- Open Access
Morphological, anatomical and physiological leaf traits of Q. ilex, P. latifolia, P. lentiscus, and M. communis and their response to Mediterranean climate stress factors
Botanical Studiesvolume 54, Article number: 35 (2013)
Limitations to plant growth imposed by the Mediterranean climate are mainly due to carbon balance in response to stress factors. In particular, water stress associated to high air temperature and irradiance in summer causes a marked decrease in CO2 assimilation. Air temperature sensitivity of photosynthesis (PN) differs from that of leaf respiration (RD). PN often decreases sharply at temperature above its optimum while RD increases exponentially over short term rises in temperature. Nevertheless, the impact of water deficit on RD is still far from clear with reports in literature including decreases, maintenance or increases in its rates. The ratio RD/PN can be considered a simple approach to leaf carbon balance because it indicates the percentage of photosynthates that is respired.
The results underline different morphological, anatomical and physiological traits of the evergreen species co-occurring in the Mediterranean maquis which are indicative of their adaptive capability to Mediterranean stress factors. The ratio RD/PN varies from 0.15 ± 0.04 in autumn, 0.24 ± 0.05 in spring through 0.29 ± 0.15 in winter to 0.46 ± 0.11 in summer. The lower RD/PN in autumn and spring underlines the highest PN rates during the favorable periods when resources are not limited and leaves take in roughly three to five times more CO2 than they lose by respiration. On the contrary, the highest RD/PN ratio in summer underlines the lowest sensitivity of respiration to drought. Among the considered species, Quercus ilex and Pistacia lentiscus have the largest tolerance to low winter temperatures while Phillyrea latifolia and Myrtus communis to drought, and Phillyrea latifolia the highest recovery capability after the first rainfall following drought.
The Mediterranean evergreen specie shows a different tolerance to Mediterranean climate stress factors. The predicted global warming might differently affect carbon balance of the considered species, with a possible change in Mediterranean shrublands composition in the long-term. Understanding the carbon balance of plants in water limited environments is crucial in order to make informed land management decisions. Moreover, our results underline the importance of including seasonal variations of photosynthesis and respiration in carbon balance models.
The Mediterranean Basin has long been recognized as a model region for studying global change effects on terrestrial ecosystems (Lavorel et al. 1998). Climatic models indicate that rainfall patterns are changing in the Mediterranean Basin as a consequence of the climate change, with a marked decrease up to 15-20% occurring mainly during summer, associated to an increase in the mean maximum air temperature of about 5.1°C by the end of the 21st century (IPCC 2007). These changes will result in extended periods of soil moisture deficit (Hlavinka et al. 2009). Limitations to plant growth imposed by the Mediterranean climate are mainly due to plant carbon balance in response to stress factors (Galmés et al. 2007). In particular, water stress associated to high air temperatures and an excess of light during summer, may result in a chronic photo-inhibition or down-regulation of photosynthesis causing a marked decrease in CO2 assimilation (Zhou et al. 2010). Carbon balance depends on the ratio between photosynthesis and respiration (Lambers et al. 1998), and both these factors change in response to climatic conditions (Baldocchi and Amthor 2001). Nevertheless, they do not necessarily respond identically to changes in these conditions (De Boeck et al. 2007). Air temperature sensitivity of photosynthesis differs from that of respiration (Morison and Morecroft 2006; Way and Sage 2008; Shen et al. 2009). Photosynthesis often decreases sharply at temperatures above its optimum (Sage and Kubien 2007; Hüve et al. 2011), with most temperate species exhibiting a broad temperature optimum in the range of 15–30°C (Atwell et al. 1999; Larcher 2004), while leaf respiration increases exponentially over short term rises in temperature (Rodríguez-Calcerrada et al. 2011). In particular, the temperature sensitivity of leaf respiration is quantified using Q10 i.e. the proportional increase in respiration for every 10°C rise in temperature (Armstrong et al. 2006). Photosynthesis provides soluble sugar as substrates for leaf respiration (Atkin et al. 2007) and availability of respiratory substrates determines the effect of temperature on respiratory enzymes and consequently on respiration temperature sensitivity (Atkin et al. 2002; Rodríguez-Calcerrada et al. 2011). Respiration decrease depends partially on the photosynthesis decrease in response to water deficit (Gimeno et al. 2010). Under water stress a lower photosynthetic activity limits the soluble sugar availability (Pinheiro and Chaves 2011). A lower soluble sugar level may reduce the temperature sensitivity of respiration and then to cause a respiration decrease (Rodríguez-Calcerrada et al. 2011). Nevertheless, the impact of water deficits on leaf respiration is still far from clear, with reports in literature including decreases, maintenance, or increases in the rates of this process (Gimeno et al. 2010).
Since the magnitude of photosynthetic and respiratory acclimation varies among species, these processes are still poorly understood, especially under field conditions (Shen et al. 2009). In dry-land forests of the Mediterranean region, the rates of carbon loss by plant respiration often equal or exceed the rate of carbon uptake by photosynthesis during the year, except in spring and autumn, when air temperatures and water availability are favorable (Zaragoza-Castells et al. 2008; Gratani et al. 2008).
Mediterranean plant species are distributed along different gradients of water availability, according to their capacity to withstand drought (Medrano et al. 2009). Nevertheless, if dry season lasts too long, plant water deficit may negatively affect plant species capacity for carbon assimilation, as a result of the lowest photosynthetic rates and leaf surface area produced (Pereira et al. 2007). Carbon assimilation is also related to stomatal conductance with a strong impact on plant water use efficiency (i.e. the amount of water used per carbon gain) that links plant performance with water availability (Craven et al. 2013). The strength and direction of the relationship between water use efficiency and plant performance can illustrate interspecific differences in drought tolerance strategies (Craven et al. 2013).
