Journal of Experimental Botany, Vol. 51, No. 347, pp. 1135-1146,
June 2000
© 2000 Oxford University Press
Comparative field water relations of three Mediterranean shrub species co-occurring at a natural CO2 vent
1 Dipartimento di Coltivazione e Difesa delle Species Legnose—Sezione Coltivazioni Arboree, Università di Pisa, Italy
2 Scuola Superiore di Studi Universitari e di Perfezionamento ‘S. Anna’, Pisa, Italy
3 Centre de Recerca Ecològica i Aplicacions Forestals (CREAF), Facultat de Ciències, Universitat Autònoma, Bellaterra (Barcelona), Spain
4 Istituto per l'Agrometeorologia e l'Analisi Ambientale applicata all'Agricoltura, Consiglio Nazionale delle Ricerche (IAT—CNR), Firenze, Italy
5 Department of Botany, Trinity College, University of Dublin, Ireland
Received 30 July 1999; Accepted 28 February 2000
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Annual variations in the water relations and stomatal response of Erica arborea, Myrtus communis and Juniperus communis occurring at a natural CO2 vent were analysed under Mediterranean field conditions. A distinct gradient of CO2concentration ([CO2]) exists between two sites near a natural CO2-emitting vent, with higher [CO2] (700 µmol mol-1) in the proximity of the CO2 spring. Plants at the CO2 spring site have been growing for generations at elevated [CO2]. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits during the summer drought were severe. Leaf conductance and water potential recovered after major rainfalls in September to predrought values. Strong relationships between leaf conductance, predawn water potential, and leaf-specific hydraulic resistance are consistent with the role of stomata in regulating plant water status. Considerable between-species variation in sensitivity of water potentials and stomatal characters to elevated [CO2] were observed. Common to all the shrubs were a reduction in leaf conductance and an increase in water potentials in response to elevated [CO2]. Elevated [CO2] decreased the sensitivity of leaf conductance to vapour pressure deficit. Morphological characters (including stomatal density and degree of sclerophylly) showed site-dependent variations, but degree and sign of such changes varied with the species and/or the season. Measurements of discrimination against 13C provided evidence for long-term decreases of water use efficiency in CO2 spring plants. Analysis of C isotope composition suggested that a downward adjustment of photosynthetic capacity may have occurred under elevated [CO2]. Elevated [CO2] effects on water relations and leaf morphology persisted in the long term, but the three shrubs growing in the same environment showed species-specific responses.
Key words: Mediterranean shrubs, natural CO2 springs, stomatal conductance, stomatal density, water relations, water use efficiency.
|
Introduction |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Evergreen sclerophylls are regarded as one of the most typical components of the Mediterranean-type vegetation (Specht, 1969
; Kummerow, 1973; Krause and Kummerow, 1977). Due to the greenhouse gas-driven predicted increase in mean global temperature at the surface of the earth (Kattenburg et al., 1996) and a concurrent decrease in precipitation at Mediterranean latitudes forecast by General Circulation Models, plant communities of Mediterranean-type ecosystems will have also to face more severe drought conditions in the future.
Current predictions about the effects of rising [CO2] on natural ecosystems are still based on data from experiments with isolated seedlings or saplings growing under highly artificial conditions and relatively short-term exposure to elevated [CO2] where acclimation may not have occurred. Questions of acclimation and adaptation to elevated [CO2] suggest the need for truly long-term elevated-[CO2] exposures. Although not ideal experiments, natural geological CO2 springs have, in some cases, exposed terrestrial ecosystems to elevated [CO2] for centuries or even millennia (Miglietta et al., 1993). The main benefit of natural CO2 springs as research tools is the long history of elevated [CO2] in an otherwise natural environment (Miglietta and Raschi, 1993; Körner and Miglietta, 1994; Jones et al., 1995; Tognetti et al., 1996; Hättenschwiler et al., 1997). Natural CO2 springs have the potential to provide information concerning long-term plant and ecosystem responses to concurrent elevated [CO2] and varying environmental factors (in particular seasonal water and temperature stress) (Amthor, 1995; Raschi et al., 1997), without some of the disadvantages of other short-term methods now in use to study the effect of elevated [CO2] (leaf cuvettes, branch chambers, growth chambers, glasshouses, open-top field chambers, and free-air CO2 enrichment facilities—FACE). Undoubtedly, some new problems and questions arise when working with such natural sites, such as fluctuations of [CO2] (van Gardingen et al., 1995).
Stomata can be considered to be integrators of all environmental factors affecting plant growth (Morison, 1998). High [CO2] has been found to improve the response of plants to water stress by inducing, in the short-term, stomatal closure and/or maintaining higher water potentials (Tyree and Alexander, 1993; Morison, 1998), although varying responses to changes in [CO2] have been reported (Amthor, 1995; Drake et al., 1997). In the long term, a change in stomatal conductance may also be caused by changes in numbers or frequency and size of stomata as individual leaves develop (Woodward and Kelly, 1995; Wagner et al., 1996). Stomatal conductance and frequency of plants grown in controlled conditions may, however, be affected by poor atmospheric coupling and an altered balance between energy supply and water loss from leaves (Morison, 1987). There is, at present, little information on physiological acclimation and morphological adjustment of stomata (Morison, 1998), particularly in natural ecosystems, because many responses are species-specific, but it is clear that acclimation and adjustment in the long term may have a significant impact on conductance and hence on gas exchange and water use efficiency (Morison, 1993).
