Species differ in individual-to-individual differences in their defense chemistry and some patterns in this variation can be only witnessed by detailed inspections of the relative composition of individual metabolites
Use clones when you can, or be prepared to using excess replicates
Clones are really nice to use in experiments where the numbers of individual plants needs to be minimized. Their defensive chemistry is quite similar – but not exactly – and thus the effects of some selected treatments can be easily spotted when compared to the control plants. Given that the clones used are no tree seedlings, but grown trees, if trees are to be used (see the section Life Stage Differences). Of course the use of clones is not possible in all cases and in these cases at least the origin of the used plants should be known and homogenized as well as possible. If all test plants are selected randomly from the forest, then the effects of a treatment may need tens and hundreds of plant individuals before differences to control plants can be spotted. These difficulties may be highly species-specific and compound-class specific, white birch and mountain birches and their hydrolysable tannins being the most difficult pairs so far we have seen at the Natural Chemistry Research Group. Exact reasons for such differences in birches remain yet to be discovered, but we expect those to be related to the controlling of the activity of the dehydroshikimate dehydrogenase enzyme that catalyzes the formation of gallic acid from dehydroshikimic acid, the intermediate of the shikimic acid pathway.
How large are the typical differences then – a species-specific question
How large are then the differences between individuals and why? This depends on many factors some of which are still unknown. If we take two plant individuals of the same species, same age and size, it is expected that their tissues contain qualitatively the same defense compounds, but that the concentrations may differ a little. Typically this means 10-30% differences in total concentrations and slight variations in the compound composition.
“This tree is much richer in hydrolysable tannins – you can see it from the leaves!”