Considering global change, variations in water supply will induce important changes in Mediterranean plant species that suffer of water scarcity, especially during drought (Llusiá et al. 2011). Different species can respond to global change by developing different mechanisms both at physiological and morphological levels. Nevertheless if the length or strength of the dry season increases, the distribution area of the species could shrink (Díaz-Barradas et al. 2010) and affect the composition of vegetation in the long-term (Gebrekirstos et al. 2011). Assessment of vegetation level vulnerability and climate change resilience require understanding of the diversity among plant species in the current vegetation, and of their growth strategies in response to fluctuating water availability (Dawson et al. 2009). A rapid adaptation to an increased aridity will be crucial for the future of many species in the Mediterranean region (Sánchez-Gómez et al. 2011). To predict how climate change might affect future Mediterranean species presence and distribution and, as a consequence, community structure and ecosystem functioning, it is essential to have a broad knowledge of which climatic factors are constraining plant species physiological traits, and how these constraints are manifested temporally (Llorens et al. 2003). Morphological adaptations as small, thick, layered leaves with high stomatal density of small size in many Mediterranean evergreen species could favor carbon gain profits over transpiration losses (Rotondi et al. 2003; Gratani and Varone 2004, 2006). Quercus ilex L., Phillyrea latifolia L., Pistacia lentiscus L., and Myrtus communis L. are evergreen shrub species largely distributed in the vegetation of the Mediterranean Basin. Q. ilex extends longitudinally from Portugal to Syria and latitudinally from Morocco to France (Valladares et al. 2000); it occurs in the Mediterranean maquis and forests, growing in different soil conditions and over a broad range of elevations, from the sea level to 1100 m a.s.l. (Khatouri 1992; Terradas and Savé 1992; Gratani et al. 2003). P. latifolia is a drought- and salt-stress-tolerant evergreen shrub species growing in the Mediterranean maquis, forests (Gratani and Bombelli 2000; Ogaya and Peñuelas 2003) and on seashore dunes, where excess soil salinity and salt spray are additional stress agents (Ogaya and Peñuelas 2003). P. lentiscus occurs in a wide variety of habitats, from open communities in garigue to closed ones in more mesic sites (Correia and Diaz Barradas 2000). M. communis is the only species of the Myrtaceae in the actual flora of the Mediterranean Basin (González-Varo 2010). It grows on fertile soils of warm habitats in the Mediterranean region (González-Varo et al. 2009) and in the maquis (Pignatti 1982).
The main objective of this research was to investigate morphological, anatomical and physiological leaf traits of Q. ilex, P. latifolia, P. lentiscus and M. communis and their involvement in carbon acquisition. Moreover, the ratio respiration to photosynthesis which is indicative of the capacity of plants to produce new biomass for growing and reproductive structures (Galmés et al. 2007; Millar et al. 2011) was analyzed over the year. Improving knowledge on carbon acquisition capability of the Mediterranean species will allow us to hypothesize their presence into the distribution area over the long-term, also in consideration of global change.
Study site and plant material
Experiments were carried out in the period from December 2009 to October 2010, on Q. ilex, P. latifolia, P. lentiscus, and M. communis shrubs (5 shrubs per species) growing in the open, under the same environmental conditions, at the Botanical Garden of Rome (41°53′53′′N, 12°28′46′′E; 53 m a.s.l.). The selected shrubs had comparable size (height = 1.36 ± 0.19 m, mean value of the considered shrubs). During the study period the selected shrubs were not watered and they received only natural rain.
The climate of the study area was of the Mediterranean type: the mean minimum air temperature (Tmin) of the coldest months (January and February) was 5.3 ± 0.2°C, the mean maximum air temperature (Tmax) of the hottest months (July and August) was 30.9 ± 0.2°C, and the yearly mean air temperature (Tm) was 16.8 ± 6.5°C. Dry period was from the beginning of June to the end of August (65.5 mm total rainfall of the period). Total annual rainfall was 708 mm, most of it occurring in autumn and in winter (Data from UCEA for the years 1995 to 2010). During the study period Tmin of the coldest month (January) was 3.8 ± 3.1°C, Tmax of the hottest month (July) 34.0 ± 2.2°C, and total rainfall was 709 mm, most of it occurring in winter.
Anatomical leaf traits
Leaf thickness (L, μm) was measured by leaf sections from fresh, fully expanded sun leaves (20 per species), collected at the end of September 2010 from the selected shrubs, and measured by light microscope. Stomatal density (SD, stomata mm-2) was measured from nail varnish impressions (n = 20 per species) of the inferior lamina, according to Sack et al. (2003), each of them 0.5 × 1.0 cm, obtained by a Zeiss Axiocam MRc 5 digital camera (Carl Zeiss), with Axiovision AC software (Release 4.5). Stomatal pore length (SPL, μm) and width (SPW, μm) were measured on the same recorded digital images. Dimension of the stomata was used to calculate the equivalent area of the ellipsoid representing the stomatal pore area (SPA) by the following formula: (π × length × width)/4, according to Minnocci et al. (1995) and Bartolini et al. (1997).
Morphological leaf traits
Measurements of leaf morphological traits were carried out on fully expanded sun leaves (n = 20 per species), collected at the end of September 2010. The following parameters were measured: projected fresh leaf surface area (LA, cm2) (excluding petioles), obtained by the Image Analysis System (Delta-T Devices, UK), and leaf dry mass (DM, mg), determined drying leaves at 80°C to constant mass. Leaf mass per unit leaf area (LMA, mg cm-2) was calculated by the ratio of DM and LA (Reich et al. 1992).
Leaf tissue density (LTD, mg cm-3) was calculated by the ratio of LMA and leaf thickness (Wright and Westoby 2002).
Gas exchange measurements
Measurements of gas exchange were carried out using an infrared gas analyser (ADC LCA4, UK), equipped with a leaf chamber (PLC, Parkinson Leaf Chamber). Measurements were made on fully expanded sun leaves (10 leaves per species per each sampling occasion) during the study period.
Net photosynthetic rate [PN, μmol (CO2) m-2 s-1], photosynthetically active radiation [PAR, μmol (photons) m-2 s-1], stomatal conductance [gs, mol (H2O) m-2 s-1], leaf temperature (Tl, °C), and leaf chamber air temperature (Tch, °C) were measured.
The PNgs, and E rates shown were the mean of the maximum rates for the four days measurement per month, carried out in comparable weather conditions. During gas exchange measurements, the leaf to air vapour pressure deficit (VPDleaf, kPa) was calculated according to Grantz (1990) as: VPD = es – ea, where es was saturated vapour pressure at leaf temperature and ea the air vapour pressure. The intrinsic water use efficiency [IWUE, μmol (CO2) mol (H2O) -1] was calculated as PN/gs ratio, according to Medrano et al. (2009).
Measurements were carried out under natural conditions, on cloud – free days (PAR ≥ 1200 μmol m-2 s-1), in the morning, from 8.00 a.m. to 12.00 p.m., according to Reich et al. (1995).
