Introduction

Isodon rubescens (Hemsley) H. Hara is a perennial subshrub belonging to a genus of the Lamiaceae family1. There are several bioactive chemical components in I. rubescens, such as oridonin and ponicidin. The dry aerial portions of I. rubescens are named rabdosiae rubescentis herba and are used in traditional Chinese medicine for the treatment of sore throats, inflammation and gastrointestinal problems2,3.

The ecological and economic cultivation of I. rubescens is currently being carried out. However, there are different environmental conditions in different places. Wild I. rubescens grows on mountains or hills. There are obvious differences between the environmental conditions of mountains and plains. The demand of I. rubescens for light intensity should be made clear. The photosynthetic and chlorophyll fluorescence characteristics of I. rubescens were studied in this research to define the most suitable environmental conditions for I. rubescens cultivation and improve cultivation techniques.

Results

Diurnal variation in I. rubescens leaf photosynthesis

The results of the diurnal variation in I. rubescens leaf photosynthesis are shown in Table 1. Based on the collected data, the curve of the diurnal variation in I. rubescens leaf photosynthesis is shown in Fig. 1.

Table 1 Diurnal variation in I. rubescens leaf photosynthesis (average).
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Figure 1
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Diurnal variation in I. rubescens leaf photosynthesis.

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The diurnal variation in I. rubescens leaf photosynthesis indicates that there was no obvious midday depression of photosynthesis. There is still a high net photosynthetic rate in I. rubescens leaves at noon with high light intensity. Leaves of I. rubescens can utilize very faint light, e.g., with a 20 µmol·m−2·s−1 intensity. The leaves of I. rubescens can photosynthesize even in the faint light of evening.

Light response curve of I. rubescens leaves

The data from the light response curve of I. rubescens leaf photosynthesis are shown in Table 2. Based on the collected data, the curve of the light response of I. rubescens leaf photosynthesis is shown in Fig. 2.

Table 2 Light response curve of I. rubescens leaf photosynthesis (average).
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Figure 2
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Light response curve of I. rubescens leaf photosynthesis.

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The light response curve of I. rubescens leaf photosynthesis indicates that the net photosynthetic rate was obviously related to the light intensity when the light intensity was low. The net photosynthetic rate of I. rubescens leaves rapidly increased as the light intensity increased from 20–400 µmol·m−2·s−1. I. rubescens leaves were able to utilize intense light. With light intensities of 400–2200 µmol·m−2·s−1, the net photosynthetic rate of I. rubescens leaves was high. However, the net photosynthetic rate of I. rubescens leaves obviously decreased when the light intensity was above 2200 µmol·m−2·s−1.

The results of the light response curve fitted with the modified rectangular hyperbola model are shown in Table 2. The fitted light saturation point and the net photosynthetic rate at this point were very similar to the observed value.

CO2 response curve of I. rubescens leaves

The CO2 response curve data of I. rubescens leaf photosynthesis are shown in Table 3.

Table 3 CO2 response curve of I. rubescens leaf photosynthesis (average).
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Based on the collected data, the curve of the CO2 response of I. rubescens leaf photosynthesis is shown in Fig. 3.

Figure 3
figure 3

CO2 response curve of I. rubescens leaf photosynthesis.

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The CO2 response curve of I. rubescens leaf photosynthesis indicates that the net photosynthetic rate was obviously related to the concentration of CO2 in the air when the CO2 concentration was below 1000 µmol·mol−1. However, the effect of the CO2 concentration on the net photosynthetic rate was not obvious when the concentration of CO2 was above 1000 µmol·mol−1.

The results of the CO2 response curve fitted with the modified rectangular hyperbola model are shown in Table 3. The fitted CO2 saturation point and the net photosynthetic rate at this point were very similar to the observed value.

Chlorophyll fluorescence characteristics of I. rubescens leaves

The results of the slow kinetics of chlorophyll fluorescence are shown in Table 4.

