Growth and Development of Horticultural Crops (1+1)

Distribution of auxin in monocot and dicot seedlings

• In monocot seedling, the highest concentration of auxin is found in the coleoptiles tip which decreases progressively toward its base. From the base of the coleoptiles, the auxin concentration increases progressively up to the root tip. However, the concentration of auxin at the tip of root is much lower than at the coleoptiles tip.
• In dicot seedling, although the pattern of auxin distribution appears to be complex, but obviously highest auxin concentrations are found in growing regions of shoot, root, young leaves and developing axillary shoots.
• Within the plant, the auxins may be present in two forms-free auxins and bound auxins. Free auxins are those which can be easily extracted by various organic solvents such as diethyl ether or those which are easily diffusible such as that obtained in agar block from cut coleoptiles tip. Bound auxins on the other hand, need more drastic methods for their extraction from plants such as hydrolysis, autolysis, enzymolysis etc., and are not easily diffusible. Bound auxins occur in plant as complexes (conjugated auxins) usually with carbohydrates such as glucose, arabinose or sugar alcohols, or proteins or amino acids such as aspartate, glutamate or with inositol.
1. The free form of auxin is biologically active form of the hormone. In bound or conjugated form (which predominates in plants), the auxin is considered to be biologically inactive.
2. The metabolism of bound or conjugated auxin might be a major contributing factor in controlling level of free auxin in plants.