Seasonal Variation and Correlation with Environmental Factors of Photosynthesis and Water Use Efficiency of Juglans regia and Ziziphus jujuba
Supported by the State Key Basic Research and Development Plan of China (2002CB111504), the Project of Turning Cropland to Forest of State Forestry Administration and the Distinguished Young Scientist Fund of the National Natural Science Foundation of China (2002002002).
Abstract
Both the photosynthetic light curves and CO2 curves of Juglans regia L. and Ziziphus jujuba Mill. var. spinosa in three seasons were measured using a LI-6400 portable photosynthesis system. The maximal net photosynthetic rate (Amax), apparent quantum efficiency(φ), maximal carboxylation rate (Vcmax) and water use efficiency (WUE) of the two species were calculated based on the curves. The results showed that Amax of J. regia reached its maximum at the late-season, while the highest values of Amax of Z. jujuba occurred at the mid-season. The Amax of J. regia was more affected by relative humidity (RH) of the atmosphere, while that of Z. jujuba was more affected by the air temperature. Light saturation point (LSP) and Light compensation point (LCP) of J. regia had a higher correlation with RH of the atmosphere, those of Z. jujuba, however, had a higher correlation with air temperature. Vcmax of both J. regia and Z. jujuba had negative correlation with RH of the atmosphere. WUE of J. regia would decrease with the rise of the air temperature while that of Z. jujuba increased. Thus it could be seen that RH, temperature and soil moisture had main effect on photosynthesis and WUE of J. regia and Z. jujuba. Incorporating data on the physiological differences among tree species into forest carbon models will greatly improve our ability to predict alterations to the forest carbon budgets under various environmental scenarios such as global climate change, or with differing species composition.
The research on plant photosynthetic physiology is always the hot issue in ecology, especially at present, because the temperature has been getting higher and the concentration of CO2 of the atmosphere has been increasing all around the globe, which could greatly affect plant photosynthesis (Cure and Acock 1986; Tissue et al. 1995; Jiang et al. 1997; Jiang and Qu 2000; Heath et al. 2005). In addition, understanding the relationships between the traits of plant photosynthesis and their large-scale patterns is essential for simulating carbon cycle in ecosystems and predicting ecosystem functioning in response to environmental changes (Mellilo et al. 1993; Nemry et al. 1996; Norby and Luo 2004). The tree photosynthesis was the most important physiological process that determined the productivity and ecohydrology of forests (Raich et al. 1991; Warnant et al. 1994; Imhoff et al. 2004), and photosynthetic performance might be a diagnostic feature of the successional and ecological restoration status of forest species (Eschenbach et al. 1998).
Drought and semiarid lands occupied more than one third of the total land area on the earth, thus researching and cultivating the plant species that were adaptive to grow on the drought and semiarid lands was very important for the survival of human beings in the future (Swindale and Bidinger 1981; Boyer 1982; Levitt 1972). Water use efficiency (WUE) of plants would rise with the decrease of soil moisture within a certain scope, which had been reported in many experiments (Seiler and Johnson 1988; Dickmann et al. 1992; Smit and van den Driessche 1992; Damesin et al. 1998; Korol et al. 1999). WUE was treated as a key functional trait under drought conditions and study on that had been another hot issue in drought and semiarid areas in recent years (Polley et al. 1993; Dewar 1997; Wang et al. 1998). The natural native arbores or shrubs which had higher WUE should be chosen as possible to restore and reconstruct the steady vegetation system (Hu and Wang 1998; Xue et al. 2003).
