M. Zunzunegui

Comparison of proline accumulation in two mediterranean shrubs subjected to natural and experimental water deficit

The effect of water stress on proline accumulation was tested in two contrasted species of Mediterranean scrub: Halimium halimifolium (L.) Willk and Pistacia lentiscus L. Leaf water potential, stomatal resistance and proline content have been measured both in experimental and in natural water stress conditions. Both species accumulated proline in their leaves when leaf water potential dropped below a threshold value of −3.0 MPa, under natural as well as under experimental conditions. In the field, however, a time-lag between decrease of leaf water potential and proline accumulation could be observed. In Halimium halimifolium, proline accumulation appeared to be associated with severe stress conditions as most plants with high proline contents suffered irreversible wilting, especially in the greenhouse. P. lentiscus showed a different pattern, accumulating proline at two different times of the year, as a response to cold or to drought. The results of our study indicated that the role of proline in this species, rather than an osmotic agent, seems to be more related to a protective action in cases of severe stress conditions.

Plant functional types and ecosystem function in Mediterranean shrubland

. This study attempts to identify Plant Functional Types (PFTs) within the scrub vegetation of the stabilized sand dunes of the Donana National Park (southwestern Spain) and to explore possible links with ecosystem function. Two 10-km long transects were sampled along a gradient of water table depth from elevated and dry dunes to a border area close to the salt marsh. A matrix of cover values for 20 woody species × 58 plots and a matrix of 24 morphological and physiological traits × 20 species were analysed by means of DCA and TWINSPAN to identify the main vegetation types and PFTs. In order to know the predominant plant traits, the matrix of 20 species × 58 plots was multiplied by the 24 traits × 20 species matrix. The resulting 24 traits × 58 plots matrix was analysed by means of DCA. The main vegetation types coincide with the previous descriptions of the Park with Juniperus phoenicea woodland and xerophytic species dominating the top of the dunes and hygrophytic species with isolated Quercus suber trees in the depressions. Previous classifications of Mediterranean woody plants resulted in two main PFTs. The present analysis confirmed these two groups: maquis versus garrigue, but added four new groups. The analysis of the traits × plots matrix revealed that the main trend of variation is related to the water table gradient, while the traits related to the species of mature plant communities had little weight in the analysis because these communities are restricted to isolated patches.

To live or to survive in Donana dunes: Adaptive responses of woody species under a Mediterranean climate

The Dune System of Doñana National Park (SW Spain) exhibit a mosaic of environmental characteristics, with different plant communities, all under the same Mediterranean climate, creating an interesting field laboratory for the study of plant responses to stressing conditions. Fourteen woody plant populations were selected, belonging to either xerophytic or hygrophytic plant communities on stabilised dunes, where topography causes differences in soil water availability. Plants were tagged and morphological and ecophysiological measurements were recorded in winter and summer of 1999. Seasonal differences in ecophysiological measurements together with morphological variables were used as plant traits to identify the main adaptive responses of the species. Cluster analysis of traits separated three groups of plant strategies: spiny legume species; sclerophyll, and semideciduous species. In addition, another two kind of strategies have been found in the semideciduous group of species, those withstanding water shortages, attaining very negative water potentials, low photochemical efficiency, and leaf proline accumulation in summer, as opposed to those tolerating water deficit, with moderate seasonal differences in water potential, proline content and photochemical efficiency. The results of this study indicate that legume species behave as a different functional group and drought-semideciduous species present different adaptive responses under the same environmental stress. Ecophysiological measurements must be used as plant traits to detect functional groups under Mediterranean climate.

Vegetation fluctuation in mediterranean dune ponds in relation to rainfall variation and water extraction

. The structure of dune ponds hygrophytic vegetation has a spatial organisation in belts around the pond basin, closely related to water level and flooding regime. Donana National Park has an important representation of temporal dune ponds, which are subjected to rainfall fluctuations and may be suffering the impact of water extraction from the neighbouring tourist resort. Permanent transects in a vegetation complex of five dune ponds have been monitored during a eight year period (1990-1997). This period was characterised by a number of dry years (annual rainfall around 300 mm), located between two wet cycles (800-900 mm). Transects were analysed in 1990 (wet period), 1994 (dry) and 1997 (wet) by hierarchical agglomera-tive clustering. During the dry period hygrophytic species showed regression, with a high mortality of some species like Ulex minor, while the xerophytic species advanced to lower areas. Seedlings of some xerophytic species colonised the dry surface of the pond basin. The lowering of the water table varied in the different ponds, ranging from 312 to 190 cm depending on topography and the distance to the pumping area. The new period of flooding during 1995-96 and 1996-97 cycles provided the opportunity for hygrophytic spe cies to re-establish themselves in their original places. This study suggest that changes in vegetation are caused by the interaction between weather conditions and human disturbance (water extractions). In our example man-made disturbance is more marked during the dry periods while wet periods tend to obscure the effects of water extractions. From a management perspective, long-term monitoring of water table and vegetation structure is revealed as a key procedure to the management of land-water ecotones on pressured areas and threatened habitats.

