayana, enhanced UV-B radiation significantly decreased leaf area and total biomass, while it significantly increased WUE and ascorbate peroxidase (APX). In contrast, such changes were not observed in P. kangdingensis. In addition, the effects of enhanced UV-B radiation on leaf area, total biomass and UV-B absorbing compounds were closely related to the nutrient status. Our results indicated that P. kangdingensis, which originates from the altitude of 3500 m and is apparently adapted to low-nutrient and high-UV-B habitats, exhibits better tolerance to enhanced UV-B radiation and greater growth under low-nutrient availability than does P. cathayana originating from the altitude of 1500 m.

Contrasting responses in the growth and energy utilization properties of sympatric Populus and Salix to different altitudes: implications for sexual dimorphism in Salicaceae

An interesting ecological and evolutionary puzzle arises from the observations of male-biased sex ratios in genus Populus, whereas in the taxonomically related Salix, females are generally more dominant. In the present study, we combined results from a field investigation into the sex ratios of the Salicaceous species along an altitudinal gradient on Gongga Mountain, and a pot experiment by monitoring growth and energy utilization properties to elucidate the mechanisms governing sexual dimorphism. At middle altitudes 2000 and 2300 m, the sex ratios were consistent with a 1:1 equilibrium in sympatric Populus purdomii and Salix magnifica. However, at the lower and higher ends of the altitudinal gradient, skewed sex ratios were observed. For example, the male:female ratios were 1.33 and 2.36 in P. purdomii at 1700 and 2600 m respectively; for S. magnifica the ratio was 0.62 at 2600 m. At 2300 m, the pot-grown seedlings of both species exhibited the highest biomass accumulation and total leaf area, simultaneously with the balanced sex ratios in the field. At 3300 m, the specific leaf area in male P. purdomii was 23.9% higher than that of females, which may be the morphological cause for the observed 19.3% higher nitrogen allocation to Rubisco, and 20.6% lower allocation to cell walls. As such, male P. purdomii showed a 32.9% higher foliar photosynthetic capacity, concomitant with a 12.0% lower construction cost. These properties resulted in higher photosynthetic nitrogen- and energy-use efficiencies, and shorter payback time (24.4 vs 40.1 days), the time span that a leaf must photosynthesize to amortize the carbon investment. Our results thus suggested that male P. purdomii evolved a quicker energy-return strategy. Consequently, these superior energy gain-cost related traits and the higher total leaf area contributed to the higher growth rate and tolerance in stress-prone environments, which might, in part, shed new light on the male-biased sex ratios in Populus. However, no significant sexual difference was observed in S. magnifica for all the above parameters, thereby implying that the female-biased sex ratios in Salix cannot be explained in terms of the energy-use properties studied here.

Growth, biomass allocation and photosynthetic responses are related to intensity of root severance and soil moisture conditions in the plantation tree Cunninghamia lanceolata

We employed the warm temperate conifer Cunninghamia lanceolata (Lamb.) Hook. as a model of plantation forest species to investigate ecophysiological responses to root treatments (control (0%), and ∼25, 50 or 75% of the initial root mass) under well-watered and water-limited conditions. Our results indicated that total root dry mass accumulation was negatively associated with the severity of root pruning, but there was evidence of multiple compensatory responses. The plants exhibited higher instantaneous and long-term (assessed by carbon isotope composition, δ(13)C) water-use efficiency in pruning treatments, especially under low water availability. Root pruning also increased the fine root/total root mass ratio, specific root length and fine root vitality in both water availability treatments. As a result of the compensatory responses, under well-watered conditions, height, stem dry mass accumulation, leaf/fine root biomass ratio (L/FR), transpiration rate, photosynthetic capacity and photosynthetic nitrogen-use efficiency (EN) were the highest under 25% pruning. Yet, all these traits except L/FR and foliage nitrogen content were severely reduced under 75% pruning. Drought negatively affected growth and leaf gas exchange rates, and there was a greater negative effect on growth, water potential, gas exchange and EN when >25% of total root biomass was removed. The stem/aboveground mass ratio was the highest under 25% pruning in both watering conditions. These results indicate that the responses to root severance are related to the excision intensity and soil moisture content. A moderate root pruning proved to be an effective means to improve stem dry mass accumulation.

Causes for the unimodal pattern of leaf carbon isotope composition in Abies faxoniana trees growing in a natural forest along an altitudinal gradient

Leaf morphological and physiological traits of Abies faxoniana growing in a natural forest along an altitudinal gradient were measured with the aim to identify the central mechanism for the marked variation in foliar δ13C determined by an isotope ratio mass spectrometer. There is a unimodal pattern of plant functional traits in these temperate and semi-humid areas. Stomatal parameters, specific leaf area, and C/N ratio increased, whereas C, N and δ13C values decreased with increasing altitude below 3000 m a.s.l.. In contrast, they exhibited opposite trends above 3000 m a.s.l.. Our results demonstrated that high-altitude plants achieve higher water use efficiency (WUE) at the expense of decreasing nitrogen use efficiency (NUE), whereas plants at 3000 m can maintain a relatively higher NUE but a lower WUE. Such intra-specific differences in the trade-off between NUE and WUE may partially explain the altitudinal distribution of the plants in relation to moisture and nutrient availability. Our results clearly indicate that the functional relations between nutritional status and the structure of leaves are responsible for the altitudinal variations associated with δ13C. The pivotal role of specific leaf area in regulating plant adaptive responses provides a potential physiological mechanism for the observed growth advantage of populations occupying the medium altitude. These adaptive responses to altitudinal gradients showed that an altitude of approximately 3000 m a.s.l. is the optimum distribution zone for A. faxoniana, allowing the most vigorous growth and metabolism. These results improve our understanding of the various roles of environmental and biotic variables upon δ13C dynamics and provide useful information for subalpine coniferous forest management.