Biology subjects

Roddy, A.

Publications and source records attributed to Roddy, A..

12 recordsLinked to original sources

bioRxiv2024

Transpirational water loss from flowers is low but unregulated

Flowers play a critical role in reproduction for most of the flowering plants, and maintaining flowers throughout their lifespan can require substantial resources, such as carbon and water. Increases in temperature and aridity due to climate change are shifting the atmospheric conditions experienced by flowers, potentially altering the costs of floral maintenance. However, little is known about floral physiology and the capacity of flowers to regulate water loss. Because many flowers have few, if any, stomata, flowers may not be able to curtail water loss when the atmospheric demand for water vapor is high. Here, we tested whether the surface conductance of of flower petals, tepals, and showy bracts responds dynamically to changes in the vapor pressure gradient driving water loss. We measure the responses of flower surface conductance (gs) to step changes in the vapor pressure gradient on nine species. Across species, gs was low among all species, and there was little, if any, response in gs to step changes in humidity. The lack of response in gs resulted in linear responses of transpirational water loss to variation in vapor pressure deficit. These results suggest that unusually hot, dry conditions could elevate water loss from flowers, leading to premature wilting and senescence, thereby shortening floral longevity.

plant biology↗

bioRxiv2023

The effects of genome size on cell size and the functional composition and morphology of leaves: a case study in Rhododendron (Ericaceae)

Background and AimsDespite the predominance of scaling photosynthetic metabolism by two-dimensional leaf surface area, leaves are three-dimensional structures composed of multiple tissues that directly and indirectly influence photosynthetic metabolism. The structure of leaf surfaces for CO2 diffusion and light transmission and the internal volume of tissues that process energy and matter work together to control rates of resource acquisition and turnover. Here we investigate the influence of cell size and packing density on resource acquisition as measured by surface conductance to CO2 and water vapor and on resource turnover as measured by leaf water turnover time. MethodsWe sampled wild and cultivated congeneric species in the genus Rhododendron (Ericaceae) and measured genome size, anatomical traits related to cell sizes and packing densities, and morphological traits related to water content and dry mass allocation. ResultsAmong Rhododendron, anatomical traits related to cell size and morphological traits related to water content and dry mass investment varied largely orthogonally to each other, allowing for many combinations of leaf traits to exist. However, there was a strong, negative relationship between the leaf water residence time ({tau}) and the maximum leaf surface conductance per leaf volume (gmax,vol), both of which are influenced by cell size and cell packing densities. ConclusionsDespite leaf function being controlled by many potential combinations of leaf cell- and tissue-level traits, cell size has a pervasive effect on leaf function. Small cells allow for higher diffusion of CO2 and water vapor per unit leaf volume (gmax,vol) even at constant leaf thickness, but small cells also result in shorter leaf water residence times ({tau}). The strong tradeoff between gmax,vol and ({tau}) illuminates how genome size-cell size allometry influences the fast-slow continuum of plant carbon and water economy.

plant biology↗

bioRxiv2023

Flowers are leakier than leaves but cheaper to build

- Producing and maintaining flowers is essential for reproduction in most angiosperms, underpinning population persistence and speciation. Although the physiological costs of flowers often oppose pollinator selection, these physiological costs have rarely been quantified. - We measured a suite of physiological traits quantifying the water and carbon costs and drought tolerance on flowers and leaves of over 100 phylogenetically diverse species, including water and dry mass contents, minimum epidermal conductance to water vapor (gmin), vein density, and dry mass per area. - Although there was substantial variation among species, flowers had significantly higher gmin and water content per unit area than leaves, but significantly lower vein density and dry mass per area than leaves. Both leaves and flowers exhibited similarly strong scaling between dry mass investment and water content. - The higher gmin of flowers offset their higher water content, suggesting that flowers may desiccate more rapidly than leaves during drought. The coordination between dry mass and water investment suggests that flowers rely on a hydrostatic skeleton to remain upright rather than on a carbon-based skeleton. For short-lived structures like flowers, water may be relatively cheaper than carbon, particularly given the relatively high amount of water loss per unit of carbon synthesized in photosynthesis.

plant biology↗

bioRxiv2022

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

Background and AimsWhile 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. MethodsHere, we examined the coordination between genome size, leaf cell sizes, and cell packing densities, and leaf size in 13 mangrove species across four sites. Four of these species occurred at more than one site, allowing us to test the effect of climate on leaf anatomy. ResultsWe found that genome sizes of mangroves were very small compared to other angiosperms, and, 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. Contrary 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. ConclusionsWhile mangroves exhibited coordination between veins and stomata and attained a maximum theoretical stomatal conductance similar to other angiosperms, the tissue-level tradeoffs underlying these similar relationships across species and environments was markedly different, perhaps indicative of the unique structural and physiological adaptations of mangroves to their stressful environments.

