What effects does auxin have on different plant organs?

When high school biology explains plant phototropism, it says: "Light can cause auxin to be distributed more on the shaded side than on the illuminated side. Therefore, the shaded side grows faster than the illuminated side. As a result, the stem bends toward the side that grows more slowly, that is, toward the light source, causing the plant stem to show phototropism." This indicates that a higher concentration of auxin makes plants grow faster, while at low concentration plants grow slowly; yet the book also says: "The effect of auxin on plant growth has a certain relationship with its concentration. Generally speaking, low concentration promotes plant growth, and high concentration inhibits plant growth." And this is used to explain the phenomenon of apical dominance, which seems to contradict the former. Is it high concentration that promotes growth or low concentration that promotes growth, and what is the optimal concentration of auxin for the growth of different plant organs? Why does the auxin concentration that makes the apical bud grow preferentially inhibit the growth of lateral buds? The main physiological role of auxin is to promote cell elongation, but its induction of plant growth has a dual positive and negative nature, that is, it can both promote growth and inhibit growth; it can both promote bud germination and inhibit bud germination; it can both preserve flowers and fruits and thin flowers and fruits. This mainly depends on the concentration applied and the plant species, organ type, and so on. Under normal circumstances, low concentrations of auxin can promote growth; exceeding the appropriate concentration, because auxin can induce ethylene production and interfere with the internal physiological balance, thereby inhibiting growth. In addition, different organs of plants differ greatly in their response to auxin concentration. Roots are the most sensitive to auxin, and the optimal concentration for promoting root growth is only about 10-11 to 10-9 mol, while stems are less sensitive to auxin, about 10-5 mol, and buds about 10-8 mol, between the two. Different plant organs also have different optimal concentrations of auxin. The optimal growth concentration for stems is exactly the concentration that inhibits the growth of roots and buds. The auxin produced by the apical stem bud of the plant is transported downward polarly and accumulates in large amounts at the lateral buds; the auxin concentration at the lateral buds rises above the suitable concentration for bud growth, while the auxin concentration in the stem is optimal, promoting cell elongation, showing rapid growth of the main stem, while the germination and growth of lateral buds are inhibited, and the plant shows apical dominance. The phototropism of plants is due to the uneven distribution of auxin caused by light. The auxin concentration on the shaded side is higher than on the illuminated side, but the auxin concentrations on both sides of the stem are within the suitable concentration range for stem growth, so the shaded side grows faster and bends toward the light source. Therefore, within a certain concentration range, high concentration promotes growth, and low concentration inhibits growth. But excessively high concentrations have toxic effects on plants, which is why auxin 2,4-D can be used as a herbicide in production. It can thus be seen that the two explanations in the book are not contradictory, but rather clarify the physiological role of auxin from different angles, and are correct. (Jiang Chengzi)