Structural and hydraulic correlates of heterophylly in Ginkgo biloba

Why incorporate plant architecture into trait-based ecology?

Trait-based ecology has improved our understanding of the functioning of organisms, communities, ecosystems, and beyond. However, its predictive ability remains limited as long as phenotypic integration and temporal dynamics are not considered. We highlight how the morphogenetic processes that shape the 3D development of a plant during its lifetime affect its performance. We show that the diversity of architectural traits allows us to go beyond organ-level traits in capturing the temporal and spatial dimensions of ecological niches and informing community assembly processes. Overall, we argue that consideration of multilevel topological, geometrical, and ontogenetic features provides a dynamic view of the whole-plant phenotype and a relevant framework for investigating phenotypic integration, plant adaptation and performance, and community structure and dynamics.

Integrating genome-wide association and transcriptome analysis to provide molecular insights into heterophylly and eco-adaptability in woody plants

Heterophylly is regard as an important adaptive mechanism in response to different environments within plants. However, the genetic mechanisms responsible for heterophylly in woody plants are still poorly understood. Herein, the divergence of heterophyllous leaves was investigated at morphogenesis and using microdissection and physiological indexes in paper mulberry, and the genetic basis of heterophylly was further revealed combined with genome-wide association study (GWAS), transcriptome analysis and weighted gene coexpression network analysis (WGCNA). Our results revealed that the flavonoid content and antioxidant activity increased gradually from the entire leaf to the palmatisect leaf, while the hormone content and net photosynthetic rate decreased. Through GWAS and transcriptome analysis, a total of 98 candidate genes and 2338 differentially expressed genes associated with heterophylly were identified. Importantly, we uncovered critical variations in the candidate genes Bp07g0981 (WOX) and Bp07g0920 (HHO), along with significant differences in haplotypes and expression levels among heterophyllous leaves. Our results also suggested that the genes involved in hormone signaling pathways, antioxidant activity, and flavonoid metabolism might be closely related to the heterophylly of paper mulberry, which could account for the physiological data. Indeed, CR-wox mutant lines showed significant changes in leaf phenotypes, and differential expression profile analysis also highlighted the expression of genes related to phytohormones and transcription factors. Together, the genetic variations and candidate genes detected in this study provide novel insights into the genetic mechanism of heterophylly, and would improve the understanding of eco-adaptability in heterophyllous woody plants.

Drought stress-induced the formation of heteromorphic leaves of Populus euphratica Oliv: evidence from gene transcriptome

Populus euphratica Oliv., a dominant species of arid desert community, grows heteromorphic leaves at different crown positions. Whether heteromorphic leaves are a strategy of plant adaptation to drought stress is rarely reported. This study sequenced the transcriptome of three typical heteromorphic leaves (lanceolate, ovate and broad-ovate leaves) of P. euphratica, and measured their drought stress. We wanted to reveal the molecular mechanisms underlying the formation of heteromorphic leaves. Drought stress was increased significantly from lanceolate to ovate to broad-ovate leaves. Gene ontology (GO) and KEGG enrichment analysis showed that the MADs-box gene regulated the expression of peroxidase (POD) in the phenylpropane biosynthetic pathway. The up-regulated expression of the chalcone synthase (CHS) gene in broad-ovate leaves significantly activated the flavonoid biosynthetic pathway. In the process of leaf shape change, the different expressions of homeodomain leucine zipper (HD-ZIP) among the three heteromorphic leaves had potential interactions on the AUX and ABA pathways. The expression of Sucrose phosphate synthase (SPS) and sucrose synthase (SUS) increased from lanceolate to broad-ovate leaves, resulting in a consistent change in starch and sucrose content. We concluded that these resistance-related pathways are expressed in parallel with leaf formation genes, thereby inducing the formation of heteromorphic leaves. Our work provided a new insights for desert plants to adapt to drought stress.

Hydraulic constraints determine the distribution of heteromorphic leaves along plant vertical height

As an interesting and important trait of some drought-tolerant species, heteromorphic leaves are distributed differentially along plant vertical heights. However, the underpinning mechanism for the formation of heteromorphic leaves remains unclear. We hypothesize that heteromorphic leaves are caused by the hydraulic constraints possibly due to the compensation of the changes in functional traits in response to water transport capacity or the reduction of ineffective water loss. In this study, differences in water transport capacity, morphological traits, anatomical structures, and cellular water relations among three typical types of heteromorphic leaves (i.e., lanceolate, ovate, and broad-ovate) of Populus euphratica Oliv. (a dominant species of desert riparian forest in Central and West Asia) and their relationships were analyzed in order to explore the forming mechanism of heteromorphic leaves. The results showed that the lanceolate, ovate, and broad-ovate leaves were growing in the lower, intermediate, and higher positions from the ground, respectively. Morphological traits, anatomical structures, cellular water relations, and water transport capacity significantly varied among the three types of heteromorphic leaves (P< 0.01). Drought stress in broad-ovate leaves was significantly higher than that in ovate and lanceolate leaves (P< 0.01). Water transport capacity has significant correlations with morphological traits, anatomical structures, and cellular water relations (R ² ≥ 0.30; P< 0.01). Our results indicated that heteromorphic leaves were used as an important adaptive strategy for P. euphratica to alleviate the increase of hydraulic constraints along vertical heights.

