Illinois Field Crop Applicators Practice Test

Which type of herbicide causes twisting and cupping of leaves?

1. Growth regulator

2. Photosynthesis inhibitors

3. Contact herbicides

4. Systemic herbicides

The correct answer is: Growth regulator

Growth regulator herbicides are known to cause twisting and cupping of leaves because they mimic natural plant hormones called auxins. These herbicides disrupt the normal growth processes in plants by promoting abnormal growth patterns. When a plant is exposed to a growth regulator herbicide, it can lead to excessive elongation or distortion of plant tissues, particularly in the leaves. This distortion manifests as curling or twisting, which can significantly impair the plant's ability to photosynthesize and develop normally. In contrast, photosynthesis inhibitors work by blocking the photosynthesis process without typically causing such noticeable morphological changes in leaf shape. Contact herbicides act by destroying the plant upon direct contact but do not affect the entire plant's growth in the same way as growth regulators. Systemic herbicides are absorbed by the plant and translocated throughout; their effects are more related to overall plant health rather than specific leaf deformation patterns.