, ,*, , ,

1. College of Resources and Environment, Tibet Agriculture & Animal Husbandry University, Nyingchi 860000, China; 2. Tibetan Featured Flower Research and Development Center of National Flower Engineering Technology Research Center, Nyingchi 860000, China; 3. National Station for Field Observation and Research of Alpine Forest Ecosystem, Nyingchi 860000, China; 4. Food Science College, Tibet Agriculture & Animal Husbandry University, Nyingchi 860000, China

Abstract　Chamaenerion spp. (Onagraceae) are a class of ecological restoration medicinal herbs. Mount Shergyla has three species and one variant of Chamaenerion. Studying their photoresponse characteristics and elucidating their adaptation and physiological response to the photosynthetic characteristics of the environment is the basis for the development and protection of the plant resources of the genus Chamaenerion. In this experiment, Li-6400 portable photosynthesis instrument was used to study the photoresponse characteristics of C. spp. in Mount Shergyla, and the photoresponse curves were fitted using a right-angle hyperbolic correction model. The results showed that Chamaenerion conspersum has low light compensation point[4.931 μmol/(m2·s)], high light saturation point[1 000 μmol/(m2·s)], and strongest adaptability to light, which is one of the determinants of its distribution at each elevation of Mount Shergyla. Chamaenerion angustifolium subsp. circumvagum has higher light compensation point [11.848 μmol/(m2·s)], low light saturation point[800 μmol/(m2·s)], and weakest adaptability to light, and it is a typical sciophilous plant. This result is consistent with the habitats (Rhododendron wardii shrub and spruce forest margin) that C. angustifolium are distributed in Mount Shergyla. C. angustifolium and Chamaenerion latifolium have higher light compensation points and higher light saturation points, suggesting that the adaptability of C. angustifolium and C. latifolium to the light environment is weak. This can also explain the narrow distribution of C. angustifolium and C. latifolium in Mount Shergyla. There are certain degree of differences in the responses of net photosynthetic rate (Pn), intercellular CO2 concentration (Ci), stomatal conductance (Gs) and transpiration rate (Tr) of the three species and one variant of Chamaenerion to different photosynthetically active radiation (PAR). With the increase of light intensity, the Pn, Gs and Tr of Chamaenerion spp. increased significantly, and the Ci decreased. The Pn of C. conspersum and C. latifolium differed insignificantly, but they were both higher than that of C. angustifolium. The Pn of C. angustifolium subsp. circumvagum was the lowest. The Gs of C. conspersum and C. latifolium differed insignificantly, but they were both higher than that of C. angustifolium subsp. circumvagum. The Gs of C. angustifolium was the lowest. There was no obvious difference in Ci between C. angustifolium and C. latifolium, of which the Ci was both higher than that of C. conspersum. The Ci of C. angustifolium subsp. circumvagum was the lowest. C. angustifolium subsp. circumvagum had the highest Tr, followed by C. angustifolium and C. conspersum (no obvious difference), and C. latifolium had the lowest Tr. The light utilization efficiency of C. spp. ranked as C. angustifolium subsp. circumvagum < C. angustifolium < C. latifolium < C. conspersum. The light suppression phenomenon was evident in C. angustifolium, C. angustifolium subsp. circumvagum and C. latifolium. C. conspersum is most suitable for application under high light intensity conditions in Tibet. C. angustifolium, C. angustifolium subsp. circumvagum and C. latifolium should be shaded properly under conditions of high light intensity in the cultivation process, while C. conspersum needs to be supplemented with light during the cultivation process.

