ORCID Profile
0000-0003-2768-6420
Current Organisation
University of the Witwatersrand Johannesburg
Does something not look right? The information on this page has been harvested from data sources that may not be up to date. We continue to work with information providers to improve coverage and quality. To report an issue, use the Feedback Form.
Publisher: Oxford University Press (OUP)
Date: 22-05-2018
DOI: 10.1104/PP.18.00103
Publisher: Proceedings of the National Academy of Sciences
Date: 03-2021
Abstract: A fundamental association between sustained water transport and downstream tissue survival should select for xylem that avoids embolism in long-lived woody plants. Previous studies suggest that long-vessel species, such as oaks and vines, are more susceptible to drought-induced loss of function than other species. We show that western North American oaks—even those occurring in wet temperate forest—possess xylem capable of tolerating substantial water stress. Evolutionary relationships between drought tolerance traits combined with plant–climate interactions yield positive hydraulic safety margins in oaks from erse habitats, demonstrating that these key species are not yet on the verge of hydraulically mediated loss of function. Quantifying physical tolerance limits to desiccation is imperative for predicting ecological consequences of future droughts.
Publisher: Proceedings of the National Academy of Sciences
Date: 20-04-2015
Abstract: Recent reports of widespread drought-induced plant die-back have enhanced the need to understand variation in plant hydraulic strategies and to determine which species are susceptible to drought-induced mortality. Finding meaningful ways to describe the many different types of responses plants might demonstrate in bio erse regions can be extremely challenging. Here, we present the architecture in which functional traits can be incorporated into a known hydraulic framework to provide additional insight into the range of hydraulic strategies present in erse communities. Merging stomatal regulation strategies that represent an index of water use behavior with xylem hydraulic strategies that represent water transport and vulnerability to water deficit behavior facilitates a more comprehensive framework to characterize plant response to drought.
Publisher: CSIRO Publishing
Date: 2012
DOI: 10.1071/BT11231
Abstract: Although pubescence has traditionally been considered to be related to the water economy of plants, the results are ambivalent and vary between different species. We tested two contrasting hypotheses for the functional significance of leaf pubescence of Proteaceae species from the Cape Floristic Region. First, we hypothesised that pubescence is a xeromorphic trait that conserves water by increasing the boundary layer resistance to diffusion. Water loss was measured in two morphotypes of Leucospermum conocarpodendron (L.) Buek that differ in the degree of leaf pubescence, using both gas exchange and gravimetric techniques. Pubescence contributed less than 5% of total leaf resistance and pubescent leaves transpired at least as rapidly as glabrous leaves due to having larger numbers of small stomata per leaf area. Although pubescence was not associated with drier sites in L. conocarpodendron, there was a weak negative correlation between rainfall and pubescence across 18 other Proteaceae species. We also hypothesised that pubescence is a radiation-protective trait. We assessed the effect of pubescence on light reflectance, leaf temperature, fluorescence and gas exchange characteristics in situ. Pubescent leaves of L. conocarpodendron were 19.2 ± 0.08% more reflective than glabrous leaves and had significantly greater pre-dawn photochemical efficiency. There was a positive association between leaf pubescence and habitat temperature in Proteaceae. We conclude that although pubescence is unlikely to be a xeric adaptation, it could serve a role in reducing photoinhibition and heat loading in Proteaceae species.
Publisher: Wiley
Date: 07-01-2016
DOI: 10.1111/NPH.13846
Abstract: Vascular plant mortality during drought has been strongly linked to a failure of the internal water transport system caused by the rapid invasion of air and subsequent blockage of xylem conduits. Quantification of this critical process is greatly complicated by the existence of high water tension in xylem cells making them prone to embolism during experimental manipulation. Here we describe a simple new optical method that can be used to record spatial and temporal patterns of embolism formation in the veins of water‐stressed leaves for the first time. Applying this technique in four erse angiosperm species we found very strong agreement between the dynamics of embolism formation during desiccation and decline of leaf hydraulic conductance. These data connect the failure of the leaf water transport network under drought stress to embolism formation in the leaf xylem, and suggest embolism occurs after stomatal closure under extreme water stress.
