Different tolerance to salinity of two populations of Oenothera drummondii with contrasted biogeographical origin.

Oenothera drummondii is a native species from the coastal dunes of the Gulf of Mexico that has nowadays extended to coastal areas in temperate zones all over the world, its invasion becoming a significant problem locally. The species grows on back beach and incipient dunes, where it can suffer flooding by seawater, and sea spray. We were interested in knowing how salinity affects this species and if invasive populations present morphological or functional traits that would provide greater tolerance to salinity than native ones. To this end, we conducted a greenhouse experiment where plants from one native and from one invading population were irrigated with five salinity treatments. We measured functional traits on photosynthetic, photochemical efficiency, water content, flowering, Na+ content, pigment content, and biomass. Although O. drummondii showed high resistance to salinity, the highest levels recorded high mortality, especially in the invasive population. Plants exhibited differences not only in response to time under salinity conditions, but also according to their biogeographic origin, the native population being more resistant to long exposure and high salt concentration than the invasive one. Native and invasive populations showed different response to salt stress in photosynthesis and transpiration rates, stomatal conductance, water use efficiency, carboxylation efficiency, electron transport rate, electron transport efficiency, energy used in photochemistry, among others. The increasing salinity levels resulted in a progressive reduction of photosynthesis rate due to both stomatal and biochemical limitations, and also in a reduction of biomass and number and size of flowers, compromising the reproductive capacity.

Field comparison of ecophysiological traits between an invader and a native species in a Mediterranean coastal dune.

Photochemical efficiency, gas exchange, leaf water potential, pigment content and free proline content of Oenothera drummondii subsp. drummondii Hook (an invasive non-native species) and Achillea maritima (L.) Ehrend. & Y.P. Guo, (an ecologically similar native species) were explored to understand the success of invasive non-native species in Mediterranean coastal dunes of southwest Spain. We have conducted a field study during a complete annual cycle, comparing both species. Fifteen pairs of neighbouring plants of the two study species of similar size were selected and measured seasonally. The results show that in spring and summer, assimilation rates of O. drummondii were significantly higher than those of the native, A. maritime, even though the native species had higher photochemical efficiency. Additionally, the non-native species presented better water content regulation than the native one, surely related to better water use efficiency and maybe linked to greater root development. The differences in leaf dry matter content values for both species might indicate a different strategy of resource use; with A. maritima displaying a more conservative strategy and O. drummondii presenting a rapid resources acquisition and use strategy as predictors of rapid growth and soil fertility. We conclude that O. drummondii utilizes light, water and probably nutrients more efficiently than the native A. maritima and suffers lower stress in Mediterranean coastal dunes where water availability is reduced (44 mm from May to October in the study area) and light radiation levels are high.

Season-dependent and independent responses of Mediterranean scrub to light conditions.

Semi-arid plant species cope with excess of solar radiation with morphological and physiological adaptations that assure their survival when other abiotic stressors interact. At the leaf level, sun and shade plants may differ in the set of traits that regulate environmental stressors. Here, we evaluated if leaf-level physiological seasonal response of Mediterranean scrub species (Myrtus communis, Halimium halimifolium, Rosmarinus officinalis, and Cistus salvifolius) depended on light availability conditions. We aimed to determine which of these responses prevailed independently of the marked seasonality of Mediterranean climate, to define a leaf-level strategy in the scrub community. Thirty six leaf response variables - involving gas exchange, water status, photosystem II photochemical efficiency, photosynthetic pigments and leaf structure - were seasonally measured in sun exposed and shaded plants under field conditions. Physiological responses showed a common pattern throughout the year, in spite of the marked seasonality of the Mediterranean climate and of species-specific differences in the response to light intensity. Variables related to light use, CO2 assimilation, leaf pigment content, and LMA (leaf mass area) presented differences that were consistent throughout the year, although autumn was the season with greater contrast between sun and shade plants. Our data suggest that in Mediterranean scrub shade plants the lutein pool could have an important role in the photoprotection of the photosynthetic tissues. There was a negative linear correlation between the ratio lutein/total chlorophylls and the majority of leaf level variables. The combined effect of abiotic stress factors (light and drought or light and cold) was variable-specific, in some cases enhancing differences between sun and shade plants, while in others leading to unified strategies in all scrub species.

Competitive effect of a native-invasive species on a threatened shrub in a Mediterranean dune system.

The canopy shade of the Retama species has been widely reported to ameliorate the environmental conditions in the understory, thus facilitating other species' establishment. The shading effect of the native-invasive leguminous shrub Retama monosperma (L.) Boiss on the endangered Thymus carnosus Boiss was analysed to determine a positive or negative net effect. Data was taken in all four seasons, representing contrasting light and water availability in a Mediterranean coastal dune ecosystem (SW Spain). The morphological and physiological status of sun-exposed T. carnosus plants growing in open areas versus shaded plants growing under R. monosperma were measured seasonally. Leaf mass area, leaf area index and pigment content showed typical sun-shade responses. In contrast, sun-exposed T. carnosus displayed higher stem water potential, transpiration rate and water use efficiency, both intrinsic and integrated, denoting low tolerance to the presence of R. monosperma. Five years after the measurements, canopy cover had decreased and mortality was higher in shaded plants, thus confirming the competitive effect of R. monosperma on T. carnosus. R. monosperma arises as a competitor for endangered T. carnosus communities, consequently reinforcing its invasive behaviour. This species-specific shrub study demonstrates that eventual beneficial effects of Retama canopy may be overridden by competition in the understory, particularly in the case of species well-adapted to high light and low water levels.

