Differential tolerance of native and invasive tree seedlings from arid African deserts to drought and shade

Efforts to understand why some species become successful invaders and why some habitats are more at risk from invasive species is an important research focus in invasion ecology. With current global climate change, evaluation of the effects of shade and drought on cohabiting native and invasive species from extreme ecosystems is especially important. Acacia tortilis subsp. raddiana is a tree taxon native to arid African deserts. Prosopis glandulosa, native to the southwestern United States and Mexico, is invading African arid and semiarid regions that are habitat for A. t. subsp. raddiana. The aim of this study was to evaluate and compare the tolerance and responses of the seedlings of these two tree species to shade, water stress and their interactions. We measured and recorded growth rates and morphological, biochemical and physiological plant traits under two radiation and two water treatments in greenhouse conditions. Radiation intensity was a stronger driver of the performance of both species than water availability. Beyond the independent effects of shade and drought, the interactions of these factors yielded synergistic effects on seedlings of both tree species, affecting key plant traits. The seedlings of A. t. subsp. raddiana were able to implement important shifts in key functional traits in response to altering abiotic stress conditions, behaving as a stress-tolerant species that is well-adapted to the habitat it occupies in hot arid African deserts. In contrast, the fast-growing seedlings of P. glandulosa were stress-avoiding. The alien P. glandulosa seedlings were highly sensitive to water and shade stress. Moreover, they were particularly sensitive to drought in shade conditions. However, although alien P. glandulosa seedlings were exposed to high stress levels, they were able to avoid permanent damage to their photosynthetic apparatus by mechanisms such as increasing energy dissipation by heat emission and by adjusting the relative allocation of resources to above- and below-ground structures. Our results are useful for conservation planning and restoration of invaded hyperarid ecosystems

Phenotypic plasticity of polyploid plant species promotes transgressive behaviour in their hybrids

Hybridization is a frequent process that leads to relevant evolutionary consequences, but there is a lack of studies regarding the relationships of the variability of the response of parental plant species to environmental gradients and the responses of their hybrids at a phenotypic level. We designed an experiment in which we exposed two reciprocal cordgrass hybrids, Spartina maritima × densiflora and S. densiflora × maritima, and their parental species to four salinity concentrations for 30 days. The main objectives were to compare the performance of the hybrids with that of their parents, to distinguish the phenotypic inheritance operating in the hybrids and to analyse the relationships between the variability in the responses of the parents and the responses of their hybrids to salinity. We characterized the responses and the degree of variability for 37 foliar traits. Both hybrids presented greater salinity tolerance than their parents, showing their highest percentage of transgressive traits at both extremes of the salinity gradient. When the parental plants themselves showed a more plastic response for a given trait, there was a greater chance that their hybrid developed a transgressive behaviour for this trait. This finding supports a new focus to be applied for the artificial development of vigorous hybrid crops

Effects of competition from the invasive Cordgrass Spartina densiflora on Native Atriplex portulacoides

Invasion by the South American cordgrass Spartina densiflora Brongn. in European salt marshes is causing concern about potential impacts to native plant communities., S. densiflora colonization of mid-marsh habitats may be slowed by native Atriplex portulacoides L., but the mechanism behind this negative interaction is not known. The aim of this work was to study the establishment and growth response of S. densiflora seedlings grow-ing in a competitive environment with mature A. portulacoides plants in controlled greenhouse conditions. With this aim we measured establishment, growth, foliar nutrients, and photosynthetic pigments of S. densiflora plants grown with and without A. portulacoides. Results showed that S. densiflora seedlings readily established in mature stands of A. portulacoides. Every Spartina clump growing with Atriplex survived, producing fewer tillers with simi-lar heights than when growing without competition, which was reflected on lower above-ground biomass. These results indicated that S. densiflora was affected at the above-ground level by the interspecific competition with Atriplex, but the invasive cordgrass was able to keep similar below-ground bio-mass with and without competition, resulting in a decrease of below-ground biomass of Atriplex when competing with Spartina. Our results in green-house controlled conditions were in agreement with our field observations where S. densiflora plants have been observed growing within A. portu-lacoides monospecific communities

Effect of Low and High Temperatures on the Photosynthetic Performance of Lantana camara L. Leaves in Darkness

