Physiological responses of two soybean cultivars to cadmium

Anthropogenic activities are increasing cadmium (Cd) concentrations in agricultural soils. Cadmium can be absorbed by plant roots and modify the physiology of the plant. Carbon exchange rate (CER) and leaf conductance to water vapor (g(s)) were measured on the fifth leaf of two soybean (Glycine max [L.] Merr.) cultivars (Illini insensitive and Richland sensitive) for 6 consecutive days; Cd(NO3)(2) was added to the hydroponic solution to achieve a final concentration of 50 mu mol. At the end of the experiment, stomata length and width, mesophyll limitation to photosynthesis, root hydraulic conductance, relative water content (RWC), and Cd concentration in leaves, stems, and roots were measured on treated and control plants. Cadmium progressively reduced CER and g(s) to about 50% after 6 d of treatment. This was more evident in Richland than in Illini and was not linked with leaf RWC and mesophyll limitation to photosynthesis. After 6 d, the apparent root hydraulic water conductivity was 67% lower in the Cd-treated plants than in controls. The primary mechanism affected by Cd-induced stress in soybean is root water uptake, and this reduction is consistent with the decrease in stomatal opening and conductance, and therefore, in photosynthesis.