Plant hydraulics at the heart of plant, crops and ecosystem functions in the face of climate change

New crops on the block: effective strategies to broaden our food, fibre, and fuel repertoire in the face of increasingly volatile agricultural systems

Climate change poses significant challenges to our ability to keep a growing global population fed, clothed, and fuelled. This review sets the scene by summarizing the impacts of climate change on production of the major grain crop species rice, wheat, and maize, with a focus on yield reductions due to abiotic stresses and altered disease pressures. We discuss efforts to improve resilience, emphasizing traits such as water use efficiency, heat tolerance, and disease resistance. We move on to exploring production trends of established, re-emerging, and new crops, highlighting the challenges of developing and maintaining new arrivals in the global market. We analyse the potential of wild relatives for improving domesticated crops, or as candidates for de novo domestication. The importance of pangenomes for uncovering genetic variation for crop improvement is also discussed. We examine the impact of climate change on non-cereals, including fruit, nut, and fibre crops, and the potential of alternative multiuse crops to increase global sustainability and address climate change-related challenges. Agave is used as an exemplar to demonstrate the strategic pathway for developing a robust new crop option. There is a need for sustained investment in research and development across the entire value chain to facilitate the exploration of diverse species and genetic resources to enhance crop resilience and adaptability to future environmental conditions.

Minimum leaf conductance during drought: unravelling its variability and impact on plant survival

Leaf water loss after stomatal closure is key to understanding the effects of prolonged drought on vegetation. It is therefore important to accurately quantify such water losses to improve physiology-based models of drought-induced plant mortality. We measured water loss of detached leaves continuously during dehydration in nine woody angiosperm species. We computed minimum leaf conductance (gmin) at different water potential thresholds along a sequence of physiological function losses, spanning from turgor loss point to hydraulic failure. A mechanistic model evaluated the impact of different gmin estimations on the time to hydraulic failure (THF). Residual conductance is not steady and decreases continuously at varying rates across species during the entire dehydration process, even after correcting for leaf shrinkage and vapor pressure deficit shifts. Different estimations of gmin had a significant impact on the THF predicted by the model, especially for drought-resistant species. We demonstrate that residual conductance is variable during dehydration, and thus, it is important to use physiological or water status boundaries for its estimation in order to determine distinct gmin values of water loss. We describe an accurate, repeatable and open-source methodology to estimate gmin. Such methodology could enhance models of plant mortality under drought.

Physiological Traits Combination Shapes Common Strategies of Water and Carbon Use Regulation Across Fruit Tree Species

Crop plants, including fruit trees, are particularly vulnerable to water scarcity because past selection prioritized productivity over drought resistance, making it challenging to maintain productivity with minimal water use in the context of climate change. This study aims to determine which trait combination of 10 fruit tree species influences their water and carbon use, with the goal of understanding their adaptability to water scarcity. The results showed that water stress traits (turgor loss point, TLP; vulnerability index, VI), a carbon-related trait (specific leaf area; SLA), and a biomass allocation trait (Huber value; Hv) define the major axis of variability and present the strongest correlations with other traits. Two distinct strategies emerged: the first, mainly around Prunus species, was characterized by high Hv, low SLA, more negative TLP, and low VI, indicating greater water-stress tolerance due to sapwood redundancy and reduced organ vulnerability. They also exhibited higher maximum photosynthetic rates, indicating greater assimilation rates. The second strategy, mainly including Citrus species, exhibited opposite traits and trends. These trait combinations were likely shaped by shared ancestry and environmental factors. Understanding these correlations can guide irrigation practices and the selection of resilient species, contributing to more robust agricultural systems in a changing climate scenario.

Decreased root hydraulic traits in German winter wheat cultivars over 100 years of breeding

Wheat (Triticum aestivum L.) plays a vital role in global food security, and understanding its root traits is essential for improving water uptake under varying environmental conditions. This study investigated how over a century of breeding has influenced root morphological and hydraulic properties in 6 German winter wheat cultivars released between 1895 and 2002. Field and hydroponic experiments were used to measure root diameter, root number, branching density, and whole root system hydraulic conductance (Krs). The results showed a significant decline in root axes number and Krs with release year, while root diameter remained stable across cultivars. Additionally, dynamic functional-structural modeling using the whole-plant model CPlantBox was employed to simulate Krs development with root system growth, revealing that older cultivars consistently had higher hydraulic conductance than modern ones. The combined approach of field phenotyping and modeling provided a comprehensive view of the changes in root traits arising from breeding. These findings suggest that breeding may have unintentionally favored cultivars with smaller root systems and more conservative water uptake strategies under the high-input, high-density conditions of modern agriculture. The results of this study may inform future breeding efforts aimed at optimizing wheat root systems, helping to develop cultivars with water uptake strategies better tailored to locally changing environmental conditions.