Since Mediterranean shrubs show convergent behaviour with respect to leaf functional characteristics (Lange et al., 1985), it is also possible that different sclerophylls may display similar physiological trends, in terms of field water relations, in response to elevated [CO2]. In the present study, assessment was made in a Mediterranean environment of the variability of stomatal response and field water relations in Erica arborea L., Juniperus communis L. and Myrtus communis L. plants under natural field conditions at two sites, near Lajatico (Pisa, Italy) characterized by different [CO2] throughout 12 months. Plants sampled in this experiment have been exposed, probably for generations, to elevated [CO2] and a large range of natural disturbances. The main hypothesis was that long-term differences in CO2 availability between the high [CO2] and a neighbouring control site would be reflected in improved field water relations of plants close to CO2 springs.
|
Materials and methods |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Site description and plant material
Field measurements of xylem pressure potential and leaf conductance were made through a whole year for E. arborea, J. communis and M. communis, three common and widespread Mediterranean macchia shrub species. Measurements were obtained from October, 1996 through September, 1997 at a natural CO2 spring near Lajatico (Pisa, Italy) called ‘I Borboi’ (43°26' N, 10°42' E). A full description of the geology of the site has been described earlier (Panichi and Tongiorgi, 1975
). The enriched area extends over an area of 0.7 ha. The area is covered by a coppiced stand dominated by Quercus ilex L., but in which Quercus pubescens Willd. and Arbutus unedo L. (as well as several other tree species, e.g. Quercus cerris L., Fraxinus ornus L., represented by scattered individuals) and the shrub level (including, other than the species studied here, Smilax aspera L., Cytisus scoparius L., Cistus salviifolius L., Genista sp., Ligustrum vulgare L., Pistacia lentiscus L., and Phyllirea latifolia L.) contribute to constitute the great bulk of the flora. The CO2 spring is located on the north-facing slope (20%) of a hill near the bottom of a small valley about 200 m above sea level (Raiesi, 1998). Almost pure CO2 emissions occur from a series of vents located along a narrow seasonal creek and the [CO2] tends to decrease upslope. The vents emit small amounts of H2S, but its concentration never exceeds 0.04 µmol mol-1 which is not considered to be harmful to plants (Raiesi, 1998). Plants around the CO2 spring are exposed to daytime [CO2] of about 700 µmol mol-1 throughout the year with short-term variations between 500 and 1000 µmol mol-1 depending on windspeed and convective turbulence. The [CO2] at different heights within the canopy varied little. The aspect limited early morning and late afternoon direct insolation. The climate is typical Mediterranean, with cool, wet winters and hot, dry summers (Fig. 1 data are from Volterra meteorological station, 500 m a.s.l. and about 10 km from the site).
|
Measurements were obtained from individuals growing in close association in the proximity of the CO2 spring and at a control site chosen along the same creek about 150 m upstream; thus roots of selected plants experienced the same soil environment and branches the same aerial environment, except for [CO2]. The area has non-calcareous, brown loamy clayey soils, developed from calcareous marl (pH 6–7) (Raiesi, 1998
). Soil samples (six per site at each date) were taken at 0.2 m for gravimetric measurements of soil moisture after drying in oven at 105 °C. At both sites, shrubs of similar exposure were selected for each species. Measurements were made at about monthly intervals on leaves in the upper or outer canopy so that they would be exposed to sunlight throughout most of the day.
Field measurements of water potential and stomatal response
Because the weather at this study site remained similar for several days at a time, it was possible to select days with clear skies throughout to make diurnal measurements of leaf conductance (gs) and water potentials (
) on the three species. Diurnal measurements (from dawn to dusk) of gs and transpiration (E) were made using a null-balance steady-state porometer (Li-1600, Li-Cor Inc., Lincoln, Nebraska, USA) on six individuals per species from each of the two sites. A conifer foliage cuvette was used for measurements on E. arborea and J. communis; foliage area (one side projection, hypostomatous leaves) was computed from total mass by regression analysis. Two to four fully developed leaves/shoots were measured on each plant on each sampling occasion. The standard leaf/shoot was defined as a sun-exposed, apparently healthy leaf/shoot. Leaves/shoots examined in consecutive measurement cycles were not necessarily the same. From October to April, recorded data were from previous spring leaves. Starting from May, gs was measured separately for both the current year's new fully expanded leaves and for older leaves. Leaf conductances of one-year-old leaves were only 5–10% below those of the current year's leaves in the spring then converged later, and the CO2 environment did not affect this difference so that data were pooled. Each measurement was completed within 30 s and humidity inside the porometer chamber was kept near ambient values to minimize the effects on subsequent measurements. For each measurement, air temperature inside the cuvette, leaf temperature (thermocouple), relative humidity, and incoming photosynthetic photon flux density (PPFD) were recorded. Cuvette overheating was minimized by shading the cuvette between measurements. The natural inclination and azimuth of the leaves/shoots were maintained during the measurements. Changes in leaf conductance were considered to reflect changes in stomatal conductance, on the assumption that boundary layer conductance inside the cuvette was constant and large.
Daily courses of shoot were determined using a pressure chamber (PMS-100, PMS Instruments Co., Corvallis, Oregon, USA) from predawn to sunset in parallel with the diurnal determinations of gs. Two to three terminal shoots per plant were chosen close to those on which gs had been measured. The absolute difference between minimum water potential (m), generally recorded at midday, and predawn water potential (pd) was termed 
.
Leaf-specific hydraulic resistance (RL) on each of the plants sampled was calculated, following Ohm's law analogy, by dividing the average by average E (Castell et al., 1994; Sala and Tenhunen, 1994), assuming that pd represents soil water potential in contact with the roots. The transpiration rate measured with the porometer does not equal that of plants in situ because of temperature and boundary layer disturbances. However, since values between treatments were compared, absolute values of E should be less important than relative values.
