Alien invasive species generally have a strong capacity to be tolerant or adaptive in varied environments (Xu et al., 2001). Since the most importance of photosynthesis, photosynthetic traits and their acclimation to the invasive habitats may be one premise for their successful colonization. Stomatal regulation in changing microenvironment facilitates the high photosynthetic production (Passioura, 1982; Bazzaz, 1996; Wang et al., 2001a). Moreover, stomata are both important for controlling the processes of photosynthesis and water utilization via transpiration, which can optimize the water use of plants (Cowan, 1982). The stomatal acclimation in short term and long term has been well demonstrated in laboratory work, whereas a few of field works are reported (Passioura, 1982; Wang et al., 2003a). Eupatorium adenophorum widely distributes in the hydrological different regions from 37°N in Spain to 35°S in South Africa and Australia (Qiang, 1998). Thus, the water utilization strategy with respect to stomatal regulation and acclimation may facilitate the colonization of this species. Therefore, we hypothesize that photosynthetic traits including stomatal regulation on photosynthesis and water use strategy may be crucial for the rampancy and prolificacy of E. adenophorum in our study sites. However, in the case of this weed, we still do not have enough information regarding its gas exchange. Existent reports showed that its photosynthetic capacity can keep at a higher level in most days of a year (Liu et al., 1988) and soil drought can sharply decrease its photosynthesis, which may lead to its distribution only in moist region (Liu et al., 1989). However, no further studies to the underlying mechanism of stomatal regulation are available.

Therefore, for testifying our hypothesis, the stomatal regulation in short-term acclimation to diurnal changing microenvironment and after long-term acclimation to xeric and hygric habitats were discussed in this paper. Moreover, a comparison with 20 other species in local region and other regions were done to find the functional difference of stomatal regulation on water utilization.

1 Materials and methods 1.1 Study sites and materials

We selected a study site near Dechang City (about 10 km), Liangshan autonomous prefecture, Sichuan Province (27°28′36″N, 102°12′8″E), which is recognized as one of the most disastrous region by the invasion of E. adenophorum (Zhou et al., 1999). The sub-tropical plateau monsoon climate in this region is typified with warm winter and cool summer. Annual mean air temperature is 17 ℃, annual precipitation is 1 121 mm. Typical year is classified as dry season from December to next March, rain season from June to September and transition period for other time of the year. The plot was set near the foot of a south-facing slope (2 to 5 degree). The altitude is 1 487 m. This region is the typical region for well growth of E. adenophorum (Zhou et al., 1999).

For studying the effect of long term hydrological difference on photosynthetic characteristics of this species, two typical habitats including xeric and hygric sites were selected. The hygric habitats include two sites in a valley with a brook and a beach of a stream. E. adenophorum grew well in these sites and even in the water of the brook. The xeric habitats include 5 sites in abandoned agricultural land, edge, gap and in the floor of a pine plantation (Pinus yunnanensis) and roadside near a village. We assumed this type of land as xeric sites since it is far away from river (about 3 km), which is similar to the condition of other xeric sites. The humidity both air and soil was substantially higher in hygric habitats (Tab. 1). Slight differences in soil organic matters and soil pH values of sieved soil, and soil available nitrogen were observed (Tab. 1), however, the depth of soil in xeric soil was obvious shallower than that of hygric soil and much stone was found in xeric soil.

For comparing the relationship between stomatal conductance and water use efficiency, another 20 species were also measured (Tab. 2). 8 species is local species accompanying E. adenophorum. Other 12 species is from other sites of China and northern Japan. One species of alien invasive vine(Mikania micrantha) in three sites of Shenzhen, Zhuhai and Dongguan of Guangdong Province (22°51′N, 113°47′E)was measured in Sep., 2003. One invasive species of perennial evergreen shrub (Sasa senanensis) in Sapporo, Japan was measured in summer of 2000 (Wang et al., 2001b; 2003a). Two annual grass (Dioscorea nipponica, Potentilla cryptotaeniae), 1 perennial deciduous shrub (Rubus sachalinensis), 5 deciduous tree species (Larix gmelinii, Juglans mandshurica, Phellodendron amurense, Franxinus mandshurica, Alnus sibirica) and 2 evergreen coniferous species (Picea koraiensis, Pinus koraiensis) in Northeast China were measured in summers of 1998 and 1999 (Wang et al., 2001a; Zu et al., 2002; Wang et al., 2003b). Detail of the sites can be found in the related references as cited above.