Publisher: Oxford University Press (OUP)
Date: 31-07-2019
DOI: 10.1104/PP.19.00774
Publisher: Elsevier BV
Date: 04-2011
Publisher: Wiley
Date: 07-01-2019
DOI: 10.1111/PCE.13486
Abstract: Despite the appeal of the iso/anisohydric framework for classifying plant drought responses, recent studies have shown that such classifications can be strongly affected by a plant's environment. Here, we present measured in situ drought responses to demonstrate that apparent isohydricity can be conflated with environmental conditions that vary over space and time. In particular, we (a) use data from an oak species (Quercus douglasii) during the 2012-2015 extreme drought in California to demonstrate how temporal and spatial variability in the environment can influence plant water potential dynamics, masking the role of traits (b) explain how these environmental variations might arise from climatic, topographic, and edaphic variability (c) illustrate, through a "common garden" thought experiment, how existing trait-based or response-based isohydricity metrics can be confounded by these environmental variations, leading to Type-1 (false positive) and Type-2 (false negative) errors and (d) advocate for the use of model-based approaches for formulating alternate classification schemes. Building on recent insights from greenhouse and vineyard studies, we offer additional evidence across multiple field sites to demonstrate the importance of spatial and temporal drivers of plants' apparent isohydricity. This evidence challenges the use of isohydricity indices, per se, to characterize plant water relations at the global scale.
Publisher: Wiley
Date: 05-02-2020
Publisher: International Society for Horticultural Science (ISHS)
Date: 11-2018
Publisher: Wiley
Date: 27-08-2018
DOI: 10.1111/ELE.13139
Abstract: Many recent studies on drought-induced vegetation mortality have explored how plant functional traits, and classifications of such traits along axes of, for ex le, isohydry-anisohydry, might contribute to predicting drought survival and recovery. As these studies proliferate, the consistency and predictive value of such classifications need to be carefully examined. Here, we outline the basis for a systematic classification of plant drought responses that accounts for both environmental conditions and functional traits. We use non-dimensional analysis to integrate plant traits and metrics of environmental variation into groups that can be associated with alternative drought stress pathways (hydraulic failure and carbon limitation), and demonstrate that these groupings predict physiological drought outcomes using both synthetic and measured data. In doing so, we aim to untangle some confounding effects of environment and trait variations that undermine current classification schemes, advocate for more careful treatment of the environmental context within which plants experience and respond to drought, and outline a pathway towards a general classification of drought vulnerability.
Publisher: Wiley
Date: 26-01-2017
DOI: 10.1111/NPH.14450
Abstract: Finding thresholds at which loss of plant functionality occurs during drought is critical for predicting future crop productivity and survival. Xylem resistance to embolism has been suggested as a key trait associated with water-stress tolerance. Although a substantial literature exists describing the vulnerability of woody stems to embolism, leaves and roots of herbaceous species remain under-represented. Also, little is known about vulnerability to embolism at a whole-plant scale or propagation of embolism within plants. New techniques to view the process of embolism formation provide opportunities to resolve long-standing questions. Here, we used multiple visual techniques, including X-ray micro-computed tomography and the optical vulnerability method, to investigate the spread of embolism within intact stems, leaves and roots of Solanum lycopersicum (common tomato). We found that roots, stems and leaves of tomato plants all exhibited similar vulnerability to embolism, suggesting that embolism rapidly propagates among tissues. Although we found scarce evidence for differentiation of xylem vulnerability among tissues at the scale of the whole plant, within a leaf the midrib embolized at higher water potentials than lower order veins. Substantial overlap between the onset of cavitation and incipient leaf damage suggests that cavitation represents a substantial damage to plants, but the point of lethal cavitation in this herbaceous species remains uncertain.
