Grace P. John

The species Cavendishia bracteata and Macleania rupestris (Ericaceae) are plants with medicinal potential used in ecological restoration in the high Andean forests of South America; however, the anatomy and histochemistry of their leaves have not yet been described. Plant material was collected from permanent plots established in the Encenillo Biological Reserve, Guasca, Colombia. For anatomical analysis, the leaf samples were embedded in paraffin, sectioned, and stained using astra blue and fuchsin staining techniques; subsequently, tissue measurements were taken. For histochemical analysis, freehand sections were made and stained with potassium dichromate, phloroglucinol, Lugol's, and Sudan black. The leaves of both species displayed meso-xeromorphic traits with a thick cuticle, smooth adaxial and irregular abaxial surfaces, an epidermis with thick outer walls, lignified adaxial and abaxial hypodermis, one or two palisade parenchyma layers, and a central collateral vascular bundle. Histochemical analysis revealed the presence of phenolic compounds, tannins, lignin, and neutral lipids. No starch granules were observed. This study describes the leaf anatomy of C. bracteata and M. rupestris under high-altitude conditions in the Colombian Andes. The qualitative presence of phenolic compounds, tannins, lignin, and neutral lipids supports their potential medicinal value.

The generality of the worldwide leaf economics spectrum (LES) has made it a pillar of trait-based ecological research. Yet, few studies have examined the processes shaping the evolution of the LES within species, in part, because most species occupy only a small portion of the LES. To address this gap, we took advantage of the distinct leaf economics strategies present in different ecotypes of the phenotypically diverse perennial grass Panicum virgatum (switchgrass) to generate a genetic mapping population, which we planted in common gardens at three sites spanning 12 degrees of latitude in the central United States. With this genetic mapping population, we evaluated two potentially interacting causes of LES evolution: 1) genetic architecture, where multiple traits are influenced by either the same gene (pleiotropy) or by genes in close physical proximity (genetic linkage), and 2) correlational selection, where selection acts on traits in combination rather than in isolation. We found that shared genetic architecture influenced covariation between photosynthetic rate ( A _MASS ) and leaf nitrogen ( N _MASS ) and between A _MASS and leaf mass per area (LMA). We also found that correlational selection favored the trait combinations predicted by the LES (e.g., high LMA with low N _MASS or low LMA with high N _MASS ) and disfavored other, mismatched trait combinations at two of the three sites. Together, these results demonstrate how the evolution of an integrated LES within species can arise from multiple evolutionary causes.

The world is experiencing a rise in global temperatures as a result of climate change. Higher temperatures along with more frequent heat waves negatively impact physiological levels and ultimately lead to plant death. Although woody species have different strategies to regulate leaf temperature, the relative importance of structural and anatomical traits in leaf temperature regulation remains unclear. We evaluated the variation in thermal tolerance (T₅₀ and T_crit) among 17 woody species in an Andean Tropical Montane Forest in Colombia. We explored how 18 leaf structural and anatomical traits can explain the variation in thermal tolerance. Our findings revealed high variability in thermal tolerance, structural, and anatomical leaf traits among species. Thick layers of upper cuticle, epidermis, and hypodermis increased T₅₀. However, increases in T_crit were only associated with upper hypodermis thickness. Higher stomatal index, a trait related to increased stomatal conductance and leaf transpiration, increased T₅₀, while traits associated with leaf water transport did not correlate with temperature tolerance. Our results highlight the contrasting influence of leaf tissues on leaf tolerance and the diverse strategies plants employ to cope with high temperatures. These findings hold significant implications for the climate‐smart restoration of Andean Tropical Montane forests. Read the free Plain Language Summary for this article on the Journal blog.

Leaf surface conductance to water vapor and CO2 across the epidermis (gleaf) strongly determines the rates of gas exchange. Thus, clarifying the drivers of gleaf has important implications for resolving the mechanisms of photosynthetic productivity and leaf and plant responses and tolerance to drought. It is well recognized that gleaf is a function of the conductances of the stomata (gs) and of the epidermis + cuticle (gec). Yet, controversies have arisen around the relative roles of stomatal density (d) and size (s), fractional stomatal opening (α; aperture relative to maximum), and gec in determining gleaf. Resolving the importance of these drivers is critical across the range of leaf surface conductances, from strong stomatal closure under drought (gleaf,min), to typical opening for photosynthesis (gleaf,op), to maximum achievable opening (gleaf,max). We derived equations and analyzed a compiled database of published and measured data for approximately 200 species and genotypes. On average, within and across species, higher gleaf,min was determined 10 times more strongly by α and gec than by d and negligibly by s; higher gleaf,op was determined approximately equally by α (47%) and by stomatal anatomy (45% by d and 8% by s), and negligibly by gec; and higher gleaf,max was determined entirely by d. These findings clarify how diversity in stomatal functioning arises from multiple structural and physiological causes with importance shifting with context. The rising importance of d relative to α, from gleaf,min to gleaf,op, enables even species with low gleaf,min, which can retain leaves through drought, to possess high d and thereby achieve rapid gas exchange in periods of high water availability.

