Cellular Photosynthetic Pigment and Structural Change in Festuca arundinacea (Tall Fescue) after Exposure to Acute and Chronic Chromium and Copper Stress

Abstract

Expanded industrial globalization has resulted in the release of high concentrations of heavy metals into environmental water sources and soils. Phytoremediation may help to remove these heavy metals from contaminated soils. Tall fescue (Festuca arundinacea Shreb.) exhibits phytoremediation potential due to its endurance and high stress tolerances. Here, we report photochemical and structural responses in tall fescue to acute and chronic doses of heavy metals, copper (Cu) and hexavalent chromium (Cr(VI)). Visual signs of stress and decreased photosynthetic yield measurements were detected for both the acute and chronic exposures. To gain insight into stress responses at the cellular level, structural and pigment changes in tall fescue in response to Cu and Cr(VI) stress were assessed with brightfield and confocal fluorescence imaging. While brightfield images showed qualitative changes in plant tissue structure, the quantification of changes were not statistically significant due to high variability between leaf blades. Fluorescence imaging confirmed decreasing total chlorophyll content in tall fescue cross-sections in response to Cr(VI) and Cu exposure. To spectrally separate the closely related chlorophyll pigments (Chl-a, Chl-b, and Chl in photosystem I) and visualize their relative localizations within the plant tissue, hyperspectral confocal fluorescence microscopy was conducted with multivariate curve resolution (MCR) analysis of the data. These results determined that Chl-a and Chl-b were more significantly reduced than Chl associated with photosystem I. Additionally, a new spectral component was identified. A broad autofluorescence (AF) feature appeared in the late stress response of both acute and chronically exposed tall fescue and was localized in globular bodies. While the identity of the broad AF feature remains to be identified, we hypothesize that it may be associated with degraded chlorophyll components in autophagic bodies. If confirmed, this would indicate that autophagy is a stress response to heavy metal exposure in tall fescue.