A Brief Discussion on the Red Leaf Phenomenon in Succulents
About succulents, this article briefly discusses the red leaf phenomenon in succulents, followed by a comprehensive introduction.
The red leaf phenomenon is a common color change associated with environmental changes in angiosperms, while it is very rare in gymnosperms. In succulents, red leaves are a common horticultural type found in many varieties, especially in the genus Echeveria, such as the Huilongyu. Some varieties also exhibit similar phenomena, but the seasonal dependency is not obvious, such as the Hongzhiyu in the Crassula family, and even the Buwenqiu in the Euphorbiaceae family.
Traditionally, red leaves refer to the changes that occur with the seasons, especially in autumn and winter. In fact, so-called red leaves are actually a result of the alternation of pigments, triggered by factors such as light, temperature, and stress, with light being the most obvious.
The red leaf phenomenon occurs due to changes in the ratio of pigments, including the fading and decomposition of normal pigments, the generation of new pigments, or changes in the chemical structure of normal pigments. This issue has been extensively researched since the 1980s, with the theory at the time suggesting that the decomposition of chlorophyll in autumn reveals the colors of pigments such as xanthophyll and anthocyanins.
Recent studies have shown that the redness of plant epidermis in autumn is due to the synthesis of a large amount of anthocyanidins, which are usually red, purple, and blue. After the beginning of autumn, the synthesis of anthocyanidins in epidermal cells is a result of the decomposition of chlorophyll, with the cold and dry conditions of autumn being the main factors promoting the synthesis of anthocyanidins.
Anthocyanidins are a type of flavonoid compound, formed by the connection of an oxazole ring and a benzene ring, creating a 15-carbon bicyclic compound. The ring can derive some hydroxyl and methyl groups, and on the corresponding hydroxyl groups, monosaccharide molecules can form glycosides. The simple 15-carbon bicyclic molecule without monosaccharides is anthocyanin. Different positions being hydroxyl, methylated, or forming glycosides will make the compound have different absorption spectra, thus producing different colors. Moreover, as the compound contains a glycoside structure and hydroxyl groups, pH will have some effect on its spatial structure, meaning that changes in acidity and alkalinity can lead to color changes.
The synthesis of anthocyanidins is the main reason for the red leaf phenomenon, and the amount and structural changes of its synthesis will affect the appearance of red leaves. Since the biochemical metabolism of anthocyanidins is complex and involves many biochemical reactions, it will not be elaborated further. However, it is worth mentioning the key enzymes in the entire metabolism: O-methyltransferase, UDPG-flavonoid glucosyltransferase, and rhamnosyltransferase. These three enzymes are key to synthesis, and any enzyme inhibitor or activator can affect the red leaf phenomenon.
Factors that affect the synthesis and metabolism of anthocyanidins are those that affect red leaves, including:
1. Genetic type
This is the main factor determining red leaves. Normal Huilongyu and red-leaf Huilongyu show rich color changes, with the latter simply deepening in color but not displaying various bright colors. The changes in red leaves between Doushu and Huilongyu are also different, with Huilongyu being more likely to change color. The gene types of the Yewu genus should also be rich in changes, but unfortunately, its vascular system structure is unique, and the epidermis is covered with a thick wax layer, which affects the impact of light and temperature on pigment metabolism, leading to low reactivity. In horticultural breeding and natural mutations of peonies, wax layer defects or increased pigment sensitivity can lead to the red leaf phenomenon in the genus. In summary, whether it's the pigment system or auxiliary system, unsuitable genotypes can affect pigment metabolism, thus affecting the red leaf phenomenon. Moreover, the degree of chlorophyll decomposition is also an important factor in reflecting pigment color. Red leaf plants all have the characteristic of starting to decompose in large amounts while synthesizing anthocyanidins, which can show bright colors; otherwise, they are superimposed colors, such as the original species of牡丹玉, Baotou, and Yanji Shitong.
2. Light
Light is universally recognized as the cause of red leaves, as the most basic change in red leaves is associated with light. In autumn, due to factors such as atmospheric concentration and the angle of direct sunlight, the transmission of light increases, and light intensity rises, especially the transmission of strong-effect ultraviolet light. These factors act on the photosystem of plant epidermal cells, thereby activating light-sensitive switches, initiating second messengers, and inducing the synthesis of anthocyanidins. The main method of artificially inducing red leaves is to use strong-effect ultraviolet (UV-B) light to induce the synthesis of anthocyanidins. In practice, we have found that red leaves grown in the open air are better than those in plastic greenhouses, and those in glass greenhouses are better than those in plastic ones. The red leaves in the (T-W) region are very beautiful, mainly because the humidity and temperature there are favorable for open-air cultivation, and the latitude results in a higher transmission of ultraviolet light, which is conducive to the production of red leaves. In the north, the red leaf phenomenon is more obvious in autumn, mainly due to the large temperature differences and changes in seasonal ultraviolet light. Unfortunately, red leaves are very unsatisfactory in household conditions, mainly because glass windows filter out most of the ultraviolet light.
