Abstract
The biomass of Isodon rubescens stems is greater than that of the leaves. The stems possess a considerable surface area, although less than that of the leaves. The photosynthetic rates, light response curves and chlorophyll fluorescence characteristics of the stems were determined in this study to clarify their photosynthetic capacity and photosynthetic potential. The results showed that the I. rubescens stems possessed considerable photosynthetic capacity, although less than that of the leaves. The shape of the light response curve of the I. rubescens stem was different from that of leaf. The light response curve of stems slowly increased during the intermediate growth period. The light saturation point of the stems was significantly greater than that of the leaves. There was clear, strong light suppression in both stems and leaves. However, I. rubescens stems could effectively photosynthesize, and the stem has a high light saturation point and can adapt to intense light.
Keywords: Non-leaf tissue photosynthesis, Electron transport rate, Rabdosiae Rubescentis Herba
Subject terms: Physiology, Plant sciences
Introduction
Isodon rubescens (Hemsley) H. Hara is a medicinal plant belonging to the genus Isodon of the Lamiaceae family1. The entire I. rubescens plant is used as the Chinese medicinal material Rabdosiae rubescentis Herba2,3. Rabdosia rubescentis herba is used for the treatment of sore throats, throat inflammation and oesophageal cancer in traditional Chinese medicine. I. rubescens is a deciduous perennial subshrub that is widely distributed in China1. The upper parts of I. rubescens stems are herbaceous and wither in winter. The middle and lower parts of I. rubescens stems are woody and can sprout the next spring. There are few branches on the upper part of the I. rubescens stem. The majority of branches grow from the lower part of the stem.
The main site of plant photosynthesis is the leaves. However, the stems of many plant species, particularly herbaceous plants, can perform photosynthesis4,5. Stem photosynthesis in Triticum aestivum L. can significantly increase yield6,7. The stems of some plants, such as Opuntia dillenii (Ker Gawl.) Haw., Acacia confusa Merr., Ruscus aculeatus L., Casuarina equisetifolia L., Ephedra sinica Stapf and Equisetum ramosissimum Desf., are the main sites of photosynthesis8,9. The biomass of I. rubescens stems is greater than that of I. rubescens leaves. The area of I. rubescens stems is approximately one third the area of I. rubescens leaves. One-year-old I. rubescens stems are green and should contribute to photosynthesis.
There are few reports on the photosynthetic rate or capacity of I. rubescens stems. The photosynthetic rates, light response curves and chlorophyll fluorescence characteristics of I. rubescens stems were determined in this study. For comparison, these indices were simultaneously determined for I. rubescens leaves. This study clarifies the photosynthetic capacity and photosynthetic potential of I. rubescens stems. The results of this study revealed the photosynthetic physiology and provide a theoretical foundation for the rational cultivation and management of I. rubescens.
Results
The photosynthetic capacity of different stems and leaves varied widely. The average photosynthetic rate of I. rubescens stems was clearly lower than that of I. rubescens leaves under the same light intensity (Fig. 1); the rates of I. rubescens stems were approximately 60% of the rate of I. rubescens leaves.
Fig. 1.
The photosynthetic rates of Isodon rubescens stems and leaves under the same light intensity.
The light response curve of I. rubescens stems was very different from that of I. rubescens leaves (Fig. 2). The light response curve of the leaves sharply increased and then stabilized with increasing light intensity in the early and middle growth periods. The light response curve of the leaves sharply declined in the later growth period. The light saturation point of I. rubescens leaves was approximately 700 µmol·m− 2·s− 1. The light response curve of I. rubescens stems rapidly increased during the early period of increasing light intensity and then slowly increased during the intermediate period of increasing light intensity. The light response curve of stems rapidly decreased in the later period of increasing light intensity (Fig. 2). The light saturation point of the stems was approximately 2000 µmol·m− 2·s− 1, which was clearly higher than that of the leaves (Table 1). The light compensation point of the stems was slightly higher than that of the leaves. The dark respiration rate and the maximum photosynthetic rate of the stems were lower than those of the leaves (Table 1).
Fig. 2.
The light response curve of Isodon rubescens stems and leaves.
Table 1.
