Do not even think of looking at the seasonal variation of plant defense compounds without measuring the leaf dry weight – the use of graphical vector analysis is useful just like the proper quantitation protocols
Proper quantitation methods are not enough – there is more
There are a few issues to consider in detail before rushing into measuring seasonal variation of any plant metabolites. We are not talking of all the issues that need to be taken into account in quantitative studies, but additional issues specifically linked to studies on seasonal variation. See below for some examples.
Are you prepared to measure the dry weight of all leaves as well?
To understand seasonal variation and further biogenesis of the plant metabolites, it is most productive to measure concentrations of known compounds that have clear structural linkages to each other. Do you then want to draw conclusions, which of these compounds is actively synthesized and at which stage of the season? If yes, then you must weigh every single leaf you use in the quantitation for its dry weight. Why is that? It is simply no good quantifying mg/g DW concentrations in the growing leaf tissue without knowing the leaf dry weight, since otherwise one cannot make any conclusions about the rate of biosynthesis.
It is in general a very sensitive issue to make any conclusions of the rate of biosynthesis on the basis of foliar metabolite concentrations. However, sometimes people interpret constant mg/g DW concentration of foliar metabolites as a proof of constant biosynthesis, i.e. equal amount of the compound is produced as is used for the production of further biosynthetic products. In spring this conclusion could not be much worse! In rapidly growing leaf a constant mg/g DW concentration may mean that the mg/leaf content of the metabolite may be more than doubled in a couple of days. One should thus calculate the mass of the compound per one leaf, not the mg/g DW concentration only. Figure 1 shows examples of such cases where mg/g and µg/leaf values give fully different impressions of seasonal variation of hydrolysable tannins.
Consider using the graphical vector analysis – it is quite visual
The above issue of increased vs. decreased biosynthesis rates can be revealed also by graphical vector analysis developed by Dr. Julia Koricheva in 1999. This analysis uses simultaneously both the foliar metabolite concentration in mg/g DW and the leaf dry weight to see how these change in time. This analysis reveals the stage of biosynthesis visually and is thus highly powerful to be used parallel with the figures shown above. Fig. 2 shows an example of graphical vector analysis.
“Do not dilute your knowledge – plants do it for you.”