Seasonality profoundly influences leaf physiology in deciduous forests (Dougherty et al. 1979). Large seasonal variation in physiological activity exists in part because temperature, humidity, light levels, soil moisture and CO2 concentration all vary through the course of the season (Jiang and Zhu 2001; Guo and Gao 2004). Physiological response to environmental conditions may also shift seasonally (Dougherty et al. 1979). The research on seasonal variations of photosynthetic characteristics and WUE could be helpful for us to identify the relative contribution of various environmental factors on photosynthesis and WUE, and further revealed that species-specific sensitivities to various environmental conditions shifted through the course of the season (Eamus 1991; Bassow and Bazzaz 1998; Zhang and Xu 2000), which will facilitate choose and cultivation of various species under various environmental conditions in the management of forestry. We focused on two tree species: Juglans regia L. and Ziziphus jujuba Mill. var. spinosa (Bunge) Hu ex. H.F. Chou. The two species are widely distributed in China, and they are usually grown in drought and semiarid lands (Wu and Raven 1998). Mature individuals of both species are frequently found together at our study site, Beichuan county, Sichuan Province, China, where was a demonstration spot of the project of returning cropland to forest or grassland in China. There J. regia and Z. jujuba were chosen in the project after actively searching in many patterns and ecological restoration based on native species.
The goals of this paper are to address the following questions: (1) What is the essentiality of the seasonal differences in the photosynthesis rates and WUE? (2) Can we explain these differences based on the seasonally shifting environmental conditions? Does the relative importance of the environmental conditions to photosynthesis and WUE shift through the course of the season? Answers to these questions will help improve understanding of atmosphere-biosphere interactions and facilitate incorporating biological processes in forest carbon-cycle models (Bassow and Bazzaz 1998).
Results
Differences in physiological characteristics in three seasons
Seasonal differences in light utilization of J. regia and Z. jujuba
Maximal net photosynthetic rate (Amax) was equal to the mean photosynthesis rate of observations taken with incident light levels above light saturation point (LSP). There were sufficient light-saturated observations of leaves for both species in our study, and Amax estimates were calculated for the leaf populations. Amax of J. regia growing at the early-season was 7.51 μmol·m−2·s−1, which had not significant difference from that at mid-season, 7.68 μmol·m−2·s−1. And at the late-season, Amax of J. regia reached its peak value, 9.45 μmol·m−2·s−1, which had significant difference from the early-season and mid-season (P < 0.05, n = 9). Unlike J. regia, the maximum of Amax of Z. jujuba occurred at the mid-season and late-season.
Apparent quantum efficiency (φ) of J. regia was highest at the late-season and that at the early-season was as low as at the mid-season (P > 0.05, n = 9). Apparent quantum efficiency of Z. jujuba achieved its maximum at the mid-season and the minimum occurred at the early-season. The figures also showed that the seasonal variation of apparent quantum of efficiency presented the same trend as Amax of both J. regia and Z. jujuba.
Both LSP and LCP of the two species had significant differences between the early-season, mid-season and late-season. LSP of J. regia reached the maximum at the mid-season while LSP of Z. jujuba achieved the maximum at the mid-season. LCP of J. regia reached the minimum at the late-season while LCP of Z. jujuba reached its maximum at the late-season (Figure 1).
Seasonal variations of maximal net photosynthetic rate (Amax), apparent quantum efficiency (φ), light saturation point (LSP) and light compensation point (LCP), for J. regia and Z. jujuba.Error bars represent ±1 SE of the mean estimate. The values of histograms under different lowercase letters are significantly different at P < 0.05.
Seasonal differences in CO2 utilization of J. regia and Z. jujuba
Vcmax of J. regia and Z. jujuba had significant differences (P < 0.05, n = 9) between the early-season, mid-season and late-season. And Vcmax of J. regia reached its maximum at the mid-season and reached the minimum at the late-season. Vcmax of Z. jujuba decreased as the processing of the season.
CO2 saturation point (CSP) of J. regia had significant differences (P < 0.05, n = 9) between the three seasons, and the value increased as the processing of the season. CSP of J. regia reached its maximum, 1 235 μmol/mol, at the late-season. Similarly, CSP of Z. jujuba reached its maximum at the late-season, 1 101 μmol/mol, and CSP hadn't significant differences between the early-season and late-season (P > 0.05, n = 9). CO2 compensation point (CCP) of both J. regia and Z. jujuba had significant differences between the three seasons (P < 0.05, n = 9) and CCP of J. regia reached its maximum at the mid-season while the maximum of Z. jujuba occurred at the late-season (Figure 2).