Publisher: Springer Science and Business Media LLC
Date: 24-01-2023
DOI: 10.1007/S00442-023-05326-9
Abstract: Understanding climate change impacts on the Cape Floristic Region requires improved knowledge of plant physiological responses to the environment. Studies examining physiological responses of mountain fynbos have consisted of c aign-based measurements, capturing snapshots of plant water relations and photosynthesis. We examine conclusions drawn from prior studies by tracking in situ physiological responses of three species, representing dominant growth forms (proteoid, ericoid, restioid), over 2 years using miniature continuous sap flow technology, long-term observations of leaf/culm water potential and gas exchange, and xylem vulnerability to embolism. We observed considerable inter-specific variation in the timing and extent of seasonal declines in productivity. Shallow-rooted Erica monsoniana exhibited steep within-season declines in sap flow and water potentials, and pronounced inter-annual variability in total daily sap flux ( J s ). Protea repens showed steady reductions in J s across both years, despite deeper roots and less negative water potentials. Cannomois congesta —a shallow-rooted restioid—was least negatively impacted. Following rehydrating rain at the end of summer, gas exchange recovery was lower in the drier year compared with the normal year, but did not differ between species. Loss of function in the drier year was partially accounted for by loss of xylem transport capacity in Erica and Cannomois , but not Protea. Hitherto unseen water use patterns, including inter-annual variability of gas exchange associated with contrasting water uptake properties, reveal that species use different mechanisms to cope with summer dry periods. Revealing physiological responses of key growth forms enhances predictions of plant function within mountain fynbos under future conditions.
Publisher: Oxford University Press (OUP)
Date: 08-2020
DOI: 10.1104/PP.20.00897
Publisher: Wiley
Date: 14-06-2022
DOI: 10.1111/AEC.13206
Abstract: Decade‐old observations of sex‐specific differences in culm δ 13 C for the erse and ecologically important Restionaceae of the Cape Florisitic Region (CFR) have yet to be explained mechanistically. In the dioecious, wind‐pollinated Cannomois congesta (Mast.), we found that sexual differences in culm δ 13 C were not explained by intrinsic differences in water use efficiency (WUE), but rather by differences in growth phenology. When growing together, instantaneous WUE (A/g) was equal between sexes. However, we noted strong phenological asynchrony between the sexes where males began to grow new culms nearly 2 months earlier than females. This initial period of growth in males corresponded to hotter, drier conditions and temporarily higher instantaneous WUE, thus driving less negative δ 13 C signals in male culms. We suspect that phenological differences might explain previously reported sexual differences in culm δ 13 C in the Restionaceae family. Despite earlier culm emergence in males, reduced growth in males prior to flowering, presumably due to costs associated with reproduction, resulted in both sexes flowering at the same height. We discuss the potential selective advantage of earlier growth in males as it relates to inflorescence heights and pollination efficiency.
Publisher: Oxford University Press (OUP)
Date: 11-2019
DOI: 10.1104/PP.19.01177
Publisher: Bio-Protocol, LLC
Date: 2017
Publisher: Wiley
Date: 12-06-2019
DOI: 10.1111/NPH.15886
Abstract: Vulnerability to embolism varies between con‐generic species distributed along aridity gradients, yet little is known about intraspecific variation and its drivers. Even less is known about intraspecific variation in tissues other than stems, despite results suggesting that roots, stems and leaves can differ in vulnerability. We hypothesized that intraspecific variation in vulnerability in leaves and stems is adaptive and driven by aridity. We quantified leaf and stem vulnerability of Quercus douglasii using the optical technique. To assess contributions of genetic variation and phenotypic plasticity to within‐species variation, we quantified the vulnerability of in iduals growing in a common garden, but originating from populations along an aridity gradient, as well as in iduals from the same wild populations. Intraspecific variation in water potential at which 50% of total embolism in a tissue is observed (P 50 ) was explained mostly by differences between in iduals (& % of total variance) and tissues (16%). There was little between‐population variation in leaf/stem P 50 in the garden, which was not related to site of origin aridity. Unexpectedly, we observed a positive relationship between wild in idual stem P 50 and aridity. Although there is no local adaptation and only minor phenotypic plasticity in leaf/stem vulnerability in Q. douglasii , high levels of potentially heritable variation within populations or strong environmental selection could contribute to adaptive responses under future climate change.
