Morphological and physiological responses of Galapagos endemic tree Croton scouleri to site conditions varying through its altitudinal range

: Galapagos is a natural laboratory offering a great opportunity to study plants functional traits. This study characterises morphological and ecophysiological responses of Croton scouleri, an endemic tree that habits from humid and upper elevations to semiarid lowlands, throughout an altitudinal gradient and in a manipulative experiment. Croton scouleri trees were gradually smaller with less total leaf area due to a gradual reduction in mean leaf size, and they folded their leaves at lower elevations. These results were also recorded after cutting every deep root. Two physiological traits that allowed Croton scouleri to avoid damages to the photosynthetic apparatus were detected between 30 and 150 m a.s.l. Lower variable fluorescence (Fv) and basal fluorescence (F0) keeping constant maximum photochemical efficiency of PSII (Fv/Fm) denoted a drop in chlorophyll concentration. Concomitantly, the recorded increase in the Quantum efficiency of PSII ( PSII) with similar Fv/Fm means that Croton scouleri could be using cyclic electron transport as photoprotective mechanism. On the other hand, a deep root system to reach the water table allowed Croton scouleri to behave as a drought-avoider, which was reflected in: (1) unvarying water status Leaf Water Content and Relative Water Content were always higher than 69 and 58%, respectively; (2) stable and low photoinhibition levels; and (3) unvarying leaf area index. However, Croton scouleri was not able to avoid drought at altitudes lower 30 m a.s.l. where similar responses to those recorded after root cutting were recorded.Junta de Andalucía AI60/0

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

Wetland Loss by Erosion in Odiel Marshes (SW Spain)

Wetland loss in many estuaries around the world, has been attributed mainly to undermining and collapse of channel banks. This study aims to quantify bank erosion and vertical erosion/accretion rates on intertidal sediments in the Odiel tidal marshes (SW Spain). Bi-monthly erosion/accretion measurements were taken on eight channels over a four year period, using markers (iron stakes) located on intertidal areas and on eroding banks. The intensity of erosion divides the Odiel marshes into two zones. The northern zone has low erosion rates (horizontal erosion c. -20 cm year-1 and vertical erosion / accretion between 0 and -1 cm year-1), and coincides with low levels of human activities. The southern zone has higher erosion rates (horizontal erosion c. -25 cm year-1 and vertical erosion / accretion between 0 and -5 cm year-1) and exhibits higher levels of anthropogenic pressure. The highest horizontal and vertical erosion rates (c. -80 cm year-1) were recorded on navigation channels. Horizontal and vertical erosion showed a positive linear relationship (r2 = 0.66; P< 0.01), indicating that sediments mobilized by bank erosion are not deposited on adjacent intertidal areas. Erosion led to mature marsh habitat loss of c. 17000 m2 year-1 and a sediment mobilization of c. 16500 m3 year-1

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

Monitoring of emerging water stress situations by thermal and vegetation indices in different almond cultivars

In recent years, the area dedicated to modern irrigated almond plantations has increased significantly in Spain. However, the legal irrigation allocations are lower than the maximum water requirements of the crop in most cases. Therefore, almond growers are forced to implement regulated deficit irrigation strategies on their farms, applying water stress in certain resistant phenological periods and avoiding it in sensitive periods. Given the need to monitor the water status of the crop, especially in the most sensitive periods to water stress, the objective of this work was to evaluate the sensitivity of two UAV-based crop water status indicators to detect early water stress conditions in four almond cultivars. The field trial was conducted during 2020 in an experimental almond orchard, where two irrigation strategies were established: full irrigation (FI), which received 100% of irrigation requirements (IR), and regulated deficit irrigation (RDI), which received 70% of IR during the whole irrigation period except during the kernel-filling stage when received 40% IR. The UAV flights were performed on four selected dates of the irrigation season. The Crop Water Status Index (CWSI) and the Normalized Difference Vegetation Index (NDVI) were derived from thermal and multispectral images, respectively, and compared to classical water status indicators, i.e., stem water potential (Ψstem ), stomatal conductance (gs ), and photosynthetic rate (AN ). Of the four flights performed, three corresponded to mild water stress conditions and a single flight was performed under moderate water stress conditions. Under mild water stress, CWSI was not able to capture the differences between FI and RDI trees that were observed with Ψstem . Under moderate stress conditions, CWSI was sensitive to the water deficit reached in the trees and showed significant differences among both irrigation treatments. No differences were observed in the CWSI and NVDI response to water stress among cultivars. Although NDVI and CWSI were sensitive to water stress, the low signal intensity observed in NDVI makes this index less robust than CWSI to monitor crop water stress. It can be concluded that UAV-based CWSI measurements are reliable to monitor almond water status, although for early (mild) levels of water stress, Ψstem seems to be the preferred option.Junta de Andalucía AVA.AVA2019.05

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