Seasonal variations of maximal carboxylation rate (Vcmax), CO2 saturation point (CSP) and CO2 compensation point (CCP) for J. regia and Z. jujuba.Error bars represent ±1 SE of the mean estimate. The values of histograms under different lowercase letters are significantly different at P < 0.05.
Seasonal variation in water use efficiency (WUE) and transpiration rate (Tr) of J. regia and Z. jujuba
WUE of J. regia and Z. jujuba reached its maximum at the late-season, and WUE of J. regia was the lowest at the mid-season while WUE of Z. jujuba had its minimum at the early-season. Tr of J. regia was the lowest at the early-season, and Tr of Z. jujuba reached its minimum at the late-season (Figure 3).
Seasonal variations of water use efficiency (WUE) and transpiration rate (Tr) for J. regia and Z. jujuba.Error bars represent ±1 SE of the mean estimate. The values of histograms under different lowercase letters are significantly different at P < 0.05.
Correlation of photosynthesis of J. regia and Z. jujuba with environmental factors
The relationships between Amax, φ, LSP or LCP and the environmental factors
Amax and φ of J. regia had apparent correlation with atmospheric relative humidity and soil moisture (r2 = 0.950 and r2 = 0.681, respectively for Amax and φ). And Amax of Z. jujuba had significant correlation with atmospheric temperature and soil moisture (r2 = 0.331), whereas apparent quantum efficiency of Z. jujuba had significant correlation only with air temperature (r2 = 0.402) (Table 1).
| Species | Parameters | Regression equation | Adjusted R2 |
|---|---|---|---|
| Juglans regia | A max | A max= 1.849 + 0.087RH + 0.158SM | 0.950*** |
| φ | φ=−0.012 + 0.001RH + 0.001SM | 0.681*** | |
| LSP | LSP = 676.563 + 10.878RH | 0.703*** | |
| LCP | LCP = 3.865 − 1.243SM + 0.298Ta+ 0.261RH | 0.950*** | |
| Ziziphus jujuba | A max | A max= 0.307 + 0.158 Ta+ 0.277SM | 0.331** |
| φ | φ= 0.001 + 0.001 Ta | 0.402*** | |
| LSP | LSP =−455.312 + 34.409 Ta+ 0.433PAR | 0.631*** | |
| LCP | LCP = 414.216 − 2.979 Ta− 0.791Ca | 0.424** |
- RH, air relative humidity; SM, soil moisture; Ta, air temperature; PAR, photosynthetic active radiation; Ca, air CO2 concentration. The effect of regression was evaluated by ANOVA, significance levels of P < 0.01, and P < 0.001 are indicated by symbols **, and ***.
Correlation of Vcmax, CSP and CCP with the environmental factors
Vcmax of J. regia and Z. jujuba presented negative correlation with atmospheric relative humidity (r2 = 0.932). In addition, Vcmax of Z. jujuba was significantly affected by PAR and RH (r2 = 0.745). CSP of J. regia was significantly correlated with soil moisture (SM) (r2 = 0.965) while CCP was correlated with atmospheric temperature (r2 = 0.916). CSP of Z. jujuba not only had significant positive correlation with SM but also had negative correlation with atmospheric temperature (r2 = 0.513). And CCP was just significantly negatively correlated with the temperature (r2 = 0.357) (Table 2).
| Species | Parameters | Regression equation | Adjusted R2 |
|---|---|---|---|
| Juglans regia | V cmax | v cmax= 43.499 − 0.391RH | 0.932*** |
| CSP | CSP = 426.888 + 60.206SM | 0.965*** | |
| CCP | CCP = 12.633 + 1.306Ta | 0.916*** | |
| Ziziphus jujuba | V cmax | v cmax= 49.148 − 0.625RH + 0.018PAR − 1.671SM | 0.745*** |
| CSP | CSP = 1 474.346 − 24.680Ta+ 29.861SM | 0.513*** | |
| CCP | CCP = 57.965 − 0.291Ta | 0.357** |
- RH, air relative humidity; SM, soil moisture; Ta, air temperature; PAR, photosynthetic active radiation. The effect of regression was evaluated by ANOVA, significance levels of P < 0.01, and P < 0.001 are indicated by symbols **, and ***.