Greater Resistance to Drought-Induced Embolism Is Linked to Higher Yield Maintenance in Soybean

With increasing drought events worldwide, crop breeding must focus on drought resistance to maintain crop yields. To ensure a high level of gas exchange and growth, plants need to maintain the integrity of their vascular system under drought conditions. While the impact of drought-induced vascular damage on tree species is well-documented, its effect on the yield of annual crops like soybean (Glycine max (L.) Merrill) remains unknown. We investigated xylem vulnerability to embolism of ten soybean cultivars with contrasting phylogenetic origins and phenology using the optical technique. With X-ray micro-tomography, we assessed xylem vulnerability across the plant vascular pathway to quantify the vulnerability segmentation. Our results revealed that soybean is moderately vulnerable to xylem embolism (mean leaf P50 = -1.85 MPa), with a significant Intraspecific variability with a difference of 1 MPa between the most extreme cultivars. Cultivars with higher leaf embolism resistance maintained higher yields in the field, particularly during dry years, highlighting the critical role of xylem hydraulic failure during drought in crop yield. This study provides new insights into the importance of hydraulic traits underlying drought tolerance in soybeans and their incorporation into breeding programmes for embolism resistance to improve yield resilience.

A low-cost protocol for the optical method of vulnerability curves to calculate P 50

Premise

The quantification of plant drought resistance, particularly embolism formation, within and across species, is critical for ecosystem management and agriculture. We developed a cost-effective protocol to measure the water potential at which 50% of hydraulic conductivity (P 50) is lost in stems, using affordable and accessible materials in comparison to the traditional optical method.

Methods and Results

Our protocol uses inexpensive USB microscopes, which are secured along with the plants to a pegboard base to avoid movement. A Python program automatized the image acquisition. This method was applied to quantify P 50 in an exotic species (Nicotiana glauca) and native species (Rhus integrifolia) of the Mediterranean vegetation in Baja California, Mexico.

Conclusions

The intra- and interspecific patterns of variation in stem P 50 of N. glauca and R. integrifolia were obtained using the low-cost optical method with widely available and affordable materials that can be easily replicated for other species.

Delayed Vegetation Mortality After Wildfire: Insights from a Mediterranean Ecosystem

Wildfires, one of the most important ecological disturbances, influence the composition and dynamics of ecosystems all around the world. Changes in fire regimes brought on by climate change are making their effects worse by increasing the frequency and size of fires. This study examined the issue of delayed mortality at the species and community levels, concentrating on Mediterranean forests dominated by Quercus ilex and Quercus suber. This research examined areas lacking spectral recovery following a megafire, which, although relatively small compared to the total burned area, represented significant ecological disturbances. The results highlighted distinct post-fire dynamics at both the woodland and species levels. Q. ilex experienced higher delayed mortality, particularly in areas of lower fire severity (NR), likely due to increased intra-specific competition. Because of its thick bark, which offers stronger fire resistance and encourages regeneration even in high-severity fire zones (HR), Q. suber showed greater resilience. Responses from the shrub layer varied, and some species, such as Pteridium aquilinum and Cytisus villosus, showed post-fire proliferation. To improve our knowledge of ecosystem resilience and guide forest management in fire-prone areas, these findings highlight the intricacy of post-fire ecological processes and the need to integrate species-specific features with more general community-level patterns.

Does Pre-Acclimation Enhance the Tolerance of Quercus ilex and Arbutus unedo Seedlings to Drought?

Mediterranean forests are severely threatened by increasing seedling mortality due to harsh environmental conditions, especially drought. In this study, we investigate whether seedlings of Quercus ilex and Arbutus unedo, previously exposed to water deficit, acquired tolerance to summer drought. Seedlings of the two species were grown from April to September in a plastic tunnel greenhouse and exposed to two irrigation regimes (control, 100% water holding capacity; water-stressed, 50% of control). In mid-August, the irrigation of all plants was suspended for three weeks. The response of the species was analyzed to evaluate survival, growth, ecological, and anatomical traits of wood produced under stressful conditions and marked through the pinning technique. The results suggest that both species show pre-acclimation to drought, with Q. ilex demonstrating a marked increase in survival percentage. This is likely due to a reduction in vessel size in response to previous water stress. In contrast, in A. unedo, the higher frequency of narrower vessels allowed safer water transport compared to Q. ilex, thus explaining the slight increase in survival. Overall results indicated that the two species adopt different strategies to overcome drought, providing valuable insights for managing seedlings in natural ecosystems and urban green spaces.

