Drought enhances stomatal closure in response to shading in sorghum (Sorghum bicolor) and in millet (Pennisetum americanum)

When field-grown sorghum (Sorghum bicolor (L.) Moench) and millet (Pennisetum americanum (L.) Leeke) plants are subjected to drought, the speed of stomatal closure in response to darkness is enhanced in comparison with the speed observed in well-irrigated control plants. This shade-induced closure is most apparent at early stages of desiccation and is not rapidly reversible. These results need to be considered when developing protocols for the measurement of photosynthetic light response curves in the field. The sensitivity to crop water status of this stomatal closure response potentially provides a very valuable means for detection of the early stages of soil drying, and may also provide opportunities for screening different varieties for their adaptation to drought conditions.