Table 4 Slow kinetics of chlorophyll fluorescence.
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The slow kinetics of chlorophyll fluorescence of I. rubescens leaves indicates that the maximal photochemical efficiency of photosystem II ((Fm-Fo)/Fm) in I. rubescens leaves reached 0.7. The electron transport rate of photosystem II in I. rubescens leaves reached 20 μmol electrons/(m2·s). The fraction of energy dissipated as heat via the regulated photoprotective NPQ mechanism (Y(NPQ)) was much more than that passively dissipated in the form of heat and fluorescence (Y(NO)).

The results of the rapid light curves of chlorophyll fluorescence in I. rubescens leaves are shown in Table 5. The rapid light curve of chlorophyll fluorescence in I. rubescens leaves is shown in Fig. 4.

Table 5 Rapid light curve of chlorophyll fluorescence in I. rubescens leaves (average).
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Figure 4
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Rapid light curve of chlorophyll fluorescence in I. rubescens leaves.

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The rapid light curve of chlorophyll fluorescence in I. rubescens leaves was automatically fitted with a PAM-2500 portable chlorophyll fluorescence apparatus according to the model of Eilers and Peeters [5]. The fitted results are shown in Table 5.

The rapid light curve of chlorophyll fluorescence in I. rubescens leaves indicates that the maximum quantum yield of PSII with a saturated pulse after dark adaptation (Fv’/Fm’ x ETR factor/2) was higher than the effective quantum yield of PSII (Y(II)). The initial slope (alpha) signifying the maximum photosynthetic efficiency was higher than the apparent quantum yield fitted in the light response curve of I. rubescens leaves.

Discussion and Conclusion

The modified rectangular hyperbola model is suitable for fitting light response curves and CO2 response curves. We compared the fit of the light response curve and CO2 response curve of Paeonia lactiflora created with different models. It was found that the fit results based on the modified rectangular hyperbola model were more similar than the results from other models to the observed values6.

I. rubescens is a heliophyte plant, which can tolerate intense light. There are very few reports about photosynthesis of I. Rubescens. There was no obvious midday depression of photosynthesis in I. rubescens leaves in terms of this study. The midday photosynthetic depression occurred in most of plants. The factors such as intense light, high air temperature, low soil moisture, low air humidity and so on can cause midday photosynthetic depression7,8,9,10. There is no midday photosynthetic depression in some other plants, such as C4 plants (Characterized by the Hatch-Slack photosynthetic pathway), CAM plants (plants with crassulacean acid metabolism) and aquatic plant11,12. Some plants perform midday photosynthetic depression in a certain environment but express no midday photosynthetic depression in another environment. Their performances are affected by environment or some chemicals13,14,15,16. The environment of I. rubescens studied in this paper was consistent with that of yield I. rubescens. It was sunny day and the light intensity was highest in a year in the locality when the data were determined. I. rubescens performed no midday photosynthetic depression in the severe environment, which indicated that it would similarly perform in suitable environment. Therefore, I. rubescens can tolerate intense light.

There was no obvious difference between the net photosynthetic rate of light saturation point and that of light intensities of 2000 µmol·m−2·s−1 although the light saturation point of I. rubescens leaves was 802.262 µmol·m−2·s−1. Therefore, there was no obvious effect of intense light above the light saturation point on the photosynthesis of I. rubescens leaves. The net photosynthetic rate of the light intensities of 1484.135 µmol·m−2·s−1 was the highest in diurnal variation of photosynthesis because the temperature was suitable for it at that time. I. rubescens can also tolerate low light. Leaves of I. rubescens can utilize low light (i.e., at an intensity of 20 µmol·m−2·s−1). Therefore, I. rubescens can grow on shady slopes. The most important factor affecting the photosynthetic efficiency in I. rubescens leaves is the concentration of CO2 in the air. Photosynthesis in I. rubescens leaves was not obviously affected by high concentrations of CO2 alone.

The maximum electron transport rate (ETRmax) in I. rubescens leaves was far higher than the observed electron transport rate (ETR). The chlorophyll fluorescence characteristics of I. rubescens leaves showed that there was very large potential for photosynthesis in I. rubescens leaves. The fraction of energy dissipated as heat via the regulated photoprotective NPQ mechanism (Y(NPQ)) was much more than that passively dissipated in the form of heat and fluorescence (Y(NO)). The minimum saturation light intensity (Ik) was far less than the light saturation point (LSP). Therefore, I. rubescens leaves can tolerate intense light.