Publisher: CSIRO Publishing
Date: 2013
DOI: 10.1071/FP12379
Abstract: There has been limited application of sapflow technology to small-stemmed species and across co-existing functional types, restricting its use in erse floras such as the Mediterranean-type shrubland in South Africa. Our main objective was to test whether sapflow may provide an alternative to traditional gas-exchange measurements, which would permit comparative evaluation of transpiration at a previously unattained temporal resolution. We tested miniature external heat ratio method (HRM) sapflow gauges on three co-occurring functional types with contrasting stem or culm anatomies and examined the relationship between sapflow and shoot- and leaf-level water loss in both a controlled and field environment. Our sapflow gauges captured dynamic patterns of transpiration in both settings for all three functional types. In a controlled environment the relationship between sapflow and transpiration was linear in all three species with r2 values ranging from 0.78 for Cannomois congesta Mast. (Restionaceae) to 0.96 for Protea repens (L.) L. (Proteaceae) and Erica monsoniana L.f. (Ericaceae). In the field, r2 values were lower, ranging from 0.59 for C. congesta to 0.74 for P. repens. We discuss the efficacy and potential of this methodology to cast light on patterns of community ecology in functionally erse shrublands by capturing continuous variation in transpiration.
Publisher: Wiley
Date: 16-06-2017
DOI: 10.1111/NPH.14652
Abstract: Drought can cause major damage to plant communities, but species damage thresholds and postdrought recovery of forest productivity are not yet predictable. We used an El Niño drought event as a natural experiment to test whether postdrought recovery of gas exchange could be predicted by properties of the water transport system, or if metabolism, primarily high abscisic acid concentration, might delay recovery. We monitored detailed physiological responses, including shoot sapflow, leaf gas exchange, leaf water potential and foliar abscisic acid ( ABA ), during drought and through the subsequent rehydration period for a s le of eight canopy and understory species. Severe drought caused major declines in leaf water potential, elevated foliar ABA concentrations and reduced stomatal conductance and assimilation rates in our eight s le species. Leaf water potential surpassed levels associated with incipient loss of leaf hydraulic conductance in four species. Following heavy rainfall gas exchange in all species, except those trees predicted to have suffered hydraulic impairment, recovered to prestressed rates within 1 d. Recovery of plant gas exchange was rapid and could be predicted by the hydraulic safety margin, providing strong support for leaf vulnerability to water deficit as an index of damage under natural drought conditions.
Publisher: Wiley
Date: 20-06-2023
DOI: 10.1111/PCE.14649
Abstract: Observations show vulnerability segmentation between stems and leaves is highly variable within and between environments. While a number of species exhibit conventional vulnerability segmentation (stem leaf ), others exhibit no vulnerability segmentation and others reverse vulnerability segmentation (stem leaf ). We developed a hydraulic model to test hypotheses about vulnerability segmentation and how it interacts with other traits to impact plant conductance. We do this using a series of experiments across a broad parameter space and with a case study of two species with contrasting vulnerability segmentation patterns: Quercus douglasii and Populus trichocarpa . We found that while conventional vulnerability segmentation helps to preserve conductance in stem tissues, reverse vulnerability segmentation can better maintain conductance across the combined stem‐leaf hydraulic pathway, particularly when plants have more vulnerable s and have hydraulic segmentation with greater resistance in the leaves. These findings show that the impacts of vulnerability segmentation are dependent upon other plant traits, notably hydraulic segmentation, a finding that could assist in the interpretation of variable observations of vulnerability segmentation. Further study is needed to examine how vulnerability segmentation impacts transpiration rates and recovery from water stress.