Low and high temperatures are known as most important factors influencing plant performance and distribution. Plants of Lantana camara L. coming from two distinct geographical populations (Iberian Peninsula and Galápagos Islands) were cultivated in a common garden experiment, and their leaves were subjected to thermal treatments (from +20.0 to –7.5°C during the winter and from +20.0 to +50.0°C during the summer) in a programmable water bath in darkness. Their photosynthetic performance and their recovery capacity after the thermal treatment were evaluated by measuring chlorophyll fluorescence, net photosynthe sis rate, and leaf necrosis. In general, L. camara photosynthetic apparatus showed a wide range of tempera ture tolerance in darkness, showing optimal functioning of its photosystem II just after exposure to tempera tures between –2.5 and +35.0°C for the Iberian population and between +10.0 and +25.0°C for the Galápa gos population. Just after exposure to low and high temperatures, gradual cold and heatinduced photoinhibition was recorded for both populations. After 24 h, leaves of L. camara demonstrated a great recovery capacity from –2.5 to +42.5°C. However, leaves of the treatments from –5.0°C down and +47.50°C up showed permanent damages to the photosynthetic apparatus and to the leaf tissues. Slight inter population differences were found only at extreme temperatures. KeywAgencia Española de Cooperación Internacional para el Desarrollo (AECID

Competition from native hydrophytes reduces establishment and growth of invasive dense-flowered cordgrass (Spartina densiflora)

Experimental studies to determine the nature of ecological interactions between invasive and native species are necessary for conserving and restoring native species in impacted habitats. Theory predicts that species boundaries along environmental gradients are determined by physical factors in stressful environments and by competitive ability in benign environments, but little is known about the mechanisms by which hydrophytes exclude halophytes and the life history stage at which these mechanisms are able to operate. The ongoing invasion of the South American Spartina densiflora in European marshes is causing concern about potential impacts to native plants along the marsh salinity gradient, offering an opportunity to evaluate the mechanisms by which native hydrophytes may limit, or even prevent, the expansion of invasive halophytes. Our study compared S. densiflora seedling establishment with and without competition with Phragmites australis and Typha domingensis, two hydrophytes differing in clonal architecture. We hypothesized that seedlings of the stress tolerant S. densiflora would be out-competed by stands of P. australis and T. domingensis. Growth, survivorship, biomass patterns and foliar nutrient content were recorded in a common garden experiment to determine the effect of mature P. australis and T. domingensis on the growth and colonization of S. densiflora under fresh water conditions where invasion events are likely to occur. Mature P. australis stands prevented establishment of S. densiflora seedlings and T. domingensis reduced S. densiflora establishment by 38%. Seedlings grown with P. australis produced fewer than five short shoots and all plants died after ca. 2 yrs. Our results showed that direct competition, most likely for subterranean resources, was responsible for decreased growth rate and survivorship of S. densiflora. The presence of healthy stands of P. australis, and to some extent T. domingensis, along river channels and in brackish marshes may prevent the invasion of S. densiflora by stopping the establishment of its seedlings

Combining a Process-Based Model of Stomatal Conductance with Leaf Turgor Pressure Related Probe Measurements to Study the Regulation of Plant Water Status and Stomatal Conductance under Drought

The recently developed plant sensor of relative changes in leaf turgor pressure (LPCP probe) was compared with the turgor pressure output simulated with a process-based stomatal model (BMF model). Our results confirm the good agreement between the simulated turgor pressures and those derived from LPCP readings. The combined use of the BMF model and LPCP probes raised new insights into the regulation of hydraulic conductivity and osmotic pressure.Ministerio de Ciencia e Innovación español. AGL2009-11310/AGRPrograma FEDER, Comisión Europe

Influence of the soil water content and distribution on both the hydraulic and transpiration performance of 'Manzanilla' olive trees

VI International Symposium on Irrigation of Horticultural Crops, celebrado del 2-6 de noviembre de 2006 en Viña del Mar, Chile ISBN-978-90-66057-13-5This work was made with mature 'Manzanilla' olive trees in an orchard of a semi-arid area in southern Spain. Three water treatments were considered: Rainfed, in which the trees had rainfall as the only source of water supply; FAO, in which the trees were under localized irrigation to replace the crop water demand, with some roots left in drying soil; Pond, in which the whole rootzones of the trees were maintained under non-limiting soil water conditions for the whole dry season. Our aim was to obtain information on the mechanisms behind the reduction of transpiration (Ep) in the FAO trees, as compared to the Pond trees. Our results show a near-isohydric behaviour of the FAO trees, i.e. those trees under localized irrigation in which some roots are left in drying showed lower stomatal conductance than the Pond trees in which all roots were in wetted soil. This helped the FAO trees to maintain similar leaf water potentials than the Pond trees. In addition, the FAO trees maintained a constant difference between the water potential of the canopy and that in the soil. This has been described as an isohydrodynamic behaviour, and it is thought to be an improvement over a typically anisohydric behaviour. These mechanisms were behind the similar values of tree hydraulic conductance (K p) found in the FAO and Pond treatments. The Rainfed trees showed lower Kp values because of the low Ep values of those trees, due to the low soil water availability in that treatment. Our results show, however, that the Rainfed trees were able to maintain similar values of Kp all throughout the dry season, which shows that the hydraulic efficiency of the xylem of those trees was little affected by embolism, despite of the high demanding conditions in the area.Ministry of Education and Science (Spain) AGL2006-04666/AGRUnión Europea STREP 327 02312

