Climate drives vein anatomy in Proteaceae

Sensitive Hydraulic and Stomatal Decline in Extreme Drought Tolerant Species of California Ceanothus

Identifying the physiological mechanisms by which plants are adapted to drought is critical to predict species responses to climate change. We measured the responses of leaf hydraulic and stomatal conductances (Kleaf and gs, respectively) to dehydration, and their association with anatomy, in seven species of California Ceanothus grown in a common garden, including some of the most drought-tolerant species in the semi-arid flora. We tested for matching of maximum hydraulic supply and demand and quantified the role of decline of Kleaf in driving stomatal closure. Across Ceanothus species, maximum Kleaf and gs were negatively correlated, and both Kleaf and gs showed steep declines with decreasing leaf water potential (i.e., a high sensitivity to dehydration). The leaf water potential at 50% decline in gs was linked with a low ratio of maximum hydraulic supply to demand (i.e., maximum Kleaf:gs). This sensitivity of gs, combined with low minimum epidermal conductance and water storage, could contribute to prolonged leaf survival under drought. The specialized anatomy of subg. Cerastes includes trichomous stomatal crypts and pronounced hypodermis, and was associated with higher water use efficiency and water storage. Combining our data with comparative literature of other California species, species of subg. Cerastes show traits associated with greater drought tolerance and reliance on leaf water storage relative to other California species. In addition to drought resistance mechanisms such as mechanical protection and resistance to embolism, drought avoidance mechanisms such as sensitive stomatal closure could contribute importantly to drought tolerance in dry-climate adapted species.

Diverse mangroves deviate from other angiosperms in their genome size, leaf cell size and cell packing density relationships

BACKGROUND AND AIMS

While genome size limits the minimum sizes and maximum numbers of cells that can be packed into a given leaf volume, mature cell sizes can be substantially larger than their meristematic precursors and vary in response to abiotic conditions. Mangroves are iconic examples of how abiotic conditions can influence the evolution of plant phenotypes.

METHODS

Here, we examined the coordination between genome size, leaf cell sizes, cell packing densities and leaf size in 13 mangrove species across four sites in China. Four of these species occurred at more than one site, allowing us to test the effect of climate on leaf anatomy.

RESULTS

We found that genome sizes of mangroves were very small compared to other angiosperms, but, like other angiosperms, mangrove cells were always larger than the minimum size defined by genome size. Increasing mean annual temperature of a growth site led to higher packing densities of veins (Dv) and stomata (Ds) and smaller epidermal cells but had no effect on stomatal size. In contrast to other angiosperms, mangroves exhibited (1) a negative relationship between guard cell size and genome size; (2) epidermal cells that were smaller than stomata; and (3) coordination between Dv and Ds that was not mediated by epidermal cell size. Furthermore, mangrove epidermal cell sizes and packing densities covaried with leaf size.

CONCLUSIONS

While mangroves exhibited coordination between veins and stomata and attained a maximum theoretical stomatal conductance similar to that of other angiosperms, the tissue-level tradeoffs underlying these similar relationships across species and environments were markedly different, perhaps indicative of the unique structural and physiological adaptations of mangroves to their stressful environments.

Leaf Economic and Hydraulic Traits Signal Disparate Climate Adaptation Patterns in Two Co-Occurring Woodland Eucalypts

With climate change impacting trees worldwide, enhancing adaptation capacity has become an important goal of provenance translocation strategies for forestry, ecological renovation, and biodiversity conservation. Given that not every species can be studied in detail, it is important to understand the extent to which climate adaptation patterns can be generalised across species, in terms of the selective agents and traits involved. We here compare patterns of genetic-based population (co)variation in leaf economic and hydraulic traits, climate–trait associations, and genomic differentiation of two widespread tree species (Eucalyptus pauciflora and E. ovata). We studied 2-year-old trees growing in a common-garden trial established with progeny from populations of both species, pair-sampled from 22 localities across their overlapping native distribution in Tasmania, Australia. Despite originating from the same climatic gradients, the species differed in their levels of population variance and trait covariance, patterns of population variation within each species were uncorrelated, and the species had different climate–trait associations. Further, the pattern of genomic differentiation among populations was uncorrelated between species, and population differentiation in leaf traits was mostly uncorrelated with genomic differentiation. We discuss hypotheses to explain this decoupling of patterns and propose that the choice of seed provenances for climate-based plantings needs to account for multiple dimensions of climate change unless species-specific information is available.