On each sampling occasion, leaf respiration rate [RD, μmol (CO2) m-2 s-1] measurements were carried out contemporary to PN ones (on the same leaves) by darkening the leaf chamber with a black paper, according to Cai et al. (2005), for 30 min prior to each measurement to avoid transient post-illumination bursts of CO2 releasing (Atkin et al. 1998a, 1998b). The RD rates shown were the mean of the maximum rates for the four days measured per month, carried out in comparable weather conditions. The ratio between RD and PN was also calculated. Q10 was calculated according to Rodríguez-Calcerrada et al. (2012) as: Q10 = e10k where k is the slope of the linear regression between Tch and the natural logarithm of RD (Atkin et al. 2005).
Differences in the considered variables were determined by the analysis of variance (ANOVA), and Tukey test for multiple comparisons, performed using a statistical software package (Statistica, Statsoft, USA). The regression analysis was carried out to evaluate correlations among the considered variables. The principal component analysis (PCA) was carried out in order to summarise the considered anatomical (L, SD, SPL, SPA), morphological (LMA, LTD) and physiological (PN, RD, gs, E, IWUE) leaf traits into major components which explained their variation in the considered species.
Anatomical and morphological leaf traits
Anatomical leaf traits of the considered species are shown in Table 1. L ranged from 311 ± 10 μm (M. communis) to 419 ± 25 μm (P. latifolia). SD varied significantly (p < 0.05): M. communis had the highest SD (508 ± 82 stomata mm-2), followed by Q. ilex and P. lentiscus (419 ± 18 stomata mm-2, mean value), then by P. latifolia (238 ± 21 stomata mm-2). P. latifolia had the highest SPL and SPA (19.5 ± 2.6 μm, 170.3 ± 31.3 μm2, respectively), and Q. ilex the lowest ones (9.1 ± 1.9 μm, 42.1 ± 12.4 μm2, respectively). LMA varied from 9 ± 1 mg cm-2 (M. communis) to 20 ± 1 mg cm-2 (P. latifolia) (Table 2). Q. ilex had the highest LTD (613 ± 40 mg cm-3) and M. communis the lowest one (304 ± 51 mg cm-3).
Seasonal gas exchange and leaf respiration variations
During the study period all the considered species had the highest PN and gs in spring (March, April and May), peaking in May, when Tmax was 23.8 ± 3.0°C, and water availability 113.8 mm (total rainfall of May). Q. ilex, P. lentiscus and P. latifolia had the significantly (p < 0.05) highest PN [15.4 ± 0.2 μmol (CO2) m-2 s-1, mean value measured in May] than M. communis [10.7 ± 0.7 μmol (CO2) m-2 s-1].
gs of M. communis [0.11 ± 0.04 mol (H2O) m-2 s-1, in May] was 52%, 39%, and 35% lower than P. latifolia, P. lentiscus and Q. ilex, respectively. P. lentiscus and P. latifolia had the lowest IWUE in May [84 ± 10 and 68 ± 6 μmol (CO2) mol (H2O)-1, respectively] while Q. ilex and M. communis in April [81 ± 9 and 98 ± 7 μmol (CO2) mol (H2O)-1, respectively].
A different RD trend was observed in spring: P. lentiscus and Q. ilex RD peaked in May [3.7 ± 0.3 μmol (CO2) m-2 s-1, mean value], while P. latifolia in March [RD = 4.3 ± 0.6 μmol (CO2) m-2 s-1]. M. communis RD was not significantly different from March to May [2.7 ± 0.1 μmol (CO2) m-2 s-1, mean value of the three months]. M. communis had the highest RD/PN ratio (0.30 ± 0.06, mean of March, April and May), followed by P. latifolia (0.23 ± 0.08), P. lentiscus (0.22 ± 0.03) and Q. ilex (0.20 ± 0.05).
During winter (December, January and February) PN, gs and RD decreased in respect to the spring maximum, reaching the lowest rates in January (Tmin 3.8 ± 3.1°C; 118.2 mm = total rainfall of the month). In particular, Q. ilex had the lowest PN decrease (68% compared to the spring maximum) followed by P. lentiscus (82%), M. communis (92%) and P. latifolia (97%). In the same month, Q. ilex had the significantly (p < 0.05) highest gs [0.06 ± 0.01 mol (H2O) m-2 s-1], followed by M. communis [0.03 ± 0.01 mol (H2O) m-2 s-1], P. lentiscus, and P. latifolia [0.02 ± 0.01 mol (H2O) m-2 s-1, mean value].
Among the species the highest IWUE was measured in P. lentiscus [127 ± 22 μmol (CO2) mol (H2O)-1, mean of December, January and February] and the lowest one in P. latifolia [45 ± 22 μmol (CO2) mol (H2O)-1]. RD in January was, on an average, 84% lower than the spring maximum. M. communis and P. latifolia had the highest RD/PN ratio (0.61 ± 0.02 and 0.75 ± 0.03, respectively) and Q. ilex and P. lentiscus had the lowest one (0.12 ± 0.02 and 0.27 ± 0.01, respectively).
In summer (June, July, August), PN significantly decreased, reaching the lowest rates in August, when Tmax was 32.3 ± 2.0°C, and the total rainfall of the month 4.4 mm. In particular, PN decreased, on an average, by 64% in P. lentiscus and Q. ilex, and 46% in P. latifolia and M. communis. IWUE was lower in P. lentiscus (24%) and P. latifolia (1%), and higher in M. communis and Q. ilex, (24% and 33%, respectively), compared to the spring values. M. communis had the lowest gs decrease (64%) compared to the spring maximum, followed by Q. ilex (70%), P. lentiscus (75%), and P. latifolia (74%).
RD was 43% and 12% higher than the spring maximum in P. lentiscus and P. latifolia, respectively, while RD was 13% lower than the spring maximum in Q. ilex. There were no significant RD differences in M. communis between spring and summer measurements. P. lentiscus and Q. ilex had the highest RD/PN rate (0.94 ± 0.05 and 0.61 ± 0.06, respectively), followed by P. latifolia (0.56 ± 0.04), and M. communis (0.43 ± 0.02).
PN recovered 64% of the spring maximum in Q. ilex, P. latifolia and M. communis (mean value), and 53% in P. lentiscus at the end of September (Tmax 28.2 ± 3.1°C) after the first rainfall (22.9 mm from the middle to the end of September) following drought. gs recovered 59% of the spring maximum in Q. ilex, 54% in M. communis, 44% in P. lentiscus, and 35% in P. latifolia.