1.2 Measurement of leaf gas exchange

About 3 to 6 years old wild E. adenophorum was studied on typical sunny days. The photosynthesis was measured on the adult healthy leaves (the 3^th to 5^th leaves from top of stem) in health individuals. We did the measurement in moist season from Aug. to Sep., 2003 and dry season (transition season) in Apr. of 2004. All photosynthesis measurements were done by Li-6400 portable photosynthesis system (LiCor, USA) with a broad-leaved chamber and an LED light source (Li6400-02B).

For discussing the stomatal regulation of photosynthesis in short term acclimation to diurnal changing microenvironments, we measured the diurnal courses of light response curve and carboxylation efficiency(A-C_i curve, photosynthesis-intercellular CO₂ curve). Six setting points, 9:00, 11:00, 12:00, 14:00, 16:00, 18:00, were selected for measurement. The measurement of light response curves was carried out by auto-program of Li-6400 from high photosynthetic active radiation(PAR) to dark with 9 setting points (1 500, 1 200, 1 000, 700, 500, 200, 100, 30 and 0 μmol·m^-2s^-1). At each setting point of PAR, the equilibration time was kept 60~300 s to ensure total coefficient of variation (CV) of the system less than 1% (This is done by the auto program of Li-6400 system). Manual adjustment of the scrub of the soda lime tube enables the A-C_i curve measurement, while light in the chamber were maintained at 1 200 μmol·m^-2s^-1 using the LED light source. The CO₂ concentration in the chamber were set at least 3 points (ambient CO₂, 180 μmol·mol^-1, and CO₂ free about 0 μmol·mol^-1). At each CO₂ setting point, 90~300 s waiting time after leaf clamping were used to equilibrate the total CV of the system less than 1%.

For discussing the stomatal regulation of gas exchange after long term acclimation to xeric and hygric habitats, 2 adult leaves from health individuals at each site were measured in the morning of typical sunny days to avoid the midday depression in photosynthesis, i.e., 4 to 10 replicates were done in xeric and hygric habitats. The photosynthesis was carried out with the same Li-6400 with similar protocol for measurement.

The gas exchange characteristics of the other 20 species (Tab. 1) were also measured by Li-6400 system. Moreover, stomatal conductance (g_s) and leaf transpiration rate (Tr) of all the measured species were simultaneously measured by Li-6400 when photosynthesis measurements were carried out.

1.3 Measurement of leaf nitrogen, soil available nitrogen

After measurement of photosynthesis, the leaves were collected for leaf nitrogen measurement. The leaf samples were oven dried at 80 ℃ for 72 h, then 5~20 mg leaf sample was scaled for automatically analyzing of nitrogen by CN Analyzer (Shimadzu, NC-900, Kyoto, Japan). Three replicates were done on each leaf sample.