Comparative anatomy of leaf petioles in temperate trees and shrubs: the role of plant size, environment and phylogeny

BACKGROUND AND AIMS

Petioles are important plant organs connecting stems with leaf blades and affecting light-harvesting ability of the leaf as well as transport of water, nutrients and biochemical signals. Despite the high diversity in petiole size, shape and anatomy, little information is available regarding their structural adaptations across evolutionary lineages and environmental conditions. To fill this knowledge gap, we investigated the variation of petiole morphology and anatomy of mainly European woody species to better understand the drivers of internal and external constraints in an evolutionary context.

METHODS

We studied how petiole anatomical features differed according to whole-plant size, leaf traits, thermal and hydrological conditions, and taxonomic origin in 95 shrubs and trees using phylogenetic distance-based generalized least squares models.

KEY RESULTS

Two major axes of variation were related to leaf area and plant size. Larger and softer leaves are found in taller trees of more productive habitats. Their petioles are longer, with a circular outline and are anatomically characterized by the predominance of sclerenchyma, larger vessels, interfascicular areas with fibres and indistinct phloem rays. In contrast, smaller and tougher leaves are found in shorter trees and shrubs of colder or drier habitats. Their petioles have a terete outline, phloem composed of small cells and radially arranged vessels, fibreless xylem and lamellar collenchyma. Individual anatomical traits were linked to different internal and external drivers. Petiole length and vessel diameter increase with increasing leaf blade area. Collenchyma becomes absent with increasing temperature, and petiole outline becomes polygonal with increasing precipitation.

CONCLUSIONS

We conclude that species' temperature and precipitation optima, plant height, and leaf area and thickness exerted a significant control on petiole anatomical and morphological structures not confounded by phylogenetic inertia. Species with different evolutionary histories but similar thermal and hydrological requirements have converged to similar petiole anatomical structures.

The Optical Properties of Leaf Structural Elements and Their Contribution to Photosynthetic Performance and Photoprotection

Leaves have evolved to effectively harvest light, and, in parallel, to balance photosynthetic CO2 assimilation with water losses. At times, leaves must operate under light limiting conditions while at other instances (temporally distant or even within seconds), the same leaves must modulate light capture to avoid photoinhibition and achieve a uniform internal light gradient. The light-harvesting capacity and the photosynthetic performance of a given leaf are both determined by the organization and the properties of its structural elements, with some of these having evolved as adaptations to stressful environments. In this respect, the present review focuses on the optical roles of particular leaf structural elements (the light capture module) while integrating their involvement in other important functional modules. Superficial leaf tissues (epidermis including cuticle) and structures (epidermal appendages such as trichomes) play a crucial role against light interception. The epidermis, together with the cuticle, behaves as a reflector, as a selective UV filter and, in some cases, each epidermal cell acts as a lens focusing light to the interior. Non glandular trichomes reflect a considerable part of the solar radiation and absorb mainly in the UV spectral band. Mesophyll photosynthetic tissues and biominerals are involved in the efficient propagation of light within the mesophyll. Bundle sheath extensions and sclereids transfer light to internal layers of the mesophyll, particularly important in thick and compact leaves or in leaves with a flutter habit. All of the aforementioned structural elements have been typically optimized during evolution for multiple functions, thus offering adaptive advantages in challenging environments. Hence, each particular leaf design incorporates suitable optical traits advantageously and cost-effectively with the other fundamental functions of the leaf.

Microanatomical traits track climate gradients for a dominant C4 grass species across the Great Plains, USA

BACKGROUND AND AIMS

Andropogon gerardii is a highly productive C4 grass species with a large geographic range throughout the North American Great Plains, a biome characterized by a variable temperate climate. Plant traits are often invoked to explain growth rates and competitive abilities within broad climate gradients. For example, plant competition models typically predict that species with large geographic ranges benefit from variation in traits underlying high growth potential. Here, we examined the relationship between climate variability and leaf-level traits in A. gerardii, emphasizing how leaf-level microanatomical traits serve as a mechanism that may underlie variation in commonly measured traits, such as specific leaf area (SLA).