IWUE ranged from 100 ± 13 μmol (CO2) mol (H2O)-1 in P. lentiscus to 125 ± 13 μmol (CO2) mol (H2O)-1 in P. latifolia. P. latifolia had the highest RD [1.7 ± 0.5 μmol (CO2) m-2 s-1], followed by P. lentiscus and Q. ilex [1.2 ± 0.3 μmol (CO2) m-2 s-1, mean value], and M. communis [0.6 ± 0.1 μmol (CO2) m-2 s-1]. In October, PN, gs increased compared to the rates monitored in September in all the considered species while RD increased in P. latifolia, P. lentiscus and M. communis and decreased in Q. ilex.
In September, P. latifolia had the highest RD/PN ratio (0.17 ± 0.02) followed by P. lentiscus (0.15 ± 0.04), Q. ilex (0.12 ± 0.01) and M. communis (0.09 ± 0.02).
Q. ilex and P. latifolia showed the highest Q10 value (1.78 ± 0.01, mean value) followed by P. lentiscus (1.47 ±0.03) and M. communis (1.44 ± 0.02).
Leaf to air vapour pressure deficit
The seasonal VPD trend of the considered species is shown in Figure 3. The considered species had similar VPD trend with the lowest values in January ranging from 0.21 ± 0.06 kPa (in P. latifolia) to 0.33 ± 0.05 kPa (in M. communis). VPD increased from February (0.62 ± 0.06 kPa, mean value) to August (1.42 ± 0.21 kPa, mean value) when Q. ilex had the highest VPD (1.70 ± 0.12 kPa) and P. lentiscus the lowest one (1.20 ± 0.09). In September and October VPD, on an average, decreased by 7% and 45% respectively, compared to August.
The PCA analysis extracted two factors accounting for 76% of the total variance among the considered species (48% and 28% for the 1st and the 2nd factor, respectively). The 1st factor was related to physiological traits (PN in summer, gs in spring, IWUE in spring and summer, RD in summer and spring) and anatomical leaf traits (L, SPL, SPA and SD). The 2nd factor was mainly related to morphological leaf traits (LMA and LTD), and to PN in winter and spring, and IWUE in winter. According to these results, the considered species were divided into three groups (Figure 6): the 1st group included P. lentiscus and Q. ilex, the 2nd group P. latifolia, and the 3rd group M. communis.
Knowledge of plant species response to limited soil moisture is important for providing insights into potential ecological impacts on wild populations (Wu et al. 2010) also in consideration of climate change scenarios which hypothesizes an increasing aridity in many regions worldwide (Canadell et al. 2007). Our results on the whole underline different morphological, anatomical and physiological leaf traits of the considered shrub species which are indicative of their adaptive capability to Mediterranean climate stress factors. Among the considered species, Q. ilex has the highest PN in spring associated to the highest R which may be related to the concomitance of vegetative activity (spring shoots production) and flowering (Gratani et al. 1996). Q. ilex has the largest tolerance to low winter air temperatures evidenced by the lowest PN decrease (68% of the maximum) and low RD rates (87% of the maximum). Under drought conditions, plants optimize carbon assimilation and minimize water loss by decreasing gs (Medrano et al. 2002), and IWUE may be considered a good indicator of carbon assimilation optimization. Q. ilex has a high responsiveness to drought showing a high gs decrease at the beginning of June (41% lower compared to the maximum) associated with a 22% PN decrease determining a 40% IWUE increase compared to the maximum. The responsive stomatal behaviour is also underlined by the significant relationship between gs and VPD (R2 = 0.46). As drought stress progresses in July, Q. ilex IWUE does not significantly increase because of PN and gs change to the same extent. At the highest drought intensity (August) IWUE decreases by 10% compared to June, due to a higher PN decrease than gs. Despite the high PN decrease in August (by 65% compared to the maximum), Q. ilex is able to recover 65% of the spring rates in September. Gratani and Varone (2003) underline the sufficiently high leaf water potential and relative water content during drought in Q. ilex. Moreover, the results underline that Q. ilex does not seem to suffer significant metabolic damage that could make a demand on respiratory products as drought stress progresses, according to the results of Rodríguez-Calcerrada et al. (2011). This is also pointed out by a 13% RD decrease in August compared to the spring rates. Due to the high PN decrease, Q. ilex shows a relatively high RD/PN ratio (0.61 ± 0.06) in August. The most important factor determining how negative the plant carbon balance becomes under water stress is the absolute and proportional change in PN rates since drought has typically a greater proportional inhibitory effect on photosynthesis than on respiration thus, resulting in a higher RD/PN ratio (Galmés et al. 2007). As regards leaf anatomy and morphology, Q. ilex high SD and low SPL and SPA, associated to a high LMA and LTD, contribute to an efficient control of gas exchange. Niinemets (2001) underlines that the adaptive significance of leaves characterised by thick cell walls and low fractions of intercellular air spaces (i.e. high LMA and LTD) lies in their large elastic module which upholds water flow from drying soils.
P. lentiscus strategy to stress factors is similar to that of Q. ilex (i.e. high PN in spring and a relatively high PN in winter associated to a high RD). In August gs and PN decrease by 72% and 63%, respectively, and RD increases by 43% resulting in a high RD/PN ratio (0.94 ± 0.05).
The similar strategy of P. lentiscus and Q. ilex is also underlined by their similar IWUE values during the study period and their PN recovery capability in September. Moreover, P. lentiscus shows a higher relationship between gs and VPD (R2 = 0.33) as well as Q. ilex. A higher PN recovery capability might be related to the capacity of this species to have low leaf water potential and relative water content variations during the year (Gratani and Varone 2004). At morphological and anatomical levels, P. lentiscus is characterised by a high LMA and LTD. In particular, the larger SPL, SPW and SPA in P. lentiscus with respect to Q. ilex may be related to its origin from the semi-arid steppes of central Asia with an exceptionally hot summer and an exceptionally cold and dry winter (Blondel and Aronson 1999). Billing et al. (1971) and Cunningham and Read (2003) hypothesize that plant species which have originated in climates with more fluctuating temperatures may have a higher gas-exchange acclimation to air temperature than those originated in more constant climate. Thus, Q. ilex and P. lentiscus capability to maintain sufficiently high photosynthetic rates both in cold and drought stress periods seem to be related to their origin under a climate characterized by a pronounced seasonality.