Publisher: Oxford University Press (OUP)
Date: 06-2019
DOI: 10.1104/PP.19.00500
Publisher: Research Square Platform LLC
Date: 14-07-2022
DOI: 10.21203/RS.3.RS-1821506/V1
Abstract: ● Understanding climate change impacts on the Cape Floristic Region requires improved knowledge of plant physiological responses to the environment. Prior studies examining physiological responses of fynbos species have consisted of c aign-based measurements, capturing snapshots in time of plant water relations and photosynthesis. We examine conclusions drawn from these studies by tracking in situ physiological responses of three species, representing three dominant growth forms (proteoid, ericoid, restioid), over two years using miniature continuous sap flow technology, in combination with long-term observations of xylem water potentials, gas exchange and assessments of xylem vulnerability to embolism. ● We observed considerable inter-specific variation in the timing and extent of dehydration-induced declines in productivity. Erica monsoniana (a shallow-rooted ericoid) exhibited steep within-season declines in sap flow and water potentials, and pronounced inter-annual variability in total daily sap flux (Js). Protea repens showed steady reductions in Js across both years, despite maintaining less negative water potentials. Cannomois congesta - a shallow-rooted restioid - was least negatively impacted. Following rehydrating rain at the end of summer we compared gas exchange recovery in the representative dry year with the normal year, finding significant seasonal, but not species, effects. Loss of function in the drier year was partially accounted for by loss of stem xylem transport capacity, but only in E. monsoniana and C. congesta. ● Hitherto unseen water-use patterns, including inter-annual variability of gas exchange that were driven by contrasting water uptake properties, reveal that mountain fynbos species use different mechanisms to cope with summer dry periods.
Publisher: Wiley
Date: 24-05-2019
DOI: 10.1111/NPH.15893
Publisher: Oxford University Press (OUP)
Date: 06-09-2023
Publisher: Oxford University Press (OUP)
Date: 04-2019
DOI: 10.1104/PP.19.00057
Publisher: Cold Spring Harbor Laboratory
Date: 21-08-2023
DOI: 10.1101/2023.08.20.553748
Abstract: Two decades of widespread drought-induced forest mortality events on every forested continent have raised the specter of future unpredictable, rapid ecosystem changes in 21 st century forests. Yet our ability to predict drought stress, much less drought-induced mortality across the landscape remains limited. This uncertainty stems at least in part from an incomplete understanding of within-species variation in hydraulic physiology, which reflects the interaction of genetic differentiation among populations (ecotypic variation) and phenotypic plasticity in response to growth environment. We examined among-population genetic differentiation in a number of morphological and hydraulic traits in California blue oak ( Quercus douglasii ) using a 30 year old common garden. We then compared this genetic trait differentiation and trait-trait integration to wild phenotypes in the field from the original source populations. We found remarkably limited among-population genetic differentiation in all traits in the common garden, but considerable site-to-site variation in the field. However, it was difficult to explain trait variation in the field using site climate variables, suggesting that gridded climate data does not capture the drivers of plasticity in drought physiology in this species. Trait-trait relationships were also considerably stronger in the field than in the garden, particularly links between leaf morphology, leaf hydraulic efficiency and stem hydraulic efficiency. Indeed, while twelve of 45 potential trait-trait relationships showed significant wild phenotypic correlations, only four relationships showed both genetic and phenotypic correlations, and five relationships showed significantly different genetic and phenotypic correlations. Collectively, our results demonstrate limited ecotypic variation in drought-related physiology but considerable geographic variation in physiology and phenotypic integration in the wild, both driven largely by plasticity.
Location: South Africa
Location: United States of America
Location: No location found
No related grants have been discovered for Robert Skelton.