Genetic Architecture of Heterophylly: Single and Multi-Leaf Genome-Wide Association Mapping in Populus euphratica

Heterophylly is an adaptive strategy used by some plants in response to environmental changes. Due to the lack of representative plants with typical heteromorphic leaves, little is known about the genetic architecture of heterophylly in plants and the genes underlying its control. Here, we investigated the genetic characteristics underlying changes in leaf shape based on the model species, Populus euphratica, which exhibits typical heterophylly. A set of 401,571 single-nucleotide polymorphisms (SNPs) derived from whole-genome sequencing of 860 genotypes were associated with nine leaf traits, which were related to descriptive and shape data using single- and multi-leaf genome-wide association studies (GWAS). Multi-leaf GWAS allows for a more comprehensive understanding of the genetic architecture of heterophylly by considering multiple leaves simultaneously. The single-leaf GWAS detected 140 significant SNPs, whereas the multi-leaf GWAS detected 200 SNP-trait associations. Markers were found across 19 chromosomes, and 21 unique genes were implicated in traits and serve as potential targets for selection. Our results provide novel insights into the genomic architecture of heterophylly, and provide candidate genes for breeding or engineering P. euphratica. Our observations also improve understanding of the intrinsic mechanisms of plant growth, evolution, and adaptation in response to climate change.

iTRAQ-based proteomic analysis of heteromorphic leaves reveals eco-adaptability of Populus euphratica Oliv

BACKGROUND

Heterophylly is regard as adaptation to different environments in plant, and Populus euphratica is an important heterophyllous woody plant. However, information on its molecular mechanism in eco-adaptability remains obscure.

RESULTS

In this research, proteins were identified by isobaric tags for relative and absolute quantitation (iTRAQ) technology in lanceolate, ovate, and dentate broad-ovate leaves from adult P. euphratica trees, respectively. Besides, chlorophyll content, net photosynthetic rate, stomatal conductance, transpiration rate and peroxidase activity in these heteromorphic leaves were investigated. A total number of 2,689 proteins were detected in the heteromorphic leaves, of which 56, 73, and 222 differential abundance proteins (DAPs) were determined in ovate/lanceolate, dentate broad-ovate/lanceolate, and dentate broad-ovate/ovate comparison groups. Bioinformatics analysis suggested these altered proteins related to photosynthesis, stress tolerance, respiration and primary metabolism accumulated in dentate broad-ovate and ovate leaves, which were consistent with the results of physiological parameters and Real-time Quantitative PCR experiments.

CONCLUSION

This research demonstrated the mechanism of the differential abundance proteins in providing an optimal strategy of resource utilization and survival for P. euphratica, that could offer clues for further investigations into eco-adaptability of heterophyllous woody plants.

Rejuvenation increases leaf biomass and flavonoid accumulation in Ginkgo biloba

Rejuvenation refers to the transition from an adult state to a juvenile state. Trunk truncation at the base of the tree can result in tree rejuvenation. However, little is known about the association of rejuvenation with leaf biomass and flavonoid accumulation. The results of this study showed that, compared with control leaves, leaves of renewed Ginkgo biloba shoots were larger, thicker, and more lobed and had higher fresh/dry weights and chlorophyll contents. The leaf biomass per hectare of rejuvenated trees was twofold higher than that of the untruncated controls. Moreover, we observed a marked increase in the accumulation of flavonol glycosides via metabolomic analysis and detected upregulated expression of genes involved in flavonoid biosynthesis, including CHS, FLS, F3'H, DFR, and LAR. Overexpression of GbCHS in ginkgo calli confirmed that GbCHS plays an important role in flavonoid biosynthesis. Interestingly, the contents of gibberellins significantly increased in the rejuvenated leaves. Moreover, exogenous gibberellin treatment significantly increased GbCHS expression and flavonoid contents. Our findings show that truncation can stimulate tree rejuvenation by altering hormone levels, representing an effective and feasible approach for enhancing the biomass and flavonoid content of G. biloba leaves.