ecology↗

bioRxiv2020

Diversification, disparification, and hybridization in the desert shrubs Encelia

O_LIThere are multiple hypotheses for the spectacular plant diversity found in deserts. We explore how different factors, including the roles of ecological opportunity and selection, promote diversification and disparification in Encelia, a lineage of woody plants in the deserts of the Americas. C_LIO_LIUsing a nearly complete species-level phylogeny along with a broad set of phenotypic traits, we estimate divergence times and diversification rates, identify instances of hybridization, quantify trait disparity, and assess phenotypic divergence across environmental gradients. C_LIO_LIWe show that Encelia originated and diversified recently (mid-Pleistocene) and rapidly, with rates comparable to notable adaptive radiations in plants. Encelia probably originated in the hot deserts of North America, with subsequent diversification across steep environmental gradients. We uncover multiple instances of gene flow between species. The radiation of Encelia is characterized by fast rates of phenotypic evolution, trait lability, and extreme disparity across environments and between species-pairs with overlapping geographic ranges. C_LIO_LIEncelia exemplifies how interspecific gene flow in combination with high trait lability can enable exceptionally fast diversification and disparification across steep environmental gradients. C_LI

evolutionary biology↗

bioRxiv2020

Natural selection maintains species despite widespread hybridization in the desert shrub Encelia

Natural selection is an important driver of genetic and phenotypic differentiation between species. A powerful way to test the role of natural selection in the formation and maintenance of species is to study species complexes in which potential gene flow is high but realized gene flow is low. For a recent radiation of New World desert shrubs (Encelia: Asteraceae), we use fine-scale geographic sampling and population genomics to determine patterns of gene flow across two hybrid zones formed between two independent pairs of species with parapatric distributions. After finding evidence for extremely strong selection at both hybrid zones, we use a combination of field experiments, high-resolution imaging, and physiological measurements to determine the ecological basis for selection at one of the hybrid zones. Our results identify multiple ecological mechanisms of selection (drought, salinity, herbivory, and burial) that together are sufficient to maintain species boundaries despite high rates of hybridization. Given that multiple pairs of species hybridize at ecologically divergent parapatric boundaries in the adaptive radiation of Encelia, such mechanisms may maintain species boundaries throughout this group. SIGNIFICANCE STATEMENTIn Baja California, the deserts meet the coastal dunes in a narrow transition visible even from satellite images. We study two species pairs of desert shrubs (Encelia) that occur across this transition. Although these species can interbreed, they remain distinct. Using a combination of genetics, field experiments, 3D-imaging, and physiological measurements, we show that natural selection counteracts the homogenizing effects of gene exchange. The different habitats of these species create multiple mechanisms of selection - drought, salinity, herbivory, and burial, which together maintain these species in their native habitats and their hybrids in intermediate habitats. This study illustrates how environmental factors influence traits and fitness and how this in turn maintain species, highlighting the importance of natural selection in speciation.

evolutionary biology↗

bioRxiv2019

Emergent honeycomb topology of the leaf spongy mesophyll

The spongy mesophyll layer in leaves is ubiquitous among vascular plants, yet its structure is relatively unknown and typically described as a disordered assemblage of isodiametric cells. We characterized spongy mesophyll structure among diverse taxa using X-ray microCT imaging and found that leaves with small cell sizes, high cell packing densities, and close vein spacing were congruent with the isodiametric paradigm. When these structural traits exceeded well-defined thresholds, the spongy mesophyll domain was instead tessellated with an emergent topological motif of an irregular honeycomb that minimizes cellular investment and obeys Eulers Law of space filling. Our data suggest spongy mesophyll is governed by allometric scaling laws, with two distinct topologies optimized for either photosynthetic performance or minimal resource investment. One Sentence SummaryConserved topological motifs in the spongy mesophyll are coordinated with leaf photosynthetic performance.

plant biology↗

bioRxiv2019

The scaling of genome size and cell size limits maximum rates of photosynthesis with implications for ecological strategies

A central challenge in plant ecology is to define the major axes of plant functional variation with direct consequences for fitness. Central to the three main components of plant fitness (growth, survival, and reproduction) is the rate of metabolic conversion of CO2 into carbon that can be allocated to various structures and functions. Here we (1) argue that a primary constraint on the maximum rate of photosynthesis per unit leaf area is the size and packing density of cells and (2) show that variation in genome size is a strong predictor of cell sizes, packing densities, and the maximum rate of photosynthesis across terrestrial vascular plants. Regardless of the genic content associated with variation in genome size, the simple biophysical constraints of encapsulating the genome define the lower limit of cell size and the upper limit of cell packing densities, as well as the range of possible cell sizes and densities. Genome size, therefore, acts as a first-order constraint on carbon gain and is predicted to define the upper limits of allocation to growth, reproduction, and defense. The strong effects of genome size on metabolism, therefore, have broad implications for plant biogeography and for other theories of plant ecology, and suggest that selection on metabolism may have a role in genome size evolution.

