Environment is the condition for the survival and development of plants. There are many factors that are not conducive to the growth of plants in alpine environment, such as large temperature difference, low air pressure, strong wind, strong solar radiation and low temperature stress. Plants living in this extreme environment will inevitably form their special physiological and ecological adaptation mechanism. Mountainous vertical zone is the epitome of horizontal zone, with large change gradient and sensitive response to climate factors, which has always been the focus of vegetation ecologists. As the survival boundary of tree species in extreme mountain environment, subtle changes of various environmental factors have great influence on the growth and yield of trees and other vegetation. Therefore, it is considered as an ideal "monitor" of global climate change and a natural laboratory to study the relationship between vegetation and climate. Since the 20th century, judging from the changes in the position and pattern of the high line, global climate change has indeed affected the altitude of the forest line, which will inevitably affect the tree species, growth and renewal of the forest line. Therefore, with the intensification of climate change, the response and adaptability of high-speed plants to environmental changes have become the focus of research, while tree planting is completely different from herbaceous plants, with a long life cycle, and its response to climate change has its special significance, and this research has also received great attention. Because climate change will first affect the individual adaptability of tall tree species, and then it will cause the change of the whole forest line, and the adaptation of individual trees in morphological anatomy and physiological ecology is very important for the survival of forest line tree species. In recent years, there have been reports about the formation mechanism of Gaolinshan line and the physiological and ecological characteristics of high altitude plants in China. Wang Xiangping also suggested that the climatic factor limiting the distribution height of forest lines in China is the temperature in the growing season. Based on the author's previous work on Picea crassifolia and Sabina przewalskii in Qilian Mountain in recent two years, the research progress of adaptation strategies of highly linear tree species was summarized from two aspects: leaf morphological anatomy and physiological ecology, which provided reference information for further understanding the physiological and ecological characteristics of highly linear tree species and studying the dynamic changes and coping strategies of highly linear tree species under climate change.
1 Morphological and anatomical adaptability of alpine plants
Due to the active response and adaptation of leaf morphology and anatomical structure to environmental factors, leaf morphology, epidermal characteristics and basic tissue structure will undergo adaptive changes. However, the environment of the high-speed line is extremely harsh, and the leaf morphological structure of trees growing near it must have its unique adaptive characteristics.
L. 1 Morphological and anatomical characteristics of leaves
Water, temperature, light and CO2 concentration are very important environmental factors that affect the variation of plant morphology and anatomical structure, so the response and adaptation of plants to them has gradually become an active field in the research work of botanists and ecologists. From the size, thickness, shape, stomata and cell arrangement of leaves or needles, it shows adaptability to the environment. At present, the research on the leaf structure characteristics of alpine plants mainly focuses on herbs and shrubs, but there are few reports on evergreen conifers. Plants that grow in alpine areas for a long time have specialized leaves in the form of scales, strips, columns or needles, and the leaves become smaller and thicker. The research shows that the small and thick leaves are the response changes of plants to drought and strong radiation (including UV-B) stress, which is the adaptation to water shortage and strong UV-B radiation caused by low temperature in alpine environment.
Stomata, as the main channel for plants to communicate with the external environment, can be adjusted to control the exchange of water and gas to adapt to the adverse environment. Water deficit caused by low temperature in alpine environment will inevitably affect the distribution characteristics, density and stomatal conductance of plants. Leontopodioides is an alpine plant. With the elevation, stomatal density increases, stomatal area decreases and stomatal opening increases, which can enhance the gas exchange ability between leaves and the external environment, increase CO2 intake and increase photosynthetic rate, which is the adaptation of plants to the low partial pressures of CO2 and O2 in alpine environment. With the elevation, the medicinal plant Sedum crassipes not only thickens its leaves and increases its stomatal density, but also increases the number of glandular hairs and non-glandular hairs on its epidermis, which is also the result of adapting to low temperature and strong radiation in alpine regions.
In addition, the study also shows that the alpine environment has changed the palisade tissue and sponge tissue of alpine plants, and their differentiation degree can indirectly reflect the water state in the environment. Similar to the leaf structure of early growing plants, palisade tissue is multi-layered, sponge tissue is reduced, and there are large cell gaps, which may be related to the low temperature in alpine regions. However, the observation of the leaf structure of Rhodiola sachalinensis shows that. Tanggutaka shows that the leaves of this plant have no obvious palisade tissue, and the intercellular part accounts for a high proportion of the total tissue volume. The anatomical study on the leaves of Sabina przewalskii also found that the mesophyll cell gap of Sabina przewalskii was large and the ventilation tissue was developed. This shows that the arrangement of cells in the leaves of high altitude plants is looser than that of low altitude plants, and the gap between cells is larger, thus increasing the internal surface area of leaves and expanding the photosynthetic area, which is conducive to improving photosynthetic efficiency, ensuring the growth and development of alpine plants in a short growing season, and plays an important role in enhancing the cold tolerance of alpine plants.