Geometric and topological approaches to shape variation in Ginkgo leaves

Leaf shape is a key plant trait that varies enormously. The range of applications for data on this trait requires frequent methodological development so that researchers have an up-to-date toolkit with which to quantify leaf shape. We generated a dataset of 468 leaves produced by Ginkgo biloba, and 24 fossil leaves produced by evolutionary relatives of extant Ginkgo. We quantified the shape of each leaf by developing a geometric method based on elastic curves and a topological method based on persistent homology. Our geometric method indicates that shape variation in modern leaves is dominated by leaf size, furrow depth and the angle of the two lobes at the leaf base that is also related to leaf width. Our topological method indicates that shape variation in modern leaves is dominated by leaf size and furrow depth. We have applied both methods to modern and fossil material: the methods are complementary, identifying similar primary patterns of variation, but also revealing different aspects of morphological variation. Our topological approach distinguishes long-shoot leaves from short-shoot leaves, both methods indicate that leaf shape influences or is at least related to leaf area, and both could be applied in palaeoclimatic and evolutionary studies of leaf shape.

Roles of alternative electron flows in response to excess light in Ginkgo biloba

Ginkgo biloba L., the only surviving species of Ginkgoopsida, is a famous relict gymnosperm, it may provide new insight into the evolution of photosynthetic mechanisms. Flavodiiron proteins (FDPs) are conserved in nonflowering plants, but the role of FDPs in gymnosperms has not yet been clarified. In particular, how gymnosperms integrate FDPs and cyclic electron transport (CET) to better adapt to excess light is poorly understood. To elucidate these questions, we measured the P700 signal, chlorophyll fluorescence and electrochromic shift signal under fluctuating and constant light in G. biloba. Within the first seconds after light increased, G. biloba could not build up a sufficient proton gradient (ΔpH). Concomitantly, photo-reduction of O₂ mediated by FDPs contributed to the rapid oxidation of P700 and protected PSI under fluctuating light. Therefore, in G. biloba, FDPs mainly protect PSI under fluctuating light at acceptor side. Under constant high light, the oxidation of PSI and the induction of non-photochemical quenching were attributed to the increase in ΔpH formation, which was mainly caused by the increase in CET rather than linear electron transport. Therefore, under constant light, CET finely regulates the PSI redox state and non-photochemical quenching through ΔpH formation, protecting PSI and PSII against excess light. We conclude that, in G. biloba, FDPs are particularly important under fluctuating light while CET is essential under constant high light. The coordination of FDPs and CET fine-tune photosynthetic apparatus under excess light.

Morphological, structural and physiological differences in heteromorphic leaves of Euphrates poplar during development stages and at crown scales

Euphrates poplar (Populus euphratica Oliv.) has heteromorphic leaves including strip, lanceolate, ovate, and broad-ovate leaves from base to top in the mature canopy. To clarify how diameter at breast height (DBH) and tree height affect the functional characteristics of all kinds of heteromorphic leaves, we measured the morphological anatomical structure and physiological indices of five crown heteromorphic leaves of P. euphratica at 2, 4, 6, 8, 10, and 12 m from the same site. We also analysed the relationships between morphological structures and physiological characteristics of heteromorphic leaves and DBH and the height of heteromorphic leaves. The results showed that the number of abnormalities regarding blade width, leaf area, leaf thickness, leaf mass per area, cuticle layer thickness, palisade tissue thickness, and palisade tissue/sponge tissue ratio increased with size order and sampling height gradient. Net photosynthetic rate, transpiration rate, stomatal conductance, instantaneous water use efficiency, stable delta carbon isotope ratio, proline and malondialdehyde (MDA) content increased with DBH and sampling height. By contrast, blade length, leaf shape index, and intercellular CO2 concentration decreased with the increase in path order and sampling height gradient. Although MDA content and leaf sponge thickness were not correlated with DBH or sampling height, other morphological structure and physiological parameters were significantly correlated with these variables. In addition, correlations were found among leaf morphology, anatomical structure, and physiological index parameters indicating that they changed with path order and tree height gradient. The differences in the morphology, anatomic structure and physiological characteristics of the heteromorphic leaves ofP. euphratica are related to ontogenesis stage and coronal position.