Compared to the considered species, P. latifolia has the lowest PN decrease in drought (45% compared to the spring maximum) associated to a 74% gs decrease resulting in a high IWUE. The lower RD/PN ratio (0.56 ± 0.04) in P. latifolia compared to P. lentiscus and Q. ilex, is due to the lowest PN decrease in drought. The high P. latifolia photosynthetic recovery capacity in September (64% of the maximum) after the first rainfall following drought attests to its greater drought tolerance through the maintenance of a high PN rate even at low leaf water potential (Bombelli and Gratani 2003). Moreover, the high P. latifolia LMA, due to the presence of thick cell walls and sclereids (Gratani and Bombelli 1999) and the high LTD (i.e. a densely packed mesophyll cells with few air spaces, Gratani and Bombelli 2000) contribute to improve drought resistance by improving water use efficiency (Niinemets 2001) and limiting photochemical damage to the photosynthetic apparatus through the reduction of the incident irradiance (Jordan et al. 2005). On the contrary, the lower P. latifolia PN in winter compared to the maximum underlines its lower tolerance to cold temperatures, according to the results of Ogaya and Peñuelas (2003), and Ogaya et al. (2011), also pointed out by the highest RD/PN (0.75 ± 0.03). The lowest RD rates in January underline the limitation of the enzyme activity of the respiratory apparatus (i.e. glycolysis, the TCA cycle and mitochondrial electron transport chain) (Atkin and Tjoelker 2003).
M. communis has a physiological response to drought similar to that of P. latifolia, which may be related to their common origin in the dry tropics of the continental Africa and adjacent regions (Blondel and Aronson 1999). M. communis has a low PN decrease (by 46%) during drought associated to stable RD rates which determine a lower RD/PN ratio (0.43 ± 0.02). In winter a 92% PN decrease associated to a 79% RD decrease results in a higher RD/PN ratio (0.61 ± 0.02). Hernández et al. (2010) underline that M. communis has a low capacity to transport water from roots to leaves also under water availability. Gratani et al. (1980) show its low biomass production capability respect to other Mediterranean shrubs which are pointed out by the significant lowest M. communis PN rates during the study period compared to the other considered species. Moreover, the low stomatal control of M. communis is pointed out by a lower relationship between gs and VPD (R2 = 0.29). Despite the highest SD, M. communis has a very small SPL and SPW which could explain the low gs. The lower LMA and LTD M. communis with respect to P. latifolia underline a lower leaf consistency. The above considerations are confirmed by the PCA showing a higher similarity between Q. ilex and P. lentiscus compared to P. latifolia and M. communis.
Chu et al. (2011) suggest that RD/PN ratio can be considered as a simple approach to leaf carbon balance because it indicates the percentage of photosynthates that is respired. Our results show that RD/PN ratio of the considered species, calculated over the study period, varies from 0.15 ± 0.04 in autumn, 0.24 ± 0.05 in spring, through 0.29 ± 0.15 in winter to 0.46 ± 0.11 in summer, and it is indicative of the different sensitivity of both RD and PN to water availability and air temperature changes, according to results of Zaragoza-Castells et al. (2008). The low RD/PN ratio in autumn and spring of the considered Mediterranean evergreen species (i.e. during vegetative activity) underlines the highest PN rates during the favorable periods, when resources are not limited, and leaves take in roughly three to five times more CO2 than they lose by dissimilatory processes during the same period of time (Larcher 2003). On the contrary, the highest RD/PN ratio in summer underlines the lower sensitivity of respiration to drought (Atkin and Macherel 2009) that is indicative of a higher proportion of fixed carbon which is respired at elevated temperature (Gratani et al. 2011; Riikonen et al. 2012). Thus, summer drought can reduce the carbon assimilation because of RD rates increasing more than PN rates.
It is known that over short-term rises in temperature, RD increases exponentially but the seasonal temperature sensitivity of RD is often lower than that observed over hours, a phenomenon known as thermal acclimation (Rodríguez-Calcerrada et al. 2012). This phenomenon involves adjustments in RD rates to compensate for changes in air temperature (Atkin et al. 2000). In particular, acclimation of RD to high temperatures can result in a lower slope (i.e. lower Q10) for the temperature-response curve of acclimated tissue (Atkin et al. 2000). Among the considered species, M. communis has the higher acclimation to high temperatures compared to the other species pointed out by the lower Q10 value (1.44 ± 0.02) and by more stable RD rates during the year. There is growing evidence that acclimation of RD to heat and drought reflects the metabolic down-regulation that reduces carbon depletion and helps plants to grow and survive in Mediterranean-type environments (Rodríguez-Calcerrada et al. 2010, 2011). Understanding the function of plant species in water limited environments is crucial in order to make informed land management decisions (Maseyk et al. 2008). Moreover, under a Mediterranean type of climate, our results underline the importance of including seasonal variations of photosynthesis and respiration in carbon balance models.
Limitations to plant growth imposed by the Mediterranean climate are mainly due to carbon balance in response to stress factors. In particular, water stress associated to high air temperature and irradiance in summer causes a marked decrease in CO2 assimilation. The results underline the response of the evergreen species co-occurring in the Mediterranean maquis to Mediterranean stress factors. In particular, the lower RD/PN in autumn and spring underlines the highest PN rates during the favorable periods while the highest RD/PN ratio in summer shows the lower sensitivity of respiration to drought. Among the considered species, Q. ilex and P. lentiscus have the largest tolerance to low winter temperatures while P. latifolia and M. communis to drought. Among the considered species, M. communis has the higher acclimation to high temperatures compared to the other species and this is underlined by the lower Q10 value and the more stable RD rates during the year. The predicted global warming might differently affect carbon balance of the considered species, with a possible change in Mediterranean shrublands composition in the long term.
Leaf dry mass
Leaf surface area
Leaf mass area
Leaf tissue density
Stomatal pore area
Stomatal pore length
Stomatal pore width
Net photosynthetic rate
Leaf chamber air temperature
Intrinsic water use efficiency.
Armstrong AF, Logan DC, Atkin OK: On the development dependence of leaf respiration: responses to short- and long-term changes in growth temperature. Amer J Bot 2006, 93: 1633–1639. 10.3732/ajb.93.11.1633
Atkin OK, Macherel D: The crucial role of plant mitochondria in orchestrating drought tolerance. Ann Bot 2009, 103: 581–597.