Climatic conditions and herbivory effects on morphological plasticity of Argania spinosa.

The main objective of this paper was to look into the morphological differentiation patterns and phenotypic plasticity in four populations of Argania spinosa with environmentally contrasted conditions. Mean response, magnitude and pattern of morphological intra- and inter-population plasticity indexes were measured and analyzed in order to identify which characters contribute the most to the acclimation of this species. Populations growing in the ecological optimum of the species presented the lowest plasticity, while those growing in the most stressed habitats showed an increased morphological variability. The study of four populations showed that human pressure seems to play an important function in the regulation of morphological characters. However, climatic conditions seem to play a significant role in the increase of morphological plasticity.

Some secrets of Argania spinosa water economy in a semiarid climate.

Argania spinosa is an example of an avoider tree growing under semi-arid conditions in Morocco. To assess what are the physiological strategies of this species, different variables were measured through an annual cycle in two populations located in the species' main distribution area. Results show the expected decrease of leaf water potential (psi) with an increase of water-use efficiency (A/gs) with the onset of the dry season. In summer, leaf conductance (g(s)) was sensitive to vapour pressure deficit (VPD), and stomatal closure occurred over 30 mbar of VPD. Surprisingly, carbon isotope discrimination (delta13C) maintained very low values over the year, with almost no relationship with any physiological or morphological variable. Hence Argania spinosa presents a complex set of mechanisms to avoid water deficit, but delta13C cannot be used as an ecological tracer of long term WUE.

Gender-specific costs of reproduction on vegetative growth and physiological performance in the dioecious shrub Corema album.

BACKGROUND AND AIMS: Reproductive costs imply trade-offs in resource distribution at the physiological level, expressed as changes in future growth and/or reproduction. In dioecious species, females generally endure higher reproductive effort, although this is not necessarily expressed through higher somatic costs, as compensatory mechanisms may foster resource uptake during reproduction. METHODS: To assess effects of reproductive allocation on vegetative growth and physiological response in terms of costs and compensation mechanisms, a manipulative experiment of inflorescence bud removal was carried out in the sexually dimorphic species Corema album. Over two consecutive growing seasons, vegetative growth patterns, water status and photochemical efficiency were measured to evaluate gender-related differences. KEY RESULTS: Suppression of reproductive allocation resulted in a direct reduction in somatic costs of reproduction, expressed through changes in growth variables and plant physiological status. Inflorescence bud removal was related to an increase in shoot elongation and water potential in male and female plants. The response to inflorescence bud removal showed gender-related differences that were related to the moment of maximum reproductive effort in each sexual form: flowering in males and fruiting in females. Delayed costs of reproduction were found in both water status and growth variables, showing gender-related differences in resource storage and use. CONCLUSIONS: Results are consistent with the existence of a trade-off between reproductive and vegetative biomass, indicating that reproduction and growth depend on the same resource pool. Gender-related morphological and physiological differences arise as a response to different reproductive resource requirements. Delayed somatic costs provide evidence of gender-related differences in resource allocation and storage. Adaptive differences between genders in C. album may arise through the development of mechanisms which compensate for the cost of reproduction.

Physiological performance and xylem water isotopic composition underlie gender-specific responses in the dioecious shrub Corema album.

Gender-specific requirements of reproduction in dioecious species can lead to different physiological responses in male and female plants, made in relation to environmental constraints, and influencing growth, survival and population structure. Gender-related physiological differences and seasonal responses, indicating the existence of compensatory mechanisms of reproduction, were examined during a drought year in the dioecious shrub species Corema album. To integrate aboveground and belowground physiological responses, chlorophyll fluorescence, leaf gas exchange, water potential and xylem water isotopic composition were monitored throughout the diurnal cycle and annual phenological sequence of the species. Sampling was carried out in Doñana Natural Park (SW Spain) in Mediterranean-type climate conditions. The gender which bore greater reproductive effort showed higher physiological stress. Intersexual differences in leaf water potential were interpreted as arising from each gender's maximum reproductive allocation; lower values were found during flowering in males and during fruit production in females. Cold temperatures during winter fostered photoinhibitory responses that were most evident in male individuals, as a response to their relatively higher investment in reproduction during flowering. Net assimilation rate was not influenced by reproductive status; however, females tended to show higher values of this parameter at midday. The integrated analysis of photosynthetic variables and water relations indicated a gender effect in the physiological response at midday. The oxygen isotopic composition of xylem water showed a lack of dependence on the water table during the drought period, and indicated intersexual differences in water catchment. Females reached deeper soil layers, suggesting mechanisms compensating for their higher reproductive effort, and giving new evidence of physiological gender dimorphism in the belowground responses of a woody species.