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.

Ecophysiology, growth timing and reproductive effort of three sexual foms of Corema album (Empetraceae)

Corema album is a dioecious shrub endemic of the Iberian Peninsula, growing in sandy coastal areas from the North of Galicia to Gibraltar in the South. Hermaphrodite individuals have been found in the southern populations of El Asperillo (Spain) and Vila Real de Santo Antonio (Portugal). In this paper, the role of hermaphroditic individuals in the driest populations of its biogeographical area is discussed and the trade-off between reproduction and vegetative growth in the three sexual forms is presented. Asperillo site where hermaphrodite individuals were more abundant was selected for this current study. The proportion of hermaphrodite individuals was 1–4%, and male/female sex ratio ranged from 0.9 to 1.1. Hermaphrodite plants had a low number of hermaphrodite inflorescences, in relation to the number of staminate inflorescences. Their ripe fruits were smaller than those of female plants. The reproductive effort was three times higher in female than in male or hermaphrodite plants. The observed interannual oscillation in vegetative elongation was not related to annual rainfall. However, differences existed in growth timing among the three sexual forms, related to the timing of peak investment in reproduction. These results suggest the existence of trade-offs between investment in reproduction and vegetative growth. There were significant differences in leaf water potential among the three sexual forms early in summer, with males showing the less negative and hermaphrodite the most negative potential values. The demand of water for fruit production may explain these differences. [Valdés B., Talavera S. and Fernández Galiano E. (eds), 1987. Flora Vascular de Andalucía Occidental, vol. 1. Ketres Editora, Barcelona.]

Seasonal Differences in Photochemical Efficiency and Chlorophyll and Carotenoid Contents in Six Mediterranean Shrub Species under Field Conditions

The effects of summer and winter stress on the chlorophyll and carotenoid contents and photosystem 2 efficiency were examined in six Mediterranean scrub species. These six species belong to two different plant functional types: drought semi-deciduous (Halimium halimifolium L., Rosmarinus officinalis L., Erica scoparia L.) and evergreen sclerophylls (Juniperus phoenicea L., Pistacia lentiscus L., Myrtus communis L.). Two sites with different water availability were chosen. In the xerophytic site, despite they belong to two different functional types, R. officinalis and J. phoenicea showed a similar response. These were the most affected species in summer. H. halimifolium showed optimal values of Fv/Fm and non-significant seasonal changes in xanthophyll content. In the mesic site, E. scoparia and M. communis were apparently the most affected species by winter climatic conditions. P. lentiscus presented a pattern similar to H. halimifolium, except for elevated F0 values. In all the studied species, lutein plus zeaxanthin content was negatively correlated with Fv/Fm in summer and with leaf water potential, thus indicating that the thermal dissipation of energy was a general pattern for all species. Under stress, plant response is more species-specific than dependent on its functional type.

Effects of Pinus pinea litter on seed germination and seedling performance of three Mediterranean shrub species

Pinus pinea L. is a coniferous tree inhabiting the Mediterranean Basin, which appears widely distributed in the Iberian Peninsula, frequently associated to Mediterranean shrub species, such as Cistus salvifolius L., C. libanotis L. and Halimium halimifolium (L.) Willk. (Cistaceae). This study investigated the effects (chemical and physical) of P. pinea needles on the seed germination and early growth of these three shrub species. Laboratory experiments were carried out in order to assess an expected negative effect by using different treatments to seeds (physical layer and two aqueous extracts concentrations). Seed germination of all species was inhibited when treated with aqueous extracts and the degree of inhibition was higher when using the extract with the highest concentration, while no effect was observed with the physical layer treatment. Seedling performance was also sensitive to the different treatments, showing the most stressed values with the high concentration extract. The results from this study highlight the importance of allelopathy of P. pinea needles as a mechanism for controlling seed germination and seedling performance of some Mediterranean shrub species.

Seasonal Changes in Photochemical Efficiency in Leaves of Halimium Halimifolium, a Mediterranean Semideciduous Shrub

The relationship between soil water availability, physiological responses [leaf chlorophyll (Chl) fluorescence, leaf water potential (Ψ), and stomatal conductance (gs)] and plant stress was studied in Halimium halimifolium (L.) Willk, Cistaceae, in three sites with contrasted water regimes (Monte Blanco - MB, Monte Intermedio - MI, and Monte Negro - MN) of Doñana National Park (SW of Spain) along the day, in September (summer period) and December (winter period) 1994. In winter, differences among the areas were not significant, with Ψ, gs, and photochemical efficiency values of −1.5 MPa, 0.200 cm s−1, and 0.70, respectively. In summer, however, high declines of gs (0.014 cm s−1 in MI and 0.021 cm s−1 in MB), photochemical efficiency (0.65 Fv/Fp in MB and MI sites) and Ψ (−3.76 in MI and −3.04 MPa in MB) were recorded. Winter-summer differences were minimum in well-watered sites (MN) and maximum in MI. The Ψ and photosystem 2 (PS2) fluorescence were correlated and showed similar seasonal patterns in all three areas.

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 genders 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.