Using a process-based stomatal model in olive and its potential application to deficit irrigation studies

Over the last years modeling plant transpiration has been pointed out as a powerful tool to optimize the management of irrigation in fruit trees. In this study we tested the hydromechanical model of stomata functioning proposed by Buckley et al. (2003), a model with a strong physiological basis. The great contribution of this model is that its parameters have direct physiological meaning. Firstly, the model was simplified to make its parameters estimation friendly and easy. Secondly, the model was fitted to data obtained in a hedgerow olive orchard under regulated deficit irrigation. The hydromechanical model fitted our data satisfactorily and allowed us to analyze the physiological parameters obtained.Ministerio de Ciencia e Innovación español AGL2009-11310/AGRJunta de Andalucí

Modulación de la actividad nitrato reductasa de hojas de cebada

1. Los estudio de modulación in vivo de la NR foliar de cebada indican que la dependencia por luz y por CO2 de la actividad NR se debe a la operación de un sistema de control que determina el grado de actividad del enzima en función de la capacidad de fijación de CO2 de la hoja. 2. Los resultados obtenidos indican que la regulación de la NR por luz y CO2 está mediada por los niveles citoplasmáticos de hexosas-fosfato, concretamente de glucosa 6-fosfato, que actuaría como señal reguladora en la coordinación entre actividad NR y capacidad fotosintética de la hoja. 3. El mecanismo de inactivación in vivo por limitación de la fijación de CO2 de la NR foliar de cebada (y de especies agrupadas en este trabajo en el Tipo I) es independiente de Mg2+, mientras que el que opera sobre la NR de espinaca (y especies agrupadas en el Tipo II) requiere Mg2+ para ser efectivo. Adicionalmente, la NR foliar de todas las especies ensayadas se inhibe in vitro por iones Mg2+, siendo más sensible a esta inhibición el enzima extraído de hojas en oscuridad. Esta inhibición podría tener significado fisiológico puesto que se observa a concentraciones del ion similares a la que existen en el citoplasma. 4. Los azúcares-fosfato. Especialmente la glucosa 6-fosfato, protegen a la NR de la inactividad in vivo, por limitación de la fijación de CO2, y de la inhibición in vitro por iones Mg2+. 5. En conjunto, los resultados permiten proponer un modelo de control por carbono de la asimilación de nitrato. En condiciones de fijación activa de CO2, con altos niveles de hexosas-fosfato, la glucosa 6-fosfato interaccionaría con la NR produciendo una conformación resistente tanto al sistema de inactivación por modificación covalente y/o unión estable a una proteína inactivadora, como a la inhibición por iones Mg2+. En condiciones de oscuridad o carencia de CO2 la NR pasaría a una forma más sensible a la inactivación y a la inhibición por Mg2+

Lantana camara L.: A weed with great light-acclimation capacity

Plant invasions may be limited by low radiation levels in ecosystems such as forests. Lantana camara has been classified among the world's 10 worst weeds since it is invading many different habitats all around the planet. Morphological and physiological responses to different light fluxes were analyzed. L. camara was able to acclimate to moderately shaded environments, showing a high phenotypic plasticity. Morphological acclimation to low light fluxes was typified by increasing leaf size, leaf biomass, leaf area index and plant height and by reduced stomatal density and leaf thickness. Plants in full sunlight produced many more inflorescences than in shaded conditions. Physiological acclimation to low radiation levels was shown to be higher stomatal conductance, higher net photosynthetic rates and higher efficiency of photosystem II (PSII). L. camara behaves as a facultative shade-tolerant plant, being able to grow in moderately sheltered environments, however its invasion could be limited in very shady habitats. Control efforts in patchy environments should be mainly directed against individuals in open areas since that is where the production of seeds would be higher and the progress of the invasion would be faster