Spectroscopic studies of photosynthetic responses of tomato plants to the interaction of zinc and cadmium toxicity

The in vivo chlorophyll (Chl) fluorescence spectra of Solanum lycopersicum leaves were recorded in the spectral region 650–800 nm using a spectroscopic method based on ultraviolet light emitting diode induced fluorescence spectroscopy (UV-LED IFS). These spectra have been used to analyze the interactive functions of cadmium (Cd2+) and zinc (Zn2+) on photosynthetic activities of S. lycopersicum plants. The fluorescence intensity ratios (F690/F735) of the chlorophyll bands at 685 and 730 nm were calculated by evaluating curve fitted parameters using a Gaussian spectral function, for control as well as treated plants. The fluorescence induction kinetics (Kautsky effect) was also measured on dark adapted intact plant leaves at the chlorophyll bands for determining the variable chlorophyll fluorescence decrease ratio (RFd values) and the stress adaptation index (Ap). In addition, metal accumulation in plants, plant growth and photosynthetic pigments content were estimated. It was found that the RFd(690), RFd(730) and Ap values decreased whereas the F690/F735 ratio increased in the case of 10 μM Cd2+ treated plants, indicating an impairment of the photosynthetic efficiency. Zn2+ supplementation, at low concentration (10 and 50 μM), in combination with Cd2+ protect the photochemical functions. However, the high Zn2+ concentration exacerbated the negative effects of Cd2+ and showed a severe decrease of RFd(690), RFd(730) and Ap values compared to Cd2+ alone. It is seen that F690/F735 ratios are strongly correlated with chlorophyll contents. The results demonstrate the usefulness of F690/F735, Ap and RFd values in determining the potential photosynthetic activity of an intact attached leaf in a non-destructive way.