Atkin OK, Tjoelker MG: Thermal acclimation and the dynamic response of plant respiration to temperature. Trends Plant Sci 2003, 8: 343–351. 10.1016/S1360-1385(03)00136-5
Atkin OK, Evans JR, Ball MC, Siebke K: Relationship between the inhibition of leaf respiration by light and enhancement of leaf dark respiration following light treatment. Aust J Plant Physiol 1998a, 25: 437–433. 10.1071/PP97159
Atkin OK, Evans JR, Ball MC, Siebke K, Pons TL, Lambers H: Light inhibition of leaf respiration: the role of irradiance and temperature. In Plant Mitochondria: from Gene to Function. Edited by: Moller IM, Gardestrom P, Gliminius K, Glaser E. Leiden: Backhuys Publishers; 1998b:567–574.
Atkin OK, Edwards EJ, Loveys BR: Response of root respiration to changes in temperature and its relevance to global warming. New Phytol 2000, 147: 141–154. 10.1046/j.1469-8137.2000.00683.x
Atkin OK, Zhang QS, Wiskich JT: Effect of temperature on rates of alternative and cytochrome pathway respiration and their relationship with the redox poise of the quinine pool. Plant Physiol 2002, 128: 212–222. 10.1104/pp.010326
Atkin OK, Bruhn D, Tjoelker MG: Response of plant respiration to changes in temperature: mechanisms and consequences of variations in Q 10 values and acclimation. In Plant Respiration. From Cell to Ecosystem. Edited by: Lambers H, Ribas- Carbó M. Dordrecht, The Netherlands: Springer; 2005:95–135.
Atkin OK, Scheurwater I, Pons TL: Respiration as a percentage of daily photosynthesis in whole plants is homeostatic at moderate, but not high, growth temperatures. New Phytol 2007, 174: 367–380. 10.1111/j.1469-8137.2007.02011.x
Atwell BJ, Kriedemann PE, Turnbull CGN (Eds): Plants in Action. Adaptation in Nature, Performance in Cultivation. South Yarra: Macmillan Education Australia Pty Ltd; 1999.
Baldocchi DD, Amthor JS: Canopy photosynthesis. In Terrestrial global productivity. Edited by: Roy J, Saugier B, Mooney HA. San Diego, CA, USA: Academic Press; 2001:9–31.
Bartolini S, Minnocci A, Vitagliano C: Influence of temperature on morpho-anatomic characteristics of “Trebbiano” gravepine leaves. Agricoltura Mediterranea 1997, 127: 37–43.
Billing WD, Godfrey PJ, Chabot BF, Bourque DP: Metabolic acclimation to temperature in arctic and alpine ecotypes of Oxyria digyna . Arct Antarct Apl Res 1971, 3: 277–289.
Blondel J, Aronson J (Eds): Biology and Wildlife of the Mediterranean Region. New York: Oxford University Press; 1999. 328 pp 328 pp
Bombelli A, Gratani L: Interspecific differences of leaf gas exchange and water relations of three evergreen Mediterranean shrub species. Photosynthetica 2003, 41: 619–625.
Cai ZQ, Slot M, Fan ZX: Leaf development and photosynthetic properties of three tropical species with delayed greening. Photosynthetica 2005, 43: 91–98. 10.1007/s11099-005-1098-3
Canadell JG, Pataki DE, Pitelka LF (Eds): Terrestrial ecosystems in a changing World. Berlin: Springer; 2007.
Chu Z, Yijun L, Chang J, Wang M, Jiang H, He J, Peng C, Ge Y: Leaf respiration/photosynthesis relationship and variation: an investigation of 39 woody and herbaceous species in east subtropical China. Trees 2011, 25: 301–310. 10.1007/s00468-010-0506-x
Correia O, Diaz Barradas MC: Ecophysiological differences between male and female plants of Pistacia lentiscus L. Plant Ecol 2000, 149: 131–142. 10.1023/A:1026588326204
Craven D, Hall JS, Ashton MS, Berlyn GP: Water-use efficiency and whole-plant performance of nine tropical tree species at two sites with contrasting water availability in Panama. Trees 2013, 27: 639–653. 10.1007/s00468-012-0818-0
Cunningham SC, Read J: Do temperate rainforest trees have a greater ability to acclimate to changing temperatures than tropical rainforest trees? New Phytol 2003, 157: 55–64. 10.1046/j.1469-8137.2003.00652.x
Dawson IK, Lengkeek A, Weber JC, Jamnadas R: Managing genetic variation in tropical trees: linking knowledge with action in agroforestry ecosystem for improved conservation and enhanced livelihood. Biodivers Conserv 2009, 18: 969–986. 10.1007/s10531-008-9516-z
De Boeck HJ, Lemmens CMHM, Vicca S, den Berge JV, Van Dongen S, Janssens IA, Ceulemans R, Nijs I: How do climate warming and species richness affect CO 2 fluxes in experimental grasslands? New Phytol 2007, 175: 512–522. 10.1111/j.1469-8137.2007.02122.x
Díaz-Barradas MC, Zunzunegui M, Ain-Lhout F, Jáuregui J, Boutaleb S, Álvarez-Cansino L, Esquivias MP: Seasonal physiological responses of Argania spinosa tree from Mediterranean to semi-arid climate. Plant Soil 2010, 337: 217–231. 10.1007/s11104-010-0518-8
Galmés J, Ribas-Carbó M, Medrano H, Flexas J: Response of leaf respiration to water stress in Mediterranean species with different growth forms. J Arid Environ 2007, 68: 206–222. 10.1016/j.jaridenv.2006.05.005
Gebrekirstos A, van Noordwijk M, Neufeldt H, Mitlöhner R: Relationships of stable carbon isotopes, plant water potential and growth: an approach to asses water use efficiency and growth strategies of dry land agroforestry species. Trees 2011, 25: 195–102.
Gimeno TE, Sommerville KE, Valladares F, Atkin OK: Homeostasis of respiration under drought and its important consequences for foliar carbon balance in a drier climate: insights from two contrasting Acacia species. Funct Plant Biol 2010, 37: 323–333. 10.1071/FP09228
González-Varo JP: Fragmentation, habitat composition and the dispersal/predation balance in interactions between the Mediterranean myrtle and avian frugivores. Ecography 2010, 33: 185–197. 10.1111/j.1600-0587.2009.06021.x
González-Varo JP, Rafael G, Albaladejo RG, Aparicio A: Mating patterns and spatial distribution of conspecific neighbours in the Mediterranean shrub Myrtus communis ( Myrtaceae ). Plant Ecol 2009, 203: 207–215. 10.1007/s11258-008-9534-7
Grantz DA: Plant response to atmospheric humidity. Plant Cell Environ 1990, 13: 667–679. 10.1111/j.1365-3040.1990.tb01082.x
Gratani L, Bombelli A: Leaf anatomy, inclination, and gas exchange relationships in evergreen sclerophyllous and drought semideciduous shrub species. Photosynthetica 1999, 37: 573–585.