METHODS

Andropogon gerardii leaves were collected in August 2017 from Cedar Creek Ecosystem Science Reserve (MN), Konza Prairie Biological Station (KS), Platte River Prairie (NE) and Rocky Mountain Research Station (SD). Leaves from ten individuals from each site were trimmed, stained and prepared for fluorescent confocal microscopy to analyse internal leaf anatomy. Leaf microanatomical data were compared with historical and growing season climate data extracted from PRISM spatial climate models.

KEY RESULTS

Microanatomical traits displayed large variation within and across sites. According to AICc (Akaike's information criterion adjusted for small sample sizes) selection scores, the interaction of mean precipitation and temperature for the 2017 growing season was the best predictor of variability for the anatomical and morphological traits measured here. Mesophyll area and bundle sheath thickness were directly correlated with mean temperature (annual and growing season). Tissues related to water-use strategies, such as bulliform cell and xylem area, were significantly correlated with one another.

CONCLUSIONS

The results indicate that (1) microanatomical trait variation exists within this broadly distributed grass species, (2) microanatomical trait variability appears likely to impact leaf-level carbon and water use strategies, and (3) microanatomical trait values vary across climate gradients, and may underlie variation in traits measured at larger ecological scales.

Similar hydraulic efficiency and safety across vesselless angiosperms and vessel-bearing species with scalariform perforation plates

The evolution of xylem vessels from tracheids is put forward as a key innovation that boosted hydraulic conductivity and photosynthetic capacities in angiosperms. Yet, the role of xylem anatomy and interconduit pits in hydraulic performance across vesselless and vessel-bearing angiosperms is incompletely known, and there is a lack of functional comparisons of ultrastructural pits between species with different conduit types. We assessed xylem hydraulic conductivity and vulnerability to drought-induced embolism in 12 rain forest species from New Caledonia, including five vesselless species, and seven vessel-bearing species with scalariform perforation plates. We measured xylem conduit traits, along with ultrastructural features of the interconduit pits, to assess the relationships between conduit traits and hydraulic efficiency and safety. In spite of major differences in conduit diameter, conduit density, and the presence/absence of perforation plates, the species studied showed similar hydraulic conductivity and vulnerability to drought-induced embolism, indicating functional similarity between both types of conduits. Interconduit pit membrane thickness (Tm) was the only measured anatomical feature that showed a relationship to significant vulnerability to embolism. Our results suggest that the incidence of drought in rain forest ecosystems can have similar effects on species bearing water-conducting cells with different morphologies.

Drought tolerance traits do not vary across sites differing in water availability in Banksia serrata (Proteaceae)

Interspecific variation in plant hydraulic traits plays a major role in shaping species distributions across climates, yet variation within species is poorly understood. Here we report on intraspecific variation of hydraulic traits in Banksia serrata (L.f.) sampled from three sites characterised by contrasting climates (warm-wet, warm-dry and cool-wet). Hydraulic characteristics including vulnerability to embolism, hydraulic conductance, pressure-volume traits and key morphological traits were measured. Vulnerability to embolism in leaf and stem, defined by the water potential inducing 50 and 88% loss of hydraulic conductivity (P50 and P88 respectively), did not differ across sites. However, plants from the warm-dry environment exhibited higher stem conductivity (Ks) than the cool-wet environment. Leaf turgor loss point (TLP) did not vary among sites, but warm-dry site plants showed lower leaf capacitance (C*FT) and higher modulus of elasticity (ε) than the other two sites. Plants from the cool-wet site had lower specific leaf area (SLA) and plants from the warm-dry site had lower sapwood density (WD). Overall, key hydraulic traits were generally conserved across populations despite differences in mean site water availability, and the safety-efficiency trade-off was absent in this species. These results suggest that B. serrata has limited ability to adjust hydraulic architecture in response to environmental change and thus may be susceptible to climate change-type drought stress.