Summary: Turgor loss point (TLP) is an important proxy for plant drought tolerance, species habitat suitability, and drought‐induced plant mortality risk. Thus, TLP serves as a critical tool for evaluating climate change impacts on plants, making it imperative to develop high‐throughput and in situ methods to measure TLP. We developed hyperspectral pressure–volume curves (PV curves) to estimate TLP using leaf spectral reflectance. We used partial least square regression models to estimate water potential (Ψ) and relative water content (RWC) for two species, Frangula caroliniana and Magnolia grandiflora. RWC and Ψ's model for each species had R² ≥ 0.7 and %RMSE = 7–10. We constructed PV curves with model estimates and compared the accuracy of directly measured and spectra‐predicted TLP. Our findings indicate that leaf spectral measurements are an alternative method for estimating TLP. F. caroliniana TLP's values were −1.62 ± 0.15 (means ± SD) and −1.62 ± 0.34 MPa for observed and reflectance predicted, respectively (P > 0.05), while M. grandiflora were −1.78 ± 0.34 and −1.66 ± 0.41 MPa (P > 0.05). The estimation of TLP through leaf reflectance‐based PV curves opens a broad range of possibilities for future research aimed at understanding and monitoring plant water relations on a large scale with spectral ecophysiology.

Allometric relationships among the dimensions of leaves and their cells hold across diverse eudicotyledons, but have remained untested in the leaves of grasses. We hypothesised that geometric (proportional) allometries of cell sizes across tissues and of leaf dimensions would arise due to the coordination of cell development and that of cell functions such as water, nutrient and energy transport, and that cell sizes across tissues would be associated with light‐saturated photosynthetic rate. We tested predictions across 27 globally distributed C₃ and C₄ grass species grown in a common garden. We found positive relationships among average cell sizes within and across tissues, and of cell sizes with leaf dimensions. Grass leaf anatomical allometries were similar to those of eudicots, with exceptions consistent with the fewer cell layers and narrower form of grass leaves, and the specialised roles of epidermis and bundle sheath in storage and leaf movement. Across species, mean cell sizes in each tissue were associated with light‐saturated photosynthetic rate per leaf mass, supporting the functional coordination of cell sizes. These findings highlight the generality of evolutionary allometries within the grass lineage and their interlinkage with coordinated development and function.

Improved estimation of climate niches is critical, given climate change. Plant adaptation to climate depends on their physiological traits and their distributions, yet traits are rarely used to inform the estimation of species climate niches, and the power of a trait‐based approach has been controversial, given the many ecological factors and methodological issues that may result in decoupling of species' traits from their native climate. For 107 species across six ecosystems of California, we tested the hypothesis that mechanistic leaf and wood traits can robustly predict the mean of diverse species' climate distributions, when combining methodological improvements from previous studies, including standard trait measurements and sampling plants growing together at few sites. Further, we introduce an approach to quantify species' trait‐climate mismatch. We demonstrate a strong power to predict species mean climate from traits. As hypothesized, the prediction of species mean climate is stronger (and mismatch lower) when traits are sampled for individuals closer to species' mean climates. Improved resolution of species' climate niches based on mechanistic traits can importantly inform conservation of vulnerable species under the threat of climatic shifts in upcoming decades. Read the free Plain Language Summary for this article on the Journal blog.

Intra‐specific trait variation (ITV) plays a role in processes at a wide range of scales from organs to ecosystems across climate gradients. Yet, ITV remains rarely quantified for many ecophysiological traits typically assessed for species means, such as pressure volume (PV) curve parameters including osmotic potential at full turgor and modulus of elasticity, which are important in plant water relations. We defined a baseline “reference ITV” (ITV_ref) as the variation among fully exposed, mature sun leaves of replicate individuals of a given species grown in similar, well‐watered conditions, representing the conservative sampling design commonly used for species‐level ecophysiological traits. We hypothesized that PV parameters would show low ITV_ref relative to other leaf morphological traits, and that their intraspecific relationships would be similar to those previously established across species and proposed to arise from biophysical constraints. In a database of novel and published PV curves and additional leaf structural traits for 50 diverse species, we found low ITV_ref for PV parameters relative to other morphological traits, and strong intraspecific relationships among PV traits. Simulation modeling showed that conservative ITV_ref enables the use of species‐mean PV parameters for scaling up from spectroscopic measurements of leaf water content to enable sensing of leaf water potential.

Summary: Water stress can cause declines in plant function that persist after rehydration. Recent work has defined ‘resilience’ traits characterizing leaf resistance to persistent damage from drought, but whether these traits predict resilience in whole‐plant function is unknown. It is also unknown whether the coordination between resilience and ‘resistance’ – the ability to maintain function during drought – observed globally occurs within ecosystems. For eight rainforest species, we dehydrated and subsequently rehydrated leaves, and measured water stress thresholds for declines in rehydration capacity and maximum quantum yield of photosystem II (F_v/F_m). We tested correlations with embolism resistance and dry season water potentials (Ψ_MD), and calculated safety margins for damage (Ψ_MD – thresholds) and tested correlations with drought resilience in sap flow and growth. Ψ thresholds for persistent declines in F_v/F_m, indicating resilience, were positively correlated with Ψ_MD and thresholds for leaf vein embolism. Safety margins for persistent declines in F_v/F_m, but not rehydration capacity, were positively correlated with drought resilience in sap flow. Correlations between resistance and resilience suggest that species' differences in performance during drought are perpetuated after drought, potentially accelerating shifts in forest composition. Resilience to photochemical damage emerged as a promising functional trait to characterize whole‐plant drought resilience.