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"Raschi, A"
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Water transport in plants under climatic stress : proceedings of an international workshop, held in Vallombrosa, Firenze, Italy
The editors have brought together contributions from a range of experts who have worked on the captivation of water in the transport system. They provide a compendium of information for those working in the plant and environmental sciences as well as for those whose interests lie in the disciplines of agriculture and forestry.
Book
Coordination of stomatal physiological behavior and morphology with carbon dioxide determines stomatal control
• Premise of the study: Stomatal control is determined by the ability to alter stomatal aperture and/or the number of stornata on the surface of new leaves in response to growth conditions. The development of stomatal control mechanisms to the concentration of CO₂ within the atmosphere ([CO₂]) is fundamental to our understanding of plant evolutionary history and the prediction of gas exchange responses to future [CO₂]. • Methods: In a controlled environment, fern and angiosperm species were grown in atmospheres of ambient (400 ppm) and elevated (2000 ppm) [CO₂]. Physiological stomatal behavior was compared with the stomatal morphological response to [CO₂]. • Key results: An increase in [CO₂] or darkness induced physiological stomatal responses ranging from reductions (active) to no change (passive) in stomatal conductance. Those species with passive stomatal behavior exhibited pronounced reductions of stomatal density in new foliage when grown in elevated [CO₂], whereas species with active stornata showed little morphological response to [CO₂]. Analysis of the physiological and morphological stomatal responses of a wider range of species suggests that patterns of stomatal control to [CO₂] do not follow a phylogenetic pattern associated with plant evolution. • Conclusions: Selective pressures may have driven the development of divergent stomatal control strategies to increased [CO₂]. Those species that are able to actively regulate guard cell turgor are more likely to respond to [CO₂] through a change in stomatal aperture than stomatal number. We propose a model of stomatal control strategies in response to [CO₂] characterized by a trade-off between short-term physiological behavior and longer-term morphological response.
Journal Article
Vulnerability of xylem to embolism in relation to plant hydraulic resistance in Quercus pubescens and Quercus ilex co-occurring in a Mediterranean coppice stand in central Italy
The seasonal patterns of xylem embolism and xylem transport properties
in Quercus pubescens Willd. and Quercus
ilex L. trees growing in a natural mixed coppice stand in
conditions of severe water stress were investigated. Xylem
embolism was evaluated in both dehydrating branches and in
apical twigs during a whole year. Measurements of
xylem water potential were conducted from predawn to sunset
on selected sunny days. On the same days, diurnal
courses of leaf conductance were monitored. Measurements of
half-hourly sap flow were made by the heat-pulse
technique throughout the summer. At the onset of summer, a
sharp decrease in water potential was observed in
both species. Full recovery of water potentials was observed
for both species after the first major rainfall event in
September. Both experienced serious embolism throughout the year,
ranging between minima of c. 60%
(expressed as percentage loss of hydraulic conductivity) after
the rains in autumn and after bud burst in spring,
and maxima of c. 80% during summer and after freezing-thawing
events during the winter season. A significant
negative linear relationship was found between water potential
and xylem embolism in branches dehydrating in
air for Q. pubescens and Q. ilex. Q. pubescens
had greater
efficiency in hydraulic transport (higher specific
conductivity and leaf specific conductivity) by the xylem than
Q. ilex. In June, leaf conductance was high early in
the morning and decreased gradually during the day. Midday
depression of leaf conductance, as a result of high
evaporative demand combined with water deficit, was observed in
both species. In August, leaf conductance of
both species was greatly reduced, as water potential dropped to
extremely low values, and the stomata were almost
completely closed during the afternoon. No hysteresis resulting
from plant capacitance was observed in the
relationship between shoot water potential and sap flow. Q.
pubescens exhibited very high values of whole-tree
hydraulic resistance between July and September, whereas
Q. ilex generally showed lower values. The effect of soil
moisture depletion on the relationship between sap flow and
shoot water potential appears as a lowering of water
potential at zero flow. A significant decrease of whole-tree
hydraulic resistance in both species was observed with
the onset of the autumn, preceding the partial recovery of twig
hydraulic conductivity. The results demonstrate
that both Q. pubescens and Q. ilex, although highly tolerant
of
severe water stress and tissue dehydration, operate
at the limits of safety which are surpassed under severe droughts,
and prolonged climatic stress might predispose
these Quercus species to decline.