The fitted results of the light response curve of Isodon rubescens stems and leaves.
| Parameter | Stem 1 | Stem 2 | Stem 3 | Leaf |
|---|---|---|---|---|
| E | 0.0028737 | 0.003666903 | 0.0034667 | 0.0450921 |
| M | 0.0002407 | 0.000169523 | 0.00020623 | 0.000245 |
| N | 0.0001450 | 0.000350839 | 0.00024919 | 0.0070127 |
| LCP (µmol·m− 2·s− 1) | 21.594453 | 20.1941838 | 22.8666455 | 19.194446 |
| LSP (µmol·m− 2·s− 1) | 1846.8869 | 2161.14263 | 1967.4344 | 684.15968 |
| DRR (µmol·m− 2·s− 1) | 0.062057 | 0.07405011 | 0.079271 | 0.865519 |
| MPSR (µmol·m− 2·s− 1) | 2.297783 | 2.82926992 | 2.688083 | 4.304935 |
| R | 0.962523 | 0.980734 | 0.981626 | 0.99251 |
E is the apparent quantum yield. M and N are parameters. LCP is the light compensation point. The DRR is the dark respiration rate, LSP is the light saturation point, and MPSR is the maximum photosynthetic rate. R is correlation coefficient.
There was strong light suppression in the rapid light curve of chlorophyll fluorescence in I. rubescens stems (Fig. 3). The maximum electron transport rates of the stems were all lower than those of the leaves (Table 2). The minimum saturation of light intensity for the stems was generally higher than that of the leaves. The initial slopes of the stems were lower than those of the leaves.
Fig. 3.
The rapid light curve of chlorophyll fluorescence in Isodon rubescens stems and leaves.
Table 2.
The fitted results of the rapid light curve of chlorophyll fluorescence in Isodon rubescens stems and leaves.
| Sample | a | b | c | Alpha | ETRmax (µmol·m− 2·s− 1) |
Ik | R |
|---|---|---|---|---|---|---|---|
| Stem 1 | 0.000008252 | 0.013782 | 4.3881 | 0.2279 | 38.733351 | 729.22 | 0.987568 |
| Stem 2 | 0.000015070 | 0.006668 | 5.1483 | 0.1942 | 41.178891 | 584.49 | 0.963187 |
| Stem 3 | 0.000007470 | 0.012072 | 4.9537 | 0.2019 | 41.257808 | 814.34 | 0.993705 |
| Leaf | 0.000007898 | 0.010883 | 3.1302 | 0.3195 | 48.013721 | 629.55 | 0.993927 |
The a, b and c are the parameters in the model, Alpha is the initial slope, Ik is the minimum saturation of the light intensity, and ETRmax is the maximum electron transport rate. R is correlation coefficient.
Discussion
The photosynthetic capacity of plant stems is generally lower than that of leaves5,10. However, the biomass of the stem is a large part of the plant biomass. The photosynthetic products of stems can provide some materials and energy for the growth and development of plants6,7,11.
There were many branches on the lower part of the I. rubescens plant. The stems were all green and could photosynthesize in the first year. The photosynthetic capacity of stems was lower than that of the leaves. The maximum photosynthetic rate of the stems was less than that of the leaves. The rapid light curves of chlorophyll fluorescence in the I. rubescens stems and leaves showed the same trend. The photosynthetic cells in the stem are located on the inner surface of the epidermis. There are some cells in the interior of I. rubescens stems that could not photosynthesize, such as vascular tissue and pith. In contrast, most of the cells in the I. rubescens leaf could photosynthesize. Therefore, the photosynthetic capacity of I. rubescens leaves was greater than that of stems.
The thickness of the I. rubescens stems was obviously greater than that of the leaves, but intense light could penetrate into the interior of the stems. Some cells in the interior of the stems could photosynthesize under intense light. Therefore, the light saturation point of I. rubescens stems was greater than that of leaves. The intense light adaptability of the stem was superior to that of the leaf. The rapid light curves of chlorophyll fluorescence in the stems and leaves confirmed that the stems could adapt to intense light. This phenomenon has been observed in other plants4. The light compensation point of the stems was slightly higher than that of the leaves. The metabolism of the stems was lower than that of leaves. Therefore, the dark respiration rate of I. rubescens stems was lower than that of leaves. The stem even could utilize the CO2 came from the respiration of internal tissues in photosynthesis12.
