The IR bands simply indicate the type of bond. The O-H indicates an alcohol, and the C=O indicates a ketone. Combined in one spectra they may indicate an organic acid or possible ester. WHAT that has to do with LEAD remediation, I don't know. If you are looking for lead (Pb) uptake, an AA or ICP would be a better analysis method. IN ALL cases, did you run BLANKS of the same plants/parts to compare with your test plants?

Princess Olivar Tuquero , as you noted previous researchers identified the IF band shift when affected by metals. Assuming that is correct, then the only thing necessary to correlate your FTIR results with the metal content is a graph of the amount of observed band shift to the amount of lead found.

Secondary correlations are not generally preferred analytically, but are sometimes necessary. If you are trying to identify the particular chemical/structure actually doing the adsorption of the lead I can see how combining both might seem simpler. However, as long as you have an AAS it would be better to use it for the metal analysis instead of trying to infer it from the IR. If that is just to explain why your IR peaks are shifted a bit, the metal content would explain that via reference to the paper you mentioned.

As normal growth in plants transports nutrients from the roots ultimately to the leaves it is not surprising to find the highest concentrations of any other compound taken up there as well. I would not expect to find them in the same concentrations evenly throughout the plant structure.