3. Temperature
Cold and large temperature differences can trigger the synthesis of anthocyanidins, with cold being the most obvious. The earliest manifestation of the red leaf phenomenon in autumn is temperature-dependent, followed by dependency on light. Crassula's Hongzhiyu easily turns red after being cold, which is a typical example. In household environments, the red leaf phenomenon is not obvious, not only due to lack of light but also because the temperature is higher and the temperature difference is smaller. The reason why low temperature can induce the synthesis of anthocyanidins is that it can enhance the plant's resistance to cold, possibly related to the absorption spectrum of anthocyanidins being conducive to better absorption of light energy. Moreover, the synthesis of anthocyanidins is accompanied by the generation of ABA (abscisic acid), which is also a major hormone for plant dormancy and resistance.
4. Salt Stress
Stress includes many factors, such as low temperature, drought, diseases and pests, salt and alkali, etc. The stress we discuss here mainly refers to salt stress. Regarding salt stress, there has been a lot of research, mainly focusing on the increase in osmotic pressure of cells or tissues leading to the synthesis of anthocyanidins. This is reflected in the fact that plants are more likely to show red leaf phenomena when they are short of water. Under this hint, we can use high-salt solutions to treat plants to induce red leaf phenomena, but this can easily damage the plants. Similarly, in household conditions, we cannot use high-salt alkaline water for plants, which is another reason why red leaves are not obvious.
5. Others
Chlorophyll, for red leaves to be clearly visible, there must be a light background, that is, the concentration of chlorophyll must be reduced to a certain level. This is closely related to the genotype, only chlorophyll mutants that can change periodically will have beautiful and obvious red leaf phenomena. Moreover, red leaves often accompany chlorophyll deficiency (variegation), which may also be due to the deficiency promoting the synthesis of anthocyanidins.
Hormones, gibberellin receptor antagonists, and growth inhibitors are all conducive to the production of red leaves, mainly because these hormones act like stress or light (in fact, light and stress are also caused by changes in hormone levels in reality). Unfortunately, the mechanism of hormones in the red leaf phenomenon is still very unclear, and the effects are not conclusive, so they are rarely used.
Infection, infection by certain pathogens may produce substances that activate the plant's dormancy system or ABA system, leading to abnormal red leaf phenomena. For example, some Doushu seedlings will show a deepening of epidermal color and purple after infection, often with root shrinkage or decay when inspected.
The difference between red leaves and variegation: Red leaves refer to the periodic or reactive synthesis and decomposition of anthocyanidins under external factors, accompanied by the decomposition of chlorophyll and other pigment metabolism changes. Variegation is due to the absence of chlorophyll accompanied by the excessive synthesis of other pigments. The common point between the two is that they are both related to the changes in chlorophyll and have changes in pigment components; the difference is that variegation has damage or defects in the chlorophyll gene, while the chlorophyll gene in red leaves is normal or periodically normal, which is their essential difference. The color production of variegation is due to the secondary hyperactivity of normal pigments, while red leaves are the primary synthesis of anthocyanidins. In genetics, the inheritance of red leaves is in the nucleus, while that of variegation is mostly in the cytoplasm (plastids). Variegation can appear simultaneously with red leaves, that is, on the basis of chlorophyll deficiency or partial deficiency, the synthesis of anthocyanidins occurs, making the color more beautiful. A typical example is the various red-leafed Lantauyu Jin.
Artificial mutation of red leaves: The preferred method is still UVB treatment, which inhibits the expression of chlorophyll genes while promoting the synthesis of anthocyanidins, with a higher mutation probability, but it is unstable and the probability of stable inheritance is less than 1%. Different from this, the mutation of variegation is preferably done with x-rays and chlorophyll blockers, which mainly destroy the chlorophyll synthesis genes in the chloroplast or the chloroplast control genes in the nucleus.
Naming: The naming of red leaves is generally "Red Leaf某某," such as Red Leaf Huilongyu; the naming of variegation is "某某Jin" or "某某Banru" (rarely used in China), such as Lantauyu Jin; when both appear, it is called "Red Leaf某某Jin" or "Red Leaf某某Banru," for example, Red Leaf Lantauyu Jin.
The above content is a brief discussion on the red leaf phenomenon in succulents. Have green plant enthusiasts understood it?