At each site near the individuals of photosynthesis measurement, 2 soil cores (0~10 cm) were sampled for analysis of soil organic matter, soil nitrogen and soil pH value (i.e. 4 to 10 replicates were measured for xeric and hygric habitats). More than 70% roots were in the superficial soil of 10 cm in depth (our unpublished data), so, the sampling soil could represent the characteristics of rhizonsphere. The samples were ground and sieved (mesh size=0.149 mm) for future analysis. Walkley-Black acid digestion method (Walkley et al., 1934; Nelson et al., 1991) was used for soil organic analysis. 0.1 to 0.2 g soil sample was oxidized by mixture of chromic acid (K₂Cr₂O₇) and sulfuric acid (H₂SO₄) in a 18 mm×180 mm curvette. The temperature was maintained at 170~180 ℃ for 5 min in a soybean oil cooker with electric furnace. The solution was poured into a flask and 6 drops of Ferroin indicator were added then titrated with 0.2 mol·L^-1 FeSO₄ to a wine red at the endpoint. In the calculation of total soil organic matter, 58% of them were assumed to be oxidized in this procedure (Nelson et al., 1991). Soil pH was measured with a basic pH meter (PB-20, Sartorius, Germany). The solution of soil and distilled water (1:10) was used (Page et al., 1991). Soil available nitrogen was estimated as the summation of NH₄⁺-N and NO₃^--N. The methods proposed in the Forestry Executive Standards of China (1999) were used to analyze NH₄⁺-N and NO₃^--N. Totally 10 samples from xeric habitats and 8 samples from hygric habitats were used.

1.4 Analysis of photosynthetic light response and A-C_i curves

Tamiya's equation (Tamiya, 1951) was used to evaluate the characteristics of photosynthetic light response curve:

(1)

where P_n (μmol·m^-2s^-1) and PAR (μmol·m^-2s^-1) are the net photosynthetic rate and photosynthetic active radiation, respectively, R_day(μmol·m^-2s^-1) is the dark respiration rate, b is the initial slope of photosynthetic light response curves, and a is the reciprocal of PAR at half asymptotic rate of gross photosynthetic rate.

Other parameters derived from Tamiya's equation are,

(2)

(3)

(4)

where L_cp(μmol·m^-2s^-1) and L_sp(μmol·m^-2s^-1) are the light compensation point and light saturation point, respectively, and A_max is the potential maximum gross photosynthetic rate under the condition of saturated sunlight. The light saturation point was assumed to be the PAR value when P_n reached 90% of A_max(Wang et al., 2001a).

Maximum apparent quantum yield(AQY)was estimated from the initial slope value of the light-photosynthesis curve when PAR is less than 100 μmol·m^-2s^-1(Koike et al., 1996; Zu et al., 1998). Carboxylation efficiency(CE) was calculated by the initial increment of intercellular CO₂ partial pressure(C_i) and net photosynthetic rate based on the assumption of uniform stomatal responses to CO₂ (Koike et al., 1996).

(5)

Where, P_n and C_i are measured by Li-6400, B and A are best-fitting constant. B value biologically indicates the CE. Similarly, substituting C_i by the CO₂ concentration in the chamber in above equation, CO₂ compensation point(C_cp) can be determined as following,

(6)

1.5 Analysis of stomatal regulation on gas exchange

The relations between stomatal conductance and gas exchange rate of CO₂ and H₂O at saturation light were fitted by hyperbolic model,

(7)

where, P_n and g_s are net photosynthetic rate at saturation light and stomatal conductance, respectively. P₁ and P₂ are best-fitting constant and their biological meanings are the potential maximum photosynthesis or transpiration rate without any stomatal limitation and stomatal conductance at half maximum gas exchange rate, respectively (Wang et al., 2003b).

Stomatal limitation(L_s) was determined according to the method of Farquhar and Sharkey(1982). Following is the equation,

(8)

where, C_l and C_i are CO₂ concentration in the leaf chamber and intercellular CO₂ concentration of leaf, respectively. C_cp is the CO₂ compensation point, which has been estimated by equation 6.

Water utilization efficiency (WUE, μmol CO₂·mmol^-1 H₂O) is determined by the ratio of net photosynthetic rate and transpiration rate(Passioura, 1982; Zu et al., 1998; Wang et al., 2003a). To exclude the effect of light difference on WUE and equations 7 and 8, the photosynthesis and transpiration under saturated PAR were used in the calculations. According to our measurement, saturation light in the morning is higher than 1 000 μmol·m^-2s^-1 and higher than 50 μmol·m^-2s^-1 at noon and in the afternoon.

