Evidence for within-species transition between drought response strategies in Nicotiana benthamiana.

Nicotiana benthamiana is predominantly distributed in arid habitats across northern Australia. However, none of six geographically isolated accessions shows obvious xerophytic morphological features. To investigate how these tender-looking plants withstand drought, we examined their responses to water deprivation, assessed phenotypic, physiological, and cellular responses, and analysed cuticular wax composition and wax biosynthesis gene expression profiles. Results showed that the Central Australia (CA) accession, globally known as a research tool, has evolved a drought escape strategy with early vigour, short life cycle, and weak, water loss-limiting responses. By contrast, a northern Queensland (NQ) accession responded to drought by slowing growth, inhibiting flowering, increasing leaf cuticle thickness, and altering cuticular wax composition. Under water stress, NQ increased the heat stability and water impermeability of its cuticle by extending the carbon backbone of cuticular long-chain alkanes from c. 25 to 33. This correlated with rapid upregulation of at least five wax biosynthesis genes. In CA, the alkane chain lengths (c. 25) and gene expression profiles remained largely unaltered. This study highlights complex genetic and environmental control over cuticle composition and provides evidence for divergence into at least two fundamentally different drought response strategies within the N. benthamiana species in < 1 million years.

The AusTraits plant dictionary.

Traits with intuitive names, a clear scope and explicit description are essential for all trait databases. The lack of unified, comprehensive, and machine-readable plant trait definitions limits the utility of trait databases, including reanalysis of data from a single database, or analyses that integrate data across multiple databases. Both can only occur if researchers are confident the trait concepts are consistent within and across sources. Here we describe the AusTraits Plant Dictionary (APD), a new data source of terms that extends the trait definitions included in a recent trait database, AusTraits. The development process of the APD included three steps: review and formalisation of the scope of each trait and the accompanying trait description; addition of trait metadata; and publication in both human and machine-readable forms. Trait definitions include keywords, references, and links to related trait concepts in other databases, enabling integration of AusTraits with other sources. The APD will both improve the usability of AusTraits and foster the integration of trait data across global and regional plant trait databases.

Stomatal response to VPD is not triggered by changes in soil-leaf hydraulic conductance in Arabidopsis or Callitris.

Stomatal closure under high VPDL (leaf to air vapour pressure deficit) is a primary means by which plants prevent large excursions in transpiration rate and leaf water potential (Ψleaf) that could lead to tissue damage. Yet, the drivers of this response remain controversial. Changes in Ψleaf appear to drive stomatal VPDL response, but many argue that dynamic changes in soil-to-leaf hydraulic conductance (Ks-l) make an important contribution to this response pathway, even in well-hydrated soils. Here, we examined whether the regulation of whole plant stomatal conductance (gc) in response to typical changes in daytime VPDL is influenced by dynamic changes in Ks-l. We use well-watered plants of two species with contrasting ecological and physiological features: the herbaceous Arabidopsis thaliana (ecotype Columbia-0) and the dry forest conifer Callitris rhomboidea. The dynamics of Ks-l and gc were continuously monitored by combining concurrent in situ measurements of Ψleaf using an open optical dendrometer and whole plant transpiration using a balance. Large changes in VPDL were imposed to induce stomatal closure and observe the impact on Ks-l. In both species, gc was observed to decline substantially as VPDL increased, while Ks-l remained stable. Our finding suggests that stomatal regulation of transpiration is not contingent on a decrease in Ks-l. Static Ks-l provides a much simpler explanation for transpiration control in hydrated plants and enables simplified modelling and new methods for monitoring plant water use in the field.

Variation in xylem vulnerability to cavitation shapes the photosynthetic legacy of drought.

