EFFECT OF CADMIUM AND ENHANCED UV-B RADIATION ON SOYBEAN ROOT EXCRETION

An increase in ultraviolet_B (UV_B, 280_315 nm) radiation reaching the earth's surface due to depletion of the stratospheric ozone layer and heavy metal pollution due to industrial waste have become serious concerns. In order to understand the influence of enhanced UV_B radiation and heavy metal pollution on plant root growth, physiological metabolism, and tolerance to heavy metals, the effect of cadmium (Cd) associated with enhanced UV_B radiation on the excretion activity of soybean root was investigated. The full amounts of nitrogen, organic carbon, and sulfur excreted by soybean ( Glycine max ) root exposed to Cd 2+ pollution, enhanced UV_B radiation and both in combination were measured using the Kjeldahl method and a CS_344 carbon and sulfur system (LECO USA). The results showed that Cd 2+ could enhance root excretion of organic carbon, but the excretion was inhibited by enhanced UV_B radiation. Excretion of organic sulfur compounds was only accelerated by severe stress, that is a combination of higher Cd 2+ concentration and stronger UV_B radiation had a synergistic effect on excretion. Nitrogen content was mainly affected by Cd 2+ . The possible reasons were: 1) Cd 2+ pollution promoted roots to excrete more organic nitrogen compounds, such as amino acids and polypeptides; 2) Cd 2+ decreased absorption ability of roots, and nitrogen uptake from rhizosphere was reduced, thereby resulting in an increase in nitrogen content in the environment of the root. Nitrogen content in the environment could also be affected by enhanced UV_B radiation, but this effect was insignificant. The results indicated that UV_B radiation could alter the composition and proportion of root excretions, and, perhaps, affect plant tolerance to Cd 2+ stress.