For comparative purposes, curves describing changes in measured maximum gs with atmospheric vapour pressure deficit (VPD) were obtained from the entire data set (Lange et al., 1987; Tognetti et al., 1998b, 1999). Maximum daily gs at light saturation (gsmax) was defined (for each species at each site) as the average hourly reading with the highest value of the day. Data from diurnal courses indicated that the time when the maximum daily light-saturated gs generally occurred was between 07.00 h and 09.00 h solar time. In this case, VPD was estimated by measuring relative humidity and temperature with the porometer chamber held open next to the foliage. Regressions of stomatal response to VPD were constructed by pooling values obtained from plants sampled at each site (each data pair corresponding to individual leaves/twigs).
Microscope observation and sclerophylly of leaves
Observations were made on portions taken from the central area of fully expanded leaves collected in May (1998). Sampling was carried out on 8–10 leaves from six plants per species from each of two sites. The leaf portions were fixed for about 24 h in a 4% glutaraldehyde solution in 0.1 M phosphate buffer, rinsed in 0.1 M phosphate buffer, post-fixed in 2% osmium tetraoxide solution in 0.1 M phosphate buffer for 1 h, dehydrated in increasing alcohol solutions, desiccated to the critical point in a Critical Point Dryer (CPD 030 Bal-Tec, Balzers, Liechtenstein), sputter-coated with 20 nm of gold (measured by quartz thin-film monitor) and observed at 15 kV in a scanning electron microscope (Philips SEM 515, The Netherlands). Slow-scan images were digitized at a resolution of 768x576 pixels (256 grey levels) and analysed with AnalySIS 2.0 (Soft-Imaging Software GmbH, Germany).
Stomatal density (SD, number of stomata mm-2) of six fields per sample (500x magnification) and pore length were determined on the recorded digital images. Stomata overlapping the margins were excluded. Dimensions (polar and equatorial) of the stomatal apparatus were then used for calculating the equivalent area of the ellipsoid according to (
xlengthxwidth)/4, representing stomatal apparatus dimension (Di, µm2) (Minnocci et al., 1995; Bartolini et al., 1997). The ratio of leaf mass (after oven drying at 70 °C for 48 h) to leaf area was calculated and termed degree of sclerophylly (Ds, kg m-2) (Lo Gullo and Salleo, 1988).
Carbon isotope analysis
On several occasions during the study period, shoots were collected from six individuals of each species (E. arborea, M. communis and J. communis) at each site (CO2 spring and control), early in the morning for C isotope discrimination analysis (10–12 leaves of the same stage of development were bulked for each plant at each sampling date). The shoots were immediately transported (in black plastic bags) to the laboratory where they were oven-dried at 60 °C to constant weight. The C isotope composition was measured on subsamples (about 1 mg) of leaves ground to a fine powder. The C isotope composition was measured by a dual inlet triple collector isotope ratio mass spectrometer (SIRA II, VG Isotech, Middlewich, UK), operated in direct inlet continuous flow mode. Before analysis, the solid samples were combusted in a Dumas-combustion elemental analyser (Model EA 1500, Carlo Erba, Milano, Italy) and CO2 was cryogenically purified. The C isotope discrimination () in foliage samples was calculated according to Farquhar et al. (Farquhar et al., 1989) from relative abundances of 13C and 12C expressed in terms of the conventional 
notation according to the relationships:
 | (1) |
 | (2) |
a is the 13C value for source atmospheric CO2 (-0.0083 and -0.0089 the average at the control and CO2 spring site, respectively; M Saurer personal communication), and p is the 13C value for plant foliage. A system check of analysis was achieved with interspersed working standards of cellulose, atropine and urea (Sigma, St Louis, MO, USA). The accuracy of the measurement was ±0.1. A simplified model relating leaf to intercellular [CO2] and atmospheric [CO2] (Farquhar et al., 1989) was used to calculate the ci/ca ratio:
 | (3) |
) and b (-27) are constants representing fractionations due to diffusion in air and carboxylation of CO2 in C3 plants, respectively (Farquhar et al., 1989).
Statistical analysis
Means were made averaging the measurements on individuals for each sampling date. Within a species, two-way analysis of variance (ANOVA) with sampling time and [CO2] as the main effects was conducted for all parameters except for those relative to the SEM analysis which were tested by one-way ANOVA with LSD test for mean separation.
|
Results |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Seasonal course of shoot water potential
The seasonal courses of
pd and m (about midday) showed a progressive drought development and indicated somewhat seasonal segregation by species (Fig. 2). Generally, during the growing season, E. arborea showed the lowest values in while M. communis the highest, with J. communis showing intermediate values. The predawn water potential remained close to zero from the fall to the following spring. Starting from June a sharp decrease in pd was observed in all species up to a minimum (between -2 and -3 MPa) reached in August. The predawn water potential recovered with the onset of rain in September. The midday water potential showed a more gradual decrease characterized by a plateau (between -1 and -2 MPa) between the onset of flowering and the period of maximum shoot elongation (spring months). As drought progressed m reached a minimum (between -3 and -4 MPa) in August. A consequence of the seasonal patterns in pd and m was that showed two maxima before and following the summer peak of water stress. The absolute minimum was recorded during winter in all the species. The differences between predawn and midday water potentials were generally greater in E. arborea than other species, but in any case did not exceed 1.5 MPa.
|
The seasonal course in
pd and m differed with CO2 exposure. In the plants growing in the proximity of the CO2 vent both pd and m were significantly higher than in those at the control site (Table 1), which implies that less water was withdrawn from the soil and/or the root system was deeper in plants at the CO2 vent. Differences in pd were more evident during the summer drought and at the beginning of recovery for E. arborea and M. communis (the interaction between sampling time and site had a P-level slightly above 0.05), while in J. communis the difference in the same period was generally less marked. The difference in m was more pronounced. The effect of elevated [CO2] on was not evident.