Key words　Mount Shergyla, Chamaenerion, Photosynthesis, Photoresponse curve

1　Introduction

Chamaenerionspp. are a type of ecological restoration medicinal ornamental plants, with extremely high application value. The inflorescences ofC. spp. are large and colorful, rich in color and long in flowering time.C. spp. can be planted in flower beds or flower arrangements, or used as cut flowers, and they are a class of rare ornamental plants[1-2].C. spp. have antibacterial, anti-inflammatory, anti-anxiety, anti-tumor growth and anti-aging effects, and have gradually become a hot topic of modern plant medicine[3-17].C. spp. are well-developed, deep in the earth, dense in branches and leaves and resistant to wind and pollution and grow quickly. They naturally grow in ecological degraded areas such as forest margins, forests, hillside grasslands, river banks, fire or cutting sites, roadside landslides, and retaining wall edges,etc[18], and have strong adaptability. Therefore,C. spp. are excellent herbal pioneers in ecological restoration. They can not only reduce the cost of conservation and management and prevent the invasion of alien species in Tibet but also form a pioneer plant community to make the damaged habitat quickly rebuild the ecological balance and form a beautiful landscape. In order to rationally develop and protect the resources ofC. spp. in Tibet, the three species and one variant ofChamaeneriongrowing in the Mount Shergyla were first cultivatedexsitu. Under the same growing environment, their photoresponse characteristics were studied to explore the differences in the photoresponse, so as to provide a theoretical support for the high-yielding cultivation ofC. spp., lay a foundation for further exploration of the production capacity of ecological products of the genusChamaenerionin Tibet and provide a basis for the application ofC. spp. in urban greening and ecological restoration in Tibet in the premise of sticking to the ecological protection red line.

2　Materials and methods

2.1OverviewofexperimentalsiteThe experiment was conducted in the Tibetan Featured Flower Research and development Center of the National Flower Engineering Technology Research Center. The experiment site is located in the rare and endangered garden plant cultivation base of Tibet Agriculture & Animal Husbandry University in Bayi Town, Bayi District, Nyingchi City. It is located in the lower reaches of the Niyang River, with an altitude of 2 970 m. It belongs to the warm and humid climate in Southeast Tibet with annual average temperature of 8.6℃, the average temperature of the hottest month of 15.6℃, the average temperature of the coldest month of 0.2℃, the extreme maximum temperature of 30.2℃, the extreme minimum temperature of -15.3℃, ≥ 10℃ accumulated temperature of 2 225.7℃ and frost-free period of 177 d. There is no month with monthly average temperature of ≤ 0℃ throughout the year. The number of days with daily average temperature of ≥ 10℃ is 159.2. The average annual rainfall is 634.2 mm. The distribution of precipitation is uneven throughout the year, mainly in June-September (71.6%). The average relative humidity, humidity coefficient, maximum snow thickness, annual hail days, annual thunderstorm days, annual strong wind (wind speed ≥ 17.0 m/s) days, annual sunshine hours, sunshine percentage, average atmospheric pressure and temperature coefficient are 71.0%, 1.01, 11 cm, 2.8 d, 28.3 d, 7.6 d, 1 988.6 h, 46%, 70.6 kPa and 8.3, respectively. The late frost appears latest in early May, and the early frost appears earliest in late September[19]. The experimental site is open, and free from shading, and the lighting, ventilation and irrigation conditions are good.

2.2ExperimentalmaterialsThe experimental materials were perennial plants ofC.angustifolium,C.angustifoliumsubsp.circumvagum,C.conspersumandC.latifolium, which were collected from the Mount Shergyla in Nyingchi, Tibet. Afterexsitucultivation and survival in the rare and endangered garden plant cultivation base in Tibet, the experiment was conducted during the flowering period.

2.3MeasurementoflightresponsecurveThe photosynthetic test was carried out during 9:00-11:30 am of a sunny day in August 2017. An open air circuit was used to automatically measure the photosynthetic light-response curves using LED red and blue light leaf chamber equipped with the Li-6400 portable photosynthesis measurement system (LiCior Inc., Lincoln, USA). The photosynthetically active radiation (PAR) was 2 000, 1 800, 1 600, 1 400, 1 200, 1 000, 800, 600, 400, 200, 100, 50, 25 and 0 μmol/(m2·s), respectively. Under different light intensity, the net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) and intercellular CO2concentration (Ci) were measured. During the measuring, the CO2concentration, chamber temperature and air flow were set as 400 μmol/mol, (25 ± 0.5)℃ and 500 μmol/s, respectively. The middle-upper mature leaves with normal morphology and without pests or diseases were selected. For each species, three plants with robust growth were selected. One leaf was selected from each plant. Three replicates were arranged for each measurement.