The links between leaf hydraulic vulnerability to drought and key aspects of leaf venation and xylem anatomy among 26 Australian woody angiosperms from contrasting climates

Background and Aims

The structural properties of leaf venation and xylem anatomy strongly influence leaf hydraulics, including the ability of leaves to maintain hydraulic function during drought. Here we examined the strength of the links between different leaf venation traits and leaf hydraulic vulnerability to drought (expressed as P50leaf by rehydration kinetics) in a diverse group of 26 woody angiosperm species, representing a wide range of leaf vulnerabilities, from four low-nutrient sites with contrasting rainfall across eastern Australia.

Methods

For each species we measured key aspects of leaf venation design, xylem anatomy and leaf morphology. We also assessed for the first time the scaling relationships between hydraulically weighted vessel wall thickness (th) and lumen breadth (bh) across vein orders and habitats.

Key Results

Across species, variation in P50leaf was strongly correlated with the ratio of vessel wall thickness (th) to lumen breadth (bh) [(t/b)h; an index of conduit reinforcement] at each leaf vein order. Concomitantly, the scaling relationship between th and bh was similar across vein orders, with a log-log slope less than 1 indicating greater xylem reinforcement in smaller vessels. In contrast, P50leaf was not related to th and bh individually, to major vein density (Dvmajor) or to leaf size. Principal components analysis revealed two largely orthogonal trait groupings linked to variation in leaf size and drought tolerance.

Conclusions

Our results indicate that xylem conduit reinforcement occurs throughout leaf venation, and remains closely linked to leaf drought tolerance irrespective of leaf size.

Mapping xylem failure in disparate organs of whole plants reveals extreme resistance in olive roots

The capacity of plant species to resist xylem cavitation is an important determinant of resistance to drought, mortality thresholds, geographic distribution and productivity. Unravelling the role of xylem cavitation vulnerability in plant evolution and adaptation requires a clear understanding of how this key trait varies between the tissues of individuals and between individuals of species. Here, we examine questions of variation within individuals by measuring how cavitation moves between organs of individual plants. Using multiple cameras placed simultaneously on roots, stems and leaves, we were able to record systemic xylem cavitation during drying of individual olive plants. Unlike previous studies, we found a consistent pattern of root > stem > leaf in terms of xylem resistance to cavitation. The substantial variation in vulnerability to cavitation, evident among individuals, within individuals and within tissues of olive seedlings, was coordinated such that plants with more resistant roots also had more resistant leaves. Preservation of root integrity means that roots can continue to supply water for the regeneration of drought-damaged aerial tissues after post-drought rain. Furthermore, coordinated variation in vulnerability between leaf, stem and root in olive plants suggests a strong selective pressure to maintain a fixed order of cavitation during drought.

Water supply and demand remain coordinated during breakdown of the global scaling relationship between leaf size and major vein density

Vein networks that disobey the global scaling of major vein density with leaf size shed light on functional constraints of vein network formation in dicotyledons. Understanding their evolution, distribution and impact on vein-stomata-climate associations is an important contribution to our global view of vein network organization. Based on vein traits of 55 species of pantropical Ochnaceae, stomata and climatic niche data, and a dated molecular phylogeny, we unveil major structural shifts in vein networks through deep time, relationships between leaf size, vein and stomata traits, and their interplay with climate. Dense 2° veins, reduction of minor veins and the associated breakdown of vein-leaf size scaling evolved multiple times independently in Ochnaceae. In spite of the drastic changes in vein architecture in this venation type, vein and stomatal densities remain correlated. Our study demonstrates that shortening the major vein-stomata distance is economically not less advantageous than by increasing minor vein density, as illustrated by the same degree of coordination between vein and stomatal densities and the similar construction costs across networks with dense 2° veins and those with 'normally' spaced 2° veins.