|
0.05, **P
Seasonal course of maximum leaf conductance
In all plants gsmax decreased as the drought cycle progressed, reaching mid-summer values below 100 mmol m-2 s-1 (Fig. 3), then after the first rainfall events of late summer partially recovered to predrought values. Another minimum was reached as plants responded to low temperature and solar radiation during winter. Maximum leaf conductances showed differences between species. Early growing season measurements showed M. communis with the highest gsmax closely followed by E. arborea and then by J. communis with consistently lower values. The differences between J. communis and the other two species almost disappeared at minimum values of gsmax in mid-winter and mid-summer.
|
Differences between CO2 vent and control sites were significant for all the three species (Table 1
), with plants at the control site showing higher gsmax than plants at the CO2 vent. However, such differences were more evident during spring and fall, associated with relatively more humid periods. M. communis showed the most pronounced differences between sites while J. communis the least. As drought progressed the decrease in gsmax was less abrupt in plants at the CO2 vent and generally in J. communis, thus differences appeared to level off.
Diurnal courses of water potential and leaf conductance, and variation in leaf conductance with vapour pressure deficit
The diurnal range of and gs were highest during spring and autumn and lowest during the summer drought for all species (Figs 4, 5). Generally, diurnal was highest for M. communis and gs was lowest for J. communis. Leaf conductance was generally relatively high in the morning and decreased gradually during the day. For the driest months of the year all species opened their stomata during the early morning hours and tended to close them for the remainder of the day. Leaf conductance was higher in the control plants than in the CO2 vent plants during the period of no or mild water stress. Site differences in gs tended to decline with the progress of drought stress and in the second half of the day, particularly for J. communis. Site differences in were less pronounced, in the opposite direction and again converging with increasing water stress. Estimates of the diurnal variation of the leaf conductance–water potential relationship showed large hysteresis early in the growing season (data not shown). Plants at the control site showed a steeper decline in the relation between and gs than CO2 vent plants.
|
|
Over the long term (data correspond to the whole year), gsmax increased in all species with VPD in the range 0–1 kPa (Fig. 6
). During the growing season gsmax decreased with increasing VPD in plants both when grown and measured at ambient [CO2] and when grown and measured at elevated [CO2]. The response, however, was steeper for control plants than for plants at the CO2 vent site; site differences were also lower at very low values of VPD.
|
Seasonal course of leaf-specific hydraulic resistance
Leaf-specific hydraulic resistance was relatively high in early spring (Fig. 7), and subsequently decreased in all species in late spring and early summer. An increase in RL occurred in late summer, coinciding with large reductions in gs, but never exceeding 3 MPa m2 s mmol-1. Seasonal course of RL differed between species, with J. communis showing generally the highest values and M. communis the lowest (steeper decline in the relation between and gs in M. communis followed by E. arborea and J. communis, respectively). Site differences were not significant in J. communis, while control plants of M. communis and secondarily E. arborea showed consistently lower RL than CO2 vent plants (Table 1).
|
Leaf and stomatal morphology
Stomatal apparatus characteristics differed among the species (Table 2). M. communis had many more stomata per unit area than J. communis. However, the stomatal apparatus of M. communis was consistently smaller than that of J. communis. Site differences were also important. Stomatal density was strongly and negatively affected by long-term elevated [CO2] exposure in M. communis, while in J. communis SD was not significantly different between sites. In both species, but more evidently in M. communis, CO2 vent plants had significantly increased pore lengths. In M. communis CO2 vent plants had significantly larger Di than control plants, while in J. communis there was no difference between sites. It was not possible to remove hairs and wax covering the stomata of E. arborea leaves without damaging the leaf and stomata, thus the measurement of stomatal apparatus was not possible.
|
The degree of sclerophylly showed segregation by species, J. communis being the most sclerophyllous and M. communis the least sclerophyllous species, with E. arborea showing intermediate values (Table 2
). Differences between CO2 spring and control plants were evident in all the three species. In J. communis CO2 spring plants had leaves with a higher Ds than control plants, while the opposite tendency was observed in E. arborea and M. communis.
Carbon isotope discrimination
There was significant variation, though not very large, in average and 13C-derived ci/ca ratio values amongst the leaves of E. arborea, M. communis and J. communis (Table 3). J. communis had relatively higher 13C-derived water use efficiency than M. communis and E. arborea. Seasonal values were approximately constant for E. arborea, relatively larger variations were observed in M. communis and J. communis, probably linked to differential use of C compounds. The C isotope analysis yielded significant higher and 13C-derived ci/ca ratio values in leaves of plants growing at the CO2 spring site, regardless of the species.
|
|
Discussion |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Seasonal and diurnal patterns of leaf conductance and xylem water potential of E. arborea, M. communis and J. communis were similar to those observed previously in Mediterranean sclerophyllous species (Nilsen et al., 1984
; Gill and Mahall, 1986; Rhizopoulou and Mitrakos, 1990; Tenhunen et al., 1987). Generally, each species exhibited a pronounced morning peak in leaf conductance. The partial closure afterwards coincided with increasing vapour pressure deficit and decreasing water potentials. Strong reductions in leaf conductance occurred when water potential approached the turgor loss point (water stress in summer), coinciding with large decreases in predawn water potential (Tognetti, 1999). The absence of marked diurnal changes in water potential during drought and the relatively fast response to new input of soil moisture (September) might indicate relatively shallow root systems in all these species (Gucci et al., 1997). The comparison of water potentials during the dry season, however, suggests that M. communis must be restricted to relatively wetter areas than E. arborea and J. communis unless it has a deeper root system (Rhizopoulou and Mitrakos, 1990).
