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International audience; Lycophytes are the earliest diverging extant lineage of vascular plants, sister to all other vas-cular plants. Given that most species are adapted to ever-wet environments, it has been hypothesized that lycophytes, and by extension the common ancestor of all vascular plants, have few adaptations to drought. We investigated the responses to drought of key fitness-related traits such as stomatal regulation , shoot hydraulic conductance (K shoot) and stem xylem embolism resistance in Selaginella haematodes and S. pulcherrima, both native to tropical understory. During drought stomata in both species were found to close before declines in K shoot , with a 50% loss of K shoot occurring at À1.7 and À2.5 MPa in S. haematodes and S. pulcherrima, respectively. Direct observational methods revealed that the xylem of both species was resistant to embolism formation, with 50% of embolized xylem area occurring at À3.0 and À4.6 MPa in S. haematodes and S. pulcherrima, respectively. X-ray microcomputed tomogra-phy images of stems revealed that the decline in K shoot occurred with the formation of an air-filled lacuna, disconnecting the central vascular cylinder from the cortex. We propose that embolism-resistant xylem and large capacitance, provided by collapsing inner cortical cells, is essential for Selaginella survival during water deficit.

Xylem embolism impairs hydraulic conductivity in trees and drives drought-induced mortality. While embolism has been monitored in vivo in potted plants, and research has revealed evidence of embolism in field-grown trees, continuous in situ monitoring of cavitation in forests is lacking. Seasonal patterns of embolism were monitored in branchlets of Aleppo pine (Pinus halepensis) trees growing in a dry Mediterranean forest. Optical visualization (OV) sensors were installed on terminal branches, in addition to monthly sampling for micro computed tomography scans. We detected 208 cavitation events among four trees, which represented an embolism increase from zero to c. 12% along the dry season. Virtually all the cavitation events occurred during daytime hours, with 77% occurring between 10:00 and 17:00 h. The probability for cavitation in a given hour increased as vapor pressure deficit (VPD) increased, up to a probability of 42% for cavitation when VPD 5 kPa. The findings uniquely reveal the instantaneous environmental conditions that lead to cavitation. The increased likelihood of cavitation events under high VPD in water-stressed pines is the first empirical support for this long hypothesized relationship. Our observations suggest that low levels of embolism are common in Aleppo pine trees at the dry edge of their distribution.

Trees of the genus Prunus produce some of the most widely consumed fruits globally. The combination of climate change-related warming and increased drought stress, scarcity of freshwater resources for irrigation, and increasing demands due to population growth creates a need for increased drought tolerance in these tree species. Recently, we have shown in the field that a native wild pear species performs better under drought than two cultivated pear species. Here, a comparative field study was conducted in Israel to investigate traits associated with drought tolerance in almond (cultivated Prunus dulcis (Mill.) D. A. Webb vs wild Prunus ramonensis Danin) and plum (cultivated Prunus domestica L. vs wild Prunus ursina Kotschy). Measurements of xylem embolism and shoot and root carbon reserves were done along a year, including seasonal drought in the wild and a 35-day drought experiment in the orchards. Synchronous measurements of native xylem embolism and shoot water potential showed that cultivated and wild almond trees lost ~50% of hydraulic conductivity at −2.3 and −3.2 MPa, respectively. Micro-CT images confirmed the higher embolism ratio in cultivated versus wild almond, whereas the two plum species were similar. Dynamics of tissue concentrations of nonstructural carbohydrates were mostly similar across species, with higher levels in cultivated versus wild plum. Our results indicate an advantage for the wild almond over its cultivated relative in terms of xylem resistance to embolism, a major risk factor for trees under drought stress. This result is in line with our previous experiment on pear species. However, the opposite trends observed among the studied plum species mean that these trends cannot be generalized. It is possible that the potential for superior drought tolerance in wild tree species, relative to their cultivated relatives, is limited to wild species from dry and hot habitats.

International audience

Summary Land surface models (LSMs) typically use empirical functions to represent vegetation responses to soil drought. These functions largely neglect recent advances in plant ecophysiology that link xylem hydraulic functioning with stomatal responses to climate.We developed an analytical stomatal optimization model based on xylem hydraulics (SOX) to predict plant responses to drought. Coupling SOX to the Joint UK Land Environment Simulator (JULES) LSM, we conducted a global evaluation of SOX against leaf‐ and ecosystem‐level observations.SOX simulates leaf stomatal conductance responses to climate for woody plants more accurately and parsimoniously than the existing JULES stomatal conductance model. An ecosystem‐level evaluation at 70 eddy flux sites shows that SOX decreases the sensitivity of gross primary productivity (GPP) to soil moisture, which improves the model agreement with observations and increases the predicted annual GPP by 30% in relation to JULES. SOX decreases JULES root‐mean‐square error in GPP by up to 45% in evergreen tropical forests, and can simulate realistic patterns of canopy water potential and soil water dynamics at the studied sites.SOX provides a parsimonious way to incorporate recent advances in plant hydraulics and optimality theory into LSMs, and an alternative to empirical stress factors., See also the Commentary on this article by Anderegg & Venturas, 226: 1535–1538.