Ultrastructural characteristics of 1.2 leaves
The research on the ultrastructure of alpine plants leaves mainly focuses on chloroplasts and mitochondria, and there are few reports on other organelles. Chloroplast and mitochondria, as the main organelles of photosynthesis and respiration in leaves, play an important role in plant growth and adaptation to the environment. When the external environment changes, the size, distribution and location of both will change. Under normal circumstances, chloroplasts in higher plants are mostly oval or spindle-shaped and distributed along the cell wall. With the elevation, the chloroplasts of alpine plants changed from regular ellipsoid to spherical or nearly spherical, the number increased, the volume decreased, and the distribution tended to move to the cell center. Light is one of the important factors affecting chloroplast development, so chloroplast rounding may be related to strong radiation. The study of alpine herbs Aaenariatapanchanesis and gymnospermum nudiflorum also found that the accumulation degree of chloroplast granules was reduced, which could prevent plants from damaging mesophyll tissue in high altitude and strong radiation environment. In the study of Sabina przewalskii, a highly linear tree species, it was found that the partial deformation of chloroplasts, the increase of lipid globules in chloroplasts, the increase of the number of lipid globules in chloroplasts and their mutual aggregation were also an adaptation of plants to the cold environment. Similarly, the smaller the individual mitochondria of alpine plants, the more the number, which can increase the relative surface area of mitochondrial membrane and expand the intima system, thus improving the respiration rate of plants and ensuring the energy supply under the stress ecological conditions. In addition, He Tao and others found that the number of mitochondria of four alpine herbs in Qinghai-Tibet Plateau not only increased generally, but also appeared the phenomenon that chloroplasts swallowed mitochondria, which was considered to be the result of adapting to the special ecological conditions in mountainous areas. However, there are relatively few reports on the ultrastructure of chloroplasts and mitochondria in tall trees, which need further study.
In addition, the distribution and quantity of starch granules in chloroplasts can indirectly reflect the environmental conditions of plants. Plants that grow in alpine areas for a long time often have more starch granules in chloroplasts, and some even have huge starch granules. At high altitude, Sabina przewalskii will accumulate a lot of starch grains in chloroplasts during the growing season, but it will decrease during dormancy, which shows that the accumulation of starch grains is an adaptation of alpine plants to low temperature environment. But k? Rner believes that the formation of starch granules in chloroplasts is the result of oversupply of carbon during photosynthesis.
2. Physiological and ecological adaptability of alpine plants
2. 1 Photosynthetic characteristics of alpine plants
Due to the coexistence of various unfavorable factors in the alpine environment, the changes of atmospheric CO2 concentration, ultraviolet radiation, low temperature and other conditions will affect the photosynthesis of individual plants, thus affecting the survival of the whole population.
CO2 is the raw material of plant photosynthesis, and its concentration change has a great influence on global ecological environment and climate change, and also has a direct effect on plant growth. The altitude near Gaolinshan Line is high, and the atmospheric CO2 content is low. In theory, the photosynthetic performance of plants should be reduced. But k? Rner study found that the maximum photosynthetic rate per unit leaf area of high-line plants was not lower than that of low-altitude plants, and the carbon fixation efficiency per unit area was higher than that of low-altitude plants. There may be two reasons for this: on the one hand, the diffusion rate of CO2 increases with the elevation, which makes up for the lack of partial pressure of CO2; On the other hand, the nitrogen content per unit area in high altitude areas increased, and the nitrogen content was positively correlated with the protein content. Most of these protein participated in the photosynthesis process. Using the δ value to study the photosynthesis of plants at different altitudes, it was found that the δ value increased with the elevation, indicating that high altitude plants had higher CO2 fixation efficiency. In addition, through the study of the CO2 compensation point of photosynthesis, it was found that the CO2 compensation point of Quercus flavescens in Gongga Mountain decreased with the elevation, which proved that one of the main reasons why high altitude plants can adapt to low CO2 partial pressure is that they have low CO2 compensation point. Thus, high altitude and low CO2 concentration are not the main factors limiting the photosynthetic performance of alpine plants. However, although the low CO2 concentration does not limit the photosynthetic capacity of forest tree species, the future climate change trend is the increase of CO2 concentration and temperature, and alpine plants will inevitably have corresponding coping strategies. The study on increasing CO2 concentration by artificially controlling larch and masson pine near forest line for 9 years shows that the increase of CO2 concentration will promote the radial growth of forest line trees and the accumulation of photosynthetic products, and larch is more responsive to CO2 than masson pine.