2.4DataprocessingThe light response curves ofC. spp. were fitted by the right-angle hyperbolic correction model[20], and the photoresponse characteristic parameters, dark respiration rate, light compensation point, light saturation point, maximum net photosynthetic rate (Pnmax) were estimated. The light response data ofC. spp. under low-light conditions[≤ 200 μmol/(m2·s)]were fitted to the linear equations. The apparent quantum efficiency (AQY) was the slope of the linear equations. Excel was used to graph, SPSS22.0 was used to analyze the data with one-way ANOVA, and LSD was used for multiple comparisons.

To further verify the accuracy of the fitted data and measured photosynthetic parameters, the relative error (RE) was defined[21]:

3　Results and analysis

3.1ComparisonofPARandPnofdifferentChamaenerionspeciesThe variation of Pn with the PAR differed among different species ofChamaenerion(Fig.1). In the light intensity range of 0-50 μmol/(m2·s), the Pn ofC. spp. increased slowly; in the light intensity range of 50-400 μmol/(m2·s), the Pn ofC. spp. increased sharply; and when the light intensity was above 400μmol/(m2·s),C.angustifoliumreached the light saturation point at 1 200 μmol/(m2·s),C.angustifoliumsubsp. circumvagum reached the light saturation point at 800 μmol/(m2·s), andC.conspersumandC.latifoliumboth reached the light saturation point at 1 000 μmol/(m2·s). The PnmaxofC.latifolium,C.conspersum,C.angustifoliumandC.angustifoliumsubsp. circumvagum was 23.067, 21.888, 18.062 and 11.862 μmol/(m2·s), respectively. After reaching the Pnmax, the Pn ofC.angustifolium,C.angustifoliumsubsp. circumvagum andC.latifoliumdeclined slowly, and the Pn ofC.conspersumstabilized with the increase in PAR.

3.2ComparisonofphotoresponsecharacteristicparametersamongdifferentspeciesofChamaenerionAQY, also known as quantum efficiency under low light intensity, is a measure of the ability of plants to use low-density light quanta. It is also a basic parameter reflecting the utilization efficiency of light energy and the production efficiency of photosynthetic substances in plants[22-23]. The initial slope obtained by fitting the photoresponse data under weak light intensity[≤ 200 μmol/(m2·s)]with the linear equation is the AQY.

As shown in Table 1 and Fig.2, the light utilization efficiency ofC. spp. ranked asC.conspersum>C.latifolium>C.angustifolium>C.angustifoliumsubsp. circumvagum. Among them, the light compensation point ofC.conspersumwas the lowest[8.519 μmol/(m2·s)], which was significantly different from those ofC.angustifoliumsubsp. circumvagum,C.angustifoliumandC.latifolium. The light compensation point ofC.latifoliumwas the highest[11.620 μmol/(m2·s)]. There was no significant difference betweenC.angustifolium[11.095 μmol/(m2·s)]andC.angustifoliumsubsp. circumvagum[11.090 μmol/(m2·s)]. It indicates thatC.conspersumhas the highest utilization efficiency for weak light, followed byC.angustifoliumandC.angustifoliumsubsp.circumvagum, andC.latifoliumhas the lowest utilization efficiency for weak light. The PnmaxofC.conspersum[22.073 μmol/(m2·s)]was the highest, followed by those ofC.latifolium[21.388 μmol/(m2·s)]andC.angustifolium[17.797 μmol/(m2·s)], and the PnmaxofC.angustifoliumsubsp.circumvagum[11.872 μmol/(m2·s)]was the lowest. It suggests that the photosynthetic capacity ofC. spp. varies. The photosynthetic capacity ofC.conspersumwas the strongest, and it showed no significant difference compared with that ofC.latifolium(P>0.05). The photosynthetic capacity ofC.angustifoliumsubsp.circumvagumwas the weakest. The light saturation point ofC.angustifoliumsubsp.circumvagum[844.411 μmol/(m2·s)]was the lowest, and that ofC.angustifolium[1 147.728 μmol/(m2·s)]was the highest. The light saturation points ofC.latifolium[1 023.592 μmol/(m2·s)]andC.conspersum[965.082 μmol/(m2·s)]ranked in the middle tier.