Increased drought conditions impact tree health, negatively disrupting plant water transport which, in turn, affects plant growth and survival. Persistent drought legacy effects have been documented in many diverse ecosystems, yet we still lack a mechanistic understanding of the physiological processes limiting tree recovery after drought. Tackling this question, we exposed saplings of a common Australian evergreen tree (Eucalyptus viminalis) to a cycle of drought and rewatering, seeking evidence for a link between the spread of xylem cavitation within the crown and the degree of photosynthetic recovery postdrought. Individual leaves experiencing >35% vein cavitation quickly died but this did not translate to a rapid overall canopy damage. Rather, whole canopies showed a gradual decline in mean postdrought gas exchange rates as water stress increased. This gradual loss of canopy function postdrought was due to a significant variation in cavitation vulnerability of leaves within canopies leading to diversity in the capacity of leaves within a single crown to recover function after drought. These results from the evergreen E. viminalis emphasise the importance of within-crown variation in xylem vulnerability as a central character regulating the dynamics of canopy death and the severity of drought legacy through time.

Hydraulic variability of tropical forests is largely independent of water availability.

Tropical rainforest woody plants have been thought to have uniformly low resistance to hydraulic failure and to function near the edge of their hydraulic safety margin (HSM), making these ecosystems vulnerable to drought; however, this may not be the case. Using data collected at 30 tropical forest sites for three key traits associated with drought tolerance, we show that site-level hydraulic diversity of leaf turgor loss point, resistance to embolism (P50 ), and HSMs is high across tropical forests and largely independent of water availability. Species with high HSMs (>1 MPa) and low P50 values (< -2 MPa) are common across the wet and dry tropics. This high site-level hydraulic diversity, largely decoupled from water stress, could influence which species are favoured and become dominant under a drying climate. High hydraulic diversity could also make these ecosystems more resilient to variable rainfall regimes.

A new fossil Acmopyle with accessory transfusion tissue and potential reproductive buds: Direct evidence for ever-wet rainforests in Eocene Patagonia.

PREMISE: Acmopyle (Podocarpaceae) comprises two extant species from Oceania that are physiologically restricted to ever-wet rainforests, a confirmed fossil record based on leaf adpressions and cuticles in Australia since the Paleocene, and a few uncertain reports from New Zealand, Antarctica, and South America. We investigated fossil specimens with Acmopyle affinities from the early Eocene Laguna del Hunco site in Patagonia, Argentina. METHODS: We studied 42 adpression leafy-shoot fossils and included them in a total evidence phylogenetic analysis. RESULTS: Acmopyle grayae sp. nov. is based on heterophyllous leafy shoots with three distinct leaf types. Among these, bilaterally flattened leaves uniquely preserve subparallel, linear features that we interpret as accessory transfusion tissue (ATT, an extra-venous water-conducting tissue). Some apical morphologies of A. grayae shoots are compatible with the early stages of ovuliferous cone development. Our phylogenetic analysis recovers the new species in a polytomy with the two extant Acmopyle species. We report several types of insect-herbivory damage. We also transfer Acmopyle engelhardti from the middle Eocene Río Pichileufú flora to Dacrycarpus engelhardti comb. nov. CONCLUSIONS: We confirm the biogeographically significant presence of the endangered West Pacific genus Acmopyle in Eocene Patagonia. Acmopyle is one of the most drought-intolerant genera in Podocarpaceae, possibly due to the high collapse risk of the ATT, and thus the new fossil species provides physiological evidence for the presence of an ever-wet rainforest environment at Laguna del Hunco during the Early Eocene Climatic Optimum.

In situ characterisation of whole-plant stomatal responses to VPD using leaf optical dendrometry.

Vapour pressure deficit (VPD) plays a crucial role in regulating plant carbon and water fluxes due to its influence on stomatal behaviour and transpiration. Yet, characterising stomatal responses of the whole plant to VPD remains challenging due to methodological limitations. Here, we develop a novel method for in situ assessment of whole-plant stomatal responses (gc ) to VPD in the herbaceous plant Tanacetum cinerariifolium. To do this, we examine the relationship between daytime VPD and the corresponding soil-stem water potential gradient (ΔΨ) monitored using the optical dendrometry in well-hydrated plants under nonlimiting light in both glasshouse and field conditions. In glasshouse plants, ΔΨ increased proportionally with the VPD up to a threshold of 1.53 kPa, beyond which the slope decreased, suggesting a two-phase response in gc . This pattern aligned with corresponding gravimetrically measured gc behaviour, which also showed a decline when VPD exceeded a similar threshold. This response was then compared with that of field plants monitored using the optical dendrometry technique over a growing season under naturally variable VPD conditions and nonlimiting light and water supply. Field plants exhibited a similar threshold-type response to VPD but were more sensitive than glasshouse individuals with a VPD threshold of 0.74 kPa. The results showed that whole-plant gc responses to VPD can be characterised optically in T. cinerariifolium, introducing a new tool for the monitoring and characterisation of stomatal behaviour in situ.

