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Using modern plant trait relationships between observed and theoretical maximum stomatal conductance and vein density to examine patterns of plant macroevolution.

ABSTRACT: Understanding the drivers of geological-scale patterns in plant macroevolution is limited by a hesitancy to use measurable traits of fossils to infer palaeoecophysiological function. Here, scaling relationships between morphological traits including maximum theoretical stomatal conductance (gmax ) and leaf vein density (Dv ) and physiological measurements including operational stomatal conductance (gop ), saturated (Asat ) and maximum (Amax ) assimilation rates were investigated for 18 extant taxa in order to improve understanding of angiosperm diversification in the Cretaceous. Our study demonstrated significant relationships between gop , gmax and Dv that together can be used to estimate gas exchange and the photosynthetic capacities of fossils. We showed that acquisition of high gmax in angiosperms conferred a competitive advantage over gymnosperms by increasing the dynamic range (plasticity) of their gas exchange and expanding their ecophysiological niche space. We suggest that species with a high gmax (> 1400 mmol m(-2) s(-1) ) would have been capable of maintaining a high Amax as the atmospheric CO2 declined through the Cretaceous, whereas gymnosperms with a low gmax would experience severe photosynthetic penalty. Expansion of the ecophysiological niche space in angiosperms, afforded by coordinated evolution of high gmax , Dv and increased plasticity in gop , adds further functional insights into the mechanisms driving angiosperm speciation.

SUBMITTER: McElwain JC

PROVIDER: S-EPMC5014202 | biostudies-literature | 2016 Jan

REPOSITORIES: biostudies-literature

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Using modern plant trait relationships between observed and theoretical maximum stomatal conductance and vein density to examine patterns of plant macroevolution.

McElwain Jennifer C JC Yiotis Charilaos C Lawson Tracy T

The New phytologist 20150731 1

Understanding the drivers of geological-scale patterns in plant macroevolution is limited by a hesitancy to use measurable traits of fossils to infer palaeoecophysiological function. Here, scaling relationships between morphological traits including maximum theoretical stomatal conductance (gmax ) and leaf vein density (Dv ) and physiological measurements including operational stomatal conductance (gop ), saturated (Asat ) and maximum (Amax ) assimilation rates were investigated for 18 extant ta ...[more]

PMID: 26230251

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Project description:Stomatal conductance (g s) in terrestrial vegetation regulates the uptake of atmospheric carbon dioxide for photosynthesis and water loss through transpiration, closely linking the biosphere and atmosphere and influencing climate. Yet, the range and pattern of g s in plants from natural ecosystems across broad geographic, climatic, and taxonomic ranges remains poorly quantified. Furthermore, attempts to characterize g s on such scales have predominantly relied upon meta-analyses compiling data from many different studies. This approach may be inherently problematic as it combines data collected using unstandardized protocols, sometimes over decadal time spans, and from different habitat groups. Using a standardized protocol, we measured leaf-level g s using porometry in 218 C3 woody angiosperm species in natural ecosystems representing seven bioclimatic zones. The resulting dataset of 4273 g s measurements, which we call STraits (Stomatal Traits), was used to determine patterns in maximum g s (g smax) across bioclimatic zones and whether there was similarity in the mean g smax of C3 woody angiosperms across ecosystem types. We also tested for differential g smax in two broadly defined habitat groups - open-canopy and understory-subcanopy - within and across bioclimatic zones. We found strong convergence in mean g smax of C3 woody angiosperms in the understory-subcanopy habitats across six bioclimatic zones, but not in open-canopy habitats. Mean g smax in open-canopy habitats (266 ± 100 mmol m-2 s-1) was significantly higher than in understory-subcanopy habitats (233 ± 86 mmol m-2 s-1). There was also a central tendency in the overall dataset to operate toward a g smax of ∼250 mmol m-2 s-1. We suggest that the observed convergence in mean g smax of C3 woody angiosperms in the understory-subcanopy is due to a buffering of g smax against macroclimate effects which will lead to differential response of C3 woody angiosperm vegetation in these two habitats to future global change. Therefore, it will be important for future studies of g smax to categorize vegetation according to habitat group.

Project description:Tree mortality during global-change-type drought is usually attributed to xylem dysfunction, but as climate change increases the frequency of extreme heat events, it is necessary to better understand the interactive role of heat stress. We hypothesized that some drought-stressed plants paradoxically open stomata in heatwaves to prevent leaves from critically overheating. We experimentally imposed heat (>40°C) and drought stress onto 20 broadleaf evergreen tree/shrub species in a glasshouse study. Most well-watered plants avoided lethal overheating, but drought exacerbated thermal damage during heatwaves. Thermal safety margins (TSM) quantifying the difference between leaf surface temperature and leaf critical temperature, where photosynthesis is disrupted, identified species vulnerability to heatwaves. Several mechanisms contributed to high heat tolerance and avoidance of damaging leaf temperatures-small leaf size, low leaf osmotic potential, high leaf mass per area (i.e., thick, dense leaves), high transpirational capacity, and access to water. Water-stressed plants had smaller TSM, greater crown dieback, and a fundamentally different stomatal heatwave response relative to well-watered plants. On average, well-watered plants closed stomata and decreased stomatal conductance (gs ) during the heatwave, but droughted plants did not. Plant species with low gs , either due to isohydric stomatal behavior under water deficit or inherently low transpirational capacity, opened stomata and increased gs under high temperatures. The current paradigm maintains that stomata close before hydraulic thresholds are surpassed, but our results suggest that isohydric species may dramatically increase gs (over sixfold increases) even past their leaf turgor loss point. By actively increasing water loss at high temperatures, plants can be driven toward mortality thresholds more rapidly than has been previously recognized. The inclusion of TSM and responses to heat stress could improve our ability to predict the vulnerability of different tree species to future droughts.

Dataset Information

Using modern plant trait relationships between observed and theoretical maximum stomatal conductance and vein density to examine patterns of plant macroevolution.

Publications

Using modern plant trait relationships between observed and theoretical maximum stomatal conductance and vein density to examine patterns of plant macroevolution.

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