The I. rubescens stems possessed a high light compensation point and could take advantage of intense light. This should be a consideration in the cultivation of I. rubescens.
Conclusion
The I. rubescens stems could effectively photosynthesize. Although the photosynthetic capacity of stems was lower than that of leaves, the photosynthetic products of stems could provide some materials and energy for the growth and development of the plant. The stem has a high light saturation point and can adapt to intense light. Photosynthesis in I. rubescens stems significantly promoted plant growth.
Materials and methods
Materials
I. rubescens is not an endangered or protected species in China. The methods for the collection of plant materials and the performance of experimental research on I. rubescens plants complied with the national guidelines of China. The seeds of I. rubescens were collected from the cultivated plants and sown at the experimental site in Xinxiang, Henan Province, China, in March 2022. These I. rubescens seeds germinated one week after sowing and developed into plants in the same year. More than 100 I. rubescens plants were transplanted at a 20 × 30 cm planting density in a sunny experimental site in March 2023. The I. rubescens plants were 40–80 cm in height in May 2024. There were 2–8 branches on the lower part of each I. rubescens stem.
Methods
More than 30 I. rubescens plants were randomly selected for sampling on May 17, 2024. Approximately 3–5 stems of each plant were randomly selected to study their photosynthetic characteristics. Adjacent 10–12 stems were placed side by side. These adjacent stems could completely fill the leaf chamber of the Li-6400 photosynthesis system or the leaf clamp of the PAM-2500 portable chlorophyll fluorescence apparatus.
The photosynthetic rates of the I. rubescens stems were determined with a Li-6400 photosynthesis system. The light intensity was set at 1000 µmol·m− 2·s− 1, the CO2 density was set at 450 µmol·mol− 1, the flow rate of gas in the leaf chamber was set at 500 µmol·s− 1, and the temperature of the leaf chamber was set at 30 °C during measurements. The determination was repeated six times with six sets of I. rubescens stems.
The light response curves of the I. rubescens stems were determined with a Li-6400 photosynthesis system. The CO2 density was set at 450 µmol·mol− 1, the flow rate of gas in the leaf chamber was set at 500 µmol·s− 1, and the temperature of the leaf chamber was set at 30 °C when the light response curves of the stems were determined. The light intensities in the leaf chamber at each stage of the light response curve were set at 2500, 2200, 2000, 1800, 1600, 1400, 1200, 1000, 800, 600, 400, 200, 150, 50, 20 and 10 µmol·m− 2·s− 1. The determination was repeated three times with three sets of I. rubescens stems.
The rapid light curves of chlorophyll fluorescence of the I. rubescens stems were determined with a PAM-2500 portable chlorophyll fluorescence apparatus. These stems were subjected to dark adaptation for 30 min before determination of the rapid light curves of chlorophyll fluorescence. The determination was repeated six times with six sets of I. rubescens stems.
Data analysis
The light response curves of the stems were fitted with a modified rectangular hyperbola model13,14. The modified rectangular hyperbola model is shown below:
The PSR is the net photosynthetic rate, LI is the light intensity in the leaf chamber, LCP is the light compensation point, E is the apparent quantum yield, and M and N are parameters in the model. The dark respiration rate (DRR) was calculated as E*LCP. The light saturation point (LSP) was calculated according to the following formula:
The maximum photosynthetic rate (MPSR, the net photosynthetic rate at the light saturation point) was calculated according to the following formula:
The rapid light curve of chlorophyll fluorescence was automatically fitted by the apparatus according to the model of Eilers and Peeters15 as shown below.
The ETR is the electron transport rate in photosynthetic system II, FI is the fluorescence intensity, and a, b and c are parameters in the model.
The initial slope (Alpha) was calculated according to the following formula15:
The minimum saturation of the light intensity (Ik) was calculated according to the following formula:
The maximum electron transport rate (ETRmax) was calculated according to the following formula:
All data were analysed with Statistical Product and Service Solutions (SPSS, International Business Machines Corporation, USA).
