Casteel notes that, “Implications of early season drought can be long-lasting or short-lived in soybean. Many of our soybean fields are limited due to drought because of reduction in photosynthetic factories (vegetative biomass) in addition to limited nodal production.” And he adds that soybeans have a longer grace period than corn in terms of responding to a drought, since there is not a brief pollination period as in corn. But he says there may be shorter production of nodes on drought-stressed soybean plants. However, he says beans may compensate for that with flowering, pod development and seed fill.
Soybeans are going through the same drought stress as is corn, but may hide that stress more than the corn plant’s leaf rolling. Beans will decrease vegetative growth initially, and then flip leaves away from the sun to reduce water loss through transpiration, and then clamp their leaves together. Early flowering in drought-stressed fields is also an indication the plant is in a survival mode.
Source: FarmGate blog