Regressions of WUE and Tr to the environmental factors
WUE of J. regia presented a significant negative correlation with atmospheric temperature (r2 = 0.814), but WUE of Z. jujuba had a significant positive correlation with RH (r2 = 0.479). Tr of J. regia had a significant positive correlation with SM and atmospheric temperature (r2 = 0.870), however, Tr of Z. jujuba presented negative correlation with RH and atmospheric CO2 concentration (r2 = 0.412) (Table 3).
| Species | Parameters | Regression equation | Adjusted R2 |
|---|---|---|---|
| Juglans regia | WUE | WUE = 9.337 − 0.096Ta | 0.814*** |
| T r | T r = 0.559 + 0.043SM + 0.010Ta | 0.870*** | |
| Ziziphus jujuba | WUE | WUE =−0.246 + 0.094RH | 0.479*** |
| T r | T r = 11.002 − 0.034RH − 0.020 Ca | 0.412** |
- RH, air relative humidity; SM, soil moisture; Ta, air temperature; Ca, air CO2 concentration. The effect of regression was evaluated by ANOVA, significance levels of P < 0.01, and P < 0.001 are indicated by symbols **, and ***.
Discussion
Seasonal and species differences of photosynthesis and WUE
Amax is an important photosynthetic parameter that represents the maximal photon utilization capability of the plants and reflects the net primary productivity and the accumulation of biomass at the subtropical forests where light level was near to or higher than LSP of the tree species on sunlit days (Gunderson et al. 1993; Bassow 1995). In our present study we found that species’ photosynthetic responses differed over the growing season. Light-saturated photosynthesis reached maximum values at different points through the growing season among the different species. Notably, J. regia did not reach maximal rates until late in the season in October, in contrast, Z. jujuba reached near maximal Amax rates in mid-season in July. For a number of deciduous tree species, Jurik (1986) found that all species he measured reached their maximal Amax in June, and he considered that the highest maximal photosynthesis rate of temperate deciduous tree species was positively correlated with the extent of the leaf expansion. However, in our study, the leaves of J. regia and Z. jujuba growing at subtropics had completed 100% leaf expansion in the early season, then their Amax both reached the minimum. At the experiment species differed in their light-saturated photosynthesis, Amax: J. regia had higher Amax values than Z. jujuba. Apparent quantum efficiency is correlative with photosynthetic electron transport rate in course of the photosynthetic phosphorylation and the regeneration of RuBP in course of the CO2 assimilation (Farquhar et al. 2001). LSP and LCP reflect the adaptability of the plants to the light condition of the natural environment. In our study J. regia had lower apparent quantum efficiency than Z. jujuba, which showed that Z. jujuba had a higher light use efficiency than J. regia. But LCP of Z. jujuba was higher than J. regia and its LSP was lower than J. regia, it revealed that Z. jujuba only made use of less narrow range of PAR than J. regia. Vcmax, CSP and CCP are the important parameters that reflect the intrinsic CO2 assimilation capability of the plants. Vcmax is maximal carboxylation rate when RuBP is saturated, and Vcmax has a positive correlation with the quantity and activity of Rubisco (Farquhar et al. 1980). In this article Vcmax of J. regia reached the maximum at the mid-season while Vcmax of Z. jujuba reached the maximum at the early-season, which reflected the variational status of the quantity and activity of Rubisco of J. regia and Z. jujuba, respectively, in one year. Photosynthetic CSP could denote the maximal CO2 capacity fixed in course of photosynthesis. In the article all of CO2 saturation points were measured under saturated light condition, and the photosynthesis of plants under saturated light was limited mostly by the quantity and activity of photosynthetic enzymes (von Caemmerer et al. 1994), especially by the quantity and activity of Rubisco under the condition of saturated CO2 and saturated light. Transpiration and WUE denote the consumption and utilization of water for the plants. WUE is the quantity of photosynthetic production when per unit water is transpired; it lies on the ratio between net photosynthetic rate and transpiration rate and denotes the drought resistance of the plants (Carmela et al. 2006). In the article WUE of J. regia and Z. jujuba reached the minimum at the mid-season when it contributes 86% of the annual rainfall in Beichuan county from May to September, which is accordant with some research results (Jiang and He 1999; Yan et al. 2001; Chen et al. 2003).
