Dr Andrea Leigh

Curtis, E.M., Leigh, A. & Rayburg, S. 2012, 'Relationships among leaf traits of Australian arid zone plants: alternative modes of thermal protection', Australian Journal Of Botany, vol. 60, no. 6, pp. 471-483.
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Despite the importance of leaf traits that protect against critically high leaf temperatures, relationships among such traits have not been investigated. Further, while some leaf trait relationships are well documented across biomes, little is known about such associations within a biome. This study investigated relationships between nine leaf traits that protect leaves against excessively high temperatures in 95 Australian arid zone species. Seven morphological traits were measured: leaf area, length, width, thickness, leaf mass per area (LMA), water content, and an inverse measure of pendulousness (LP). Two spectral properties were measured: reflectance of visible and near infrared radiation. Three key findings emerged: 1) LP decreased (pendulousness increased) with leaf size and LMA, the former relationship suggesting that pendulousness affords thermal protection when leaves are large; 2) LMA increased with thickness and decreased with water content, indicating alternative means for protection through increasing thermal mass; 3) spectral reflectance increased with LMA and thickness and decreased with water content. The consistent co-variation of thermal protective traits with LMA, a trait not usually associated with thermal protection, suggests that these traits fall along the leaf economics spectrum, with leaf longevity increasing through protection not only against structural damage but also against heat stress.

Leigh, A., Sevanto, S., Ball, M.C., Close, J.D., Ellsworth, D.S., Knight, C., Nicotra, A. & Vogel, S. 2012, 'Do thick leaves avoid thermal damage in critically low wind speeds?', New Phytologist, vol. 194, no. 2, pp. 477-487.
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Summary Transient lulls in air movement are rarely measured, but can cause leaf temperature to rise rapidly to critical levels. The high heat capacity of thick leaves can damp this rapid change in temperature. However, little is known about the extent to which increased leaf thickness can reduce thermal damage, or how thick leaves would need to be to have biological significance. We evaluated quantitatively the contribution of small increases in leaf thickness to the reduction in thermal damage during critically low wind speeds under desert conditions. We employed a numerical model to investigate the effect of thickness relative to transpiration, absorptance and leaf size on damage avoidance. We used measured traits and thermotolerance thresholds of real leaves to calculate the leaf temperature response to naturally occurring variable low wind speed. Our results demonstrated that an increase in thickness of only fractions of a millimetre can prevent excursions to damaging high temperatures. This damping effect of increased thickness was greatest when other means of reducing leaf temperature (transpiration, reflectance or reduced size) were lacking. For perennial desert flora, we propose that increased leaf thickness is important in decreasing the incidence of extreme heat stress and, in some species, in enhancing long-term survival.

Leigh, A., Zwieniecki, M.A., Rockwell, F.E., Boyce, C.K., Nicotra, A. & Holbrook, N.M. 2011, 'Structural and hydraulic correlates of heterophylly in Ginkgo biloba', New Phytologist, vol. 189, no. 2, pp. 459-470.
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Summary This study investigates the functional significance of heterophylly in Ginkgo biloba, where leaves borne on short shoots are ontogenetically distinct from those on long shoots. Short shoots are compact, with minimal internodal elongation; their leaves are supplied with water through mature branches. Long shoots extend the canopy and have significant internodal elongation; their expanding leaves receive water from a shoot that is itself maturing. Ô+ó Morphology, stomatal traits, hydraulic architecture, Huber values, water transport efficiency, in situ gas exchange and laboratory-based steady-state hydraulic conductance were examined for each leaf type. Ô+ó Both structure and physiology differed markedly between the two leaf types. Short-shoot leaves were thinner and had higher vein density, lower stomatal pore index, smaller bundle sheath extensions and lower hydraulic conductance than long-shoot leaves. Long shoots had lower xylem area : leaf area ratios than short shoots during leaf expansion, but this ratio was reversed at shoot maturity. Longshoot leaves had higher rates of photosynthesis, stomatal conductance and transpiration than short-shoot leaves. Ô+ó We propose that structural differences between the two G. biloba leaf types reflect greater hydraulic limitation of long-shoot leaves during expansion. In turn, differences in physiological performance of short- and long-shoot leaves correspond to their distinct ontogeny and architecture.

Nicotra, A., Leigh, A., Boyce, C.K., Jones, C.S., Niklas, K.J., Royer, D.L. & Tsukaya, H. 2011, 'The evolution and functional significance of leaf shape in the angiosperms', Functional Plant Biology, vol. 38, pp. 535-552.
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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.

