Detailed Explanation of the Growth, Development and Functions of Rice Roots

After the seeds are pre-germinated, they sprout and grow roots. At first, only one root grows, called the seminal root (primary root), which develops from the radicle of the seed. The fibrous roots that later grow from the root primordia at the stem nodes are called adventitious roots. The coleoptile node and the incomplete leaf node produce about 5 roots, the first, second and third leaf nodes produce about 10 roots each, and the nodes above the fourth leaf position produce about 10 to 20 roots each. This is because the leaves increase in size in sequence from bottom to top, so the nutrients supplied for root growth increase in sequence, and the number of roots produced also increases. The nodes of the aboveground part generally do not produce roots. The nutrients produced by the leaves are transported through the vascular bundles of the leaf sheaths and stem nodes to the root primordia, promoting root growth. The increase or decrease in the number of vascular bundles is affected by the environment and cultivation conditions. When the soil is moist and oxygen is sufficient, more roots are produced, and they have root hairs. Under deep-flooding conditions, the soil is in a reduced state and lacks oxygen, which affects root respiration; there are no root hairs, and black roots and rotten roots appear. For root growth, the most suitable temperature is 32°C; when the soil is at 25°C, many roots are produced but they are shorter; at 5°C, very few roots are produced. In cold-bottom fields or rice fields irrigated with cold water from the bottom layer of reservoirs, the amount of root growth is also very small. When the transplanting depth reaches 3 to 4 cun, thin and long elongated internodes appear, resulting in two-stage rooting. Plants with two-stage rooting often have few tillers or no tillers, or tiller late and produce ineffective tillers. Moreover, the stems are thin and weak and prone to lodging. Therefore, rice seedlings should not be transplanted too deep.

The roots at the coleoptile node enable the young shoot to take root, the germinated seed to turn around and the needle-like shoot to emerge. The roots born at the incomplete leaf node and the first complete leaf node play a role in absorbing nutrients during the weaning period of the seedlings. The roots born at nodes above the second leaf position play a role in seedling growth and growth after transplanting. Under normal conditions, root growth is vigorous 7 days after transplanting. The roots that first grow out have a smaller angle; the roots at the middle nodes have a larger angle; the roots growing from the upper nodes are distributed horizontally, like a net. These roots are relied upon for water and fertilizer uptake in the later period. Therefore, during the booting stage, it is inadvisable to walk in the field, so as to avoid breaking the horizontal roots that play a major role in flowering and grain setting.

Root growth is adapted to the aboveground part. The roots play a role in fixation and support, and can synthesize organic substances such as amino acids and transport them to the aboveground part for protein synthesis. Of the inorganic nutrients absorbed by the roots, 75 to 95% are supplied to the stems and leaves, with only a small portion remaining in the roots. The organic substances needed by the roots are supplied by the aboveground part. In this way, a division of labor in nutritional function between the roots and the stems and leaves is formed, and they also promote each other.

The soil environment in which rice roots live lacks oxygen. The oxygen transported from the leaves to the roots, in addition to meeting the needs of the roots' own respiration, is also partly secreted outward from the root epidermis; the oxygen then surrounds the roots, protecting them from poisoning by reducing substances in the soil. This phenomenon is called oxygen secretion. New roots secrete more oxygen and are white, with a strong ability to absorb fertilizer and water. Old roots often show rust spots, because soluble ferrous hydroxide in the paddy field is oxidized into insoluble ferric hydroxide, which accumulates around the rhizosphere and even attaches to the root surface, causing rust spots to appear on the roots. Long-term deep flooding of paddy fields produces hydrogen sulfide. When the ferric hydroxide around the rhizosphere is insufficient to combine with hydrogen sulfide, the rice roots can be harmed by toxic substances such as hydrogen sulfide, and root rot occurs. When black roots appear but do not yet rot, draining the field to expose it or drying the field increases the oxygen in the soil to strengthen oxidation, which can change the color of the black roots and prevent root rot.

As for the strength of rice roots, in the case of a single root, the root tip has the strongest vitality. In the case of a single rice plant, new roots have stronger vitality than old roots. Roots with strong vitality have a strong ability to absorb water and nutrients, and their respiration is also strong. The strength of respiration is mainly affected by the supply of oxygen and carbohydrates. Heavy clay lacking oxygen will take away large amounts of the oxygen secreted by rice roots, reducing the oxygen supplied for root respiration and lowering root vitality. If too much nitrogen fertilizer is applied, the leaves will bend and droop and become covered with dew-like moisture, so that the lower leaves supplying nutrients to the rice roots cannot receive direct sunlight and produce less carbohydrate, and root vitality declines. Some seedlings grow well in the early stage, but in the later stage the stems die early, and good plants do not produce good grain. This is mainly because the oxygen and carbohydrates needed by the rice roots are insufficient, the roots age prematurely, vitality declines, and the ability to absorb water and nutrients decreases; because the leaves cannot obtain sufficient water and nutrients, they also dry up prematurely, thereby affecting the filling of the grains.