Generally, if stomata are exposed to increased [CO2] aperture declines (Morison, 1993). The effect of increased [CO2] can be large or small and part of this variation in sensitivity may be a characteristic of particular species or perhaps particular functional groups (Bunce, 1992; Heath and Kerstiens, 1997). The present study also indicated that stomatal conductance decreased under elevated [CO2] and that the effect is not transitory but persists over the long-term and through plant generations, though even in the natural condition of a CO2 spring environment such a response is not universal and may differ with the species and/or weather conditions (Jones et al., 1995; Bettarini et al., 1998; Tognetti et al., 1998b, 1999). Maximum leaf conductance decreased in a similar fashion at both sites as soil moisture decreased. However, the reduction in stomatal conductance due to elevated [CO2] is more evident at low vapour pressure deficit, that is during the early morning and relatively wet season (spring and autumn), while the stomatal sensitivity to vapour pressure deficit appears to be reduced under elevated [CO2]. Indeed, the difference in maximum leaf conductance between sites decreased consistently at the peak of water stress in mid-summer when vapour pressure deficit was at its maximum and shoot water potential at its minimum. This observation was also partially true at very low vapour pressure deficit during the winter, when solar radiation and temperature limited stomatal functions. Similar results are reported for other woody species irrespective of the enrichment method (Roden and Ball, 1996; Will and Teskey, 1997; Heath, 1998; Tognetti et al., 1998b, 1999). This species-specific modulated response makes predictions of the effects of elevated [CO2] on stomatal conductance and water use difficult. It is possible to hypothesize, however, that the proportional effect of CO2 enrichment on stomatal conductance becomes negligible as stomata close in response to environmental stresses.
Leaf-specific hydraulic resistance was generally high in winter and early spring, possibly due to low soil temperature cooling of roots (Teskey et al., 1978) and/or delayed growth of fine roots (Calkin and Pearcy, 1984), and in mid-summer, because of changes in root water status. Decreases of leaf-specific hydraulic resistance in the three shrubs coincided with periods of growth. Different responses were evident among the three species. J. communis had the highest leaf-specific hydraulic resistance values, which is consistent with the lower hydraulic conductivity of tracheids of this shrub compared to the vessels of the other two species (Tognetti, 1999). M. communis had lower leaf-specific hydraulic resistance than E. arborea particularly in mid-summer, which may reflect the different root distribution in soil layers. These results show that stomata regulate leaf water status in coordination between soil water potential, hydraulic resistance and stomatal conductance.
Leaf-specific hydraulic resistance was higher in the CO2 spring than control plants of M. communis and secondarily E. arborea, irrespective of the predawn water potential, but this was not the case of J. communis. Higher leaf-specific hydraulic resistance in plants growing at elevated [CO2] could represent a response to the growth (and measurement) [CO2] (Bunce and Ziska, 1998), reducing leaf conductance and transpiration more consistently in one species than in others. Lower leaf-specific hydraulic conductance may lead to lower assimilation rate and thus have an impact on photosynthesis under long-term elevated [CO2]. In general, CO2 spring plants showed lower maximum leaf conductance for a given value of leaf-specific hydraulic resistance, thus suggesting that they may more effectively avoid cavitation by means of stomatal control of water loss and/or through changes in the conducting tissue.
Decreased conductance under elevated [CO2] in M. communis was associated with changes in the anatomy of the leaves. The frequency of stomata after long-term exposure to elevated [CO2] decreased in this species, according to the hypothesis of adaptive modifications of stomatal number (Woodward, 1987; Peñuelas and Matamala, 1990). On the other hand, in J. communis such a relationship was not evident and stomatal frequency was not affected by elevated [CO2], even though stomatal conductance declined, but to a lesser extent than in M. communis. Many experiments suggest only minor responses or no significant change to stomatal frequency even after long-term exposure to [CO2] exceeding the current ambient levels (Bettarini et al., 1998). The increase in pore length, indicating pore size (Wagner et al., 1996), in CO2 spring plants (which was not expected) could contribute to counteract the reduced pore aperture. However, the physiological effects of stomatal frequency reduction did not seem to be offset by the concomitant increase of pore size in M. communis. The lack of a clear relationship among stomatal density, size and conductance was confirmed for J. communis. It has been shown that the differences in stomatal conductance between [CO2] treatments may be only due to aperture and not to stomatal frequency and/or geometry (Morison, 1998). The larger size of stomatal apparatus in CO2 spring plants of M. communis may be a consequence of an amelioration of water status of plants and avoidance of water stress (Bettarini et al., 1998). The smaller influence of elevated [CO2] on stomatal characters in J. communis is, in part, consistent with this hypothesis.
Higher values of degree of sclerophylly in CO2 spring plants of J. communis could contribute to better drought tolerance (Mooney, 1982; Oertli et al., 1990), when this is not severe, and to better herbivore resistance (Turner, 1994). On the other hand, the lower degree of sclerophylly in CO2 spring plants of E. arborea and M. communis may be interpreted as an adaptation to variation in water availability or, alternatively, as an acclimation mechanism. Nevertheless, the degree of sclerophylly may change in the course of the year (Salleo and Lo Gullo, 1990).
J. communis had relatively higher 13C-derived water use efficiency than M. communis and E. arborea, which may be related to its more xeric distribution (Valentini et al., 1992). Variations in stomatal conductance in response to water availability or evaporative demand are partly responsible for variations in C isotope discrimination. However, in Mediterranean habitats (with summer-dry climates) high seasonal rainfall is not positively correlated with atmospheric evaporative demand, and a relationship between C isotope discrimination and measures of water availability (summer or total annual precipitation) may not be found.
