Metabolite analysis by GC-MS of P. vulgaris leaf AWF typically yields 40 - 60 identifiable metabolites and an approximately equal number of unidentifiable compounds (Figure 2). Organic acids, simple sugars, and amino acids represent the bulk of the identifiable metabolites, however, secondary plant metabolites have also been detected and quantified from AWF11. Several example peaks from these different molecule classes are labeled in Figure 2. Further downstream analytical techniques are applicable to these AWF samples to quantify various molecules, for example: ICP-MS, NMR, HPLC, atomic absorption spectroscopy, protein mass spectrometry. The protein component of the apoplast is also represented in the AWF as shown in the Coomassie Brilliant Blue stained SDS-PAGE gel in Figure 3. Among other roles, the apoplastic enzymes are responsible for the synthesis of the cell wall and the creation of extracellular reactive oxygen species. Proteomic studies of AWF from various species have identified dozens of individual proteins and shown that the protein component of the apoplast responds to environmental stress13,19. Ideally AWF extract should be free from contamination by cytoplasmic proteins such as Rubisco; however in practice this is difficult to achieve. The presence of a Rubisco protein band at ~53 kDa following SDS-PAGE provides a further qualitative assay for the integrity of AWF samples. For example, the sample loaded in lane 2 in Figure 3 contains a greater amount of Rubisco contamination that of lane 1.