Natural selection favours drought escape and an acquisitive resource‐use strategy in semiarid Mediterranean shrubs

Seasonal Shifts in Tree Water Use and Non-Structural Carbohydrate Storage in a Tropical Dry Forest

Predictions of increased drought frequency and intensity have the potential to threaten to forest globally. The key to trees response to drought is an understanding of tree water use and carbohydrate storage. Our objective was to evaluate sap velocity and dynamics of non-structural carbohydrates (NSC) in native trees of a dry tropical forest, during rainy and drought periods. We evaluated six key species of the Caatinga: three deciduous species with low wood density (WD), two deciduous species with high WD and one evergreen species during the rainy and dry periods. We measured sap velocity, xylem water potential, stomatal conductance, phenology and NSC. We found that the evergreen specie had higher sap velocity and frequent NSC production. While the low deciduous WD species showed low sap velocity, store water and NSC mainly in the stem and roots, and have leaf sprouting and flowering at the end of the dry period. The deciduous high WD also showed low sap velocity, however, with low stored NSC. These results suggest that under longer dry seasons and an irregular rainy seasons, species with low WD that use part of the stored NSC to resprout still during dry season may be the most affected.

Trait coordination and trade-offs constrain the diversity of water use strategies in Mediterranean woody plants

The diversity of water-use strategies among dryland plants has been the focus of extensive research, but important knowledge gaps remain. Comprehensive surveys of water-use traits encompassing multiple species growing at contrasting sites are needed to further advance current understanding of plant water use in drylands. Here we show that ecohydrological niche segregation driven by differences in water uptake depth among coexisting species is widespread across Mediterranean plant communities, as evidenced by soil and stem water isotopes measured in 62 native species growing at 10 sites with contrasting climatic conditions. Foliar carbon and oxygen isotopes revealed that leaf-level stomatal regulation stringency and water-use efficiency also differ markedly among coexisting species, and are both coordinated with water uptake depth. Larger and taller woody species use a greater proportion of deeper soil water, display more conservative water use traits at leaf level ("water-savers") and show greater investment in foliage relative to shoots. Conversely, smaller species rely mainly on shallow soil water, exhibit a more profligate water use strategy ("water-spenders") and prioritize investment in shoots over foliage. Drought stress favours coordination between above and belowground water-use traits, resulting in unavoidable trade-offs that constrain the diversity of whole-plant water use strategies in Mediterranean plant communities.

The flower does not open in the city: evolution of plant reproductive traits of Portulaca oleracea in urban populations

BACKGROUND AND AIMS

The impact of urbanization on plant evolution, particularly the evolution of reproductive traits, remains largely unknown. In this study, we aimed to investigate the consequences of urbanization on the reproductive traits of Portulaca oleracea in the Kantō region of Japan. Portulaca oleracea has a unique cleistogamous reproductive system, which consists of genetically determined chasmogamous (open, CH) and cleistogamous (closed, CL) plants.

METHODS

We collected seeds of P. oleracea from ten populations in rural areas and ten populations in urban areas. In a common garden experiment, we recorded the type of flowers (CH or CL), reproductive phenology and seed production.

KEY RESULTS

All individuals produced either CH or CL flowers, allowing us to classify them as either CH or CL plants. We observed a significant difference in the prevalence of CH and CL plants between rural and urban populations: the number of CH plants was generally low and was particularly low among urban individuals. Compared to CH plants, CL plants showed earlier phenology and produced heavier seeds, which is consistent with stress avoidance in response to heat and drought stress conditions in urban areas.

CONCLUSIONS

Our findings suggest that urbanization may drive an evolutionary change in the cleistogamous reproductive system of P. oleracea. CL plants with earlier phenology and larger seeds might be better adapted to urban environments, where they are subjected to harsh heat and drought stress.

Dehydration tolerance rather than avoidance explains drought resistance in zoysiagrass

Irrigation of grasses dominates domestic water use across the globe, and better understanding of water use and drought resistance in grasses is of undeniable importance for water conservation. Breeding programs have released cultivars with improved drought resistance, but the underlying mechanisms remain unknown. We sought to characterize the mechanisms driving drought resistance in four zoysiagrass cultivars (Lobo, Zeon, Empire, and Meyer) reported to exhibit contrasting levels of drought resistance. A dry-down was performed through deficit irrigation until 70% decline in evapotranspiration. All cultivars exhibited similar drought avoidance as they dehydrated similarly throughout the drought. Lobo and Zeon, however, exhibited a 70% decline in evapotranspiration two to three days after Empire and Meyer, thus experiencing lower water potentials. Regarding drought tolerance, Lobo and Zeon maintained higher normalized difference vegetation index (NDVI) and lower perceived canopy mortality at higher dehydration levels than Empire and Meyer. We use "perceived" because visual assessments of canopy mortality are influenced by drought-induced leaf rolling. During the recovery, leaves rehydrated and unrolled, so the "actual" canopy mortality could be evaluated. All cultivars exhibited similar mortality on the first recovery day despite Lobo and Zeon experiencing more severe dehydration. Throughout the recovery, Lobo and Empire exhibited faster re-growth and showed the lowest canopy mortality, and Lobo exhibited the highest NDVI. The improved drought resistance of Lobo and Zeon results from greater dehydration tolerance rather than avoidance. This study has implications for lawn owners selecting the best cultivars and for breeding programs aiming at improving drought resistance of zoysiagrasses.