I. rubescens performs no midday photosynthetic depression and can tolerate intense light. It can utilize low light and possesses high value of Fv/ Fm (the maximal photochemical efficiency of photosystem II). This indicated that I. rubescens leaves have a strong photoprotective capacity. However, the growth and cultivation of I. rubescens are affected by many factors such as light, air temperature, rainfall, soil, and so on17,18. This study is aimed at the photosynthetic and chlorophyll fluorescence characteristics of I. rubescens. The suitable environment for the growth and cultivation of I. rubescens still needs to study.

Materials and Methods

Instruments

Li-6400 Photosynthesis system (LI-6400 Inc., Lincoln, NE, USA). PAM-2500 portable chlorophyll fluorescence apparatus (PAM-2500, Walz, Germany).

Materials

Approximately 60 I. rubescens plants were dug up from Taihang Mountain and evenly planted in 12 flowerpots (30 cm in diameter and 35 cm in depth) in March 2018. Then, the plants were irrigated to ensure that they grew well.

Determination of photosynthetic characteristics

The photosynthetic characteristics of mature leaves on the I. rubescens plants were determined on June 5–7 (sunny day, the light intensity is highest in a year), 2019. The concentration of CO2 in the air was approximately 370 µmol·mol−1 when the diurnal variation of photosynthesis was determined. The temperature of the leaf chamber was set at 30 °C, and the concentration of CO2 in the leaf chamber was set at 400 µmol·mol−1 when the light response curve was determined. The light intensity in the leaf chamber was set at 1200 µmol·m−2·s−1, and the temperature of the leaf chamber was set at 30 °C when the CO2 response curve was determined. These photosynthetic characteristics were determined with the Li-6400 Photosynthesis system. Each determination was repeated three times.

Determination of chlorophyll fluorescence characteristics

The fluorescence characteristics of mature leaves on the I. rubescens plants were determined on June 7–8, 2019. The leaves were under dark adaptation for 30 min before the determination of the chlorophyll fluorescence characteristics. The slow kinetics of chlorophyll fluorescence were determined before determining the light curve of chlorophyll fluorescence. The tests were repeated three times.

Data analysis

The light response curve and CO2 response curves were analysed with SPSS (Statistical Product and Service Solutions, International Business Machines Corporation, USA). The light response curve and CO2 response curve were all fitted with a modified rectangular hyperbola model4.

Modified rectangular hyperbola model:

$${\rm{Photo}}={\rm{E}}\cdot (1-{\rm{M}}\cdot {\rm{PAR}})\cdot ({\rm{PAR}}-{\rm{LCP}})/(1+{\rm{N}}\cdot {\rm{PAR}})$$

PAR is the value of light intensity in the light response curve (or the value of concentration of CO2 in CO2 response curve). Photo is the net photosynthetic rate. LCP is the light compensation point (or CO2 compensation point). E, M and N are parameters. E is also the apparent quantum yield. The dark respiration rate under the light compensation point = E·LCP. The light saturation point is calculated as follows:

$$({\rm{LSP}})=((({\rm{M}}+{\rm{N}})\cdot (1+{\rm{N}}\cdot {\rm{LCP}})/{{\rm{M}})}^{1/2})/-1)/{\rm{N}}.$$

The net photosynthetic rate under the light saturation point (LSP) or CO2 saturation point (CSP) can be calculated according to the model.

The data related to the determination of the light curve of chlorophyll fluorescence were automatically fitted according to the model of Eilers and Peeters5.

The model of Eilers and Peeters is as follows:

$${\rm{ETR}}\,={\rm{PAR}}/({\rm{a}}\cdot {{\rm{PAR}}}^{2}+{\rm{b}}\cdot {\rm{PAR}}+{\rm{c}})$$

ETR is the electron transport rate of photosynthetic system II. PAR is the fluorescence intensity. The letters a, b and c are parameters.