Morphogenesis and vitality of seedlings of Ginkgo biloba in outdoor conditions

Ginkgo biloba L. is a relic tree that is preserved in natural conditions only in China. In the North-East Ukraine ginkgo is only in the process of introduction, but in certain places ginkgo trees are grown for decorative purposes which show high vitality. Therefore a relevant and promising task is the creation of industrial plantations of ginkgo for collection of its leaves. The objective of the study was morphological peculiarities of ginkgo plants and their vital condition depending on the calendar age of the seedlings at the stage when they were transplanted into the soil. For growing ginkgo, in our studies, a combined two-stage system was approbated, accor­ding to which seedlings were grown in greenhouses during 2–3 years, and then moved into soil outdoors. In our study, as planting material we used seedlings of ginkgo from a plant nursery (greenhouse) of Sumy National Agrarian University planted in 2014 and 2015. Seedlings were planted in the soil in early spring. Seedlings for growth completion were planted in rows with inter-row distance of 0.8 m and distance between the plants measuring 2.5–3.0 m. We studied the condition of the seedlings in two variants: variant 1 – three-year old seedlings obtained in the greenhouse were transplanted in spring 2018; variant 2 – two-year old seedlings obtained in greenhouse were transplanted in spring 2017. Comparative morphometric analysis and evaluation of vitality of these plants was performed in August 2019. Both groups of seedlings were five years old at the time of survey. For evaluation of morphometric structure of the ginkgo seedlings and their vital condition we used randomized selections of specimens in amount of up to 30 from plantations of variants 1 and 2. Analysis of the plants was conducted during the end of the vegetative period of 2019, when active growth of the plants stopped. Four main parameters of the plants were recorded: height, value of annual increment, diameter of the stem near the root neck and the number of leaves. Vital condition of the seedlings was evaluated on the basis of algorithm of vital analysis. Vitality of each seedling was evaluated by the coefficient of Q vitality, value of which was in interval from 0 to 1.0. Depending on the value of Q the seedlings were divided into five classes of vitality: 0–0.2 – cc (specimens of the lowest vitality), 0.2–0.4 – c (low vita­lity), 0.4–0.6 – b (average vitality), 0.6–0.8 – a (high vitality), 0.8–1.0 – aa (specimens with highest vitality). Comparative analysis of morphological peculiarities of the ginkgo seedlings revealed that two-year plants transplanted outdoors from the greenhouse had higher annual increment of the main stem, diameter of stem and number of leaves. Vital condition of such seedlings was the highest and was evaluated using Q index, which ranges between vitality of classes b to aa. This allows recommending a two-stage scheme of growing seedlings of ginkgo in North-East Ukraine. The optimum age for transplanting seedlings from greenhouses to open soil is two years.

Conjoint Analysis of Genome-Wide lncRNA and mRNA Expression of Heteromorphic Leavesin Response to Environmental Heterogeneityin Populus euphratica

Heterophylly is the phenomenon of leaf forms varying along the longitudinal axis within a single plant. Populus euphratica, a heterophyllous woody plant, develops lanceolate leaves and dentate broad-ovate leaves on the bottom and top of the canopy, respectively, which are faced with different intensities of ambient solar radiation. However, the mechanism of the heteromorphic leaf response to the microenvironment in P. euphratica remains elusive. Here, we show that the dentate broad-ovate leaves have advantages in tolerating high light intensity, while lanceolate leaves are excellent at capturing light. Compared with lanceolate leaves, more trichomes, higher stomatal density, thicker lamina, and higher specific leaf weight were observed in dentate broad-ovate leaves. Furthermore, high-throughput RNA sequencing analysis revealed that the expression patterns of genes and long noncoding RNAs (lncRNAs) are different between the two heteromorphic leaves. A total of 36,492 genes and 1725 lncRNAs were detected, among which 586 genes and 54 lncRNAs were differentially expressed. Based on targets prediction, lncRNAs and target genes involved in light adaption, protein repair, stress response, and growth and development pathways were differentially expressed in heteromorphic leaves, 10 pairs of which were confirmed by quantitative real-time PCR. Additionally, the analysis of interactions indicated that lncRNA-mRNA interactions were involved in the response to the microenvironment of heteromorphic leaves. Taken together, these results suggest that the morphological features and joint regulation of lncRNA-mRNA in heteromorphic leaves may serve as survival strategies for P. euphratica, which could lead to optimal utilization of environmental factors.

Scaling Relationships between Leaf Shape and Area of 12 Rosaceae Species

Leaf surface area (A) and leaf shape have been demonstrated to be closely correlated with photosynthetic rates. The scaling relationship between leaf biomass (both dry weight and fresh weight) and A has been widely studied. However, few studies have focused on the scaling relationship between leaf shape and A. Here, using more than 3600 leaves from 12 Rosaceae species, we examined the relationships of the leaf-shape indices including the left to right side leaf surface area ratio (AR), the ratio of leaf perimeter to leaf surface area (RPA), and the ratio of leaf width to length (RWL) versus A. We also tested whether there is a scaling relationship between leaf dry weight and A, and between PRA and A. There was no significant correlation between AR and A for each of the 12 species. Leaf area was also found to be independent of RWL because leaf width remained proportional to leaf length across the 12 species. However, there was a negative correlation between RPA and A. The scaling relationship between RPA and A held for each species, and the estimated scaling exponent of RPA versus A approached −1/2; the scaling relationship between leaf dry weight and A also held for each species, and 11 out of the 12 estimated scaling exponents of leaf dry weight versus A were greater than unity. Our results indicated that leaf surface area has a strong scaling relationship with leaf perimeter and also with leaf dry weight but has no relationship with leaf symmetry or RWL. Additionally, our results showed that leaf dry weight per unit area, which is usually associated with the photosynthetic capacity of plants, increases with an increasing A because the scaling exponent of leaf dry weight versus A is greater than unity. This suggests that a large leaf surface area requires more dry mass input to support the physical structure of the leaf.

