Difference in Cd2+ Flux Around the Root Tips of Different Soybean (Glycine Max L.) Cultivars and Physiological Response Under Mild Cadmium Stress

Abstract

The purpose of the study was to compare differences in Cd²⁺ flux in the vicinity of root tips of various soybean cultivars under mild Cd stress conditions using non-invasive micro-test technology (NMT). The results indicated that Cd²⁺ influx to the root tips under mild Cd treatment was higher compared to controls. Cd²⁺ influx showed an obvious spatial distribution, with the highest Cd²⁺ influx measured 300 μm from the root tips, and a gradually decrease above and below this site. The cultivar Liaodou32 had a lower Cd uptake (3.40 pmol cm^-2 s^-1), while Liaodou23 had a relatively higher Cd uptake (66.37 pmol cm^-2 s^-1). Cluster analysis showed that the order of the average Cd²⁺ influx of the cultivars at a distance of 300 μm from the root tips was as follows: high-uptake cultivars (61.80 pmol cm^-2 s^-1)＞medium-high-uptake cultivars (33.92 pmol cm^-2 s^-1)＞medium-low-uptake cultivars (19.78 pmol cm² s^-1)＞low-uptake cultivars (4.84 pmol cm^-2 s^-1). We also analyzed physiological responses of different soybean cultivars to mild Cd stress. The results indicated that mild Cd stress could inhibit soluble protein production and root vigor among individual soybean cultivars. Moreover, stress increased SOD, CAT and POD activities and MDA content in root tissues. It should be noted that the physio-biochemical indicators of low-uptake cultivars did not change significantly after exposure to mild Cd stress compared to controls. Pearson’s correlation analyses showed that all physio-biochemical indicators were significantly positively associated with influx, except of root SP and biomass. PCA analysis demonstrated that root vigor was a dominant factor causing the differences in Cd tolerance among different soybean seedling cultivars. Soybean seedling cultivars with parallel Cd uptake had similar physio-biochemical characteristics under Cd stress.

Keywords: Cd, soybean, influxes, root tips, root vigor, POD.