Author contributions
JIAN Zaiyou designed the study, implemented the experiment and wrote the manuscript.JIAN Susu wrote and revised the manuscript.TANG Xiaomin performed the experiment. HU Genhai participated in the data analysis.
Data availability
The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.
Declarations
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
References
- 1.Flora of China Committee. Flora of China (Volume 66) Chinese Science. 457–458. (1979).
- 2.Chinese Pharmacopoeia Committee. Pharmacopoeia of the People’s Republic of China. Part I. Chinese Chemical Industry Press, Beijing. 115–116, (2020).
- 3.Sun, H. D., Huang, S. X. & Han, Q. B. Diterpenoids from Isodon species and their biological activities. Nat. Prod. Rep. 23, 673–698 (2006). [DOI] [PubMed] [Google Scholar]
- 4.Yuangang, Z. U., Zhonghua, Z. H. A. N. G. & Wenjie, W. A. N. G. HE Haisheng. Different characteristics of photosynthesis in stems and leaves of Mikania micranth. J. Plant. Ecol. 30 (6), 998–1004 (2006). [Google Scholar]
- 5.Wei Luping, Z., Qingping, C. & Youjun, ShaoYuqiao, T. Wang Hui. Research progress on photosynthetic contribution of non-leaf green organs in plants. Plant Sci. J. 40 (2), 269–280 (2022). [Google Scholar]
- 6.Chao, W. A. N. G., Jichun, T. I. A. N., Zhaoguo, Q. I. A. N. & Xinjin, C. O. N. G. WANG Ruixi. Relationship of different photosyntheitc organ with grain yield of different wheat varieties. Shandong Agric. Sci. 3, 25–29 (2011). [Google Scholar]
- 7.Weiguo, C. H. E. N., YANGJinwen, S. H. I., Yugang, WANGSuguang, L. I. H. & SUN Daizhen. Effects of defoliating or shielding photosynthesis organs on yield-related traits and kernel morphological traits in wheat. J. Shanxi Agric. Sci. 48 (8), 1227–1230 (2020). [Google Scholar]
- 8.Zengyan, C. H. E. N. et al. Effects of salt and phosphorus fertilization on photosynthesis and nutrient characteristics of Acacia confuse. Guihaia 43 (4), 596–605 (2023). [Google Scholar]
- 9.Ming, L. I. N. Y., QiRong, G. U. O., GongFu, Y. E., Ling, L. I. N. & LIN Peng. Characteristics of matter and energy of some Casuarinaceae species in Dongshan County, Fujian Province. Acta Ecol. Sin.. 24 (10), 2217–2224 (2004). [Google Scholar]
- 10.Aili, W. E. I. & YingHua, Z. H. A. N. G. WANG ZhiMin. Dynanlic characteristics of photosynthetic rate and carbon assimilation enzyme activities of different green organs in dffierent genotypes of wheat. Acta Agron. Sin. 33 (9), 1426–1143 (2007). [Google Scholar]
- 11.Haolin, L. I. A. N. G., Yaping, Z. H. E. N. G., Zhaoyang, J. I. A. N. G. & LI Weihua. Stem elongation characteristics of Mikania micrantha and its physiological basis under low light condition. J. Trop. Subtrop. Bot. 30 (1), 70–78 (2022). [Google Scholar]
- 12.Kocurek, M., Tokarz, K. & Pilarski, J. Stem photosynthesis and its importance to the carbon budgets of plants. Wiad. Bot. 53, 7–20 (2009). [Google Scholar]
- 13.YE ZP. Application of light-response model in estimating the photosynthesis of super-hybrid rice combi nation-II youming 86. Chin. J. Ecol. 26 (8), 1323–1326 (2007). [Google Scholar]
- 14.Zaiyou, J. I. A. N. et al. Comparison of photosynthetic and fluorescence characteristics among taxa in Paeonia sect. Paeonia. Chin. J. Plant. Ecol. 34 (12), 1463–1471 (2010). [Google Scholar]
- 15.Eilers, P. H. C. & Peeters, J. C. H. A model for the relationship between light intensity and the rate of photosynthesis in phytoplankton. Ecol. Model. 42, 199–215 (1988). [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.