Journal Article
Impacts of droughts and extreme temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones
Extreme climatic events, such as droughts and heat stress, induce anomalies in ecosystem–atmosphere CO2 fluxes, such as gross primary production (GPP) and ecosystem respiration (Reco), and, hence, can change the net ecosystem carbon balance. However, despite our increasing understanding of the underlying mechanisms, the magnitudes of the impacts of different types of extremes on GPP and Reco within and between ecosystems remain poorly predicted. Here we aim to identify the major factors controlling the amplitude of extreme-event impacts on GPP, Reco, and the resulting net ecosystem production (NEP). We focus on the impacts of heat and drought and their combination. We identified hydrometeorological extreme events in consistently downscaled water availability and temperature measurements over a 30-year time period. We then used FLUXNET eddy covariance flux measurements to estimate the CO2 flux anomalies during these extreme events across dominant vegetation types and climate zones. Overall, our results indicate that short-term heat extremes increased respiration more strongly than they downregulated GPP, resulting in a moderate reduction in the ecosystem's carbon sink potential. In the absence of heat stress, droughts tended to have smaller and similarly dampening effects on both GPP and Reco and, hence, often resulted in neutral NEP responses. The combination of drought and heat typically led to a strong decrease in GPP, whereas heat and drought impacts on respiration partially offset each other. Taken together, compound heat and drought events led to the strongest C sink reduction compared to any single-factor extreme. A key insight of this paper, however, is that duration matters most: for heat stress during droughts, the magnitude of impacts systematically increased with duration, whereas under heat stress without drought, the response of Reco over time turned from an initial increase to a downregulation after about 2 weeks. This confirms earlier theories that not only the magnitude but also the duration of an extreme event determines its impact. Our study corroborates the results of several local site-level case studies but as a novelty generalizes these findings on the global scale. Specifically, we find that the different response functions of the two antipodal land–atmosphere fluxes GPP and Reco can also result in increasing NEP during certain extreme conditions. Apparently counterintuitive findings of this kind bear great potential for scrutinizing the mechanisms implemented in state-of-the-art terrestrial biosphere models and provide a benchmark for future model development and testing.
Journal Article
Joint control of terrestrial gross primary productivity by plant phenology and physiology
Terrestrial gross primary productivity (GPP) varies greatly over time and space. A better understanding of this variability is necessary for more accurate predictions of the future climate–carbon cycle feedback. Recent studies have suggested that variability in GPP is driven by a broad range of biotic and abiotic factors operating mainly through changes in vegetation phenology and physiological processes. However, it is still unclear how plant phenology and physiology can be integrated to explain the spatiotemporal variability of terrestrial GPP. Based on analyses of eddy–covariance and satellite-derived data, we decomposed annual terrestrial GPP into the length of the CO ₂ uptake period (CUP) and the seasonal maximal capacity of CO ₂ uptake (GPP ₘₐₓ). The product of CUP and GPP ₘₐₓ explained >90% of the temporal GPP variability in most areas of North America during 2000–2010 and the spatial GPP variation among globally distributed eddy flux tower sites. It also explained GPP response to the European heatwave in 2003 ( r ² = 0.90) and GPP recovery after a fire disturbance in South Dakota ( r ² = 0.88). Additional analysis of the eddy–covariance flux data shows that the interbiome variation in annual GPP is better explained by that in GPP ₘₐₓ than CUP. These findings indicate that terrestrial GPP is jointly controlled by ecosystem-level plant phenology and photosynthetic capacity, and greater understanding of GPP ₘₐₓ and CUP responses to environmental and biological variations will, thus, improve predictions of GPP over time and space.