All above analysis was done by Mircocal Origin 7.0 (Microcal software Inc., USA) and Excel 2002 (Microsoft, USA). Statistical analysis was done by Excel 2002 (Microsoft, USA).

2 Results 2.1 Stomatal regulation in short term acclimation to diurnal changing microenvironment

The diurnal courses of light response curves could be classified as three groups (Fig. 1), morning group (9:00 and 11:00), noon group (12:00 and 14:00) and afternoon group (16:00 and 18:00). Morning group had the highest capacity and the lowest was observed at noon, intermediate was observed in the afternoon group.

In the light response curves, photosynthesis was compensated around 20 μmol·m^-2s^-1 to 50 μmol·m^-2s^-1, and peaked in late afternoon. However, no typical pattern was found in its diurnal course (Tab. 3). Photosynthesis was saturated in wide range of light regime from 91 μmol·m^-2s^-1 at noon to 1 293 μmol·m^-2s^-1 in the morning, and the saturation light sharply declined at noon and kept in a relative higher value in the morning and afternoon. In average, saturation light in the morning(ca. 1 150 μmol·m^-2s^-1) was substantially higher than that at noon and afternoon (ca. 400 μmol·m^-2s^-1) (Tab. 3).

In the photosynthetic response to CO₂ variation, CO₂ compensation point was generally maintained around 85 μmol·mol^-1 in most of the day time with an exceptional higher value, 125 μmol·mol^-1 at 14:00 (Tab. 3). Carboxylation efficiency ranged from 0.016 mol·m^-2s^-1 to 0.087 mol·m^-2s^-1, and peaked in the morning then substantially declined (75%) at noon with following recover in the afternoon (Tab. 3). Similar pattern was observed in maximum apparent quantum yield, which varied in a conservative range from 0.05 mol·mol^-1 to 0.06 mol·mol^-1. No obvious difference was found between morning and afternoon, but 10% decrease at noon was observed (Tab. 3).

We found that photosynthetic rate was positive correlated with carboxylation efficiency (Fig. 2a). Moreover, carboxylation efficiency significantly correlated with quantum yield (Fig. 2b). Furthermore, stomatal conductance also showed midday-depression (Fig. 3a). Stomatal conductance and carboxylation efficiency were closely correlated with the photosynthetic rate in a diurnal scale (Fig. 3b).

2.2 Stomatal regulation on photosynthesis and transpiration after long term acclimation to xeric and hygric habitats and interspecies comparison

For testing different pattern in stomatal regulation on gas exchange in hygric habitats and xeric habitats, all the gas exchange data under saturated light regime were pooled to avoid the impact from light differences. We found that stomatal regulation on photosynthesis and transpiration in two types of contrasting habitats are obvious different (Fig. 4). In higher stomatal conductance regime, photosynthesis of E. adenophorum in hygric habitats was substantially higher, but this difference was slight in lower stomatal conductance regime (Fig. 4a). Similar pattern was found in the transpiration of leaf (Fig. 4b). Similarly, the relationship between photosynthesis, transpiration and stomatal conductance of the other 20 species also showed a hyperbola shapes (Fig. 5). According to the biological significance of the hyperbola function (Equation 6), the maximum photosynthetic rate without stomatal limitation (P₁ value of the equation 7) of the 20 species was 50% lower than that of E. adenophorum in hygric habitats, but slightly different (4%) to that in xeric habitats.

2.3 Stomatal regulation on water use efficiency after long term acclimation to xeric and hygric habitats and interspecies comparison

In wide ranges of 20 species, water use efficiency kept constant when the conductance was high. But in low stomatal conductance, an increasing tendency in water use efficiency was observed (Fig. 6a). However, in the case of E. adenophorum, different patterns were observed in hygric and xeric habitats. When water was abundantly supplied, the unnecessary for saving water resulted in higher transpiration even in low stomatal conductance, which resulted in the positive correlation between stomatal conductance and water use efficiency in hygric habitats (Fig. 6b). But in the xeric habitats, E. adenophorum could decrease stomatal conductance to water more than that of CO₂ and the water use efficiency was increased (Fig. 6b).

