Fig. 1.

AquaDust as an in situ reporter of water potential ($\psi$). (A) Schematic representation of a maize plant undergoing transpiration ($E$) in a dynamic environment driven by solar thermal radiation ($Q_{\mathrm{r}\mathrm{a}\mathrm{d}}$) and photosynthetically active radiation ($PAR$), wind speed ($u$), temperature ($T$), vapor pressure deficit ($VPD$), and soil water potential (${\psi }^{\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}}$). Water flows through the plant (blue arrows) along a gradient in water potential ($\overrightarrow{\nabla }\psi$). Zones on the leaves infiltrated with AquaDust serve as reporters of the local leaf water potential, ${\psi }^{\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{f}}$, via a short ($\sim 30$ s), minimally invasive measurement of FRET efficiency ($\zeta$) with a leaf clamp. (B) Schematic representations of infiltration of a suspension of AquaDust and of the distribution of AquaDust within the cross-section of a leaf. AquaDust passes through the stomata and localizes in the apoplastic spaces within the mesophyll; the particles are excluded from symplastic spaces and the vascular bundle. (C) Schematic diagrams showing mechanism of AquaDust response: The swollen, “wet” state when water potential in its local environment, ${\psi }^{\mathrm{e}\mathrm{n}\mathrm{v}}=0$ (i.e., no stress condition), results in low FRET between donor (green circles) and acceptor (yellow circles) dye (Upper); and the shrunken, “dry” state when ${\psi }^{\mathrm{e}\mathrm{n}\mathrm{v}}<0$ (i.e., stressed condition) results in high FRET between fluorophores, thereby altering the emission spectra (Lower). (D) Fluorescent dyes were chosen to minimize reabsorption of AquaDust emission from chlorophyll; comparison of representative fluorescent emission from AquaDust (donor peak at 520 nm and acceptor peak at 580 nm) with the absorption spectra of chlorophyll and autofluorescence of maize leaf.