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Aims In plant eco-physiology, less negative (enriched) carbon 13 (13C) in the leaves indicates conditions of reducing leaf gas exchange through stomata, e.g. under drought. In addition, 13C is expected to be less negative in non-photosynthetic tissues as compared with leaves. However, these relationships in δ 13C from leaves (photosynthetic organs) to branches, stems and roots (non-photosynthetic organs) are rarely tested across multiple closely related tree species, multiple compartments, or in trees growing under extreme heat and drought. Methods We measured leaf-to-root 13C in three closely related desert acacia species (Acacia tortilis, A. raddiana and A. pachyceras). We measured δ 13C in leaf tissues from mature trees in southern Israel. In parallel, a 7-year irrigation experiment with 0.5, 1.0 or 4.0 L day−1 was conducted in an experimental orchard. At the end of the experiment, growth parameters and δ 13C were measured in leaves, branches, stems and roots. Important Findings The δ 13C in leaf tissues sampled from mature trees was ca. −27‰, far more depleted than expected from a desert tree growing in one of the Earth’s driest and hottest environments. Across acacia species and compartments, δ 13C was not enriched at all irrigation levels (−28‰ to ca. −27‰), confirming our measurements in the mature trees. Among compartments, leaf δ 13C was unexpectedly similar to branch and root δ 13C, and surprisingly, even less negative than stem δ 13C. The highly depleted leaf δ 13C suggests that these trees have high stomatal gas exchange, despite growing in extremely dry habitats. The lack of δ 13C enrichment in non-photosynthetic tissues might be related to the seasonal coupling of growth of leaves and heterotrophic tissues.

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.

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.

Tree growth enhancement under elevated [CO2] is much smaller than originally anticipated; yet carbon overabundance can lead to increased wood carbon storage and to stomatal downregulation and hence reduced water-use. Notably, all three outcomes increase tree drought resistance. Here we studied growth, water relations, and nonstructural carbohydrates of 60 lemon saplings growing in CO2-controlled rooms at the same greenhouse, under 400, 650, and 850 ppm [CO2]. At each [CO2] level, 10 saplings were exposed to 1-month dry-down after 2 months of standard irrigation, followed by re-watering for another month. The other 10 saplings served as controls. Under drought, tree growth was maintained at elevated, but not ambient, CO2, linked with mild vs. severe tree water stress (leaf water potential of −3.5 at elevated and −5.5 MPa at ambient [CO2]). Stomatal downregulation with increasing [CO2] meant that leaf transpiration and diurnal plant water-use were 13–46% lower at elevated vs. ambient [CO2] but photosynthesis was still 15–25% higher. CO2-induced increases in root and shoot starch were transient but significant. Our results suggest that when predicting tree growth in a warmer and drier future, concomitant atmospheric CO2 concentration must be considered. In young lemon trees, elevated CO2 partially compensated for drought effects on tree growth and water status, and might delay some of the effects of the anthropogenic climate change.

Lemon (Citrus limon) is a fruit tree with major agricultural importance around the Mediterranean basin and is considered to be highly drought resistant. In this study, we tested the effect of two months summer-desiccation on physiological and yield parameters of mature lemon trees growing under Mediterranean climate during three consecutive years. We also examined the efficiency of current irrigation regime, which is based on reference evapo-transpiration. We measured leaf gas exchange and water potential (Ѱl), monitored sap flow and soil moisture and followed flowering, fruit set and fruit size. Lemon trees showed an isohydric stomatal regulation, as stomata maintained leaf water potential >-2 MPa. Summer desiccation caused a gradual decrease in diurnal tree water use, starting immediately after cessation of irrigation, with leaf gas exchange practically halted at the end of the drought period. Tree function recovered following re-irrigation, and fruit yields were not reduced, but even mildly increased during the first year. In contrast, summer desiccation during two consecutive years caused long-term effects of tree activity decrease, significantly lower yield, main branch collapse and even tree mortality. Irrigation amounts matched closely tree water-use amounts; soil moisture was maintained around 26% (v/v); and irrigation responded dynamically to meteorological changes, indicating that current irrigation regime represents highly efficient water management. The lemon desiccation protocol relied on the physiological capacity of this species to avoid short-term drought effects through stomatal closure. Still, this protocol must be managed carefully, to reduce risk to trees and save yields.