Effects of Salt Stress on the Leaf Shape and Scaling of Pyrus betulifolia Bunge

Leaf shape can reflect the survival and development of plants in different environments. In particular, leaf area, showing a scaling relationship with other leaf-shape indices, has been used to evaluate the extent of salt stress on plants. Based on the scaling relationships between leaf area and other leaf-shape indices in experiments at different levels of salt stress, we could examine which leaf-shape indices are also related to salt stress. In the present study, we explored the effects of different salt concentration treatments on leaf dry mass per unit area (LMA), the quotient of leaf perimeter and leaf area (QPA), the quotient of leaf width and length (QWL), the areal quotient (AQ) of left and right sides of a leaf and the standardized index (SI) for bilateral symmetry. We treated Pyrus betulifolia Bunge under NaCl salt solution of 2‰, 4‰ and 6‰, respectively, with fresh water with no salt as the control. The reduced major axis (RMA) was used to fit a linear relationship of the log-transformed data between any leaf trait measures and leaf area. We found that leaf fresh weight and dry weight decrease with salt concentration increasing, whereas the exponents of leaf dry weight versus leaf area exhibit an increasing trend, which implies that the leaves expanding in higher salt environments are prone to have a higher cost of dry mass investment to increase per unit leaf area than those in lower salt environments. Salt concentration has a significant influence on leaf shape especially QWL, and QWL under 6‰ concentration treatment is significantly greater than the other treatments. However, there is no a single increasing or decreasing trend for the extent of leaf bilateral symmetry with salt concentration increasing. In addition, we found that the scaling exponents of QPA versus leaf area for four treatments have no significant difference. It indicates that the scaling relationship of leaf perimeter versus leaf area did not change with salt concentration increasing. The present study suggests that salt stress can change leaf functional traits especially the scaling relationship of leaf dry weight versus leaf area and QWL, however, it does not significantly affect the scaling relationships between leaf morphological measures (including QPA and the extent of leaf bilateral symmetry) and leaf area.

Proportional Relationship between Leaf Area and the Product of Leaf Length and Width of Four Types of Special Leaf Shapes

The leaf area, as an important leaf functional trait, is thought to be related to leaf length and width. Our recent study showed that the Montgomery equation, which assumes that leaf area is proportional to the product of leaf length and width, applied to different leaf shapes, and the coefficient of proportionality (namely the Montgomery parameter) range from 1/2 to π/4. However, no relevant geometrical evidence has previously been provided to support the above findings. Here, four types of representative leaf shapes (the elliptical, sectorial, linear, and triangular shapes) were studied. We derived the range of the estimate of the Montgomery parameter for every type. For the elliptical and triangular leaf shapes, the estimates are π/4 and 1/2, respectively; for the linear leaf shape, especially for the plants of Poaceae that can be described by the simplified Gielis equation, the estimate ranges from 0.6795 to π/4; for the sectorial leaf shape, the estimate ranges from 1/2 to π/4. The estimates based on the observations of actual leaves support the above theoretical results. The results obtained here show that the coefficient of proportionality of leaf area versus the product of leaf length and width only varies in a small range, maintaining the allometric relationship for leaf area and thereby suggesting that the proportional relationship between leaf area and the product of leaf length and width broadly remains stable during leaf evolution.

Competition for epidermal space in the evolution of leaves with high physiological rates

Leaves with high photosynthetic capacity require high transpiration capacity. Consequently, hydraulic conductance, stomatal conductance, and assimilation capacities should be positively correlated. These traits make independent demands on anatomical space, particularly due to the propensity for veins to have bundle sheath extensions that exclude stomata from the local epidermis. We measured density and area occupation of bundle sheath extensions, density and size of stomata and subsidiary cells, and venation density for a sample of extant angiosperms and fossil and living nonangiosperm tracheophytes. For most nonangiosperms, even modest increases in vein density and stomatal conductance would require substantial reconfigurations of anatomy. One characteristic of the angiosperm syndrome (e.g. small cell sizes, etc.) is hierarchical vein networks that allow expression of bundle sheath extensions in some, but not all veins, contrasting with all-or-nothing alternatives available with the single-order vein networks in most nonangiosperms. Bundle sheath modulation is associated with higher vein densities in three independent groups with hierarchical venation: angiosperms, Gnetum (gymnosperm) and Dipteris (fern). Anatomical and developmental constraints likely contribute to the stability in leaf characteristics - and ecophysiology - seen through time in different lineages and contribute to the uniqueness of angiosperms in achieving the highest vein densities, stomatal densities, and physiological rates.