The relative importance of the environmental conditions to photosynthesis and WUE shift through the course of the season
In situ patterns of leaf-level photosynthesis consist of interactions between the suite of ambient environmental conditions and the species-specific sensitivity to the combination of those factors (Bassow and Bazzaz 1998). In other words, the environmental conditions may have been similar surrounding the trees studied here, but the trees’ photosynthetic responses were different. The results of stepwise regressions demonstrated that environmental conditions had some similar and some different effects on the two species. These different environmental effects on species, while relatively subtle, may have led to the significant variation among years in forest carbon and water exchange (Goulden et al. 1996).
Our results showed that atmospheric relative humidity and soil moisture had more effects on Amax of J. regia. And at the late-season in October when the rainy season had ended in Beichuan, there was higher soil moisture, lower average radiation intensity, lower atmospheric temperature and higher atmospheric relative humidity. Higher atmospheric relative humidity would be favorable for increasing stomatic conductance of the leaves (Tezara et al. 1999), therefore photosynthesis would rise and Amax increased as a result. Unlike J. regia, Amax of Z. jujuba presented a significant positive correlation with atmospheric temperature. There is a significant positive correlation between photosynthetic capability of the leaves and the activity of photosynthetic enzymes too, and one of the most important environmental factors that affect the activity of the enzymes is the temperature. At the mid-season, there was higher atmospheric temperature that promoted photosynthesis of Z. jujuba. It was concluded that Z. jujuba was more adaptable to high temperature than J. regia. The leaves of Z. jujuba was less sensitive to atmospheric humidity than J. regia, which could prove that the leaves of Z. jujuba was more adaptable to dry atmosphere than J. regia. Soil moisture had apparent effect on Amax of J. regia and Z. jujuba and lower water content in the soil would inhibit physiological activities including photosynthesis. Difference between the phases of J. regia and Z. jujuba when the maximum of Amax occurred reflected that the maximal accumulation of productivity of the two species at different seasons, which was beneficial to the management of choosing tree species in the project of “returning cropland to forest or grassland”.
Apparent quantum efficiency of J. regia is mostly influenced by atmospheric relative humidity and soil moisture, while apparent quantum efficiency of Z. jujuba is mainly correlative with air temperature. In the article LSP and LCP of J. regia was mainly correlative with atmospheric humidity while LSP and LCP of Z. jujuba was mostly correlative with atmospheric temperature, which accorded with the opinion of some articles published (Hu and Wang 1998).
In this article CSP of J. regia and Z. jujuba had a positive correlation with soil moisture, which reflected the important effect of soil water content on the activity and quantity of photosynthetic enzymes. CCP had a significant relation with dark respiration and respiration in light (Farquhar et al. 2001). In this article CCP of J. regia had a positive correlation with atmospheric temperature while CCP of Z. jujuba had a negative correlation with air temperature, which showed the activity of enzymes in different plant species had a different correlation with air temperature. Respiration of J. regia reached the maximum at the mid-season when the air temperature was the highest, while Z. jujuba presented higher respiration intensity at the late-season when the atmospheric temperature was lower.
The results in the article indicated that WUE of J. regia was mainly affected by air temperature while WUE of Z. jujuba was mostly influenced by atmospheric humidity. In the project of returning cropland to forest or grassland, different managements should be taken to improve WUE of different tree species.
