Tobacco Cultivation

Author: Wang Dongsheng et al. / Country:
Publisher:
Publishing Date: 2002-01-01
Features: (IV) Leaf Growth
1. Leaf Differentiation
Tobacco leaves are formed by the differentiation of stem cell growth point cells from the apical bud or axillary bud. At the stem tip during the vegetative growth stage, the protoderm, procambium, and ground tissue structures are distinct. The protoderm located outside the stem tip consists of two layers of cells, while the procambium inside the protoderm is composed of a cluster of cells. During leaf primordium differentiation, the outer layer cells of the procambium first undergo periclinal division, forming two layers of cells. Subsequently, these two layers of cells and the inner layer cells of the protoderm undergo divisions in various directions, producing a cluster of cells that protrude outside the stem tip, forming the leaf primordium. Leaf primordia typically appear around the stem tip and are arranged according to the leaf arrangement pattern. When first appearing, leaf primordia are conical and composed entirely of meristematic cells. When the length of the leaf primordium reaches 500 μm, dense trichomes appear on its surface, and these cells differentiate into the basic tissue, while only the central cells retain the meristematic state. As the leaf primordium continues to grow, when its length exceeds 1 mm, the sides begin to protrude outward in a lateral wing-like shape, with the midrib and blade becoming distinct, forming a young or tender leaf. When the young leaf is about 9 mm long, the base of the blade is approximately 2 mm wide and semicircular. On leaves with lateral wings, the secondary lateral veins are fully formed. At this stage, the cells of the spongy tissue increase, air spaces begin to appear, and the meristematic cells have largely ceased division. Subsequently, further differentiation completes the internal structure of the leaf. After the leaf reaches 1 cm in length, it still undergoes a period of cell division, but the timing and intensity of active cell division and the stabilization of cell numbers vary among different regions of the tobacco leaf. Research indicates that the marginal growth of lower leaves is highly active when the leaf length is 1–4 cm, weakens significantly by 8 cm, and ends by 12 cm (10 days after emergence). The marginal growth of middle leaves is highly active when the leaf length is 1–8 cm, weakens significantly by 12 cm, and ends by 18 cm (13 days after emergence). The marginal growth of upper leaves is highly active when the leaf length is 1–15 cm, weakens significantly by 20 cm, and ends by 25 cm (15 days after emergence). Active marginal growth indicates rapid cell division and proliferation, and if prolonged, results in a larger single leaf area. The period when cell numbers stabilize varies by leaf position: lower leaves are approximately 18 cm, middle leaves are approximately 25 cm, and upper leaves are approximately 30 cm.
2. Growth of a Single Leaf
(1) Divisions of Growth Periods
To understand the growth process and characteristics of a single leaf, its growth is typically divided into six stages.
① Embryonic Differentiation Stage. From the formation of the leaf primordium to the emergence of the young leaf with a length of about 1 cm, this stage is called the embryonic differentiation stage. The main characteristics of this stage are: rapid cell division and differentiation, formation of various tissues, minimal absolute growth, and a dense covering of trichomes on the leaf surface.
② Young Leaf Growth Stage. From a leaf length of 1 cm to 1/6 to 1/4 of its final length (approximately 10–15 cm for flue-cured tobacco varieties), this stage is the young leaf growth stage. During this stage, young leaves continue to divide and begin to enlarge and elongate. Nitrogen content in the leaves increases, but chlorophyll content is not high, resulting in a lighter leaf color. The leaves stand upright, grow slowly, and leaf area increases little.
③ Vigorous Growth Stage. From the end of the young leaf growth stage to 70–80% of the final leaf length, this stage is the vigorous growth stage. During this stage, tobacco leaves expand rapidly due to the rapid enlargement and elongation of cells, leading to vigorous growth and a rapid increase in leaf area. Accompanying this rapid growth, the nitrogen or nitrogen compounds and chlorophyll content in the leaves increase, and photosynthesis significantly enhances. However, since the photosynthetic products at this stage are primarily used for the leaf's own expansion, the accumulation of dry matter is minimal. Leaves during the vigorous growth stage have high water content, a larger angle with the stem, and an oblique upright state.
④ Slow Growth Stage. From 70–80% of the final leaf length to when the leaf size is largely determined, this stage is the slow growth stage. As the enlargement and elongation of mesophyll cells decrease and cease gradually, the expansion growth of the leaf slows down. However, during this stage, chlorophyll content in the leaves approaches its peak, and photosynthesis remains strong. Along with a small amount of nitrogen export, the accumulation of dry matter gradually increases, and leaf weight and thickness also increase. From an external perspective, the leaf color becomes dark green, the angle between the stem and leaf increases, and the leaf tip begins to droop.