ecology↗

bioRxiv2019

A physiological approach to the ecology and evolution of flowers

For most angiosperms, flowers are critical to reproduction because they increase rates of outcrossing. Flowers are highly variable in numerous traits, including size, shape, and color. Most of this variation is thought to have arisen due to selection by pollinators. Yet, non-pollinator selection is increasingly being recognized as contributing to floral trait evolution. One such non-pollinator agent of selection that often opposes pollinator selection includes the physiological and resource costs of producing and maintaining flowers. Yet, our understanding of the physiological function of flowers remains limited. Here I argue for a comparative, physiological framework for studying flowers and highlight recent work elucidating some of the basic physiological structure-function relationships of flowers and apply an energy balance model to show how two pollination traits (flower size and flower color) can interact with physiological traits. This modeling shows that (1) physiologically meaningful estimates of flower size can be easily gleaned from available floras, (2) the range of flower size most common in the California flora is the range in which complex, non-linear dynamics in flower energy balance occur, and (3) that intraspecific variation in flower color can, by these conservative estimates, cause large (up to 8{degrees}C) changes in flower temperature. The strong interaction between pollination traits (flower size and color) and physiological traits (surface conductance to water vapor) suggests that a more unified framework for understanding the evolution of floral form and function would include physiological traits that represent the costs of flower production and maintenance.

ecology↗

bioRxiv2018

Hydraulic traits are more diverse in flowers than in leaves

Introduction Introduction Methods Results Discussion References The appearance of modern flowers was a turning point in the evolution of flowering plants, enhancing the intimacy and extent of interactions with their animal pollinators and increasing the likelihood of successful outcrossing (Crane et al., 1995; Fenster et al., 2004). Flowers evolved new morphological traits and rapidly diversified, effectively increasing the number of axes along which species could differentiate (Stebbins, 1951; Crepet & Niklas, 2009). Flowers are often noted for their incredible diversity of form, with high amounts of variation existing across phylogenetic scales, from within species to among major clades (Chartier et al., 2014, ...

evolutionary biology↗

bioRxiv2018

Rehydration rates and the prevalence of xylem-hydration of flowers

Angiosperm flowers are remarkably diverse anatomically and morphologically, yet they all must satisfy the physiological constraints of supplying sufficient amounts of water and carbon effectively promote pollination. Flowers often occur in the hottest, driest parts of the plant canopy and can face harsh abiotic conditions. Prior evidence suggests that extant species vary dramatically in how water is delivered to flowers, with some evidence that water may be imported into flowers by the phloem. Here we measured midday water potential gradients between flowers, leaves, and stems often phylogenetically diverse species. We further tested the likelihood of xylem-hydration by measuring rates of rehydration after experimentally induced desiccation. There was no significant difference in rehydration rates between leaves and flowers. These results are consistent with xylem-hydration of flowers and suggest that there has been little modification to the mechanisms of water transport despite the diversity of floral form.

plant biology↗

bioRxiv2017

Hydraulic constraints of reproduction do not explain sexual dimorphism in the genus Leucadendron (Proteaceae)

Because of the importance of reproduction in plant life history, the physiological costs of reproduction often influence vegetative structure and function. In dioecious species, these effects can be quite obvious, as different costs of male and female reproductive functions are entirely separated among different individuals in a population. In fire-prone ecosystems, in which recruitment is driven by fire frequency, many plants will maintain their seeds in the canopy, only to be released after a fire. The dioecious genus Leucadendron is a notable case of this, as females can maintain their seed cones for years, and, even more interestingly, species in the genus differ substantially in the degree to which males and females are sexually dimorphic. A recent study (Harris and Pannell 2010) argued that the hydraulic costs of maintaining seed cones for many years would effect the degree of sexual dimorphism among species. However, this assumed that shoot hydraulic architecture would be related to traits exhibiting sexual dimorphism. Here we explicitly test this hypothesis on two Leucadendron species. We found (1) that metrics of branch ramification used in the previous study to characterize dimorphism do not conform to known scaling relationships and (2) that sexual dimorphism in shoot architecture has no effect on hydraulic efficiency. Both of these results seriously question the pattern described by Harris and Pannell (2010) and suggest that the hydraulic costs of prolonged seed retention in Leucadendron do not significantly affect branch architecture.

ecology↗