Significance Terrestrial gross primary productivity (GPP), the total photosynthetic CO ₂ fixation at ecosystem level, fuels all life on land. However, its spatiotemporal variability is poorly understood, because GPP is determined by many processes related to plant phenology and physiological activities. In this study, we find that plant phenological and physiological properties can be integrated in a robust index—the product of the length of CO ₂ uptake period and the seasonal maximal photosynthesis—to explain the GPP variability over space and time in response to climate extremes and during recovery after disturbance.
Journal Article
Soil Respiration in European Grasslands in Relation to Climate and Assimilate Supply
Soil respiration constitutes the second largest flux of carbon (C) between terrestrial ecosystems and the atmosphere. This study provides a synthesis of soil respiration (R s) in 20 European grasslands across a climatic transect, including ten meadows, eight pastures and two unmanaged grasslands. Maximum rates of R s ( [graphic removed] ), R s at a reference soil temperature (10°C; [graphic removed] ) and annual R s (estimated for 13 sites) ranged from 1.9 to 15.9 μmol CO₂ m⁻² s⁻¹, 0.3 to 5.5 μmol CO₂ m⁻² s⁻¹ and 58 to 1988 g C m⁻² y⁻¹, respectively. Values obtained for Central European mountain meadows are amongst the highest so far reported for any type of ecosystem. Across all sites [graphic removed] was closely related to [graphic removed] . Assimilate supply affected R s at timescales from daily (but not necessarily diurnal) to annual. Reductions of assimilate supply by removal of aboveground biomass through grazing and cutting resulted in a rapid and a significant decrease of R s. Temperature-independent seasonal fluctuations of R s of an intensively managed pasture were closely related to changes in leaf area index (LAI). Across sites [graphic removed] increased with mean annual soil temperature (MAT), LAI and gross primary productivity (GPP), indicating that assimilate supply overrides potential acclimation to prevailing temperatures. Also annual R s was closely related to LAI and GPP. Because the latter two parameters were coupled to MAT, temperature was a suitable surrogate for deriving estimates of annual R s across the grasslands studied. These findings contribute to our understanding of regional patterns of soil C fluxes and highlight the importance of assimilate supply for soil CO₂ emissions at various timescales.
Journal Article
Decomposition and nutrient dynamics of Quercus pubescens leaf litter in a naturally enriched CO2 Mediterranean ecosystem
1. The chemical composition (i.e. N, P, C, lignin and polyphenol concentrations) of Quercus pubescens leaf litter derived from a natural CO2 spring in Tuscany (Italy) was analysed and compared to litter from a nearby reference site. Litter was incubated for 25 months at both the natural CO2 spring and the reference site, and monitored for decomposition rates, nutrient and lignin concentrations. 2. Long-term exposure to elevated CO2 concentrations from the natural spring was associated with a change in the chemical composition of the Oak leaf litter, with decreases in P and polyphenol concentrations and increases in lignin. No differences in N concentrations were observed between the enriched CO2 litter from the natural spring and the reference litter. 3. Decomposition was reduced in the CO2 spring, with the lower P concentration of the native litter, combined with the lack of soil fauna observed at that site, being the factors most probably responsible for the measured decreases in mass loss. However, litter from the CO2 spring and reference litter decomposed at the reference site showed similar rates of decomposition. 4. All litter showed similar N concentrations during decomposition, with N being mineralized throughout the incubation period from both litter regardless of the site of incubation. In contrast, P dynamics differed between litter, with P being immobilized in the litter derived from the spring, and mineralized from the reference litter. When the litter from the spring was incubated at the reference site, there was a trend for net P uptake from the surrounding environment. The chemical composition of decomposing litter from the spring appeared to match that of the reference litter after 3 months of incubation at the reference site. 5. The results from the CO2 spring suggest that litter decomposition may be retarded under elevated levels of atmospheric CO2. However, results from field surveys around CO2 vents should be viewed with caution because differences may relate to factors other than the known differences in CO2 concentrations.