Trade-Offs Between Hydraulic Efficiency and Safety in Cotton (Gossypium hirsutum L.) Stems Under Elevated CO2 and Salt Stress

Plants respond to environmental changes by altering the anatomical structure of the xylem and its hydraulic properties. While numerous studies have explored the effects of individual environmental factors on crops, the combined interactions of these factors remain underexplored. As climate change intensifies, the occurrence of salt stress is becoming more frequent, alongside a rise in atmospheric CO2 concentration. This study aims to investigate the effects of elevated CO2 and salt stress on the hydraulic traits and xylem anatomical structures of cotton stems. Potted cotton plants were exposed to different CO2 concentrations (aC: 400 ppm; eC: 800 ppm) and salinity levels (aS: 0‱ soil salinity; eS: 6‱ soil salinity). The study found that under eC and eS conditions, a trade-off exists between hydraulic efficiency and safety in cotton stems, which may be partially attributed to xylem anatomical structures. Specifically, eS significantly reduced stem hydraulic conductivity under aC conditions and decreased vessel diameter but increased the proportion of small-diameter vessels and enhanced implosion resistance ((t/b)2), which strengthened the xylem's resistance to salt-induced embolism. eC altered the response pattern of xylem hydraulic conductivity and embolism resistance to salt stress, with increased vessel diameter enhancing hydraulic conductivity but reducing xylem resistance to embolism. These findings enhance our comprehension of plant hydraulic adaptation under future climatic conditions and provide new insights into the trade-offs between xylem structure and function.

Contrasting drought tolerance traits of woody plants is associated with mycorrhizal types at the global scale

It is well-known that the mycorrhizal type of plants correlates with different modes of nutrient cycling and availability. However, the differences in drought tolerance between arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) plants remains poorly characterized. We synthesized a global dataset of four hydraulic traits associated with drought tolerance of 1457 woody species (1139 AM and 318 EcM species) at 308 field sites. We compared these traits between AM and EcM species, with evolutionary history (i.e. angiosperms vs gymnosperms), water availability (i.e. aridity index) and biomes considered as additional factors. Overall, we found that evolutionary history and biogeography influenced differences in hydraulic traits between mycorrhizal types. Specifically, we found that (1) AM angiosperms are less drought-tolerant than EcM angiosperms in wet regions or biomes, but AM gymnosperms are more drought-tolerant than EcM gymnosperms in dry regions or biomes, and (2) in both angiosperms and gymnosperms, variation in hydraulic traits as well as their sensitivity to water availability were higher in AM species than in EcM species. Our results suggest that global shifts in water availability (especially drought) may alter the biogeographic distribution and abundance of AM and EcM plants, with consequences for ecosystem element cycling and ultimately, the land carbon sink.

Seasonal changes in hydraulic functions of eight temperate tree species: divergent responses to freeze-thaw cycles in spring and autumn

Freeze-thaw cycles (FTCs) are the major seasonal environment stress in the temperate and boreal forests, inducing hydraulic dysfunction and limiting tree growth and distribution. There are two types of FTCs in the field: FTCs with increasing temperature from winter to spring (spring FTCs); and FTCs with decreasing temperature from autumn to winter (autumn FTCs). While previous studies have evaluated the hydraulic function during the growing season, its seasonal changes and how it adapts to different types of FTCs remain unverified. To fill this knowledge gap, the eight tree species from three wood types (ring- and diffuse-porous, tracheid) were selected in a temperate forest undergoing seasonal FTCs. We measured the branch hydraulic traits in spring, summer, autumn, and early, middle and late winter. Ring-porous trees always showed low native hydraulic conductance (Kbranch), and high percentage loss of maximum Kbranch (PLCB) and water potential that loss of 50% maximum Kbranch (P50B) in non-growing seasons (except summer). Kbranch decreased, and PLCB and P50B increased in diffuse-porous trees after several spring FTCs. In tracheid trees, Kbranch decreased after spring FTCs while the P50B did not change. All sampled trees gradually recovered their hydraulic functions from spring to summer. Kbranch, PLCB and P50B of diffuse-porous and tracheid trees were relatively constant after autumn FTCs, indicating almost no effect of autumn FTCs on hydraulic functions. These results suggested that hydraulic functions of temperate trees showed significant seasonal changes, and spring FTCs induced more hydraulic damage (except ring-porous trees) than autumn FTCs, which should be determined by the number of FTCs and trees' vitality before FTCs. These findings advance our understanding of seasonal changes in hydraulic functions and how they cope with different types of FTC in temperate forests.