The C isotope composition measurements made in leaves suggested a reduction of photosynthetic capacity (down-regulation) in these shrubs under elevated [CO2] (Miglietta et al., 1998). The depletion of 13C, as measured in the products of photosynthesis, is modulated by the rate at which CO2 diffuses into the leaf and by the rate at which CO2 is fixed by ribulose-1,5-bisphosphate carboxylase/oxygenase. Both the observed diurnal and seasonal responses of leaf conductance might be expected to increase instantaneous water use efficiency in plants at the CO2 spring site, and also the potential for considerably greater C gain over the year (Chaves et al., 1995; Jones et al., 1995; Johnson et al., 1997; Tognetti et al., 1998a). However, the limited response of stomatal conductance during drought between control and CO2 spring plants might be accompanied by reduced differences in photosynthesis during summer. The cost for a higher C gain during summer could be a general reduction in water use efficiency. The Mediterranean sclerophylls studied in this experiment may have an intrinsic growth strategy that prioritizes water saving over C uptake.
Provided that stomatal conductance of plants grown at the CO2 spring site was significantly lower than in control plants, an increased C isotope discrimination and 13C-derived ci/ca ratio, indicated that carboxylation became even more restricted (relatively to diffusion through stomata), and thus, significant downward adjustment of photosynthetic capacity should have occurred under elevated [CO2] (Miglietta et al., 1998). This should have resulted in lower 13C-derived water use efficiency values, regardless of the species. Higher C isotope discrimination values are, generally, associated with higher intercellular [CO2] values, which are determined by the balance between influx through the stomata and consumption through C fixation during photosynthesis. Plants grown under elevated [CO2] may show reduced responsiveness to increasing [CO2] at intercellular [CO2] below 700 µmol mol-1, however, stomatal sensitivity to intercellular [CO2] may be extended to higher intercellular [CO2] with the result that stomata continue to close with rising [CO2], thus exibiting constantly higher 13C-derived ci/ca ratio than plants grown at ambient [CO2] (
antrucek and Sage, 1996). Contrasting results are reported in studies of historical reconstructions of past rates of CO2 uptake and water loss of leaves, with both increasing and decreasing intercellular [CO2], or nearly constant ci/ca ratio (Ehleringer and Cerling, 1995; Beerling, 1996). Different climates in which the plants grow and/or differences in the sensitivity of stomatal conductance and stomatal density (and hydraulic properties) to elevated [CO2] may account for such discrepancies. The down-regulated photosynthetic rates might reduce the usually observed benefit of increased water use efficiency associated with growth in elevated [CO2].
In conclusion, these shrubs appear to be limited in their metabolic activity by the summer drought on the one hand or/and cool temperature on the other. Stomata of these shrubs continue to respond to elevated [CO2] after long-term exposure and may or may not display morphological and/or anatomical adjustments. Improved drought-tolerating strategies may allow plants to endure severe periodic drought. A low 13C-derived water use efficiency might reflect overall improved water relations (less water was withdrawn from the soil and/or deeper root systems) in E. arborea, M. communis and J. communis under elevated [CO2], which showed higher field water potential and turgor pressure (Tognetti, 1999). The species-specific response of co-occuring shrubs and trees may result in compensatory regulation of gas exchange, thus minimizing interspecific variation, but also in altered community composition on a larger scale in the Mediterranean environment.
|
Acknowledgments |
|---|
The research was in part supported by the National Research Council of Italy (CNR). We thank Chiara Del Guerra for collaboration during field measurements.
|
Notes |
|---|
6 To whom correspondence should be addressed at: Dipartimento di Scienze Animali, Vegetali e dell'Ambiente (SAVA), Università degli Studi del Molise, via De Sanctis I-86100 Campobasso, Italy. E-mail: tognetti{at}unimol.it
|
References |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Amthor JS.1995. Terrestrial higher-plant response to increasing atmospheric [CO2] in relation to the global carbon cycle. Global Change Biology 1, 243–274.[Web of Science]
Bartolini S, Minnocci A, Vitagliano C.1997. Influence of temperature on morpho-anatomic characteristics of ‘Trebbiano’ grapevine leaves. Agricoltura Mediterranea 127, 37–43.
Beerling DJ.1996. Ecophysiological responses of woody plants to past CO2 concentrations. Tree Physiology 16, 389–396.[Abstract]
Bettarini I, Vaccari FP, Miglietta F.1998. Elevated CO2 concentrations and stomatal density: observations from 17 plant species growing in a CO2 spring in central Italy. Global Change Biology 4, 17–22.
Bunce JA.1992. Stomatal conductance, photosynthesis and respiration of temperate deciduous tree seedlings grown outdoors at an elevated concentration of carbon dioxide. Plant, Cell and Environment 15, 541–549.
Bunce JA, Ziska LH.1998. Decreased hydraulic conductance in plants at elevated carbon dioxide. Plant, Cell and Environment 21, 121–126.
Calkin HW, Pearcy RW.1984. Leaf conductance and transpiration, and water relations of evergreen and deciduous perennials co-occurring in a moist chaparral site. Plant, Cell and Environment 7, 339–346.