Gratani L, Bombelli A: Correlation between leaf age and other leaf traits in three Mediterranean maquis shrub species: Quercus ilex , Phillyrea latifolia and Cistus incanus . Environ Exp Bot 2000, 43: 141–153. 10.1016/S0098-8472(99)00052-0
Gratani L, Varone L: Drought-adaptive responses of the Mediterranean shrub species. Atti del Tredicesimo Congresso Nazionale della S.IT.E. Como (I), 8–10 Settembre. In Processing of the 13th National Congress of the Italian Ecology Society (S.IT.E). Como, Italy: ; 2003:8–10.
Gratani L, Varone L: Adaptive photosynthetic strategies of the Mediterranean maquis species according to their origin. Photosynthetica 2004, 42: 551–558. 10.1007/S11099-005-0012-3
Gratani L, Varone L: Long-time variations in leaf mass and area of Mediterranean evergreen broad-leaf and narrow-leaf maquis species. Photosynthetica 2006, 44: 161–168. 10.1007/s11099-006-0001-1
Gratani L, Amadori M, Veri L, Bruno F, Porri M: Determinazione di un metodo di stima della Biomassa nella macchia di Castelporziano (Lazio). Annali di Botanica XXXXI 1980, 1: 131–151.
Gratani L, Tisi F, Crescente MF, Pesoli P, Larcher W: Phenology, leaf morphology and trends in CO 2 uptake of Quercus ilex L. in the climax area and at its northern distribution limit in Italy. Arch Geobot 1996, 2: 13–18.
Gratani L, Meneghini M, Pesoli P, Crescente MF: Structural and functional plasticity of Quercus ilex seedlings of different provenances in Italy. Trees 2003, 17: 515–521. 10.1007/s00468-003-0269-8
Gratani L, Varone L, Catoni R: Relationship between net photosynthesis and leaf respiration in Mediterranean evergreen species. Photosynthetica 2008, 46: 567–573. 10.1007/s11099-008-0095-8
Gratani L, Catoni R, Varone L: Photosynthetic and leaf respiration activity of Malcolmia littorea (L.) R. Br. in response to air temperature. Photosynthetica 2011, 49: 65–74. 10.1007/s11099-011-0007-1
Hernández EI, Vilagrosa A, Pausas JG, Bellot J: Morphological traits and water use strategies in seedlings of Mediterranean coexisting species. Plant Ecol 2010, 207: 233–244. 10.1007/s11258-009-9668-2
Hlavinka P, Trnka P, Semeradova D, Dubrosky M, Zalud Z, Mozny M: Effect of drought on yield variability of key crops in Czech Republic. Agric Forest Meteorol 2009, 149: 431–442. 10.1016/j.agrformet.2008.09.004
Hüve K, Bichele I, Rasulovo B, Niinemets Ü: When it is too hot for photosynthesis: heat-induced instability of photosynthesis in relation to respiratory burst, cell permeability changes and H 2 O 2 formation. Plant Cell Environ 2011, 34: 113–126. 10.1111/j.1365-3040.2010.02229.x
IPCC: Climate change 2007: impacts, adaptation and vulnerability. Fourth Assessment Report. Summary for Policymakers. Working Group II. Cambridge: University Press; 2007.
Jordan GJ, Dillon RA, Weston PH: Solar radiation as a factor in the evolution of scleromorphic leaf anatomy in Proteaceae. Am J Bot 2005, 92: 789–796. 10.3732/ajb.92.5.789
Khatouri M: Growth and yield of young Quercus ilex coppice stands in the Tafferte forest (Morocco). Vegetatio 1992, 99–100: 77–82.
Lambers H, Chapin FS III, Pons TL (Eds): Plant Physiological Ecology. New York: Springer; 1998.
Larcher W (Ed): Physiological plant ecology. Heidelberg: Springer; 2003:513.
Larcher W (Ed): Physiological Plant Ecology: Ecophysiology and Stress Physiology of Functional Groups. 4th edition. Berlin, Germany: Springer-Verlag; 2004.
Lavorel S, Canadell J, Rambal S, Terradas J: Mediterranean terrestrial ecosystem: research priorities on global change effects. Global Ecol Biogeogr Letters 1998, 7: 157–166.
Llorens L, Peñuelas J, Filella I: Diurnal and seasonal variations in the photosynthetic performance and water relations of two co-occurring Mediterranean shrubs, Erica multiflora and Globularia alypum Physiol. Plantarum 2003, 118: 84–95. 10.1034/j.1399-3054.2003.00101.x
Llusiá J, Peñuelas J, Alessio GA, Ogaya R: Species-specific, seasonal, inter-annual, and historically-accumulated changes in foliar terpene emission rates in Phillyrea latifolia and Quercus ilex submitted to rain exclusion in the Prades mountains (Catalonia). Russ J Plant Physl 2011,58(1):126–132. 10.1134/S1021443710061020
Maseyk K, Grünzweig JM, Rotenberg E, Yakir D: Respiration acclimation contributes to high carbon use efficiency in a seasonally dry pine forest. Global Change Biol 2008, 14: 1553–1567. 10.1111/j.1365-2486.2008.01604.x
Medrano H, Escalona JM, Bota J, Gulías J, Flexas J: Regulation of photosynthesis of C 3 plants in response to progressive drought: the interest of stomatal conductance as a reference parameter. Ann Bot (Lond) 2002, 89: 895–905. 10.1093/aob/mcf079
Medrano H, Flexas J, Gálmes J: Variability in water use efficiency at the leaf level among Mediterranean plants with different growth forms. Plant Soil 2009, 317: 17–29. 10.1007/s11104-008-9785-z
Millar AH, Whelan J, Soole KL, Day DA: Organization and regulation of mitochondrial respiration in plants. Annu Rev Plant Biol 2011, 62: 79–104. 10.1146/annurev-arplant-042110-103857
Minnocci A, Panicucci A, Vitagliano C: Gas exchange and morphological stomatal parameters in olive plants exposed to ozone. In Responses of plants to air pollution: biological and economic aspects. Edited by: Lorenzini G, Soldatini GF. Pacini, Pisa: Agricoltura Medieterranea (Special Volume); 1995:77–81.
Morison JIL, Morecroft MD: Plant growth and climate change. Significance of temperature in plant life. Boston, MA, U.S.A: Blackwell Publishing; 2006.
