Auxins help primarily in influencing cell elongation and enlargement hence sometimes referred to as growth hormones. Actions of auxins are to stimulate or inhibit growth hence they are regarded as growth regulators. The most abundant natural auxin manufactured in plants is indoleacetic acid (IAA), IAA is secreted most abundantly in the tips of shoots, developing leaves, flowers, fruits, cambial cells and root tips. Auxins have been artificially synthesised. An example of a synthetic auxin is Dichlorophenoxy ethanoic acid (2,4-D).
The roles of auxins
- Phototropism of shoot: When light shines on one side of the shoot of a plant, the auxin on that side is reduced, and consequently the growth of that side slows down. The shaded side (side not receiving light) receives more auxins resulting in faster growth. This unequal growth produces bending of the shoot towards the side receiving light.
- Auxin as inhibitor of lateral buds: Auxins produced in the terminal bud stimulate the growth or elongation of the main stem while at the same time move downward to the shoot to inhibit the growth of lateral buds. Thus, the terminal bud dominates the rest of the shoot so as to ensure that energy for the growth of the main stem is not wasted thereby producing a tall plant with short lateral branches.
- Auxins and fruit development: Once a flower is fertilized, auxins are produced and sent to the ovary causing rapid development of the ovary as it develops into a fruit. The presence of auxins also prevents the formation of abscission layer at the base of the flower stalk. This will help the fruit to reach ripening stage. When the fruits are ripe, auxin supply to the flower stalk is reduced and abscission layer is formed leading to the dropping of the fruit.
- Auxins and leaf abscission: Shedding of leaves is due to the formation of abscission layer at the base of the petiole. Reduced amount of auxins stimulates the formation of abscission layer. Shedding of leaves is an adaptation usually carried out during adverse condition by shrubs and some trees such as rubber and cotton trees.
- Auxins and cell division: Auxins are involved in the process of cell division.
Modern applications of auxins
- Weed control: Synthetic auxin called 2,4-D is used in killing weeds. When applied to a lawn, it kills the weeds having broad leaves without killing grasses.
- Development of seedless fruits: When auxins are applied to flowers that have not been pollinated, fruits develop. Seedless water cucumbers, watermelon and tomatoes have been produced in this way.
- Preservation: Yams and potatoes have been preserved for a long time by the application of auxins to tubers.
- Harvesting: If a farmer has a storage problem or the market price of his fruits is low due to surplus supply by other farmers, he can apply auxins to the plants to make the fruits stay longer on the plants. Reduced amount of auxins stimulates the formation of abscission layer and when auxins are applied to plants, abscission layers will not be formed hence a longer stay of fruits on the plants. Tomato and orange fruits have been prolonged on plants in this way.
- Stem cutting: Auxins have been used to pro mote root growth in stem cuttings. A stem of desired plant is cut into pieces. Auxins are sprayed on the pieces of the stem. Adventitious roots spring up from the stems. In this way the propagation of the plant is ensured.