Responses of soybean to oxygen deficiency and elevated root-zone carbon dioxide concentration

Root flooding is damaging to the growth of crop plants such as soybean (Glycine max L.). Field flooding for 3 d often results in leaf chlorosis, defoliation, cessation of growth and plant death. These effects have been widely attributed solely to a lack of oxygen in the root-zone. However, an additional damaging factor may be CO2, which attains levels of 30 % (v/v) of total dissolved gases. Accordingly, the effects of root-zone CO2 on oxygen-deficient soybean plants were investigated in hydroponic culture. Soybean plants are shown to be very tolerant of excess water and anaerobiosis. No oxygen (100 % N-2 gas) and low oxygen (non-aerated) treatments for 14 d had no effect on soybean survival or leaf greenness, but plants became severely chlorotic and stunted when the roots were exposed to no oxygen together with CO2 concentrations similar to those in flooded fields (equilibrium concentrations of 30 %). When root-zone CO2 was increased to 50 %, a quarter of soybean plants died. Those plants that survived showed severe symptoms of chlorosis, necrosis and root death. In contrast, rice (Oryza sativa L.) plants were not affected by the combination of no oxygen and elevated root-zone CO2. A concentration of 50 % CO2 did not affect rice plant survival or leaf colour. These results suggest that the high susceptibility of soybean to soil flooding, compared with that of rice, is an outcome of its greater sensitivity to Col. (c) 2003 Annals of Botany Company.