⑤ Physiological Maturity Stage. From when the leaf size is largely determined to when the leaf weight reaches its maximum, this stage is the physiological maturity stage. During this stage, the accumulation of organic matter in the leaves exceeds consumption, and with further nitrogen export, chlorophyll breaks down rapidly, while the accumulation of carbohydrates reaches its peak. The tissue becomes firmer, and leaf weight and thickness gradually reach their maximum. Leaves entering the physiological maturity stage show a significant reduction in chlorophyll content, a fading green to yellow leaf color, reduced water content, drooping leaf tips, an increased angle between the stem and leaf, and most trichomes falling off. At this stage of harvesting, tobacco leaf yield is highest, but the chemical composition is difficult to balance, and the structure is tight.
⑥ Process Maturity Stage. From when the leaf weight reaches its maximum to when the tobacco leaf reaches process maturity, this stage is the process maturity stage. One of the characteristics of tobacco leaves during this stage is the beginning of the decomposition and transformation of organic matter, such as the slight decomposition of starch and the further conversion of nitrogen compounds into nicotine, increasing the nicotine content. Along with the decomposition and transformation of organic matter, leaf weight, thickness, and water content of the tobacco leaf decrease, while the intercellular space rate of the mesophyll tissue increases, and the tissue structure tends to become looser. This is another significant characteristic of tobacco leaves entering the process maturity stage. At this stage of harvesting, although the tobacco leaf yield is not the highest, the leaves are easy to cure, and the cured tobacco leaves have a well-balanced chemical composition, a loose structure, and high quality.
When tobacco leaves reach the process maturity stage, the leaf color further turns yellow, the leaf tip gradually becomes yellowish-white, the main and lateral veins turn white and shiny, and most trichomes fall off. From the above description of the growth conditions in the six stages, it is evident that the growth of a single leaf actually involves three aspects: expansion growth, accumulation and maturation, and decomposition and transformation. Among these, expansion growth is completed in a "slow-fast-slow" rhythm. Generally, expansion growth is a type of cell division and elongation growth based on nitrogen. When leaves receive sufficient nitrogen before rapid growth, not only is the rate of subsequent rapid growth high, but the duration is also long, resulting in larger leaves. This is why it is said that "the final size of a leaf depends on the nitrogen content before rapid growth."
The accumulation and maturation process occurs as the leaf growth rate slows and nitrogen begins to export from the leaves, with the main external influencing factor being light. The timing of this process—whether early or late—affects the amount of accumulated substances (primarily starch), leading to an imbalance between leaf growth and accumulation. This imbalance not only affects the maturity, weight, and yield of the tobacco leaves but also their chemical composition and other quality traits.
Decomposition and transformation are signs of the aging of tobacco leaves. For the process maturity of tobacco leaves, this aging or change can only be in its early stages, and it can be controlled by adjusting the harvesting time.
(2) General Progress of Single Leaf Growth
The growth process of a single leaf is influenced by various factors, such as variety, position, nutritional level, and environmental conditions. However, under normal cultivation conditions, certain tobacco types or varieties follow a certain pattern. Based on observations of some low-leaf-number flue-cured tobacco varieties in various regions of China over the years, the general growth process of a single leaf is as follows:
- Embryonic differentiation requires 14–16 days.
- Young leaf growth requires 10–15 days.
- Vigorous growth requires 10–15 days.
- Slow growth to when the leaf size is largely determined requires 15–25 days.
- From when the leaf size is largely determined to process maturity requires 10–20 days.
If the time for invisible embryonic differentiation is excluded, the time range for each leaf position from emergence (1 cm) to final process maturity is 45–75 days. We refer to this period as the leaf age required for tobacco leaf growth. The variation in leaf age for a single tobacco variety is primarily influenced by position and nutritional level. According to domestic observation data, for low-leaf-number flue-cured tobacco varieties without topping, the leaf age decreases from lower to upper positions. Early or low topping can significantly extend the leaf age, especially for upper leaves. Nitrogen application also has a significant effect on leaf age, with an increase in nitrogen application extending the leaf age for all positions. Generally, the leaf age of lower leaves is 45–55 days, middle leaves are 55–65 days, and upper leaves are 65–75 days or slightly longer. Leaf age can serve as an indicator of the nutritional level of the tobacco plant.
To further understand the growth process of a single leaf, the following changes in tobacco leaf traits are described based on leaf age:
① Expansion Growth. The "slow-fast-slow" rhythm of leaf expansion growth appears as leaf age increases. According to a 1960 survey by the Chinese Academy of Agricultural Sciences Tobacco Research Institute on the middle leaves of the Yizha 7 variety, the growth rate was relatively slow in the early stages of leaf emergence. As the leaf area increased, the growth rate rapidly accelerated, reaching a peak at 15 days of age, with an average daily increase of 3.4 cm in leaf length and 2 cm in leaf width. However, the maximum expansion of leaf area occurred on the 20th day, with an average daily increase of 8.2 cm.