Which of the following is NOT a factor affecting the susceptibility of plants to certain herbicides?

The susceptibility of plants to certain herbicides is influenced by several factors, and leaf angle or size is generally considered less critical compared to the other options listed. While leaf angle and size can somewhat affect herbicide deposition and uptake, they are not fundamental factors in the same way that adjuvants, application rate, and product formulation are.

Adjuvants can enhance the effectiveness of herbicides by improving their ability to spread and adhere to plant surfaces, thereby increasing absorption. The application rate is crucial because it determines how much of the herbicide reaches the plant, influencing its effectiveness and potential for damage. Product formulation also plays a vital role, as different formulations can affect the herbicide's stability, solubility, and interaction with plant tissues.

In contrast, leaf angle and size may impact how well a herbicide adheres to the leaf surface or penetrates the leaf tissue but do not significantly change the inherent susceptibility of the plant to the herbicide's active ingredients. Therefore, while leaf characteristics sometimes play a role, they are not primary factors driving susceptibility as the other options are.