What are the characteristics and functions of the cotton root system?
Cotton originated in tropical and subtropical regions and is a perennial plant. Through long-term artificial selection and breeding, it gradually moved northward to temperate zones and evolved into an annual crop. It is sown, buds, flowers, sets bolls, and matures its seeds within a single year, completing its growth cycle. During growth and development, cotton still retains several characteristics of its original perennial nature, possessing an indeterminate growth habit. Under suitable conditions, it can continuously produce more fruiting branches, and those fruiting branches can in turn continuously produce more fruiting nodes, endlessly forming new flower buds, flowers, and bolls.
Cotton is a taproot crop with a well-developed root system, a deep main root, and widely distributed lateral roots, forming a powerful absorption network in the soil; it is a relatively drought-tolerant crop. The cotton root system consists of the main root, lateral roots, branch roots (small roots), hair roots (small branch roots), and root hairs. When the cottonseed germinates, the radicle emerges and grows downward, becoming the main root. In the early stage of cotton growth, first-order lateral roots emerge from the main root at about 10 cm from the main root's growing point, initially growing almost horizontally, then later growing obliquely downward into lower layers. Second-order lateral roots emerge at about 5 cm from the growing point of the first-order lateral roots. Under suitable conditions, third-order, fourth-order, and even fifth-order lateral roots can continue to form. The main root, lateral roots of all orders, and the large number of root hairs near their tips together constitute cotton's massive root system. Cotton roots have strong regenerative capacity; if the main root is injured or broken during transplanting, it promotes abundant lateral root growth. The smaller the cotton plant, the stronger its root regenerative capacity.
Under suitable ecological and cultivation conditions, cotton roots can generally penetrate to a depth of about 2 m, and some can exceed 3 m. The upper lateral roots extend farther, spreading horizontally up to 60-100 cm, while the lower lateral roots extend less far. How early or late lateral roots emerge and the number of first-order lateral roots have an obvious effect on boll-setting traits. Deep, loose soil layers help cotton form a strong, robust root system and improve resistance to soilborne diseases. Therefore, deep plowing is very important for root development, and when machines prepare the land, care should be taken to avoid compacting the soil too firmly; both measures can reduce the occurrence of cotton seedling diseases and Fusarium and Verticillium wilts.
The formation of the cotton root system can be roughly divided into four periods:
1. Root development period, from seed germination to squaring. At this time, root growth reaches 2 cm per day, while aboveground growth is very slow, increasing only 0.4-0.5 cm per day. Early intertillage at this stage can raise the soil temperature around the roots and promote earlier emergence of branch roots. After that, intertillage should be deepened progressively to encourage roots to grow downward; before squaring, intertillage depth should reach 7-8 cm.
2. Peak root growth period. The squaring stage is a period of vigorous growth for the main root and lateral roots; the main root elongates 2.5 cm per day and can exceed 1 m in depth. Lateral roots also spread rapidly sideways, reaching up to 50 cm. Intertillage during this period can reach its maximum depth. For cotton fields showing a tendency to excessive vegetative growth, deep intertillage has the effect of controlling vegetative growth and can reach more than 10 cm. At this time, combining intertillage with the first hilling-up helps promote root development, also helps with drought resistance and drainage, and extends the functional period of the roots.
3. Peak root absorption period. During the flowering and boll-setting stage, the cotton plant's root network is basically established and absorbs the most water and nutrients, but root-forming capacity gradually declines, so roots should not be injured during the flowering and boll-setting stage, and intertillage should become progressively shallower. Before the rows close, further hill up the soil in combination with intertillage. By strengthening water and fertilizer management, it is possible both to meet the needs of flowering and boll setting and to maintain root vigor and delay root senescence.
4. Period of declining root activity. During the boll-opening stage of the cotton plant, the roots decline and die off, and their capacity to absorb water and nutrients drops markedly. If the roots decline and die too early, it leads to premature senescence of the cotton plant. If the weather is dry and the soil is dry at this time, watering should still be carried out, which can both meet the needs of autumn boll development and prevent premature senescence. Later foliar fertilizer spraying can make up for the insufficiency of root nutrient uptake and also provide the roots with necessary nutrition.