Phytoxicants are harmful substances produced by plants that can negatively affect the growth, development, or health of other plants, animals, or humans. These compounds can serve as defense mechanisms against herbivores, pathogens, or competing vegetation. Examples include certain alkaloids, flavonoids, and terpenes that may inhibit seed germination or disrupt cellular processes. Understanding phytoxicants is important in fields like agriculture, ecology, and pharmacology.