Density-dependent factors affect population size in relation to the population’s density.

Density-dependent factors down into two categories. 

Negative density dependence 

Positive density dependence (inverse density dependence

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Glycosylated triterpenoid compound avenacin A1, found in oat (Avena sativa) roots The wheat root pathogen Gaeumannomyces graminis var. tritici is extremely sensitive to avenacins

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Tomato plants produce tomatine, a compound that is closely related to avenacin. Only certain pathovars of the fungal pathogen Septoria lycopersici can cause disease on tomatoes, and these pathovars produce a glucosidase that can detoxify tomatine.

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According to Optimal Defense Hypothesis, 

Young tissues not only contain the highest levels of constitutive secondary compounds, they are also more responsive to herbivory. Thus, younger leaves exhibit more robust inducible defenses to herbivores than older leaves.

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Constitutive defenses are costly to the plant. The production of secondary metabolites requires a significant energy investment derived from primary metabolism, which is then unavailable for use in growth and reproduction.

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Inducible defense against pests or pathogens, involve both a wound response and the recognition of certain compounds abundant in the insect’s saliva or regurgitant. 

These compounds belong to a broad group of chemicals called elicitors

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Elicitors are HAMPs or herbivore- associated molecular patterns