Solar radiation is more intense at high altitude, which is another important factor affecting photosynthesis of alpine plants. Due to the production of a large number of greenhouse gases, the temperature and cloud cover have changed abnormally, which has broken the balance of solar radiation in the global atmosphere and caused serious secondary radiation threats to plant growth. However, some alpine plants (such as Quercus huangbei in Gongga Mountain) can make full use of light energy for photosynthesis because of their low light compensation point and high light saturation point, so high light intensity will not cause "photosynthetic lunch break" of low-altitude plants. At the same time, the unique photosynthetic pigment composition characteristics of alpine plants also promote their adaptation to the alpine environment. The ratio of chlorophyll a to chlorophyll b of alpine dicotyledonous herbs is generally between 4.2 and 5.3, while that of lowland plants is generally between 3.6 and 4. L, its carotenoid content also increased with the elevation. Through the study of Lathyrus spinulosus at altitudes of 4300 m and 4500m, it was found that its UV-B absorption of pigments, carotenoids, chlorophyll A and B contents and chlorophyll a/b value were higher in winter than in summer, which indicated that the change of plant photosynthetic pigments played a very important role in the adaptation of alpine plants to strong radiation environment. The author also found that the contents of chlorophyll a and chlorophyll b increased with the elevation, and the a/b value increased (data to be published). These studies show that the change of photosynthetic pigment composition is a typical feature of alpine plants adapting to strong light and high radiation habitats. In addition, different wavelengths of light will also affect the growth and development of plants and the formation of chlorophyll. Because of the complex light environment in which tall linear plants live, light has great influence on their morphogenesis. However, the research in this field mostly focuses on the influence of light intensity on plant growth, while the influence of light quality on alpine plants is rarely reported, such as the development process and expression difference of chloroplast protein. At present, the author's research group is conducting research in this field.
As the main climatic factor affecting the height of forest line distribution, temperature plays an important role in plant photosynthetic physiology. Alpine low temperature affects the activities of photosynthesis-related enzymes, but alpine plants have low photosynthetic optimum temperature and critical temperature because of their super cold tolerance. It is found that although the temperature in mountainous areas is very low, the temperature in the growing season is not the main limiting factor affecting plant photosynthesis. The low optimum temperature of photosynthesis in alpine plants is one of the main reasons why they can maintain the photosynthetic rate in alpine low temperature environment. It has been proved that the adaptation of the optimum temperature of plant photosynthesis to low temperature is mainly realized by the change of plant photosynthetic organelles, and it is related to the electron transfer in thylakoid membrane, especially to photosystem II. In addition, the adaptation of alpine plants to low temperature is also manifested in photosynthetic enzyme activity and photosynthetic potential. The photosynthetic characteristics of Polygonum viviparum at different altitudes were studied. It was found that the photosynthetic enzyme activity and the values of FV/F0 and FV/Fm of chloroplasts were higher at high altitude than at low altitude. This shows that altitude does not affect the photosynthetic potential of alpine plants.
However, because the field photosynthesis of highly linear tree species, especially arbor tree species, is limited by many conditions, there are relatively few reports in this regard. Liu Hongyan and others studied spruce, Larix principis-rupprechtii and Sequoia Taibai near the warm temperate forest line in eastern China, and found that the photosynthesis of tall trees was affected by light and temperature, and temperature was the main limiting factor when the light was weak. However, this research result has also been questioned by relevant researchers, which also shows the difficulty of studying photosynthetic characteristics of tall tree species. Recently, a study on Abies Abies along the Gaolin Mountain in Sejila, southeastern Tibet, found that under natural conditions, the diurnal process of photosynthesis showed a bimodal curve, and stomata was the dominant factor leading to lunch break. This is completely contrary to the previously reported phenomenon that alpine herbs have no obvious "photosynthetic lunch break". Therefore, the photosynthetic characteristics of conifers in the forest line need a lot of experimental data to prove.