The pipe model theory half a century on: a review

Background

More than a half century ago, Shinozaki et al. (Shinozaki K, Yoda K, Hozumi K, Kira T. 1964a. A quantitative analysis of plant form - the pipe model theory. I. Basic analyses. Japanese Journal of Ecology B: 97-105) proposed an elegant conceptual framework, the pipe model theory (PMT), to interpret the observed linear relationship between the amount of stem tissue and corresponding supported leaves. The PMT brought a satisfactory answer to two vividly debated problems that were unresolved at the moment of its publication: (1) What determines tree form and which rules drive biomass allocation to the foliar versus stem compartments in plants? (2) How can foliar area or mass in an individual plant, in a stand or at even larger scales be estimated? Since its initial formulation, the PMT has been reinterpreted and used in applications, and has undoubtedly become an important milestone in the mathematical interpretation of plant form and functioning.

Scope

This article aims to review the PMT by going back to its initial formulation, stating its explicit and implicit properties and discussing them in the light of current biological knowledge and experimental evidence in order to identify the validity and range of applicability of the theory. We also discuss the use of the theory in tree biomechanics and hydraulics as well as in functional-structural plant modelling.

Conclusions

Scrutinizing the PMT in the light of modern biological knowledge revealed that most of its properties are not valid as a general rule. The hydraulic framework derived from the PMT has attracted much more attention than its mechanical counterpart and implies that only the conductive portion of a stem cross-section should be proportional to the supported foliage amount rather than the whole of it. The facts that this conductive portion is experimentally difficult to measure and varies with environmental conditions and tree ontogeny might cause the commonly reported non-linear relationships between foliage and stem metrics. Nevertheless, the PMT can still be considered as a portfolio of properties providing a unified framework to integrate and analyse functional-structural relationships.

The hydraulic architecture of Ginkgo leaves

PREMISE OF THE STUDY

The hydraulics of xylem has been widely studied in numerous species and organ types. However, comparatively little is known about how phloem and xylem are hydraulically coupled or about many of the basic structural properties of phloem (such as conducting cell numbers and conductive areas), which nevertheless have direct bearing on understanding phloem loading and unloading.

METHODS

Using a combination of light, epifluorescence, confocal, and transmission electron microscopy, we quantified the hydraulic architecture of Ginkgo biloba leaf laminae and examined the scaling relationships between phloem and xylem in five fully mature leaves.

KEY RESULTS

The conductive areas and lengths of sieve cells and tracheids increase basipetally toward the petiole in a manner that is consistent with Münch's pressure flow hypothesis for phloem transport. This trend holds true for individual veins, the sum of conductive areas across all veins at any distance from the petiole, and for individual sieve cells and tracheids. Further, the conductive areas of phloem and xylem are isometrically correlated across the entire vasculature of the leaf lamina. The data for conducting cell areas do not conform with the predictions of the hydraulic models of da Vinci and Murray.

CONCLUSIONS

The scaling of Ginkgo lamina hydraulics complies with that observed in leaves of other gymnosperms and most angiosperms and is inconsistent with theoretical models that assume that the volume of transported incompressible fluids is conserved.

The influence of leaf size and shape on leaf thermal dynamics: does theory hold up under natural conditions?

Laboratory studies on artificial leaves suggest that leaf thermal dynamics are strongly influenced by the two-dimensional size and shape of leaves and associated boundary layer thickness. Hot environments are therefore said to favour selection for small, narrow or dissected leaves. Empirical evidence from real leaves under field conditions is scant and traditionally based on point measurements that do not capture spatial variation in heat load. We used thermal imagery under field conditions to measure the leaf thermal time constant (τ) in summer and the leaf-to-air temperature difference (∆T) and temperature range across laminae (T_range ) during winter, autumn and summer for 68 Proteaceae species. We investigated the influence of leaf area and margin complexity relative to effective leaf width (w_e ), the latter being a more direct indicator of boundary layer thickness. Normalized difference of margin complexity had no or weak effects on thermal dynamics, but w_e strongly predicted τ and ∆T, whereas leaf area influenced T_range . Unlike artificial leaves, however, spatial temperature distribution in large leaves appeared to be governed largely by structural variation. Therefore, we agree that small size, specifically w_e , has adaptive value in hot environments but not with the idea that thermal regulation is the primary evolutionary driver of leaf dissection.

