Which type of herbicide can lead to epinasty of shoot tips, stems, leaves and petioles?

• A. Contact herbicides
• B. Photosynthesis inhibitors
• C. Herbicides affecting cell walls
• D. Growth Regulators

Answer: (D)

Growth regulators act by mimicking plant hormones, particularly auxins, and disrupt normal growth patterns rather than just damaging tissue. When these synthetic hormones perturb growth, one common result is epinasty—the downward curling or bending of shoots, leaves, and their petioles. This happens because the herbicide creates uneven growth on opposite sides of a stem or leaf, causing it to bend downward. Because epinasty across multiple plant parts is a hallmark of auxin-type, growth-regulating herbicides, this type best explains the symptom described.

Other modes of action cause different symptoms. Contact herbicides injure tissue at the point of contact without systemic hormone disruption, so they don’t produce widespread epinasty. Photosynthesis inhibitors disrupt energy production and typically lead to chlorosis or bleaching rather than the characteristic curling. Herbicides that affect cell walls weaken or disrupt cell structure in a way that doesn’t produce the uniform epinastic response seen with growth regulators.