Niinemets Ü: Global-scale climatic controls of leaf dry mass per area, density, and thickness in trees and shrubs. Ecology 2001,82(2):453–469. 10.1890/0012-9658(2001)082[0453:GSCCOL]2.0.CO;2
Ogaya R, Peñuelas J: Comparative seasonal gas exchange and chlorophyll fluorescence of two dominant woody species in a Holm Oak Forest. Flora 2003, 198: 132–141. 10.1078/0367-2530-00085
Ogaya R, Peñuelas J, Asensio D, Llusià J: Chlorophyll fluorescence responses to temperature and water availability in two co-dominant Mediterranean shrub and tree species in long-term field experiment simulating climate change. Environ Exp Bot 2011, 71: 123–127. 10.1016/j.envexpbot.2010.10.016
Pereira JS, Mateus JA, Aires LM, Pita G, Pio C, David JS, Andrade V, Banza J, David TS, Paço TA, Rodrigues A: Net ecosystem carbon exchange in three contrasting Mediterranean ecosystem—the effect of drought. Biogeosciences 2007, 4: 791–802. 10.5194/bg-4-791-2007
Pignatti S (Ed): Flora d’Italia. Bologna: Edagricole; 1982.
Pinheiro C, Chaves MM: Photosynthesis and drought: can we make metabolic connections from available data? J Exp Bot 2011,62(3):869–882. 10.1093/jxb/erq340
Reich PB, Walters MB, Ellsworth DS: Leaf life-span in relation to leaf, plant and stand characteristics among diverse ecosystem. Ecol Monogr 1992, 62: 365–392. 10.2307/2937116
Reich PB, Kloeppel BD, Ellsworth DS, Walters MB: Different photosynthesis-nitrogen relations in deciduous hardwood and evergreen coniferous tree species. Oecologia 1995, 104: 24–30. 10.1007/BF00365558
Riikonen J, Kontunen-Soppela S, Ossipov V, Tervahauta A, Tuomainen M, Oksanen E, Vapaavuori E, Heinonen J, Kivimäenpää M: Needle metabolome, freezing tolerance and gas exchange in Norway spruce seedlings exposed to elevated temperature and ozone concentration. Tree Physiol 2012, 32: 1102–1112. 10.1093/treephys/tps072
Rodríguez-Calcerrada J, Atkin OK, Robson TM, Zaragoza-Castells J, Gil L, Aranda I: Thermal acclimation of leaf dark respiration of beech seedlings experiencing summer drought in high and low light environments. Tree Physiol 2010, 30: 214–224. 10.1093/treephys/tpp104
Rodríguez-Calcerrada J, Jaeger C, Limousin JM, Ourcivall JM, Joffre R, Rambal S: Leaf CO 2 efflux is attenuated by acclimation of respiration to heat and drought in a Mediterranean tree. Funct Ecol 2011, 25: 983–995. 10.1111/j.1365-2435.2011.01862.x
Rodríguez-Calcerrada J, Limousin J-M, Martin-StPaul NK, Jaeger C, Rambal S: Gas exchange and leaf aging in an evergreen oak: causes and consequences for leaf carbon balance and canopy respiration. Tree Physiol 2012, 32: 464–477. 10.1093/treephys/tps020
Rotondi A, Rossi F, Asunis C, Cesaraccio C: Leaf xeromorphic adaptations of some plants of a coastal Mediterranean macchia ecosystem. J Medit Ecol 2003,4(3/4):25–35.
Sack L, Grubb PJ, Marañón T: The functional morphology of juvenile plants tolerant of strong summer drought in shaded firest understories in sourthern Spain. Plant Ecol 2003, 168: 247–259.
Sage RF, Kubien DS: The temperature response of C 3 and C 4 photosynthesis. Plant Cell Environ 2007, 30: 1086–1106. 10.1111/j.1365-3040.2007.01682.x
Sánchez-Gómez D, Velasco-Conde T, Cano-Martín FJ, Guevara MA, Cervera MT, Aranda I: Inter-clonal variation in functional traits in response to drought for a genetically homogeneous Mediterranean conifer. Environ Exp Bot 2011, 70: 104–109. 10.1016/j.envexpbot.2010.08.007
Shen H, Klein JA, Zhao X, Tang Y: Leaf photosynthesis and simulated carbon budget of Gentiana straminea from a decade-long warming experiment. J Plant Ecology 2009,2(4):207–216. 10.1093/jpe/rtp025
Terradas J, Savé R: The influence of summer and winter stress and water relationships on the distribution of Quercus ilex L. Vegetatio 1992, 100: 137–145.
Valladares F, Martinez-Ferri E, Balaguer L, Perez- Corona E, Manrique E: Low leaf-level response to light and nutrients in Mediterranean evergreen oaks: a conservative resource-use strategy? New Phytol 2000, 148: 79–91. 10.1046/j.1469-8137.2000.00737.x
Way DA, Sage RF: Thermal acclimation of photosynthesis in black spruce ( Picea mariana (Mill.) BSP). Plant Cell Environ 2008, 31: 1250–1262. 10.1111/j.1365-3040.2008.01842.x
Wright IJ, Westoby M: Leaves at low versus high rainfall: coordination of structure, lifespan and physiology. New Phytol 2002, 155: 403–416. 10.1046/j.1469-8137.2002.00479.x
Wu CA, Lowry DB, Nutter LI, Willis JH: Natural variation for drought-response traits in the Mimulus guttatus species complex. Oecologia 2010, 162: 23–33. 10.1007/s00442-009-1448-0
Zaragoza-Castells J, Sánchez-Gómez D, Hartley IP, Matesanz S, Valladares F, Lloyd J, Atkin OK: Climate-dependent variations in leaf respiration in a dry-land, low productivity Mediterranean forest: the importance of acclimation in both high-light and shaded habitats. Funct Ecol 2008, 22: 172–184.
Zhou HH, Chen YN, Li WH, Chen YP: Photosynthesis of Populus euphratica in relation to groundwater depths and high temperature in arid environment, northwest China. Photosynthetica 2010,48(2):257–268. 10.1007/s11099-010-0032-5
This paper was supported by the grants from Ministry of Agricultural, alimentary and Forestry politicians (MIPAF) for the years 2007–2010.
The authors declare that they have no competing interests.
LG wrote the manuscript. RC carried out the experimental research and contributed to statistical analysis. LV carried out the experimental research and statistical analysis. All authors read and approved the final manuscript.