Leaf size serves as a proxy for xylem vulnerability to cavitation in plantation trees

Hybrid poplars are an important renewable forest resource known for their high productivity. At the same time, they are highly vulnerable to water stress. Identifying traits that can serve as indicators for growth performance remains an important task, particularly under field conditions. Understanding which trait combinations translate to improved productivity is key in order to satisfy the demand for poplar wood in an uncertain future climate. In this study, we compared hydraulic and leaf traits among five hybrid poplar clones at 10 plantations in central Alberta. We also assessed the variation of these traits between 2- to 3-year-old branches from the lower to mid-crown and current-year long shoots from the mid to upper crown. Our results showed that (1) hybrid poplars differed in key hydraulic parameters between branch type, (2) variation of hydraulic traits among clones was relatively large for some clones and less for others, and (3) strong relationships between measured hydraulic traits, such as vessel diameter, cavitation resistance, xylem-specific and leaf-specific conductivity and leaf area, were observed. Our results suggest that leaf size could serve as an additional screening tool when selecting for drought-tolerant genotypes in forest management and tree improvement programmes.

Evolution of a unique anatomical precision in angiosperm leaf venation lifts constraints on vascular plant ecology

The main role of leaf venation is to supply water across the photosynthetic surface to keep stomata open and allow access to atmospheric CO2 despite evaporative demand. The optimal uniform delivery of water occurs when the distance between veins equals the depth of vein placement within the leaf away from the evaporative surface. As presented here, only angiosperms maintain this anatomical optimum across all leaf thicknesses and different habitats, including sheltered environments where this optimization need not be required. Intriguingly, basal angiosperm lineages tend to be underinvested hydraulically; uniformly high optimization is derived independently in the magnoliids, monocots and core eudicots. Gymnosperms and ferns, including available fossils, are limited by their inability to produce high vein densities. The common association of ferns with shaded humid environments may, in part, be a direct evolutionary consequence of their inability to produce hydraulically optimized leaves. Some gymnosperms do approach optimal vein placement, but only by virtue of their ability to produce thick leaves most appropriate in environments requiring water conservation. Thus, this simple anatomical metric presents an important perspective on the evolution and phylogenetic distribution of plant ecologies and further evidence that the vegetative biology of flowering plants-not just their reproductive biology-is unique.

Evolution of a unique anatomical precision in angiosperm leaf venation lifts constraints on vascular plant ecology

The main role of leaf venation is to supply water across the photosynthetic surface to keep stomata open and allow access to atmospheric CO2 despite evaporative demand. The optimal uniform delivery of water occurs when the distance between veins equals the depth of vein placement within the leaf away from the evaporative surface. As presented here, only angiosperms maintain this anatomical optimum across all leaf thicknesses and different habitats, including sheltered environments where this optimization need not be required. Intriguingly, basal angiosperm lineages tend to be underinvested hydraulically; uniformly high optimization is derived independently in the magnoliids, monocots and core eudicots. Gymnosperms and ferns, including available fossils, are limited by their inability to produce high vein densities. The common association of ferns with shaded humid environments may, in part, be a direct evolutionary consequence of their inability to produce hydraulically optimized leaves. Some gymnosperms do approach optimal vein placement, but only by virtue of their ability to produce thick leaves most appropriate in environments requiring water conservation. Thus, this simple anatomical metric presents an important perspective on the evolution and phylogenetic distribution of plant ecologies and further evidence that the vegetative biology of flowering plants-not just their reproductive biology-is unique.

Branch architecture in Ginkgo biloba: wood anatomy and long shoot-short shoot interactions

PREMISE

Ginkgo, centrally placed in seed plant phylogeny, is considered important in many phylogenetic and evolutionary studies. Shoot dimorphism of Ginkgo has been long noted, but no work has yet been done to evaluate the relationships between overall branch architecture and wood ring characters, shoot growth, and environmental conditions. •

METHODS

Branches, sampled from similar canopy heights, were mapped with the age of each long shoot segment determined by counting annual leaf-scar series on its short shoots. Transverse sections were made for each long shoot segment and an adjacent short shoot; wood ring thickness, number of rings, and number of tracheids/ring were determined. Using branch maps, we identified wood rings for each long shoot segment to year and developmental context of each year (distal short shoot growth only vs. at least one distal long shoot). Climate data were also analyzed in conjunction with developmental context. •

KEY RESULTS

Significantly thicker wood rings occur in years with distal long shoot development. The likelihood that a branch produced long shoots in a given year was lower with higher maximum annual temperature. Annual maximum temperature was negatively correlated with ring thickness in microsporangiate trees only. Annual minimum temperatures were correlated differently with ring thickness of megasporangiate and microsporangiate trees, depending on the developmental context. There were no significant effects associated with precipitation. •

CONCLUSIONS

Overall, developmental context alone predicts wood ring thickness about as well as models that include temperature. This suggests that although climatic factors may be strongly correlated with wood ring data among many gymnosperm taxa, at least for Ginkgo, correlations with climate data are primarily due to changes in proportions of shoot developmental types (LS vs. SS) across branches.

