Effects of auxin on different organs of plants

When explaining the phototropism of plants, high school biology said: "Light can make auxin distribute more in the backlight side than in the light side." Therefore, the backlight side grows faster than the light side. As a result, the stem bends toward the slow-growing side, that is, toward the side of the light source, making the stem of the plant show phototropism. " This shows that higher concentrations of auxin make plants grow faster, while plants grow slowly at low concentrations. "the effect of auxin on plant growth has a certain relationship with its concentration," the book said. In general, low concentrations promote plant growth, while high concentrations inhibit plant growth. " And use this to explain the phenomenon of top advantage, which seems to contradict the former. Does high concentration or low concentration promote growth? what is the optimum concentration of auxin for the growth of different organs of plants? Why did the concentration of auxin that gave priority to apical growth inhibit the growth of lateral buds? The main physiological function of auxin is to promote cell elongation, but the induction of plant growth has a positive and negative nature, that is, it can not only promote growth, but also inhibit growth; it can not only promote bud germination, but also inhibit bud germination; it can not only protect flowers and fruits, but also sparse flowers and fruits. This mainly depends on the concentration applied, the type of plant, the type of organ and so on. In general, low concentration of auxin can promote growth, more than the appropriate concentration, because auxin can induce the production of ethylene and interfere with the internal physiological balance, thus inhibiting growth. In addition, the response of different plant organs to the concentration of auxin is very different, and the root is the most sensitive to auxin. The optimum concentration to promote root growth is only about 10-11 ~ 10-9mol. The sensitivity of stem to auxin is about 10-5mol, and that of bud is about 10-8mol, which is between them. The optimum concentration of auxin varies with different plant organs, and the optimal concentration of stem growth is precisely the concentration of inhibiting the growth of roots and buds. The auxin produced by the top stem buds of plants is transported to the downward polarity and accumulates in large quantities in the lateral buds. The concentration of auxin in the lateral bud increased more than the suitable concentration for bud growth, and the auxin concentration in the stem was the most suitable, which promoted cell elongation, showing the rapid growth of the main stem, the germination and growth of lateral buds was inhibited, and the plant showed apical dominance. The phototropism of plants is due to the uneven distribution of auxin caused by light. The auxin concentration on one side of the backlight is higher than that on the light-facing side, but the auxin concentration on both sides of the stem is within the range of suitable concentration for stem growth, so that the backlight side grows faster and bends to the light source. Therefore, in a certain concentration range, high concentration promotes growth and low concentration inhibits growth. However, too high concentration is toxic to plants, which is why auxin 2,4-D can be used as a herbicide in production. Thus it can be seen that the two explanations in the book are not contradictory, but clarify the physiological function of auxin from different angles, which is correct. (Jiang Chengzi)