Most forests are mixtures of some different tree species. Global climate change or other major disturbance may lead to substantial shifts in species composition, which in turn will have implications for forest carbon cycles (Bazzaz et al. 1996). Incorporating data on the physiological differences among tree species into forest carbon models will greatly improve our ability to predict alterations to the forest carbon budgets under various environmental scenarios or with differing species composition. The clarity of the dynamic parameters in photosynthesis and water uses of the important cultivated species will provide baseline information to forecasting carbon storage of the artificial forests in China.
Materials and Methods
Study area
The study was carried out in southwestern China (Beichuan county, province of Sichuan, 31°58.439′N, 104°36.233′E, 808 m a.s.l.). Environmental parameters as photosynthetic active radiation (PAR), atmospheric relative humidity (RH), soil moisture (SM), air CO2 concentration (Ca), air temperature (Ta) were obtained (Table 4).
| Seasons | Sites of species | PAR (μmol·m−2·s−1) | Air CO2 concentration (μmol/mol) | Relative humidity (%) | Soil moisture (%) | Air temperature (°C) |
|---|---|---|---|---|---|---|
| Early season (in April) | Juglans regia | 1 036 ± 32† | 375.4 ± 0.7 | 52.68 ± 0.41 | 6.3 ± 0.8 | 24.57 ± 0.31 |
| Ziziphus jujuba | 1 011 ± 43 | 371.3 ± 0.9 | 47.11 ± 0.39 | 5.5 ± 1.3 | 22.72 ± 0.09 | |
| Mid-season (in July) | J. regia | 1 023 ± 46 | 369.3 ± 0.6 | 49.29 ± 0.80 | 9.8 ± 1.1 | 37.13 ± 0.10 |
| Z. jujuba | 1 332 ± 46 | 369.3 ± 0.6 | 49.29 ± 0.80 | 8.4 ± 1.5 | 34.13 ± 0.10 | |
| Late-season (in October) | J. regia | 1 019 ± 11 | 380.0 ± 0.1 | 63.01 ± 0.05 | 13.7 ± 0.9 | 20.44 ± 0.02 |
| Z. jujuba | 1 040 ± 27 | 371.7 ± 0.3 | 59.77 ± 0.23 | 8.7 ± 2.1 | 27.99 ± 0.04 |
- †Values shown are mean ± SE (n = 9).
Beichuan county is at the west-northern edge of Sichuan basin, located at the up reach of Peijiang river, 2 829 km2 of total area. It was the transition zone of west-southern alpine gorge at Hubei-Chongqing-Sichuan mountainous region in geology, which was affected by Longmen rupture and cut by Peijiang water system. Mountains were freely located across the area and ravines interlaced. The highest altitude is 4 796 m and the lowest is 540 m, which makes 4 229 m of relative difference. Beichuan county belongs to humid subtropical monsoon climatic region, thus has a mild whether all-year. The mean annual temperature was 15.6 °C and the light period was 1 100 h a year. Although Sichuan is a famous rainstorm collecting area, which is enriched in precipitation rain fall and has an average of 1 400 mm, it was distributed unevenly in temporal and spatial scale. Most of the rain fall occurs in May to September, which occupies 86 percent of a year. The land surface was eroded severely, and so was the soil loss. Flood and mud-rock flow happen frequently. The complex physiognomy characteristics and climate conditions contribute to the formation of the catastrophic weather in this area, including winter dry, spring drought, summer flood and autumn waterlogging. The type of soil in this area is mainly consisted of the yellow soil, and pH value of the soil is around 5.4–8.4 and the soil water capacity is around 5.5%–13.7%. But, northeastern Beichuan, where the study site located in, has a drought environment since the steep slope and low soil capacity for effective water storage existed. And it is disadvantaged for the growth of the trees and the crops.
