Question

Briefly explain (a) the difference between quantitative and qualitative herbivore inhibitors produced by plants and (b) describe 2 major herbivore defense strategies (theories) that potentially explain why quantitative or qualitative inhibitors may be utilized by particular plants as a herbivore defense

Answer 1

The evolution of chemical defenses in plants is linked to the emergence of chemical substances that are not involved in the essential photosynthetic and metabolic activities. These substances, secondary metabolites, are organic compounds that are not directly involved in the normal growth, development or reproduction of organisms, and often produced as by-products during the synthesis of primary metabolic products. Although these secondary metabolites have been thought to play a major role in defenses against herbivores, a meta-analysis of recent relevant studies has suggested that they have either a more minimal (when compared to other non-secondary metabolites, such as primary chemistry and physiology) or more complex involvement in defense.

Qualitative and quantitative metabolites

Secondary metabolites are often characterized as either qualitative or quantitative. Qualitative metabolites are defined as toxins that interfere with a herbivore's metabolism, often by blocking specific biochemical reactions. Qualitative chemicals are present in plants in relatively low concentrations (often less than 2% dry weight), and are not dosage dependent. They are usually small, water-soluble molecules, and therefore can be rapidly synthesized, transported and stored with relatively little energy cost to the plant. Qualitative allelochemicals are usually effective against non-adapted generalist herbivores.

Quantitative chemicals are those that are present in high concentration in plants (5 – 40% dry weight) and are equally effective against all specialists and generalist herbivores. Most quantitative metabolites are digestibility reducers that make plant cell walls indigestible to animals. The effects of quantitative metabolites are dosage dependent and the higher these chemicals’ proportion in the herbivore’s diet, the less nutrition the herbivore can gain from ingesting plant tissues. Because they are typically large molecules, these defenses are energetically expensive to produce and maintain, and often take longer to synthesize and transport.

The geranium, for example, produces a unique chemical compound in its petals to defend itself from Japanese beetles. Within 30 minutes of ingestion the chemical paralyzes the herbivore. While the chemical usually wears off within a few hours, during this time the beetle is often consumed by its own predators.

Carbon:nutrient balance hypothesis

The carbon:nutrient balance hypothesis, also known as the environmental constraint hypothesis or Carbon Nutrient Balance Model (CNBM), states that the various types of plant defenses are responses to variations in the levels of nutrients in the environment. This hypothesis predicts the Carbon/Nitrogen ratio in plants determines which secondary metabolites will be synthesized. For example, plants growing in nitrogen-poor soils will use carbon-based defenses (mostly digestibility reducers), while those growing in low-carbon environments (such as shady conditions) are more likely to produce nitrogen-based toxins. The hypothesis further predicts that plants can change their defenses in response to changes in nutrients. For example, if plants are grown in low-nitrogen conditions, then these plants will implement a defensive strategy composed of constitutive carbon-based defenses. If nutrient levels subsequently increase, by for example the addition of fertilizers, these carbon-based defenses will decrease.

Plant Apparency Hypothesis

In 1976, Feeny suggested that conspicuousness or apparency, which can affect the vulnerability of a plant to insect herbivores, is likely to influence the evolution of defences (plant apparency hypothesis, PAH). Thus, plants that are not easily detected by herbivores are less likely to suffer herbivore damage and so have less need for defences. Such plants would invest in what are known as qualitative defences, e.g. small molecules such as glucosinolates, which are relatively inexpensive to synthesize. In contrast, apparent plants would possess a range of dosage‐dependent or quantitative defences, which would interfere with the ability of herbivores to acquire nutrients (Feeny, 1976). Plants would need heavy investment in such defences, because:

• the compounds, such as tannins, are large and biosynthetically expensive to make; and
• a lot of the particular defence would be required to be effective.

On the other hand, specialist herbivores can evolve tolerance to certain defence compounds, and this is more common for qualitative defences than for quantitative defences. Qualitative defences would reduce herbivore growth rates, thereby subjecting the herbivores to increased predation risk.

Although there is some support for the apparency hypothesis, the true apparency of a plant to its herbivores is extremely difficult to quantify and it changes with changing biotic contexts (such as season, surrounding vegetation, etc.). Being apparent can also have some benefits that may counterbalance costs. For example, some volatile chemical traits of plants that contribute to their apparency to herbivores also attract their natural enemies (Halitschke et al., 2008). Apparent plants may also generally grow in less competitive environments, in which induced resistance and tolerance mechanisms may function more optimally.