Differences in the scaling of area and mass of Ginkgo biloba (Ginkgoaceae) leaves and their relevance to the study of specific leaf area

PREMISE OF THE STUDY

The manner in which the area of the leaf lamina (A) scales with respect to the dry mass of the lamina (M) is an important functional trait that is correlated with whole-plant growth rates and habitat preferences across diverse species. However, the extent to which the scaling between these two variables differs among leaves collected from different types of shoots within the canopy of a tree is poorly understood. Should they exist, significant differences in the A vs. M scaling relationship within canopies would raise a number of important questions, in particular what constitutes an adequate sampling procedure to determine the whole-canopy A vs. M relationship.

METHODS

To address this issue, we used a large data set representing 13 biologically distinct categories of leaves sampled from mega- and microsporangiate trees of the dioecious gymnosperm Ginkgo biloba.

KEY RESULTS

Analyses of the data for these 13 categories of leaves identify seven statistically significantly different modes of A vs. M scaling that result in significant differences in how specific leaf area (SLA) changes as M varies within the canopies of Ginkgo.

CONCLUSIONS

These results indicate that the protocols used to sample leaves for the analysis of foliar functional traits such as specific leaf area need to acknowledge and cope with the effects of leaf and shoot polymorphisms on the quantification of functional traits (and on the construction and testing of hypotheses about these traits).

Patterns of diversity in leaves from canopies of Ginkgo biloba are revealed using Specific Leaf Area as a morphological character

PREMISE OF THE STUDY

The difference reported in the literature for the Specific Leaf Area (SLA, cm(2)/g) of leaves on short- and long-shoots of Acer rubrum could mean that SLA can serve as a quantitative morphological trait. Our survey of SLA in canopies of Ginkgo biloba sampled a different clade of seed plants to investigate this morphological phenomenon. Such a survey in this dioecious taxon, and one in which a single canopy may have juvenile and reproductive portions, as well as one where canopies bear leaves of several shapes, examine these additional morphological factors as well as any long-shoot short-shoot differences.

METHODS

We measured SLA for a set of 642 dried leaves, a sampling across all morphological levels in canopies of large landscape specimens. The tabulated values were analyzed as distributions.

KEY RESULTS

Populations of leaves of G. biloba, sorted by morphological features of canopy structure, differ between long- and short-shoots (175%), on the two genders of tree (131%), in the juvenile and reproductive portions of a canopy (183%), and with the presence or absence of seed on short-shoots in the reproductive portion of megasporangiate canopies (114%). Basipetal leaves of long-shoots and leaves of short-shoots have similar values of SLA.

CONCLUSIONS

With the exception of the acropetal decrease in SLA along long-shoots, the differences among the several classes of leaf seem to reflect local sink strength, even though the sink itself develops after leaves mature. The large overall range in the values of SLA in Ginkgo underscores the relevance of the details of canopy structure to parsing ecological phenomena.

The ABA-mediated switch between submersed and emersed life-styles in aquatic macrophytes

Hydrophytes comprise aquatic macrophytes from various taxa that are able to sustain and to complete their lifecycle in a flooded environment. Their ancestors, however, underwent adaptive processes to withstand drought on land and became partially or completely independent of water for sexual reproduction. Interestingly, the step backwards into the high-density aquatic medium happened independently several times in numerous plant taxa. For flowering plants, this submersed life-style is especially difficult as they need to erect their floral organs above the water surface to be pollinated. Moreover, fresh-water plants evolved the adaptive mechanism of heterophylly, which enabled them to switch between a submersed and an emersed leaf morphology. The plant hormone abscisic acid (ABA) is a key factor of heterophylly induction in aquatic plants and is a major switch between a submersed and emersed life. The mechanisms of ABA signal perception and transduction appear to be conserved throughout the evolution of basal plants to angiosperms and from terrestrial to aquatic plants. This review summarizes the interplay of environmental factors that act through ABA to orchestrate adaptation of plants to their aquatic environment.

The evolution and functional significance of leaf shape in the angiosperms

Angiosperm leaves manifest a remarkable diversity of shapes that range from developmental sequences within a shoot and within crown response to microenvironment to variation among species within and between communities and among orders or families. It is generally assumed that because photosynthetic leaves are critical to plant growth and survival, variation in their shape reflects natural selection operating on function. Several non-mutually exclusive theories have been proposed to explain leaf shape diversity. These include: thermoregulation of leaves especially in arid and hot environments, hydraulic constraints, patterns of leaf expansion in deciduous species, biomechanical constraints, adaptations to avoid herbivory, adaptations to optimise light interception and even that leaf shape variation is a response to selection on flower form. However, the relative importance, or likelihood, of each of these factors is unclear. Here we review the evolutionary context of leaf shape diversification, discuss the proximal mechanisms that generate the diversity in extant systems, and consider the evidence for each the above hypotheses in the context of the functional significance of leaf shape. The synthesis of these broad ranging areas helps to identify points of conceptual convergence for ongoing discussion and integrated directions for future research.