Castell C, Terradas J, Tenhunen JD.1994. Water relations, gas exchange, and growth of resprouts and mature plant shoots of Arbutus unedo L. and Quercus ilex L. Oecologia 98, 201–211.[Web of Science]
Chaves MM, Pereira JS, Cerasoli S, Clifton-Brown J, Miglietta F, Raschi A.1995. Leaf metabolism during summer drought in Quercus ilex trees with lifetime exposure to elevated CO2. Journal of Biogeography 22, 255–259.[Web of Science]
Drake BG, Gonzàlez-Meler MA, Long SP.1997. More efficient plants: a consequence of rising atmospheric CO2? Annual Review of Plant Physiology and Plant Molecular Biology 48, 607–637.
Ehleringer JR, Cerling TE.1995. Atmospheric CO2 and the ratio of internal to ambient CO2 concentrations in plants. Tree Physiology 15, 105–111.[Abstract]
Farquhar GD, Ehleringer JR, Hubick KT.1989. Carbon isotope discrimination and photosynthesis. Annual Review of Plant Physiology and Plant Molecular Biology 40, 503–537.[Web of Science]
Gill DS, Mahall BE.1986. Quantitative phenology and water relations of an evergreen and a deciduous chaparral shrub. Ecological Monographs 56, 127–143.
Gucci R, Massai R, Casano S, Gravano E, Lucchesini M.1997. The effect of drought on gas exchange and water potential in leaves of seven Mediterranean woody species. In: Mohren GMJ et al., eds. Impacts of global change on tree physiology and forest ecosystems. Dordrecht, The Netherlands: Kluwer Academic Publishers, 225–231.
Hättenschwiler S, Miglietta F, Raschi A, Körner C.1997. 30 years in situ tree growth under elevated CO2: a model for future forest responses? Global Change Biology 3, 463–471.[Web of Science]
Heath J.1998. Stomata of trees growing in CO2-enriched air show reduced sensitivity to vapour pressure deficit and drought. Plant, Cell and Environment 21, 1077–1088.
Heath J, Kerstiens G.1997. Effects of elevated CO2 on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech. Plant, Cell and Environment 20, 57–67.
Johnson JD, Michelozzi M, Tognetti R.1997. Carbon physiology of Quercus pubescens growing in the Bossoleto CO2 spring of central Italy. In: Raschi A, Miglietta F, Tognetti R, Van Gardingen PR, eds. Plant responses to elevated carbon dioxide: evidence from natural springs. Cambridge, UK: Cambridge University Press, 148–164.
Jones MB, Clifton-Brown J, Raschi A, Miglietta F.1995. The effects on Arbutus unedo L. of long-term exposure to elevated CO2. Global Change Biology 1, 295–302.[Web of Science]
Kattenburg A, Giorgi F, Grassil H, Meehl GA, Mitchell JFB, Stouffer RJ, Tokioka T, Weaver AJ, Wigley TML.1996. Climate-models-projections of future climate. In: Houghton JT, Meira Filho LG, Callander BA et al, eds. Climate change, 1995—The science of climate change. Contribution of Working Group I to the Second Assesment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press, 285–357.
Körner C, Miglietta F.1994. Long-term effects of naturally elevated CO2 on mediterranean grassland and forest trees. Oecologia 99, 343–351.[Web of Science]
Krause D, Kummerow J.1977. Xeromorphic structure and soil moisture in the chaparral. Oecologia Plantarum 12, 133–148.[Web of Science]
Kummerow J.1973. Comparative anatomy of schlerophylls of mediterranean climate areas. In: di Castri F, Mooney HA, eds. Mediterranean-type ecosystems: origin and structure. Berlin, Germany: Springer-Verlag, 157–167.
Lange LO, Tenhunen DJ, Beyschlag W.1985. Effect of humidity during diurnal courses on the CO2 and light-saturated rate of net CO2 uptake in the sclerophyllous leaves of Arbutus unedo. Oecologia 67, 301–304.
Lange OL, Ullmann I, Tenhunen JD, Bannister P.1987. Stomatal conductance and transpiration of the two faces of Acacia phyllodes. Trees 1, 110–122.
Lo Gullo MA, Salleo S.1988. Different strategies of drought resistance in three Mediterranean sclerophyllous trees growing in the same environmental conditions. New Phytologist 108, 267–276.[Web of Science]
Miglietta F, Bettarini I, Raschi A, Körner C, Vaccari FP.1998. Isotope discrimination and photosynthesis of vegetation growing in the Bossoleto CO2 spring. Chemosphere 36, 771–776.[Web of Science]
Miglietta F, Raschi A.1993. Studying the effect of elevated CO2 in an open naturally enriched environment in Central Italy. Vegetatio 104/105, 391–402.[Web of Science]
Miglietta F, Raschi A, Bettarini I, Resti R, Selvi F.1993. Natural CO2 springs in Italy: a resource for examining long-term response of vegetation to rising atmospheric CO2 concentrations. Plant, Cell and Environment 16, 873–878.
Minnocci A, Panicucci A, Vitagliano C.1995. Gas exchange and morphological stomatal parameters in olive plants exposed to ozone. Agricoltura Mediterranea (Special Volume) 77–81.
Mooney HA.1982. Habitat, plant form and plant water relations in Mediterranean climate regions. Ecologia Mediterranea 8, 287–296.
Morison JIL.1987. Intercellular CO2 concentration and stomatal response to CO2. In: Zeiger E, Cowan IR, Farquhar GD, eds. Stomatal function. Stanford, USA: Stanford University Press, 229–251.
Morison JIL.1993. Response of plants to CO2 in water-limited conditions. Vegetatio 104/105, 193–209.[Web of Science]
Morison JIL.1998. Stomatal response to increased CO2 concentration. Journal of Experimental Botany 49, 443–452.[Abstract]
Nilsen ET, Sharifi MR, Rundel PW.1984. Comparative water relations of phreatophytes in the Sonoran Desert of California. Ecology 65, 767–778.[Web of Science]
Oertli JJ.1990. The plant cell's resistance to consequences of negative turgor pressure. In: Kreeb KH, Richter H, Hinckley TM, eds. Structural and functional responses to environmental stresses. The Hague, The Netherlands: SPB Academic Publishing, 73–87.