Summerhayes, S.A., Bishop, M.J., Leigh, A. & Kelaher, B.P. 2009, 'Effects of oyster death and shell disarticulation on associated communities of epibiota', Journal of Experimental Marine Biology and Ecology, vol. 379, no. 1-2, pp. 60-67.
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Oyster mortality and subsequent degradation of shell matrices may influence associated epibiota by modifying processes of filtration and biodeposition and by changing habitat structure. In the Hawkesbury River, NSW, Australia, QX disease devastated aquaculture populations of the native Sydney rock oyster, Saccostrea glomerata, and threatened wild populations. To ascertain effects of this oyster mortality on associated epibiota, we compared epifaunal communities occupying 100% oyster shell cover among 5 sites along the estuary, ranging from 25 ppt salinity and live oyster density of 96 ¦ 13 m- 2, to 32 ppt salinity and live oyster density of 3187 ¦ 233 m- 2. Epifaunal richness was greatest closest to the estuarine mouth, where live oyster abundance was greatest. Epifaunal abundance, by contrast, generally increased with distance upstream, as oyster matrices were increasingly dominated by the shells of dead oysters. To ascertain the role of live and dead oysters as habitat providers, we carried out a manipulative experiment to test the hypothesis that the epifaunal assemblages that settle over a 4 month period will significantly differ among constructed matrices comprised of live, whole dead and/or degraded (single valve) oysters. Treatments containing a component of single valves, which increase the availability of interstitial spaces and provide greater surface area for attachment, supported the greatest numbers of epibiota. Matrices comprised solely of live oysters supported fewest species and numbers of organisms. Results demonstrate that death and degradation of oysters alter the structure of associated communities, even where 100% cover of shell matrix is maintained. These results have important ramifications for management strategies and retaining estuarine biodiversity in the event that disease such as QX causes local oyster extinctions.

Carpenter, R.J., Jordan, G.J., Leigh, A. & Brodribb, T.J. 2007, 'Giant cuticular pores in Eidothea zoexylocarya (Proteaceae) leaves', American Journal of Botany, vol. 94, no. 8, pp. 1282-1288.
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Ubiquitous, large diameter pores have not previously been adequately demonstrated to occur in leaf cuticles. Here we show conclusively that such structures occur in Eidothea zoexylocarya, a rainforest tree species of Proteaceae restricted to the Australian Wet Tropics. The pores are abundant, large-diameter apertures (~1 Ám), that extend perpendicularly most of the way through the cuticle from the inside. They occur on both sides of the leaf, but are absent from the cuticle associated with stomatal complexes on the abaxial side. No such pores were found in any other species, including the only other species of Eidothea, E. hardeniana from New South Wales, and other species that have previously been purported to possess cuticular pores. To determine whether these pores made the cuticles more leaky to water vapor, we measured astomatous cuticular conductances to water vapor for E. zoexylocarya and seven other Proteaceae species of the Wet Tropics. Cuticular conductance for E. zoexylocarya was relatively low, indicating that the prominent pores do not increase conductance. The function of the pores is currently obscure, but the presence of both pores and an adaxial hypodermis in E. zoexylocarya but not E. hardeniana suggests evolution in response to greater environmental stresses in the tropics.

Leigh, A., Close, J.D., Ball, M.C., Siebke, K. & Nicotra, A. 2006, 'Leaf cooling curves: measuring leaf temperatures in sunlight', Functional Plant Biology, vol. 33, no. 5, pp. 515-519.
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Despite the obvious benefits of using thermography under field conditions, most infrared studies at the leaf level are generally conducted in the laboratory. One reason for this bias is that accuracy can potentially be compromised in sunlight because reflected radiation from the leaf might affect the calculation of the temperature measurement. We have developed a method for measuring leaf temperature in sunlight by using thermal imagery to generate cooling curves from which the time constant for cooling, ?, can be calculated. The original temperature of the sunlit leaf may be determined by extrapolating backwards in time. In the absence of specular reflection, there is close agreement between the extrapolated sunlit temperature and the sunlit temperature recorded by the camera. However, when reflected radiation is high, the difference between the initial (incorrect) temperature determined from the sunlit image and the temperature extrapolated from the cooling curve can be > 2¦C. Notably, our results demonstrate a close agreement between the extrapolated sunlit temperature and the temperature of the leaf approximately 1 s after being shaded, suggesting that this shaded image provides a good estimate of the original sunlit temperature. Thus, our technique provides two means for measuring leaf surface temperature in sunlight