3 Discussion 3.1 Stomatal and non-stomatal regulation, which control the short term acclimation to the diurnal changing microenvironments?

The reason induced midday depression in photosynthesis usually can be attributed to internal and external factors (Xu et al., 1997). We found that carboxylation efficiency play an important role in the diurnal pattern of photosynthesis changes (Fig. 2a). Moreover, carboxylation efficiency significantly correlated with quantum yield (Fig. 2b). One possible explanation for these results is that enhancing carbon fixation and assimilation improves the rate of photon utilization in the chloroplasts, whereas the down-regulation in Rubisco in photosynthetic midday depression induces lower level of photon utilization by leaf. Therefore, non-stomatal regulation at midday depression may be partially responsible for the diurnal pattern of gas exchange of E. adenophorum. Similar conclusions have reported in some other species both in semiarid environment (Deng et al., 2000) and humid habitats (Wang et al., 2003a) on different species.Therefore, although physical environment factors positively correlates with gas exchange (Guo et al., 1999), the biochemical (non-stomatal) regulation in this process cannot be negligible.

In general, stomatal conductance could effectively regulate leaf photosynthesis(Jones, 1992; Wang et al., 2001b). Its importance is not only in gas exchange but also in explaining biomass accumulation and productivity. One possible reason for the lower productivity of old forest is attributed to stomatal limitation (Gower et al., 1996) and stomata regulation has resulted in the different production of Korean Pine in different habitats (Wang et al., 2001a; 2003b).We also discussed the pattern of stomatal diurnal changes and its relation to photosynthesis (Fig. 3). We found that stomatal regulation may be also responsible for the diurnal pattern of gas exchange of this species. Therefore, in the diurnal changing microenvironments, E. adenophorum could fine regulate its photosynthetic processes by stomatal and non-stomatal adjustments. This kind of strategy can optimize this weed fix more carbon by photosynthesis in the suitable microenvironments.

Photosynthesis of many species was generally regulated by these two types of stomatal and non-stomatal regulations (Larcher, 2003). Thus, which one is more important for adjusting the photosynthesis of this weed? This question becomes important for understanding the photosynthetic traits of this weed in instant changing environments. Therefore, stomatal limitation and intercellular CO₂ concentration were discussed for clarifying the importance of these two regulations (Xu et al., 1997; Xu, 1997).

According to Farquhar and Sharkey (1982), one prerequisite to adjudge stomatal limitation on photosynthesis is the intercellular CO₂ concentration decline. When stomatal limitation increases and intercellular CO₂ concentration decreases are concurrently occurred, we can soundly conclude that stomata significantly limit photosynthesis. In other cases (such as stomatal limitation increases but intercellular CO₂ concentration increase co-occurred), it is that non-stomatal but not stomatal regulation is responsible for the photosynthesis decline. In fact, discussion of other species on this point has been reported in detail(Xu & Shen, 1997; Xu, 1997).For identifying the importance of stomatal and non-stomatal regulation on the photosynthesis of E. adenophorum, we found that the intercellular CO₂ concentration minimized at noon, which was coincided with the maximum value observed in stomatal limitation and midday depression in photosynthesis (Fig. 7). Therefore, comparing to non-stomatal regulation (as showed by the CE diurnal change in Fig. 3a), stomatal regulation is the most significant factor influencing the pattern of diurnal photosynthetic changes of E. adenophorum.