Panichi C, Tongiorgi E.1975. Carbon isotopic composition of CO2 from springs, fumaroles, mofettes, and travertines of Central and Southern Italy: a preliminary prospection method of geothermal area. In: Second United Nations Symposium on the Development and Use of Geothermal Resources. San Francisco, USA: Vol. 1, 815–825.
Peñuelas J, Matamala R.1990. Changes in N and S leaf content, stomatal density and specific leaf area of 14 plant species during the last three centuries of CO2 increase. Journal of Experimental Botany 41, 1119–1124.
Raiesi FG.1998. Effects of elevated atmospheric CO2 on soil organic carbon dynamics in a Mediterranean forest ecosystem. PhD dissertation, Wageningen Agricultural University, The Netherlands.
Raschi A, Miglietta F, Tognetti R, van Gardingen PR, eds.1997. Plant responses to carbon dioxide: evidence from natural springs. Cambridge, UK: Cambridge University Press.
Rhizopoulou S, Mitrakos K.1990. Water relations of evergreen sclerophylls. I. Seasonal changes in water relations of eleven species from the same environment. Annals of Botany 65, 171–178.
Roden JS, Ball MC.1996. The effect of elevated [CO2] on growth and photosynthesis of two Eucalyptus species exposed to high temperatures and water deficits. Plant Physiology 111, 909–919.[Abstract]
Sala A, Tenhunen JD.1994. Site-specific water relations and stomatal response of Quercus ilex in a Mediterranan watershed. Tree Physiology 14, 601–617.[Abstract]
Salleo S, Lo Gullo MA.1990. Sclerophylly and plant water relations in three Mediterranean Quercus species. Annals of Botany 65, 259–270.
antrucek J, Sage RF.1996. Acclimation of stomatal conductance to a CO2-enriched atmosphere and elevated temperature in Chenopodium album. Australian Journal of Plant Physiology 23, 467–478.[Web of Science]
Specht RL.1969. A comparison of the sclerophyllous vegetation characteristic of mediterranean type climates in France, California and southern Australia. I. Structure, morphology, and succession. Australian Journal of Botany 17, 277–292.
Tenhunen JD, Catarino FM, Lange OL, Oechel WC, eds.1987. Plant responses to stress. Functional analysis in Mediterranean ecosystems. Nato ASI Series, Vol. G15. Berlin, Germany: Springer-Verlag.
Teskey RO, Hinckley TM, Grier CC.1978. Effect of interruption of flow path on stomatal conductance of Abies amabilis. Journal of Experimental Botany 34, 1251–1259.
Tognetti R.1999. The impact of long-term elevated CO2 concentrations on the water relations, stomatal behaviour and hydraulic architecture of shrubs, and stem growth of trees in a Mediterranean forest ecosystem. PhD dissertation, Trinity College, University of Dublin.
Tognetti R, Giovannelli A, Longobucco A, Miglietta F, Raschi A.1996. Water relations of oak species growing in the natural CO2 spring of Rapolano (central Italy). Annales des Sciences Forestières 53, 475–485.
Tognetti R, Johnson JD, Michelozzi M, Raschi A.1998a. Foliage metabolism in Quercus pubescens and Quercus ilex mature trees with lifetime exposure to local greenhouse effect. Environmental and Experimental Botany 39, 233–245.[Web of Science]
Tognetti R, Longobucco A, Miglietta F, Raschi A.1998b. Stomatal behaviour and transpiration of Quercus ilex plants during summer and in a Mediterranean carbon dioxide spring. Plant, Cell and Environment 21, 613–622.
Tognetti R, Longobucco A, Miglietta F, Raschi A.1999. Water relations, stomatal response and transpiration of Quercus pubescens trees during summer in a Mediterranean carbon dioxide spring. Tree Physiology 19, 261–270.[Web of Science][Medline]
Turner IM.1994. Sclerophylly: primarily protective? Functional Ecology 8, 669–675.[Web of Science]
Tyree MT, Alexander JD.1993. Plant water relations and the effects of elevated CO2: a review and suggestions for future research. Vegetatio 104/105, 47–62.[Web of Science]
Valentini R, Scarascia Mugnozza GE, Ehleringer JR.1992. Hydrogen and carbon isotope ratios of selected species of a mediterranean macchia ecosystem. Functional Ecology 6, 627–631.[Web of Science]
van Gardingen PR, Grace J, Harkness DH, Miglietta F, Raschi A.1995. Carbon dioxide emissions at an Italian mineral spring: measurements of average CO2 concentration and air temperature. Agricultural and Forest Meteorology 73, 17–27.[Web of Science]
Wagner F, Below R, De Klerk P, Dilcher DL, Joosten H, Kürschner WM, Visscher H.1996. A natural experiment on plant acclimation: lifetime stomatal frequency response of an individual tree to annual atmospheric CO2 increase. Proceedings of the National Academy of Sciences, USA 93, 11705–11708.
Will RE, Teskey RO.1997. Effect of irradiance and vapour pressure deficit on stomatal response to CO2 enrichment of four tree species. Journal of Experimental Botany 48, 2095–2102.
Woodward FI.1987. Stomatal numbers are sensitive to increasing in CO2 from preindustrial levels. Nature 327, 617–618.[Web of Science]
Woodward FI, Kelly CK.1995. The influeance of CO2 concentration on stomatal density. New Phytologist 131, 311–327.[Web of Science]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||