Environmental adaptation in stomatal size independent of the effects of genome size

Cell sizes are linked across multiple tissues, including stomata, and this variation is closely correlated with genome size. These associations raise the question of whether generic changes in cell size cause suboptimal changes in stomata, requiring subsequent evolution under selection for stomatal size. We tested the relationships among guard cell length, genome size and vegetation type using phylogenetically independent analyses on 67 species of the ecologically and structurally diverse family, Proteaceae. We also compared how genome and stomatal sizes varied at ancient (among genera) and more recent (within genus) levels. The observed 60-fold range in genome size in Proteaceae largely reflected the mean chromosome size. Compared with variation among genera, genome size varied much less within genera (< 6% of total variance) than stomatal size, implying evolution in stomatal size subsequent to changes in genome size. Open vegetation and closed forest had significantly different relationships between stomatal and genome sizes. Ancient changes in genome size clearly influenced stomatal size in Proteaceae, but adaptation to habitat strongly modified the genome-stomatal size relationship. Direct adaptation to the environment in stomatal size argues that new proxies for past concentrations of atmospheric CO2 that incorporate stomatal size are superior to older models based solely on stomatal frequency.

Inferring climate from angiosperm leaf venation networks

Leaf venation networks provide an integrative linkage between plant form, function and climate niche, because leaf water transport underlies variation in plant performance. Here, we develop theory based on leaf physiology that uses community-mean vein density to predict growing season temperature and atmospheric CO2 concentration. The key assumption is that leaf water supply is matched to water demand in the local environment. We test model predictions using leaves from 17 temperate and tropical sites that span broad climatic gradients. We find quantitative agreement between predicted and observed climate values. We also highlight additional leaf traits that may improve predictions. Our study provides a novel approach for understanding the functional linkages between functional traits and climate that may improve the reconstruction of paleoclimate from fossil assemblages.

Leaf mass per area is independent of vein length per area: avoiding pitfalls when modelling phenotypic integration (reply to Blonder et al. 2014)

It has been recently proposed that leaf vein length per area (VLA) is the major determinant of leaf mass per area ( MA), and would thereby determine other traits of the leaf economic spectrum (LES), such as photosynthetic rate per mass (A(mass)), nitrogen concentration per mass (N(mass)) and leaf lifespan (LL). In a previous paper we argued that this 'vein origin' hypothesis was supported only by a mathematical model with predestined outcomes, and that we found no support for the 'vein origin' hypothesis in our analyses of compiled data. In contrast to the 'vein origin' hypothesis, empirical evidence indicated that VLA and LMA are independent mechanistically, and VLA (among other vein traits) contributes to a higher photosynthetic rate per area (A(area)), which scales up to driving a higher A(mass), all independently of LMA, N(mass) and LL. In their reply to our paper, Blonder et al. (2014) raised questions about our analysis of their model, but did not address our main point, that the data did not support their hypothesis. In this paper we provide further analysis of an extended data set, which again robustly demonstrates the mechanistic independence of LMA from VLA, and thus does not support the 'vein origin' hypothesis. We also address the four specific points raised by Blonder et al. (2014) regarding our analyses. We additionally show how this debate provides critical guidance for improved modelling of LES traits and other networks of phenotypic traits that determine plant performance under contrasting environments.

When smaller is better: leaf hydraulic conductance and drought vulnerability correlate to leaf size and venation density across four Coffea arabica genotypes

Leaf hydraulic conductance (Kleaf) and drought vulnerability in terms of leaf water potential inducing 50% loss of Kleaf (P50), were assessed in four genotypes of Coffea arabica L. We tested three hypotheses: (1) leaf P50 is lower in small leaves with higher vein densities; (2) lower P50 translates into lower Kleaf, limiting gas exchange rates and higher leaf mass per unit area (LMA); (3) P50 values are coordinated with symplastic drought tolerance. We found partial support for Hypotheses 1 and 3, but not for Hypothesis 2. Significant correlations existed among leaf size, vein network and drought resistance. Smaller leaves displayed higher major vein density, higher Kleaf and more negative P50. Kleaf was correlated with leaf gas exchange rates. A negative relationship was observed between Kleaf and LMA, whereas P50 was found to be positively correlated with LMA. Across coffee genotypes, reduced leaf surface area and increased vein density shifts P50 towards more negative values while not translating into higher LMA or lower Kleaf. Breeding crop varieties for both increased safety of the leaf hydraulic system towards drought-induced dysfunction and high gas exchange rates per unit of leaf area is probably a feasible target for future adaptation of crops to climate change scenarios.