Photosynthetic and Chlorophyll Fluorescence Characteristics of Isodon rubescens (Hemsley) H. Hara

The ecological and economic cultivation of Isodon rubescens is currently being carried out. The demand of I. rubescens for light intensity should be made clear to estimate whether the environmental conditions of an area are suitable for cultivating I. Rubescens and improve cultivation techniques. The photosynthetic and chlorophyll fluorescence characteristics of I. rubescens were determined with a Li-6400 photosynthesis system and PAM-2500 portable chlorophyll fluorescence apparatus. The results showed that there was no obvious midday depression of photosynthesis in I. rubescens leaves. The light compensation point and light saturation point of I. rubescens leaves were 21.83482 µmol·m−2·s−1 and 802.262 µmol·m−2·s−1, respectively. The CO2 compensation point and CO2 saturation point of I. rubescens leaves were 101.7199 µmol·mol−1 and 1674.514 µmol·mol−1, respectively. 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). I. rubescens can tolerate intense light above the light compensation point and utilize low light. I. rubescens leaves have a strong photoprotective capacity. I. rubescens can grow in both sunny and shady places. The most important factor affecting photosynthetic efficiency in I. rubescens leaves is the concentration of CO2 in air.

www.nature.com/scientificreports www.nature.com/scientificreports/ 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. co 2 response curve of I. rubescens leaves. The CO 2 response curve data of I. rubescens leaf photosynthesis are shown in Table 3.
Based on the collected data, the curve of the CO 2 response of I. rubescens leaf photosynthesis is shown in Fig. 3.
The CO 2 response curve of I. rubescens leaf photosynthesis indicates that the net photosynthetic rate was obviously related to the concentration of CO 2 in the air when the CO 2 concentration was below 1000 µmol·mol −1 . However, the effect of the CO 2 concentration on the net photosynthetic rate was not obvious when the concentration of CO 2 was above 1000 µmol·mol −1 .
The results of the CO 2 response curve fitted with the modified rectangular hyperbola model are shown in Table 3. The fitted CO 2 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.
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/(m 2 ·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.
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.  www.nature.com/scientificreports www.nature.com/scientificreports/ 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 CO 2 response curves. We compared the fit of the light response curve and CO 2 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 values 6 .
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,       www.nature.com/scientificreports www.nature.com/scientificreports/ high air temperature, low soil moisture, low air humidity and so on can cause midday photosynthetic depression 7-10 . There is no midday photosynthetic depression in some other plants, such as C 4 plants (Characterized by the Hatch-Slack photosynthetic pathway), CAM plants (plants with crassulacean acid metabolism) and aquatic plant 11,12 . 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 chemicals [13][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  Table 5. Rapid light curve of chlorophyll fluorescence in I. rubescens leaves (average). Note: Fv'/Fm' x ETR factor/2 is the maximum quantum yield of PSII with a saturated pulse after dark adaptation. Alpha is the initial slope. ETRmax is the maximum electron transport rate. Ik is the minimum saturation of the light intensity. www.nature.com/scientificreports www.nature.com/scientificreports/ efficiency in I. rubescens leaves is the concentration of CO 2 in the air. Photosynthesis in I. rubescens leaves was not obviously affected by high concentrations of CO 2 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 on 17,18 . 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. The concentration of CO 2 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 CO 2 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 CO 2 response curve was determined. These photosynthetic characteristics were determined with the Li-6400 Photosynthesis system. Each determination was repeated three times. ETR is the electron transport rate of photosynthetic system II. PAR is the fluorescence intensity. The letters a, b and c are parameters.