The root of the problem: diverse vulnerability to xylem cavitation found within the root system of wheat plants.

The propagation of xylem embolism throughout the root systems of drought-affected plants remains largely unknown, despite this process being comparatively well characterized in aboveground tissues. We used optical and X-ray imaging to capture xylem embolism propagation across the intact root systems of bread wheat (Triticum aestivum L. 'Krichauff') plants subjected to drying. Patterns in vulnerability to xylem cavitation were examined to investigate whether vulnerability may vary based on root size and placement across the entire root system. Individual plants exhibited similar mean whole root system vulnerabilities to xylem cavitation but showed enormous 6 MPa variation within their component roots (c. 50 roots per plant). Xylem cavitation typically initiated in the smallest, peripheral parts of the root system and moved inwards and upwards towards the root collar last, although this trend was highly variable. This pattern of xylem embolism spread likely results in the sacrifice of replaceable small roots while preserving function in larger, more costly central roots. A distinct pattern of embolism-spread belowground has implications for how we understand the impact of drought in the root system as a critical interface between plant and soil.

Combined heat and water stress leads to local xylem failure and tissue damage in pyrethrum flowers.

Flowers are critical for angiosperm reproduction and the production of food, fiber, and pharmaceuticals, yet for unknown reasons, they appear particularly sensitive to combined heat and drought stress. A possible explanation for this may be the co-occurrence of leaky cuticles in flower petals and a vascular system that has a low capacity to supply water and is prone to failure under water stress. These characteristics may render reproductive structures more susceptible than leaves to runaway cavitation-an uncontrolled feedback cycle between rising water stress and declining water transport efficiency that can rapidly lead to lethal tissue desiccation. We provide modeling and empirical evidence to demonstrate that flower damage in the perennial crop pyrethrum (Tanacetum cinerariifolium), in the form of irreversible desiccation, corresponds with runaway cavitation in the flowering stem after a combination of heat and water stress. We show that tissue damage is linked to greater evaporative demand during high temperatures rather than direct thermal stress. High floral transpiration dramatically reduced the soil water deficit at which runaway cavitation was triggered in pyrethrum flowering stems. Identifying runaway cavitation as a mechanism leading to heat damage and reproductive losses in pyrethrum provides different avenues for process-based modeling to understand the impact of climate change on cultivated and natural plant systems. This framework allows future investigation of the relative susceptibility of diverse plant species to reproductive failure under hot and dry conditions.

Cold temperature and aridity shape the evolution of drought tolerance traits in Tasmanian species of Eucalyptus.

Perennial plant species from water-limiting environments (including climates of extreme drought, heat and freezing temperatures) have evolved traits that allow them to tolerate these conditions. As such, traits that are associated with water stress may show evidence of adaptation to climate when compared among closely related species inhabiting contrasting climatic conditions. In this study, we tested whether key hydraulic traits linked to drought stress, including the vulnerability of leaves to embolism (P50 leaf) and the minimum diffusive conductance of shoots (gmin), were associated with climatic characteristics of 14 Tasmanian eucalypt species from sites that vary in precipitation and temperature. Across species, greater cavitation resistance (more negative P50 leaf) was associated with increasing aridity and decreasing minimum temperature. By contrast, gmin showed strong associations with aridity only. Among these Tasmanian eucalypts, evidence suggests that trait variation is influenced by both cold and dry conditions, highlighting the need to consider both aspects when exploring adaptive trait-climate relationships.

Resistant xylem from roots to peduncles sustains reproductive water supply after drought-induced cavitation of wheat leaves.

