What do chemical barriers in plants do to protect against pathogens?

Chemical barriers in plants primarily function to inhibit the growth or kill pathogens, making this the correct choice. These barriers produce a range of secondary metabolites, such as phenolics, terpenoids, and alkaloids, which can be toxic to pathogens or impede their growth. These compounds often have antimicrobial properties that allow plants to respond swiftly to infections, thus preventing the spread of disease within their tissues.

Plants may also use specific signaling compounds to activate defense mechanisms in response to pathogen detection, leading to the production of additional anti-pathogenic substances. This is a critical aspect of a plant's innate immune system, which allows it to defend itself against various microbial threats.

While preventing the entry of pathogens is a relevant protective measure, it is primarily physical barriers such as the cuticle and cell walls that perform this function rather than chemical means. Engaging leukocytes is a process more relevant to animals and not a feature of plant defense mechanisms. Enhancing photosynthesis does not directly correlate with pathogen defense and is unrelated to the function of chemical barriers against pathogens.