Plant materials
The two plant species studied in this paper were Juglans regia L. and Ziziphus jujuba Mill. var. spinosa (Bunge) Hu ex. H.F. Chou, which was chosen for the project of returning cropland to forest in Beichuan county both to protect the soil and advance the local farmers’ living level. J. regia is a species of deciduous arbor or shrub, which is an economic cultivated species with strong adaptabilities, wide distribution range and wide utilization. It usually grows in humid fertile soil and is often planted in plains or foothills. Z. jujuba is a deciduous shrub or small arbor, which usually grows on natural hillside, roadside or in the field. And it is mainly distributed in Liaoning, Inner-Mongolia, Hebei, Shanxi, Shandong, Anhui, Henan, Hubei, Gansu, Shanxi, Sichuan provinces. It adapts to chill or dry habitat and is commonly used for the conservation of soil and water. Both J. regia and Z. jujuba are C3 plants (Le Roux et al. 1999; Zheng and Shangguan 2005). One yr old tree seedlings of J. regia and Z. jujuba were chosen in the present study and during each measuring period, we measured the photosynthesis rate of three randomly selected leaves of each of three target trees in no particular order for each species.
Gas exchange measurement
Measurements were taken in April, July and October, 2004, respectively and only uniformly sunlit days were selected to minimize sources of diurnal heterogeneity. Three seedlings of each species were randomly selected, and the third to fifth fully expanded leaves from the apical meristem of each selected seedling were used for gas exchange measurement (Loik and Holl 1999). LI-6400P portable photosynthesis system was used at the experiment and the gas analyzer was calibrated daily and checked periodically throughout the measurement days. Photosynthetic light saturation points of the two species were calculated by light curve which was measured at the free relatively constant CO2 concentration in atmosphere. Then CO2 curve was measured at photosynthetic active radiation (PAR) that was slightly higher than light saturation point (Brooks et al. 2003). The value of PAR was reflected by photon flux density in the unit of μmol·m−2·s−1. It was controlled by red-blue light source of 6400-02B, and several light treatments was set at the range of 0–2 500 μmol·m−2·s−1. The concentration of CO2 was controlled by 6400-01 CO2 injector during the measurement of A-Ci curve, and the unit was μmol/mol. Several CO2 concentration levels were set at the range of 50–2 000 μmol/mol. The parameters directly measured by LI-6400P included net photosynthetic rate (Pn), intercellular CO2 concentration (Ci), transpiration rate (Tr) and photosynthetic active radiation (PAR) for the research goals.
Data analysis
Light curve equation
(1)
(2)
(3)In the two equations 2 and 3 above, Amax denotes maximal net photosynthetic rate, φ denotes apparent quantum efficiency, C0 denotes an index that reflects net photosynthetic rate approaching zero at a very weak irradiance.
A-Ci equation
(4)In the equation above, Ac denotes RuBP-saturated CO2 assimilation rate, Rd denotes mitochondrial respiration in the light, Vcmax denotes maximal carboxylation rate, Ci* denotes intercellular CO2 photocompensation point, Ci denotes intercellular CO2 concentration, Km(CO2)i denotes apparent Michaelis-Menten constant for CO2 evaluated at Ci.
Maximal carboxylation rate (Vcmax), CO2 saturation point (CSP) and CO2 compensation point (CCP) could be obtained from the fitting between the equation and A-Ci curve measured.
Water use efficiency (WUE)
(5)We used ANOVA test at P= 0.05 and stepwise multiple linear regression analyses to quantify the relative effects of seasonally varying micro-environmental conditions on leaf-level photosynthesis. Multiple regression is a flexible method of data analysis that may be appropriate whenever a quantitative dependent variable is to be examined in relationship to any other independent factors. Relationships may be nonlinear, independent variables may be quantitative or qualitative, and one can examine the effects of a single variable or multiple variables with or without the effects of other variables taken into account (Cohen et al. 2003). This was done across the seasons to quantify the impact on photosynthesis of seasonal variation in environmental conditions. Further, we used stepwise regression analyses to quantify the effects of light levels, air temperatures, CO2 concentrations, atmospheric relative humidities and soil moistures (Bassow and Bazzaz 1998).
(Handling editor: Da-Yong Zhang)
Acknowledgements
We thank Mr. Ji-Hong Ren, En-De He, Jin Zhou, Xing-Yuan Zhao, Xiao-Jun Liu, Jian-Guo He, Xue-Mei Gong et al. of Beichuan Forestry Bureau of Sichuan Province for providing aids in field work.