BACKGROUND AND AIMS: Many annual grasses exhibit drought-avoiding life cycles in which rapid reproduction must be completed before soil water is exhausted. This strategy would seem to require a hydraulic system capable of sustaining reproduction at all costs to the rest of the plant, yet little is known about the whole-plant structure of hydraulic vulnerability in grasses. METHODS: We examine vulnerability to water-stress-induced xylem cavitation in roots, flag leaves, and basal and apical regions of peduncles of wheat (Triticum aestivum L. 'Krichauff') to understand the staged failure of xylem function in severe drought. The functionality of segmented vulnerabilities is tested by conducting rehydration experiments after acute dehydration. KEY RESULTS: We show that water supply to peduncles is more drought resistant than in leaves due to greater xylem cavitation resistance, ensuring a pathway of water can be maintained from the roots to the reproductive tissues even after severe water deficit. Differential rehydration of peduncles compared to leaves following drought confirmed the functionality of xylem supply from roots to seed after water stress sufficient to completely cavitate flag leaf vessels. CONCLUSIONS: These results demonstrate that a proportion of the hydraulic pathway between roots and seeds remains functional under extreme dehydration, suggesting that vulnerability traits in this key grass species reflect its reproductive strategy.

Evidence for a trade-off between growth rate and xylem cavitation resistance in Callitris rhomboidea.

The ideal plant water transport system is one that features high efficiency and resistance to drought-induced damage (xylem cavitation), however, species rarely possess both. This may be explained by trade-offs between traits, yet thus far, no proposed trade-off has offered a universal explanation for the lack of water transport systems that are both highly drought-resistant and highly efficient. Here, we find evidence for a new trade-off, between growth rate and resistance to xylem cavitation, in the canopies of a drought-resistant tree species (Callitris rhomboidea). Wide variation in cavitation resistance (P50) was found in distal branch tips (<2 mm in diameter), converging to low variation in P50 in larger diameter stems (>2 mm). We found a significant correlation between cavitation resistance and distal branchlet internode length across branch tips in C. rhomboidea canopies. Branchlets with long internodes (8 mm or longer) were significantly more vulnerable to drought-induced xylem cavitation than shorter internodes (4 mm or shorter). This suggests that varying growth rates, leading to differences in internode length, drive differences in cavitation resistance in C. rhomboidea trees. The only distinct anatomical difference found between internodes was the pith size, with the average pith to xylem area in long internodes being five times greater than in short internodes. Understanding whether this trade-off exists within and between species will help us to uncover what drives and limits drought resistance across the world's flora.

Deadly acceleration in dehydration of Eucalyptus viminalis leaves coincides with high-order vein cavitation.

Xylem cavitation during drought is proposed as a major driver of canopy collapse, but the mechanistic link between hydraulic failure and leaf damage in trees is still uncertain. Here, we used the tree species manna gum (Eucalyptus viminalis) to explore the connection between xylem dysfunction and lethal desiccation in leaves. Cavitation damage to leaf xylem could theoretically trigger lethal desiccation of tissues by severing water supply under scenarios such as runaway xylem cavitation, or the local failure of terminal parts of the leaf vein network. To investigate the role of xylem failure in leaf death, we compared the timing of damage to the photosynthetic machinery (Fv/Fm decline) with changes in plant hydration and xylem cavitation during imposed water stress. The water potential at which Fv/Fm was observed to decline corresponded to the water potential marking a transition from slow to very rapid tissue dehydration. Both events also occurred simultaneously with the initiation of cavitation in leaf high-order veins (HOV, veins from the third order above) and the analytically derived point of leaf runaway hydraulic failure. The close synchrony between xylem dysfunction and the photosynthetic damage strongly points to water supply disruption as the trigger for desiccation of leaves in this hardy evergreen tree. These results indicate that runaway cavitation, possibly triggered by HOV network failure, is the tipping agent determining the vulnerability of E. viminalis leaves to damage during drought and suggest that HOV cavitation and runaway hydraulic failure may play a general role in determining canopy damage in plants.

Ethylene constrains stomatal reopening in Fraxinus chinensis post moderate drought.

