PHYSIOLOGICAL-RESPONSES OF 2 SOYBEAN [GLYCINE-MAX (L) MERR] CULTIVARS TO SHORT-TERM FLOODING

Flooding of soybean plants [Glycine max (L.) Merr] significantly reduces crop growth and yield, but the underlying physiological responses are poorly documented. The short-term physiological effects of flooding on soybean were studied in the field on a relatively impermeable soil classified as Typic Albaqualf. Flooding treatments were imposed on two soybean cultivars, Essex and Forrest, at a vegetative (V4) and a reproductive (R2) growth stage. Diurnal measurements of net photosynthesis (Pn), stomatal conductance (gs), and components of leaf water potential were recorded on 4 consecutive days following flood application, and again at 14 days after the flood was removed. Photosynthesis of Essex was reduced significantly within 48 h of flooding by 33 and 32% for the V4 and R2 growth stages, respectively, while reductions of 16 and 22% in Pn of Forrest were evident. Reductions in gs of 46 and 24% occurred within 48 h for Essex and Forrest in the V4 stage, although in the R2 growth stage, both cultivars experienced an approximate 48% reduction in gs. However, the decline in Pn with flooding was only partially explained by changes in gs. Photosynthesis was correlated with stomatal closure at low gs (< 0.5 mol m-2 sec-1), but a higher values of gs an approximate 20% reduction in Pn was observed presumably due to non-stomatal limitations. Flooding did not affect components of leaf water potential indicating that the decreases in Pn and gs were not associated with plant water-deficit stress. Flooding of Essex and Forrest at either the V4 or R2 growth stage significantly reduced the dry matter accumulation during the flooding treatment and the subsequent growth. Final seed yields were reduced significantly by a mean of 52 and 40% for Essex and Forrest, respectively. Overall, Forrest appeared more tolerant to excess water than Essex. © 1989.