2.2 Characteristics of absorption and utilization of mineral elements by alpine plants
Mineral elements, including macroelements and microelements, are very important for the growth and development of plants. In alpine regions, nitrogen and phosphorus are usually considered as important factors affecting the distribution of plant communities and limiting primary productivity. The growth of plants near the high line is limited by the supply of soil nutrients, especially nitrogen and phosphorus, so the influence of soil available nutrients on the distribution position of forest line has always been concerned. The study of tropical tall plants in Ecuador showed that the contents of nitrogen and phosphorus in leaves decreased significantly with the increase of altitude. However, the content of nutrient elements available to plants in soil is not affected by altitude, but the utilization rate of N and P by plants decreases significantly with the increase of altitude. This also shows that the content of mineral nutrients in plant tissues is not directly related to the supply of nutrients, but largely depends on the way plants absorb nutrients, that is, whether plants can make full use of them is the key issue. Even if plants occupy a nutrient-rich area, if the absorption or transportation of nutrients is blocked, plants will be undernourished. The low soil temperature at high altitude limits the ability of roots to absorb nutrients, so even if the soil nutrient content near the forest line is high, plant growth will be limited. In addition to low temperature, different vegetation types will also affect the absorption of mineral elements. Zhang Lin and others studied the forms of P in soil under different vegetation types near the forest line, and found that the average content of total P in the soil in the study area was high, but the content of active P only accounted for 65,438+00%, indicating that the forms of soil organic phosphorus were quite different under different vegetation types. Therefore, after climate change affects the soil temperature and vegetation distribution pattern near the forest line, the content and form of mineral elements in the soil may change, which will eventually affect the absorption and utilization of mineral elements by plants near the forest line.
On the other hand, the role of nitrogen and phosphorus content in alpine plants is mainly manifested in their adaptation to their growth and development and alpine adversity. Some studies show that plants in arid and cold regions generally have higher nitrogen and phosphorus contents. Because N is the catalyst of all biochemical reactions, accumulating higher contents of N and P in leaves can improve the biochemical reaction rate in low temperature environment, so it is also an adaptation of alpine plants to harsh environment. For example, the increase of N will directly affect the content and activity of photosynthesis-related enzymes (such as RuBP carboxylase), further affect the assimilation rate of CO2, and thus improve photosynthetic capacity. Han et al. measured and analyzed the contents of nitrogen and phosphorus in leaves of 753 terrestrial plants in China. The results showed that with the increase of latitude or the decrease of annual average temperature, the contents of nitrogen and phosphorus in leaves increased, and the ratio of nitrogen to phosphorus did not change obviously, but it was found that the ratio of nitrogen to phosphorus was higher than the global average. He et al. also found that the nitrogen content and photosynthetic capacity of plants in Qinghai-Tibet Plateau were higher than the global average. This may indicate that the adaptation and demand of alpine plants in different regions are different.
At present, the research on the adaptability of alpine plants to mineral elements mainly focuses on a large number of elements N, P and K. However, trace elements (Cu, Zn, Mn, Fc, Ni, Mo, B, Cl, al, n a, etc. ) also plays an important role in plant growth, development and morphogenesis, but there are few reports on this aspect. Through the study of Sabina przewalskii, a forest tree species in Qilian Mountains, the author's research group found that the content of trace elements in Sabina przewalskii leaves changed significantly with altitude (data to be published). Since many trace elements are important catalysts for biochemical reaction enzymes, the author thinks that the adaptive effects of trace elements on tall trees should be further studied in the future.
2.3 Characteristics of carbohydrate content in alpine plants
The storage forms of carbon in woody plants are non-structural carbohydrates (NSCs) and fats. The former includes starch, sucrose, glucose and fructose. High-line trees not only need enough NSC to maintain their growth in low temperature environment, but also need enough soluble sugar to improve their ability to resist low temperature. The seasonal dynamic change of NSC can reflect the supply of plant carbohydrates.
NSC metabolism affects plant growth and its response to the environment, which makes it play an important role in the adaptation of alpine plants to bad environment. Among them, the mutual transformation between soluble sugar and starch is recognized as an effective mechanism of plant stress resistance, and the proportional relationship between them plays an important role in the resistance of alpine plants to low temperature stress. Recent studies show that the survival and growth of trees in high altitude areas depend not only on the existing NSC content in tree tissues, but also on the ratio of soluble sugar to starch throughout winter. It is considered that the ratio of NSC to starch exceeds 3 to ensure the normal overwintering and the next year's growth of trees. The experiment on the influence of ultraviolet radiation on the growth of bentgrass showed that the anti-ultraviolet substances (produced by shikimic acid) produced under ultraviolet radiation were obtained through the transformation of photosynthetic products and stored carbohydrates, which indicated that the utilization and distribution of carbohydrates played an important role in the process of plant growth and ultraviolet defense.