Tree drought-mortality risk depends more on intrinsic species resistance than on stand species diversity

Increasing tree diversity is considered a key management option to adapt forests to climate change. However, the effect of species diversity on a forest's ability to cope with extreme drought remains elusive. In this study, we assessed drought tolerance (xylem vulnerability to cavitation) and water stress (water potential), and combined them into a metric of drought-mortality risk (hydraulic safety margin) during extreme 2021 or 2022 summer droughts in five European tree diversity experiments encompassing different biomes. Overall, we found that drought-mortality risk was primarily driven by species identity (56.7% of the total variability), while tree diversity had a much lower effect (8% of the total variability). This result remained valid at the local scale (i.e within experiment) and across the studied European biomes. Tree diversity effect on drought-mortality risk was mediated by changes in water stress intensity, not by changes in xylem vulnerability to cavitation. Significant diversity effects were observed in all experiments, but those effects often varied from positive to negative across mixtures for a given species. Indeed, we found that the composition of the mixtures (i.e., the identities of the species mixed), but not the species richness of the mixture per se, is a driver of tree drought-mortality risk. This calls for a better understanding of the underlying mechanisms before tree diversity can be considered an operational adaption tool to extreme drought. Forest diversification should be considered jointly with management strategies focussed on favouring drought-tolerant species.

The multidimensionality of plant drought stress: The relative importance of edaphic and atmospheric drought

Drought threatens plant growth and related ecosystem services. The emergence of plant drought stress under edaphic drought is well studied, whilst the importance of atmospheric drought only recently gained momentum. Yet, little is known about the interaction and relative contribution of edaphic and atmospheric drought on the emergence of plant drought stress. We conducted a gradient experiment, fully crossing gravimetric water content (GWC: maximum water holding capacity-permanent wilting point) and vapour pressure deficit (VPD: 1-2.25 kPa) using five wheat varieties from three species (Triticum monococcum, T. durum & T. aestivum). We quantified the occurrence of plant drought stress on molecular (abscisic acid), cellular (stomatal conductance), organ (leaf water potential) and stand level (evapotranspiration). Plant drought stress increased with decreasing GWC across all organizational levels. This effect was magnified nonlinearly by VPD after passing a critical threshold of soil water availability. At around 20%GWC (soil matric potential 0.012 MPa), plants lost their ability to regulate leaf water potential via stomata regulation, followed by the emergence of hydraulic dysfunction. The emergence of plant drought stress is characterized by changing relative contributions of soil versus atmosphere and their non-linear interaction. This highly non-linear response is likely to abruptly alter plant-related ecosystem services in a drying world.

Ecological stress memory in wood architecture of two Neotropical hickory species from central-eastern Mexico

BACKGROUND

Drought periods are major evolutionary triggers of wood anatomical adaptive variation in Lower Tropical Montane Cloud Forests tree species. We tested the influence of historical drought events on the effects of ecological stress memory on latewood width and xylem vessel traits in two relict hickory species (Carya palmeri and Carya myristiciformis) from central-eastern Mexico. We hypothesized that latewood width would decrease during historical drought years, establishing correlations between growth and water stress conditions, and that moisture deficit during past tree growth between successive drought events, would impact on wood anatomical features. We analyzed latewood anatomical traits that developed during historical drought and pre- and post-drought years in both species.

RESULTS

We found that repeated periods of hydric stress left climatic signatures for annual latewood growth and xylem vessel traits that are essential for hydric adaptation in tropical montane hickory species.

CONCLUSIONS

Our results demonstrate the existence of cause‒effect relationships in wood anatomical architecture and highlight the ecological stress memory linked with historical drought events. Thus, combined time-series analysis of latewood width and xylem vessel traits is a powerful tool for understanding the ecological behavior of hickory species.