Just as above discussion, stomata are fine sensitive apparatus to response the changes in microenvironment in the diurnal course. Stomata elaborately adjust the gas exchange by way of opening/closing its aperture in patchiness or on whole leaves (Larcher, 2003). In the case of E.adenophorum, we found that stomatal but not photo-biochemical factor is the main limiting factor controlling pattern of photosynthetic variation in the diurnal changing environment. Stomata of evergreen conifers (Pinus koraiensis) can regulate their photosynthesis although its conductance is obviously lower than that of E. adenophorum (Wang et al., 200la; 2003b).With a similar range of stomatal conductance, evergreen invasive shrub, Sasa senanensis enable their stomata quite sensitive to the fluctuation of microenvironment above leaf surface (Wang et al., 2003a). All these reports enhance the full understanding of stomatal regulation on gas exchange in a short-term scale. Moreover, the stomatal regulation in diurnal changing environments is similar in different species of local and invasive species, i.e. midday depression accompanying with decrease in stomatal conductance and intercellular CO₂ concentration. Therefore, the rampancy of this species in this region can not be attributed to this strategy.

3.2 Stomata function plastically at xeric and hygric habitats to maximize photosynthetic capacity in suitable region

Stomata are fine sensitive apparatus to response the changes in microenvironment in the diurnal changing microenvironment. However, limited messages are available on the long-term acclimation. Existent documents showed that stomatal conductance substantially declines after long-term exposure in high CO₂ and O₃ (Noormets et al., 2001), which enabled the practice in determining ancient CO₂ levels by stomatal index method (Woodward, 1987). However, the possible impacts on the relations between stomata, photosynthesis and transpiration after long-term hydrological treatment in field condition is not available (Stewart et al., 1995).

When comparing E. adenophorum with a wide range of 20 species, we found that, both on CO₂ and H₂O exchanges, stomata regulate them in a similar way although slight difference within species are observed (Fig. 4, Fig. 5). However, when we classified total data into xeric and hygric habitats, photosynthetic rate and transpiration rate in hygric habitats were substantially higher than those in xeric habitats with respect to same stomatal conductance (Fig. 4). Therefore, the stomatal regulation functioned plastically when E. adenophorum was acclimated to different hydrological habitats. This plasticity in stomatal regulation may maximize the carbon fixation by leaves when this weed grows in suitable habitats. E. adenophorum is an invasive species. This kind of photosynthetic strategy is important for their quick colonization in new habitats when suitable habitats are available.

What makes this kind of plasticity in stomatal regulation? On the other hand, it indicates that other regulation system (non-stomatal regulation) keeps this plasticity in existence. Leaf nitrogen status is directly related to capacity of leaf photosynthesis (Hikosaka et al., 1998) and their allocation to different photosynthetic apparatus shapes the pattern of non-stomatal regulation (Wullschleger, 1993; Wang et al., 200lb). Nitrogen can be allocated more to chlorophyll with increase in light in the vertical profile of a forest (Koike et al., 2001). Moreover, the matches between their allocation the enzymes of carboxylation and RuBP regeneration made plants harmonize in varied environments (Wullschleger, 1993; Wang et al., 2001b).Thus, we also analyze their foliar nitrogen differences as an indicator for the non-stomatal regulation of E.adenophorum. Our result showed that leaf nitrogen was significantly higher in hygric habitats (Tab. 1). This was coincided with the higher photosynthesis and transpiration value (Fig. 4) and about 30% higher CE value(Tab. 1). Therefore, nitrogen related non-stomatal regulation can explain the plasticity of stomatal regulation on gas exchange at higher regime of stomatal conductance, whereas it slightly influences the stomatal regulation when stomatal conductance is low. So, both stomatal and non-stomatal regulation controlled the pattern of leaf gas exchange of this weed in different habitats. Similar finding has reported in tree species, such as Teskey et al.(1986) found that stomatal and non-stomatal regulations on gas exchange have made Pinus taeda survival in different sites. Furthermore, higher water contents in hygric sites resulted in the higher ability of nitrogen absorption although their soil organic matter, soil pH value and soil available nitrogen was slightly different (Tab. 1).

