Effects of cadmium on the photosynthetic activity in mature and young leaves of soybean plants

Cadmium (Cd) is a widely spread pollutant and can be easily taken up by crop from soil, resulting in a serious health issue for humans. The objective of this study was to comparatively investigate the photosynthetic activity, chlorophyll a fluorescence, chlorophyll contents, and spectral reflectance in mature and young leaves of soybean plants after being treated with different concentrations of Cd for 10 days. The photosynthetic rate, chlorophyll contents, actual photochemical efficiency of PSII, and photochemical quenching in the young leaves decreased more significantly with increasing concentrations of Cd in the nutrient solution, compared with those in the mature leaves, though the young leaves had less Cd concentrations. Thus, there was more excessive excited energy produced in the young leaves than that in the mature leaves. In the young leaves, due to more excessive excited energy, more reactive oxygen species may be generated, which further damaged the photosynthetic apparatus. It was supported by the fact that the decrease of reflectance in near-infrared wavelengths of the young leaves was more noticeable than that of the mature leaves. In addition, the chlorophyll a fluorescence transients of the young leaves was significantly different from that in the mature leaves, indicating that the electron transport of young leaves were inhibited much more severely than that of the mature leaves. These observations imply that the responses of photosynthetic activity of soybean leaves to Cd stress depend on their growth stage, and the Cd-induced inhibition of photosynthetic activity might be attributed to the decrease in chlorophyll contents and the decrease in mesophyll CO2 assimilation ability cause by the Cd, which further decreased the consumption of ATP and NADPH, leading to accumulation of NADPH on the acceptor sides of the PSI, and then feedback inhibited electron transport in chloroplasts.