Stomata biosilica and Equisetum photosynthesis: ionic tomography insight using a PDMPO silicaphilic probe

Confocal microscopy using silicaphilic molecular probes is a promising approach to identify the ionic character of silica interfaces. Using ab initio and density functional theory we model structural and electronic properties of the (2-(4-pyridyl)-5(4-(2-dimethyl-aminoethyl-aminocarbamoyl)-methoxy)phenyl)-oxazole (PDMPO) chromophore at different protonation states, in vacuum, and when next to silica of different ionicity. For protonated chromophores next to anionic silica sites, theory suggests strong emission in the visible spectral range from higher excited states and the probability of weaker near infrared fluorescence from a lower energy manifold. Using theory insights, we conduct single- and two-color confocal microscopy in the visible and in the near infrared, respectively, to study open and closed stomata of Equisetum arvense, a heavily silicified primitive plant. Three-dimensional ionic tomography resolves sub-micron neighbouring regions of high and low ionic charges of exo/endo-skeletal silica components according to whether they are open or closed. Considering the variance of methane and carbon dioxide levels prior to, during and after the Silurian, we discuss the observed high ionic contrast of stomatal apertures upon opening as a signature of bioinorganic machinery able to moderate methane and carbon dioxide transport for optimal growth under a range of atmospheric conditions.