Conclusionly, the plasticity of stomatal regulation maximize the photosynthetic capacity in suitable (hygric) habitats and may be one potential reason for its rampancy in new colonization region.

3.3 Water use strategy difference enable the rampancy of E. adenophorum in different habitats

Water use strategy is important for survival of an invasive species in new colonized habitats. Water use efficiency of shrubs and herbs ranged from 0.2 to 4 μmol·mmol^-1 (Zu et al., 1998; Jiang et al., 1999; Yu et al., 2003), which was similar to our results of E. adenophorum in moist season (Fig. 6b). When stomatal conductance kept at high value, the wide ranges of 20 species had similar ranges. However, a wider range(0.4~13 μmol·mmol^-1) of water use efficiency was observed when stomatal conductance was low (Fig. 6a), which was similar to our results of E. adenophorum in dry season (Fig. 6b). Similarly, water use efficiency from 3 μmol·mmol^-1 to 10 μmol·mmol^-1 was also reported in a wide ranges of herbs, trees and succulents species (Larcher, 2003).

Stomata regulate photosynthesis and transpiration in a similar pattern, i.e. when stomata are in low conductance, it strongly limits both water and CO₂ exchanges, however, it gives slight limitation when stomatal conductance is in high regime. This made it possible to keep a constant water use efficiency (Passioura, 1982), which was observed in our results. In wide ranges of 20 species, water use efficiency kept constant with stomatal conductance when the conductance was high. But in low stomatal conductance, an increasing tendency in water use efficiency was observed (Fig. 6a).Similar pattern in xeric habitats was observed in E. adenophorum. However, completely different pattern in hygric habitats was observed, i.e. with the increase of stomatal conductance, water use efficiency slightly increased (Fig. 6b). When water was abundantly supplied, the unnecessary for saving water resulted in higher transpiration even in low stomatal conductance, which resulted in the positive correlation between stomatal conductance and water utilization in hygric habitats. This weed only distributes rampantly in moist regions and this type of water use strategy may be one reason for its common distribution in drought region with similar thermo-conditions (Qiang, 1998).

According to Passioura (1982), plants can be classified into conservative and prodigal plants by their water utilization strategy. If a drought is likely long, conservative behavior is a appropriate, for it will improve the water use efficiency without prejudicing the amount of water ultimately transpired. But if early relief of drought is likely, or if there is a series of short droughts or no droughts, prodigal behaviors will probably produce higher yield (Passioura, 1982). Considering the water use strategy of E. adenophorum grown in different habitats, this species possesses both conservative traits in xeric habitats and prodigal traits in hygric habitats. Stomatal aperture and function can adaptively change cross species (Larcher, 2003) and to the long term habitats within species (Woodward, 1987). Our findings indicate that stomata of E. adenophorum function plastically in different hydrological regime, which enable this weeds maximize their growth potential both by conservative and prodigal water utilization. Considering the widespread of this weed, this may be of significance for their rampancy and prolificacy in the suitable habitats.

4 Conclusion

Eupatorium adenophorum in China is a typical alien and toxic invasive species. However, no ecophysiological hints about its gas exchange characteristics enable us understanding the reason of its prolificacy in these regions. In the diurnal changing microenvironments, this weed can adjust its photosynthetic processes by stomatal and non-stomatal regulation, and stomatal regulation controls more of its diurnal photosynthetic pattern. This strategy is common for most species and contributes slightly to its rampancy. However, comparing to a wide range of 20 local accompanying species and species of other locations, stomatal regulation was different after long-term field acclimation in different hydrological habitats, i.e. gas exchange rate kept at high level when grown in hygric habitats when stomatal conductance was same. Moreover, the water use strategy of E. adenophorum was prodigal in hygric habitats and conservative in xeric habitats. This type of plasticity in stomatal function and water utilization difference enable their rampancy and prolificacy in different sites.