Journal Article
Soil respiration and microbial activity in a Mediterranean grassland exposed to Free Air CO 2 Enrichment (FACE)
The effects of elevated atmospheric CO 2 on in situ soil respiration and belowground biomass were studied in a FACE (Free Air CO 2 Enrichment) facility. A Mediterranean grassland community was exposed to elevated and ambient CO 2 concentrations in a mini-FACE system in Tuscany (Italy). We quantified litter mass and chemistry, root growth and turnover, CO 2 efflux from soils, and soil microbial biomass. Elevated CO 2 caused limited increases in aboveground production. Litter quality, fine root turnover, microbial biomass, root growth, and root biomass were not significantly affected by elevated CO 2 , except during some periods. Our results suggest that elevated atmospheric CO 2 might moderately accelerate inputs of organic matter to soil carbon pools in Mediterranean grasslands, but it may also partially accelerate losses of carbon from belowground by stimulating soil respiration.
Journal Article
Stomatal index responses of Agrostis canina to CO₂ and sulphur dioxide: implications for palaeo‐CO₂ using the stomatal proxy
• Stomatal index values of fossil plants are widely used in reconstructing palaeo‐[CO₂]. This depends upon the assumption that the stomatal index is determined by the atmospheric concentration of CO₂ ([CO₂]). This study investigates whether fumigation with, and resistance to, sulphur dioxide (SO₂) induces a reduction in the stomatal index that may affect stomatal reconstructions of palaeo‐[CO₂] coinciding with episodes of global‐scale volcanism. • Agrostis canina from Mefite di Ansanto, Italy, grow in atmospheres of elevated‐[CO₂], SO₂ and hydrogen sulphide (H₂S). Mefite A. canina were compared with a control population in a ‘common‐garden' experiment and a controlled‐environment study under elevated‐[CO₂] and SO₂ fumigation. • In A. canina, resistance to toxic volcanic gases is not associated with reduced stomatal index, and fumigation with SO₂ does not cause a decrease in stomatal initiation. The two populations of A. canina analyzed in this study exhibit different stomatal index-[CO₂] ‘responses', with control plants showing a reduction in stomatal index and Mefite plants showing no response. • Stomatal reconstructions of palaeo‐[CO₂] during past episodes of global‐scale volcanism probably reflect atmospheric [CO₂] and not [SO₂]. The lack of a reduction in the stomatal index in response to elevated [CO₂] in the Mefite plants, suggests that resistance to toxic gases and/or long‐term growth at high [CO₂] reduces, or negates, sensitivity of the stomatal index-[CO₂] relationship, or that stomatal index-[CO₂] in the Mefite plants is attuned to [CO₂] fluctuations at much higher concentrations.
Journal Article
Physiological performance and biomass production of two ornamental shrub species under deficit irrigation
KEY MESSAGE : This study shows that deficit irrigation can be encouraged in productive activities like plant nursing, especially with the most resistant species. Drought acclimation is attained by avoidance mechanisms in the short term and by adaptation changes in the long term. In this study, we applied deficit irrigation at nursery stage to elicit species-specific physiological responses and biomass production in two ornamental shrub species, comparing two settings: open air (OA) and greenhouse (GH). Viburnum opulus L. and Photinia x fraseri ‘Red robin’ were compared under three irrigation levels: severe water deficit (SWD), moderate water deficit (MWD) and control (C). In both experiments, SWD induced lower values of water potentials in both species whereas MWD had similar effects to C. In OA, SWD reduced earlier stomatal conductance in V. opulus and photosynthesis rate in P. x fraseri and in GH V. opulus showed a reduction of gas exchange even in MWD. P. x fraseri showed greater stomatal control capacity. In contrast, treatments did not affect PSII efficiency even if P. x fraseri proved a greater capacity to differentiate the I–P phase of the fluorescence transient in SWD conditions. In both experiments, SWD and MWD affected leaf area by lowering the number of leaves in P. x fraseri and reducing leaf surface in V. opulus. Moreover, V. opulus showed early leaf senescence and premature fall. Deficit irrigation had effects also on leaf characteristics: smaller leaf area unit in V. opulus, reduced biomass and leaf mass per area and succulence in both species, increase of spongy tissue thickness. Eventually, we can state that P. x fraseri can withstand 30 % evapotranspiration, while maintaining functionality.
Journal Article