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How are synthetic auxins used as selective weed killers?

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fern life cycle — Plant development The life cycle of the fern. (1) Clusters (sori) of sporangia (spore cases) grow on the undersurface of mature fern leaves. (2) Released from its spore case, the haploid spore is carried to the ground, where it germinates into a tiny, usually heart-shaped, gametophyte (gamete-producing structure), anchored to the ground by rhizoids (rootlike projections). (3) Under moist conditions, mature sperm are released from the antheridia and swim to the egg-producing archegonia that have formed on the gametophyte's lower surface. (4) When fertilization occurs, a zygote forms and develops into an embryo within the archegonium. (5) The embryo eventually grows larger than the gametophyte and becomes a sporophyte.

Synthetic auxins function as selective weed killers by exploiting differences in how various plant species respond to these growth-regulating hormones. While natural auxins like indoleacetic acid are easily broken down by plant enzymes, synthetic versions are designed to resist this enzymatic destruction, allowing their growth-promoting effects to be prolonged and intensified within the plant.12

When applied, these compounds disrupt the normal development of dicotyledonous plants (broad-leaved plants). By causing uncontrolled and abnormal growth, they effectively kill the target weeds. Because monocotyledonous crops, such as cereal grains or grasses, are generally less sensitive to these specific synthetic auxins, they can be used to selectively clear weeds from fields or lawns without harming the desired plants. Additionally, these compounds are sometimes utilized as defoliating agents to remove leaves from dicotyledonous species.12