Fig.1PhotoresponsecurveofChamaenerionspp.withphotosyntheticallyactiveradiation

Table 1　Comparisons of photoresponse characteristic parameters among Chamaenerion spp.

Fig.2ApparentquantumefficiencyofChamaenerionspp.

3.3ChangesofphotosyntheticphysiologicalparameterswithPARindifferentChamaenerionspeciesWith the increase in PAR, the Gs ofC. spp. tended to show an upward trend overall (Fig.3). The increase in Gs ofC.conspersumandC.latifoliumwas greater with the increase of PAR. In the light intensity range set by the experiment, the Gs ofC.conspersumincreased continuously, and the Gs ofC.angustifolium,C.angustifoliumsubsp.circumvagumandC.latifoliumreached the peak at 800[0.124 mol H2O/(m2·s)], 1 400[0.136 mol H2O/(m2·s)]and 1 600[0.154 mol H2O/(m2·s)]μmol/(m2·s), respectively and then slowly decreased.

Fig.3StomatalconductanceofChamaenerionspp.withphotosyntheticallyactiveradiation

The changes in the Ci ofC. spp. were basically the same. The Ci ofC. spp. decreased gradually as PAR increased (Fig.4). When the PAR increased from 0 to 400 μmol/(m2·s), the Ci decreased sharply. When the PAR was above 400 μmol/(m2·s), the Ci ofC.angustifoliumandC.angustifoliumsubsp.circumvagumdeclined continuously, and the decrease was getting smaller and smaller. The Ci ofC.angustifoliumandC.angustifoliumsubsp.circumvagumtended to be stable around 1 400 μmol/(m2·s). The Ci ofC.conspersumandC.latifoliumdeclined slowly during the PAR range of 400-1 200 μmol/(m2·s). After the PAR reached 1 200 μmol/(m2·s), the Ci ofC.conspersumandC.latifoliumslowly increased again.

Fig.4IntercellularCO2concentrationofChamaenerionspp.withphotosyntheticallyactiveradiation

The Tr ofC. spp. increased with the increase of PAR, showing an upward trend (Fig.5). In the light intensity range of 0-200 μmol/(m2·s), the Tr ofC. spp. increased slowly, and the increase was small; in the range of 200-1 600 μmol/(m2·s), the Tr increased greatly; and in the range of 1 600-2 000 μmol/(m2·s), the Tr ofC.conspersumincreased continuously, and that ofC.angustifolium,C.angustifoliumsubsp.circumvagumandC.latifoliumstopped rising or showed a stable trend.

Fig.5TranspirationrateofChamaenerionspp.withphotosyntheticallyactiveradiation

4　Discussions

4.1SelectionoflightresponsecurvefittingmodelsofC.spp.The fitting results of different photoresponse curve models are significantly different[24-25]. In this study, the photoresponse curves ofC. spp. were fitted using right-angle hyperbolic model, non-right-angle hyperbolic model, exponential model, and right-angle hyperbolic correction model. ForC. spp., the means of the relative errors between the fitted values and the measured values ranked as right-angle hyperbolic correlation model (RE = 0.216) < non-right-angle hyperbolic model (RE = 0.305) < exponential model (RE = 0.372) > right-angle hyperbolic model (RE = 0.616). Among different models, the right-angle hyperbolic correction model performed well in fitting the photoresponse curves ofC. spp. (Fig.6). The average value of the coefficient of determination was 0.998, and the fitted value was the closest to the measured value. The relative error between the fitted value and the measured value ofC.angustifolium,C.angustifoliumsubsp.circumvagum,C.conspersumandC.latifoliumwas 0.073 3, 0.230 5, 0.296 3 and 0.179 4, respectively, with an average of 0.179 4. Therefore, the right-angle hyperbolic correction model is a better model to fit the photoresponse curves ofC. spp.