Leigh, A., Cosgrove, M.J. & Nicotra, A. 2006, 'Reproductive allocation in a gender dimorphic shrub: anomalous female investment in Gynatrix pulchella?', Journal of Ecology, vol. 94, no. 6, pp. 1261-1271.
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1 In gender dimorphic species, reproductive allocation (RA, the ratio of reproductive to vegetative biomass) is predicted to be greater in female plants than in male plants. Empirical research on dimorphic plant species supports this hypothesis. To date, of 44 dimorphic angiosperms for which RA has been reported in the literature, RA is greater in females than males in 40 species, is equal in four, and in no species is it greater in males. 2 In many instances where differential RA occurs, sexual dimorphism in morphological or physiological traits has been reported. This dimorphism is often attributed to the differing costs of reproduction or to selection to counteract such costs. 3 We investigated RA and other morphological and physiological characters in Gynatrix pulchella, a dimorphic species that we found ranges from dioecious to subdioecious or gynodioecious, depending on season and locality. Our results showed that contrary to our predictions functionally male plants allocated significantly more biomass to reproduction than female plants across three populations. Greater male RA was due to a combination of larger, more numerous flowers and lower leaf biomass per branch than females. 4 There were no detectable costs of greater RA in males in terms of decreased overall growth or increased mortality. Additionally, leaf nitrogen content was greater in males than in females and there were no between-sex differences in gas exchange. 5 The finding that male plants allocate significantly more resources to reproduction than females in G. pulchella is apparently a unique case.

Zwieniecki, M.A., Stone, H.A., Leigh, A., Boyce, C.K. & Holbrook, N.M. 2006, 'Hydraulic design of pine needles: one-dimensional optimization for single-vein leaves', Plant Cell and Environment, vol. 29, no. 5, pp. 803-809.
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Single-vein leaves have the simplest hydraulic design possible, yet even this linear water delivery system can be modulated to improve physiological performance. We determined the optimal distribution of transport capacity that minimizes pressure drop per given investment in xylem permeability along the needle for a given length without a change in total water delivery, or maximizes needle length for a given pressure difference between petiole and needle tip. This theory was tested by comparative analysis of the hydraulic design of three pine species that differ in the length of their needles [ Pinus palustris (Engl.) Miller, í½ 50 cm; Pinus ponderosa Lawson & Lawson, í½ 20 cm and Pinus rigida Miller, í½ 5 cm]. In all three species, the distribution of hydraulic permeability was similar to that predicted by the optimum solution. The needles of P. palustris showed an almost perfect match between predicted and actual hydraulic optimum solution, providing evidence that vein design is a significant factor in the hydraulic design of pine leaves.

Leigh, A. & Nicotra, A. 2003, 'Sexual dimorphism in reproductive allocation and water use efficiency in Maireana pyramidata (Chenopodiaceae), a dioecious, semi-arid shrub', Australian Journal Of Botany, vol. 51, no. 5, pp. 509-514.
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Sexual dimorphism in dioecious plant species is widely attributed to the differential impacts of reproduction on male v. female plants. We investigated sexual dimorphism in reproductive, morphological and physiological traits of Maireana pyramidata (Benth.) Paul G.Wilson (Chenopodiaceae), a dioecious, semi-arid shrub endemic to Australia. We estimated reproductive allocation for each sex by calculating the relative biomass allocated to flowers and fruits per gram of leaf tissue, based on one branch per sample plant. Morphological measurements included leaf mass, stem mass, specific leaf area, plant height and plant leaf area index. We also measured leaf nitrogen and chlorophyll, gas exchange and ?13C. Reproductive allocation was nine times greater in females than in males. No significant difference between the sexes in photosynthetic rate or transpiration could be detected but instantaneous water use efficiency (photosynthesis/transpiration) was significantly lower in females than in males during the fruiting period. ?13C did not differ between the sexes. The results indicate that greater reproductive allocation in females has an immediate impact on their capacity for conservative water use but does not lead to long-term differences in water use efficiency.

Cook, A., Rayburg, S.C., Capon, S.J., Leigh, A. 2011, 'Variation in seedling emergence and mortality amongst key habitat types in a semi-arid floodplain-wetland complex', 34th IAHR World Congress - Balance and Uncertainty, 33rd Hydrology & Water Resources Symposium, 10th Hydraulics Conference, Brisbane, June 2011 in Balance and Uncertainty Water in a Changing World, Proceedings of the 34th IAHR World Congress, Engineers Australia, ed Eric M. Valentine, IAHR, Brisbane, pp. 3044-3051.
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The presence of water in semi-arid environments is both spatially and temporally unpredictable and variable, creating a harsh and patchy environment. When present, water stimulates the germination and growth of plants but can also result in plant (or seedling) mortality. This study investigates how seedling emergence and mortality differ between various key habitats in a semi-arid floodplain wetland complex that are characterised by contrasting wetting regimes. Sediment samples were collected from five habitats (red soils, box-hollows, riparian areas, floodplains and lakes) and used in a germination experiment. Over a 12 week period, each habitat (except lakes) showed a peak in seedling emergence in the second week after initial experimental wetting, with boxhollows showing the largest emergence response in all weeks and riparian areas showing the lowest mortality (as a percent of emerged plants). Meanwhile the lowest overall emergence occurred in lakes and the highest mortality was found in red soils. Each habitat displayed a unique emergence and mortality response over the 12 weeks of the experiment suggesting that habitat has a strong influence on seedling emergence and survivability.