Stomatal response to environmental conditions forms the backbone of all ecosystem and carbon cycle models, but is largely based on empirical relationships. Evolutionary theories of stomatal behaviour are critical for guarding against prediction errors of empirical models under future climates. Longstanding theory holds that stomata maximise fitness by acting to maintain constant marginal water use efficiency over a given time horizon, but a recent evolutionary theory proposes that stomata instead maximise carbon gain minus carbon costs/risk of hydraulic damage. Using data from 34 species that span global forest biomes, we find that the recent carbon-maximisation optimisation theory is widely supported, revealing that the evolution of stomatal regulation has not been primarily driven by attainment of constant marginal water use efficiency. Optimal control of stomata to manage hydraulic risk is likely to have significant consequences for ecosystem fluxes during drought, which is critical given projected intensification of the global hydrological cycle.

Climate change is expected to lead to increases in drought frequency and severity, with deleterious effects on many ecosystems. Stomatal responses to changing environmental conditions form the backbone of all ecosystem models, but are based on empirical relationships and are not well-tested during drought conditions. Here, we use a dataset of 34 woody plant species spanning global forest biomes to examine the effect of leaf water potential on stomatal conductance and test the predictive accuracy of three major stomatal models and a recently proposed model. We find that current leaf-level empirical models have consistent biases of over-prediction of stomatal conductance during dry conditions, particularly at low soil water potentials. Furthermore, the recently proposed stomatal conductance model yields increases in predictive capability compared to current models, and with particular improvement during drought conditions. Our results reveal that including stomatal sensitivity to declining water potential and consequent impairment of plant water transport will improve predictions during drought conditions and show that many biomes contain a diversity of plant stomatal strategies that range from risky to conservative stomatal regulation during water stress. Such improvements in stomatal simulation are greatly needed to help unravel and predict the response of ecosystems to future climate extremes.

Changes in leaf size, specific leaf area (SLA), transpiration and tissue water relations were studied in leaves of rooted cuttings of selected clones of Eucalyptus globulus Labill. subjected to well-watered or drought conditions in a greenhouse. Significant differences between clones were found in leaf expansion and transpiration. There was a significant clone x treatment interaction on SLA. Water stress significantly reduced osmotic potential at the turgor loss point (Pi0) and at full turgor (Pi100), and significantly increased relative water content at the turgor loss point and maximum bulk elastic modulus. Differences in tissue water relations between clones were significant only in the mild drought treatment. Among clones in the drought treatments, the highest leaf expansion and the highest increase in transpiration during the experiment were measured in those clones that showed an early and large decrease in Pi0 and Pi100.

Combining hydraulic- and carbon-related measurements helps to understand drought-induced plant mortality. Here, we investigated the role that plant respiration (R) plays in determining carbon budgets under drought. We measured the hydraulic conductivity of stems and roots, and gas exchange and nonstructural carbohydrate (NSC) concentrations of leaves, stems and roots of seedlings of two resprouting species exposed to drought or well-watered conditions: Ulmus minor (riparian tree) and Quercus ilex (dryland tree). With increasing water stress (occurring more rapidly in larger U. minor), declines in leaf, stem and root R were less pronounced than that in leaf net photosynthetic CO uptake (P ). Daytime whole-plant carbon gain was negative below -4 and -6 MPa midday xylem water potential in U. minor and Q. ilex, respectively. Relative to controls, seedlings exhibiting shoot dieback suffered c. 80% loss of hydraulic conductivity in both species, and reductions in NSC concentrations in U. minor. Higher drought-induced depletion of NSC reserves in U. minor was related to higher plant R, faster stomatal closure, and premature leaf-shedding. Differences in drought resistance relied on the ability to maintain hydraulic conductivity during drought, rather than tolerating conductivity loss. Root hydraulic failure elicited shoot dieback and precluded resprouting without root NSC reserves being apparently limiting for R.

Selecting for improved growth in drought-prone environments may lead to a trade-off between traits related to both growth and drought resistance. This paper provides a review of the use of physiological traits in breeding for yield improvement in water-limited environments. Plant responses to water stress can be grouped into strategies, which are species-specific. Once a species drought resistance strategy is known, several physiological traits can be selected to be used in the species breeding program. Despite there are multiple physiological traits involved in plant responses to drought, only a few of them have been actually used to improve both growth and drought resistance. This review highlights the relevance of stomatal conductance and water-use efficiency as well as hydraulic traits in breeding for improved growth in drought-prone environments. It is illustrated with unpublished results obtained in several research programmes that share a common objective: improving Eucalyptus globulus growth and survival in south-western Spain throughout early selection based on morpho-physiological traits.Key words: Drought tolerance strategies, stomatal conductance, water use efficiency, hydraulic conductivity, xylem vulnerability, Tasmanian blue gum. ResumenUtilización de parámetros fisiológicos en programas de mejora del crecimiento bajo déficit hídrico La selección de genotipos con el objetivo de mejorar el crecimiento bajo suministro hídrico limitante exige buscar un equilibrio entre aquellos caracteres que permiten optimizar el crecimiento y los que confieren mayor resistencia a la sequía. En el presente artículo se realiza una revisión del uso de parámetros fisiológicos en programas de mejora establecidos para zonas donde el agua es factor limitante. Las plantas muestran un amplio abanico de respuestas frente al déficit hídrico, que se pueden agrupar en estrategias. Una vez conocida la estrategia de respuesta al estrés hídrico en una especie, se puede abordar la selección parámetros fisiológicos a utilizar como herramientas de mejora. Entre estos parámetros destacan por su interés la conductancia estomática, la eficiencia en el uso del agua y parámetros hidráulicos. La revisión del uso de estos parámetros se ha ilustrado con datos bibliográficos y datos obtenidos en el marco de varios proyectos de investigación, establecidos con un objetivo primordial: mejorar la productividad de las plantaciones de Eucalyptus globulus en el suroeste español, mediante programas de mejora que incorporen criterios de selección precoz basados en parámetros morfo-fisiológicos.Palabras clave: Estrategias de tolerancia a la sequía, conductancia estomática, eficiencia en el uso del agua, conductividad hidráulica, vulnerabilidad a la cavitación, eucalipto.