In this experiment, the photoresponse data under weak light intensity[≤ 200 μmol/(m2·s)]were fitted to obtain the AQY:C.angustifoliumsubsp. circumvagum, 0.047 3;C.angustifolium, 0.052 9;C.conspersum, 0.076 0;C.latifolium, 0.067 7. The AQY of the general plant under natural conditions ranges from 0.03 to 0.05, suggesting that the fitted data were more reasonable. The AQY reflects the light energy utilization efficiency of photosynthesis, especially the ability to use weak light of plants. The high AQY value indicates that the leaf light energy conversion efficiency is high. The test results showed that the AQY value ofC. spp. was positively correlated with the Pnmax. This is consistent with the findings of Dong Zhixinetal.[26]and Zhang Liwenetal.[25]. The ability ofC. spp. for light energy utilization was in the ascending order:C.angustifoliumsubsp.circumvagum

Fig.6FittingphotoresponsecurvesofChamaenerionspp.withright-anglehyperboliccorrectionmodel

4.2RelationshipbetweenlightresponsecharacteristicsandadaptabilityofC.spp.Light is an important environmental factor that affects photosynthesis. The broad adaptation to light is an important manifestation of the ability of plants to adapt to the environment. Plants with lower light compensation points and higher light saturation points have greater adaptability to the light environment, while the plants with higher light compensation points and lower light saturation points have weaker adaptability to light[22]. In this study,C.conspersumhas lower light compensation point[4.931 μmol/(m2·s)]and higher light saturation point[1 000 μmol/(m2·s)], and its light adaptability is the strongest. It is distributed in almost all the elevations on the east and west slopes of Mount Shergyla.C.angustifoliumsubsp.circumvagumhas higher light compensation point[11.848 μmol/(m2·s)]and low light saturation point[800 μmol/(m2·s)], and its light adaptability is the weakest.C.angustifoliumsubsp.circumvagumis a typical sciophilous plant. This result is consistent with the distribution habitats (R.wardiishrub and spruce forest margin) ofC.angustifoliumsubsp.circumvagumin Mount Shergyla.C.angustifoliumandC.latifoliumhave higher light compensation points and higher light saturation points, indicating weak adaptability to the light environment. This can also explain the narrow distribution ofC.angustifoliumandC.latifoliumin Mount Shergyla.

With the increase of light intensity, the Pn, Gs and Tr ofC. spp. increased significantly, and the Ci declined. The increase of Pn is favorable for CO2assimilation, producing more dry matter. The experimental results also show that the changes of Gs are basically consistent with the changes of Pn and Tr. This is because the plant can adjust the opening and closing of the stomata in response to changes in the external environmental conditions and the plant’s own conditions, and maintain maximum CO2fixation and minimum water loss under the premise of maintaining normal plant activity. Tr is the amount of water that a plant transpires per leaf area over a given period of time. Illumination is the most important external condition affecting transpiration. It can not only increase the temperature of the atmosphere but also increase the temperature of leaves. The increase in atmospheric temperature enhances the evaporation rate of water.

In general, there are certain differences in the responses of photosynthetic physiological parameters ofC. spp. to different PARs. The Pn ofC.conspersumandC.latifoliumdiffers insignificantly, but it is higher than that ofC.angustifolium. The Pn ofC.angustifoliumsubsp.circumvagumis the lowest. The Gs ofC.conspersumandC.latifoliumdiffers insignificantly, but it is higher than that ofC.angustifoliumsubsp.circumvagum. The Gs ofC.angustifoliumis the lowest. There is no significant difference in Ci betweenC.angustifoliumandC.latifolium, of which the Ci is higher than that ofC.conspersum. The Ci ofC.angustifoliumsubsp.circumvagumis the lowest.C.angustifoliumsubsp.circumvagumshows the highest Tr, followed byC.angustifoliumandC.conspersum(no significant difference), andC.latifoliumhas the lowest Tr.