Table 4.

A framework briefly outlining the available models, the experimental setup required and the future improvements needed to predict and validate in planta, intraspecific and interspecific variation in plant chemical defences

	In planta variation	Intraspecific variation
Types of experimental setup	○ Sampling of different tissue types from the same individuals. ○ Repeated sampling of a population over time.	○ Sampling of the same species between different populations. ○ Sampling single species over environmental gradients.
Relevant theories	○ Optimal defence theory (ODT).	○ Growth rate hypothesis (GRH). ○ Growth–differentiation balance hypothesis (GDBH). ○ Resource exchange model of plant defence (REMPD). ○ Intraspecific framework for variation in plant defences (ISF).
Current status	○ The ODT is the only major theory to predict in planta variation. ○ The level of chemical defences is hypothesized to vary with the value of the tissue to the plant, as well as with the level of herbivory. ○ This ‘value’ is not well defined, but generally reproductive tissue is thought to be of higher value than leaves and is therefore better defended. ○ Its applicability to below-ground defences has not been tested thoroughly.	○ The ISF is the most advanced theory in predicting chemical defences at the intraspecific level, expanding upon the GRH by partitioning induced and constitutively active defences. ○ However, the ISF suggests a linear relationship between defences and resources, whilst the GDBH suggests there is an optimum. ○ The specific environmental factors that regulate chemical defences remain poorly defined.
Future improvement	○ More studies are needed to determine the relative value of root tissue to plants. ○ Further quantification of below-ground herbivory rates are required. ○ At the intraspecific level, other theories are considered superior to the ODT, therefore the in planta aspects of the ODT should be amalgamated with these more sophisticated models of intraspecific variation.	○ Further experimental observations are required to determine whether partitioning-induced and constitutive defences are valid. ○ Experimental in situ observations should be performed to identify putative environmental determinants of chemical defences, before validation within glasshouse studies.