NSC not only plays an important role in studying the stress resistance of plants, but also plays an important role in studying the formation mechanism of high lines. So far, based on the NSC of Gaoshu, the hypothesis of insufficient carbohydrate supply (low temperature, drought and short growth period make carbohydrate production less than "demand") and the hypothesis of growth inhibition (low temperature directly inhibits cell division and tissue differentiation) have been put forward. These two hypotheses comprehensively consider the physical environment of high-speed wire and the physiological and ecological adaptation and response of plants to these environmental factors. This paper attempts to explain the phenomenon of global Gaolinshan line from the mechanism.
In addition, NSC not only plays an osmotic adjustment role in plant stress resistance, but also may be used as a signal substance for plants to adapt to the environment.
At present, "sugar signal" has become a research hotspot in the field of world botany. Among them, glucose, fructose, sucrose and fructan with low polymerization degree may be used as signal substances, and they can also interact with other growth regulators, and the research has gone deep into the molecular level. However, the research in this area is mainly focused on herbs or model plants, and there is little research on woody plants. However, as the role of NSC in the adaptation of high-strain trees to extreme environment has been recognized, whether NSC also plays a signal regulation role in the physiological and ecological adaptation of high-strain trees needs further study.
2.4 Characteristics of antioxidant system of high-strain plants
Stress leads to the accumulation of reactive oxygen species in plant cells, which leads to oxidative damage. However, in the process of adapting to adversity, plants have also formed a set of antioxidant systems to protect cells from excessive ROS damage, including antioxidant enzyme systems and non-enzymatic antioxidant systems. The ability of plants to remove excessive ROS (that is, antioxidant capacity) is the key factor to reflect the stress resistance of plants.
At present, the research on the antioxidant system of alpine plants is mostly concentrated on herbs or shrubs. The total content of antioxidant substances increases with the elevation, but the response modes of protective substances in different plants to altitude are different. For example, the ascorbic acid content of Phyllanthus urinaria increased significantly with the elevation; However, with the elevation of Picea crassifolia and Sabina vulgaris, antioxidant protective enzymes (superoxide dismutase, peroxidase and catalase) and non-enzymatic antioxidants (proline, ascorbic acid and reduced glutathione) all showed an obvious upward trend. Because the main organs that produce ROS are chloroplasts and mitochondria, the dynamic balance of ROS between them is very important for plants to adapt to low temperature stress. Zhang studied Sabina vulgaris and Sabina vulgaris at different altitudes, and found that their antioxidant contents increased (reduced glutathione and carotenoids, etc.). ) and the decrease of superoxide anion content in chloroplast are very important to improve the cold resistance of these two cypresses. However, at present, the research on the antioxidant system in mitochondria is mostly concentrated on low-altitude herbs, and the research on the adaptability of alpine plants (especially tree species) has not been reported.
In addition, ROS also has dual functions, not only as a strong oxidant to damage plant cells, but also as a signal substance to regulate plant growth and development and participate in the response of plants to adversity. As a special extreme habitat near the Gaolinshan line, it is still blank whether ROS accumulated by trees can also regulate signals and make alpine plants adapt to the changeable adverse environment.
3 outlook
The research and monitoring of the physiological ecology of high-altitude plants is of great significance for predicting the influence trend of global climate change on alpine plants. As the main constructive species near the forest line, the trees of the high line are completely different from the herbs. Because of its long life cycle, it can experience many years of external environmental changes, and its response to climate change is of special significance. However, in recent years, although domestic scholars pay more and more attention to the adaptability of tree species in Gaolinshan, there are still many data obtained through field investigation or short-term observation. Therefore, on the basis of the existing research on the adaptation and response of tree species to the physiological ecology of alpine environment, the following aspects can be further strengthened in the future: (1) the changes of cell structure of tree species, such as the development characteristics of chloroplasts and mitochondria in the environment of strong ultraviolet radiation; (2) As an important catalyst of enzymatic reaction, trace elements play a role in the growth and development of forest trees; (3) The role of signal substances in the adaptability of forest tree species, such as whether sugar and ROS play a signal regulation role in the adaptation of forest tree species to the environment; (4) Photosynthesis is a key physiological activity affecting the survival and adaptation of tall tree species, and how to carry out and improve this research needs further discussion; (5) Study the adaptation mechanism of tree species at the molecular level, so as to better explain the physiological and ecological adaptation of tree species to the cold environment. In addition, the growth, behavior and physiological and ecological responses of plants under various environmental factors can be simulated under artificially controllable conditions, and combined with macro-positioning observation data, it is helpful to predict the response trend of high-speed plants to future climate change more accurately.