Ecotypic differentiation of leaf silicon concentration in the grass Brachypodium hybridum along a rainfall gradient

Ecotypic differentiation, reflected in substantial trait differences across populations, has been observed in various plant species distributed across aridity gradients. Nevertheless, ecotypic differentiation in leaf silicon concentration, known to alleviate drought stress in plants, remained hardly explored. Here, we provide a systematic test for ecotypic differentiation in leaf silicon concentration along two aridity gradients in the grass Brachypodium hybridum in Israel. Seed material was sampled in 15 sites along a macroclimatic aridity gradient (89 - 926 mm mean annual rainfall) and from corresponding north (moister) and south (more arid) exposed slopes (microclimatic gradient) at similar altitudes (mean north: 381 m a.s.l., mean south: 385 m a.s.l.). Plants were subsequently grown under common conditions and their leaf silicon concentration was analysed. Leaf silicon concentration increased with increasing aridity across the macroclimatic gradient, but did not differ between north and south slopes. The higher leaf silicon concentrations under more arid conditions can enhance the ability of plants to cope with more arid conditions by two mutually not exclusive mechanisms: (i) withstanding drought by reducing water loss and increasing water uptake or (ii) escaping drought by facilitating fast growth. Our study highlights that leaf silicon concentration contributes to ecotypic differentiation in annual grasses along macroclimatic aridity gradients.

Transgenerational plasticity to drought: contrasting patterns of non-genetic inheritance in two semi-arid Mediterranean shrubs

BACKGROUND AND AIMS

Intra- and transgenerational plasticity may provide substantial phenotypic variation to cope with environmental change. Since assessing the unique contribution of the maternal environment to the offspring phenotype is challenging in perennial, outcrossing plants, little is known about the evolutionary and ecological implications of transgenerational plasticity and its persistence over the life cycle in these species. We evaluated how intra- and transgenerational plasticity interplay to shape the adaptive responses to drought in two perennial Mediterranean shrubs.

METHODS

We used a novel common garden approach that reduced within-family genetic variation in both the maternal and offspring generations by growing the same maternal individual in two contrasting watering environments, well-watered and drought, in consecutive years. We then assessed phenotypic differences at the reproductive stage between offspring reciprocally grown in the same environments.

KEY RESULTS

Maternal drought had an effect on offspring performance only in Helianthemum squamatum. Offspring of drought-stressed plants showed more inflorescences, less sclerophyllous leaves and higher growth rates in both watering conditions, and heavier seeds under drought, than offspring of well-watered maternal plants. Maternal drought also induced similar plasticity patterns across maternal families, showing a general increase in seed mass in response to offspring drought, a pattern not observed in the offspring of well-watered plants. In contrast, both species expressed immediate adaptive plasticity, and the magnitude of intragenerational plasticity was larger than the transgenerational plastic responses.

CONCLUSIONS

Our results highlight that adaptive effects associated with maternal drought can persist beyond the seedling stage and provide evidence of species-level variation in the expression of transgenerational plasticity. Such differences between co-occurring Mediterranean species in the prevalence of this form of non-genetic inheritance may result in differential vulnerability to climate change.

Early human selection of crops' wild progenitors explains the acquisitive physiology of modern cultivars

Crops have resource-acquisitive leaf traits, which are usually attributed to the process of domestication. However, early choices of wild plants amenable for domestication may also have played a key role in the evolution of crops' physiological traits. Here we compiled data on 1,034 annual herbs to place the ecophysiological traits of 69 crops' wild progenitors in the context of global botanical variation, and we conducted a common-garden experiment to measure the effects of domestication on crop ecophysiology. Our study found that crops' wild progenitors already had high leaf nitrogen, photosynthesis, conductance and transpiration and soft leaves. After domestication, ecophysiological traits varied little and in idiosyncratic ways. Crops did not surpass the trait boundaries of wild species. Overall, the resource-acquisitive strategy of crops is largely due to the inheritance from their wild progenitors rather than to further breeding improvements. Our study concurs with recent literature highlighting constraints of crop breeding for faster ecophysiological traits.

Long-term rain exclusion in a Mediterranean forest: response of physiological and physico-chemical traits of Quercus pubescens across seasons

With climate change, an aggravation in summer drought is expected in the Mediterranean region. To assess the impact of such a future scenario, we compared the response of Quercus pubescens, a drought-resistant deciduous oak species, to long-term amplified drought (AD) (partial rain exclusion in natura for 10 years) and natural drought (ND). We studied leaf physiological and physico-chemical trait responses to ND and AD over the seasonal cycle, with a focus on chemical traits including major groups of central (photosynthetic pigments and plastoquinones) and specialized (tocochromanols, phenolic compounds, and cuticular waxes) metabolites. Seasonality was the main driver of all leaf traits, including cuticular triterpenoids, which were highly concentrated in summer, suggesting their importance to cope with drought and thermal stress periods. Under AD, trees not only reduced CO2 assimilation (-42%) in summer and leaf concentrations of some phenolic compounds and photosynthetic pigments (carotenoids from the xanthophyll cycle) but also enhanced the levels of other photosynthetic pigments (chlorophylls, lutein, and neoxanthin) and plastochromanol-8, an antioxidant located in chloroplasts. Overall, the metabolomic adjustments across seasons and drought conditions reinforce the idea that Q. pubescens is highly resistant to drought although significant losses of antioxidant defenses and photoprotection were identified under AD.

How to improve scaling from traits to ecosystem processes

Scaling approaches in ecology assume that traits are the main attributes by which organisms influence ecosystem functioning. However, several recent empirical papers have found only weak links between traits and ecosystem functioning, questioning the usefulness of trait-based ecology (TBE). We argue that these studies often suffer from one or more widespread misconceptions. Specifically, these studies often (i) conflict with the conceptual foundations of TBE, (ii) lack theory- or hypothesis-driven selection and use of traits, (iii) tend to ignore intraspecific variation, and (iv) use experimental or study designs that are not well suited to make strong tests of TBE assumptions. Addressing these aspects could significantly improve our ability to scale from traits to ecosystem functioning.