Measuring heavy metal stress in ryegrass using helium-cadmium excitation-based photoluminescence

Abstract

As the chemical analysis of heavy metal uptake and toxicity in plants is time-consuming, expensive, and destructive, a simple and reliable method for detecting heavy metal transfer from the soil to plants is thus necessary. We aimed to measure copper (Cu), lead (Pb), and cadmium (Cd) stress in ryegrass in vivo using plant photoluminescence based on a helium (He)-cadmium excitation source (wavelength 325 nm). The soils were combined with Cu, Pb and Cd, respectively. Fifteen ryegrass seedlings (Lolium multiflorum) were planted in each pot. After 30 days, leaves from seedlings at the fourth-leaf stage were collected and the fluorescence excitation spectra were detected using a He-Cd laser at an excitation wavelength of 325 nm. Three emission peaks that constitute known chlorophyll wavelengths, namely 450, 690, and 735 nm, were detected. These three peaks were strongly influenced by Cu, Pb, and Cd concentrations in the soil. Higher peak heights at 450 nm were observed with increasing Cu, Pb, and Cd concentrations in the soil, whereas no changes were noted at 690 and 735 nm. The P450/P690 and P450/P735 ratios were positively correlated with Cu, Pb, and Cd soil concentrations, ryegrass uptake, and DTPA-extractable Cu, Pb, and Cd in the soil. The related coefficients were all greater than 0.9. However, no correlation between the P690/P735 ratio, Cu, Pb, and Cd ryegrass uptake, and DTPA-extractable Cu, Pb, and Cd in the soil was observed. The measurement of plant photoluminescence in vivo using a He-Cd excitation source (wavelength 325 nm) may be utilized as an approach for monitoring the response of plants to specific stressors.