Clarifying the mechanisms underlying the recovery of gas exchange following drought is the key to providing insights into plant drought adaptation and habitat distribution. However, the mechanisms are still largely unknown. Targeting processes known to inhibit gas exchange during drought recovery, we measured leaf water potential, the leaf hydraulic conductance, stomatal reopening, abscisic acid (ABA) and the ethylene emission rate (EER) following moderate drought stress in seedlings of the globally pervasive woody tree Fraxinus chinensis. We found strong evidence that the slow stomatal reopening after rehydration is regulated by a slow decrease in EER, rather than changes in leaf hydraulics or foliar ABA levels. This was supported by evidence of rapid gas exchange recovery in plants after treatment with the ethylene antagonist 1-methylcyclopropene. These findings provide evidence to rigorously support ethylene as a key factor constraining stomatal reopening from moderate drought directly, thereby potentially opening new windows for understanding species drought adaptation.

Physiological trait networks enhance understanding of crop growth and water use in contrasting environments.

Plant function arises from a complex network of structural and physiological traits. Explicit representation of these traits, as well as their connections with other biophysical processes, is required to advance our understanding of plant-soil-climate interactions. We used the Terrestrial Regional Ecosystem Exchange Simulator (TREES) to evaluate physiological trait networks in maize. Net primary productivity (NPP) and grain yield were simulated across five contrasting climate scenarios. Simulations achieving high NPP and grain yield in high precipitation environments featured trait networks conferring high water use strategies: deep roots, high stomatal conductance at low water potential ("risky" stomatal regulation), high xylem hydraulic conductivity and high maximal leaf area index. In contrast, high NPP and grain yield was achieved in dry environments with low late-season precipitation via water conserving trait networks: deep roots, high embolism resistance and low stomatal conductance at low leaf water potential ("conservative" stomatal regulation). We suggest that our approach, which allows for the simultaneous evaluation of physiological traits, soil characteristics and their interactions (i.e., networks), has potential to improve our understanding of crop performance in different environments. In contrast, evaluating single traits in isolation of other coordinated traits does not appear to be an effective strategy for predicting plant performance.

Constant hydraulic supply enables optical monitoring of transpiration in a grass, a herb, and a conifer.

Plant transpiration is an inevitable consequence of photosynthesis and has a huge impact on the terrestrial carbon and water cycle, yet accurate and continuous monitoring of its dynamics is still challenging. Under well-watered conditions, canopy transpiration (Ec) could potentially be continuously calculated from stem water potential (Ψstem), but only if the root to stem hydraulic conductance (Kr-s) remains constant and plant capacitance is relatively small. We tested whether such an approach is viable by investigating whether Kr-s remains constant under a wide range of daytime transpiration rates in non-water-stressed plants. Optical dendrometers were used to continuously monitor tissue shrinkage, an accurate proxy of Ψstem, while Ec was manipulated in three species with contrasting morphological, anatomical, and phylogenetic identities: Tanacetum cinerariifolium, Zea mays, and Callitris rhomboidea. In all species, we found Kr-s to remain constant across a wide range of Ec, meaning that the dynamics of Ψstem could be used to monitor Ec. This was evidenced by the close agreement between measured Ec and that predicted from optically measured Ψstem. These results suggest that optical dendrometers enable both plant hydration and Ec to be monitored non-invasively and continuously in a range of woody and herbaceous species. This technique presents new opportunities to monitor transpiration under laboratory and field conditions in a diversity of woody, herbaceous, and grassy species.

Conduit position and connectivity affect the likelihood of xylem embolism during natural drought in evergreen woodland species.

BACKGROUND AND AIMS: Hydraulic failure is considered a main cause of drought-induced forest mortality. Yet, we have a limited understanding of how the varying intensities and long time scales of natural droughts induce and propagate embolism within the xylem. METHODS: X-ray computed tomography (microCT) images were obtained from different aged branch xylem to study the number, size and spatial distribution of in situ embolized conduits among three dominant tree species growing in a woodland community. KEY RESULTS: Among the three studied tree species, those with a higher xylem vulnerability to embolism (higher water potential at 50 % loss of hydraulic conductance; P50) were more embolized than species with lower P50. Within individual stems, the probability of embolism was independent of conduit diameter but associated with conduit position. Rather than the occurrence of random or radial embolism, we observed circumferential clustering of high and low embolism density, suggesting that embolism spreads preferentially among conduits of the same age. Older xylem also appeared more likely to accumulate embolisms than young xylem, but there was no pattern suggesting that branch tips were more vulnerable to cavitation than basal regions. CONCLUSIONS: The spatial analysis of embolism occurrence in field-grown trees suggests that embolism under natural drought probably propagates by air spreading from embolized into neighbouring conduits in a circumferential pattern. This pattern offers the possibility to understand the temporal aspects of embolism occurrence by examining stem cross-sections.

Hydraulic vulnerability segmentation in compound-leaved trees: Evidence from an embolism visualization technique.

The hydraulic vulnerability segmentation (HVS) hypothesis implies the existence of differences in embolism resistance between plant organs along the xylem pathway and has been suggested as an adaptation allowing the differential preservation of more resource-rich tissues during drought stress. Compound leaves in trees are considered a low-cost means of increasing leaf area and may thus be expected to show evidence of strong HVS, given the tendency of compound-leaved tree species to shed their leaf units during drought. However, the existence and role of HVS in compound-leaved tree species during drought remain uncertain. We used an optical visualization technique to estimate embolism occurrence in stems, petioles, and leaflets of shoots in two compound-leaved tree species, Manchurian ash (Fraxinus mandshurica) and Manchurian walnut (Juglans mandshurica). We found higher (less negative) water potentials corresponding to 50% loss of conductivity (P50) in leaflets and petioles than in stems in both species. Overall, we observed a consistent pattern of stem > petiole > leaflet in terms of xylem resistance to embolism and hydraulic safety margins (i.e. the difference between mid-day water potential and P50). The coordinated variation in embolism vulnerability between organs suggests that during drought conditions, trees benefit from early embolism and subsequent shedding of more expendable organs such as leaflets and petioles, as this provides a degree of protection to the integrity of the hydraulic system of the more carbon costly stems. Our results highlight the importance of HVS as an adaptive mechanism of compound-leaved trees to withstand drought stress.

Canopy damage during a natural drought depends on species identity, physiology and stand composition.

Vulnerability to xylem cavitation is a strong predictor of drought-induced damage in forest communities. However, biotic features of the community itself can influence water availability at the individual tree-level, thereby modifying patterns of drought damage. Using an experimental forest in Tasmania, Australia, we determined the vulnerability to cavitation (leaf P50 ) of four tree species and assessed the drought-induced canopy damage of 2944 6-yr-old trees after an extreme natural drought episode. We examined how individual damage was related to their size and the density and species identity of neighbouring trees. The two co-occurring dominant tree species, Eucalyptus delegatensis and Eucalyptus regnans, were the most vulnerable to drought-induced xylem cavitation and both species suffered significantly greater damage than neighbouring, subdominant species Pomaderris apetala and Acacia dealbata. While the two eucalypts had similar leaf P50 values, E. delegatensis suffered significantly greater damage, which was strongly related to the density of neighbouring P. apetala. Damage in E. regnans was less impacted by neighbouring plants and smaller trees of both eucalypts sustained significantly more damage than larger trees. Our findings demonstrate that natural drought damage is influenced by individual plant physiology as well as the composition, physiology and density of the surrounding stand.

In vivo monitoring of drought-induced embolism in Callitris rhomboidea trees reveals wide variation in branchlet vulnerability and high resistance to tissue death.

Damage to the plant water transport system through xylem cavitation is known to be a driver of plant death in drought conditions. However, a lack of techniques to continuously monitor xylem embolism in whole plants in vivo has hampered our ability to investigate both how this damage propagates and the possible mechanistic link between xylem damage and tissue death. Using optical and fluorescence sensors, we monitored drought-induced xylem embolism accumulation and photosynthetic damage in vivo throughout the canopy of a drought-resistant conifer, Callitris rhomboidea, during drought treatments of c. 1 month duration. We show that drought-induced damage to the xylem can be monitored in vivo in whole trees during extended periods of water stress. Under these conditions, vulnerability of the xylem to cavitation varied widely among branchlets, with photosynthetic damage only recorded once > 90% of the xylem was cavitated. The variation in branchlet vulnerability has important implications for understanding how trees like C. rhomboidea survive drought, and the high resistance of branchlets to tissue damage points to runaway cavitation as a likely driver of tissue death in C. rhomboidea branch tips.