Changes in Grain Sorghum Stomatal and Photosynthetic Response to Moisture Stress across Growth Stages1

Abstract

The dynamics of stomatal response and canopy apparent photosynthesis in grain sorghum [Sorghum bicolor (L.) Moench] were studied throughout a growing season in relation to water stress. Treatments received irrigation weekly at 100% soil water deficit replacement or no irrigation on the extremes of a sprinkler irrigation gradient conducted on a fine sand (Typic Ustipsamment). Stomatal resistance was sensitive to small reductions in leaf water potential during the vegetative period. During the reproductive period the stomata became nearly insensitive to leaf water potential and remained open at low leaf water potentials. Maximum midday deficits in leaf water potential due to drought stress were in the range of 0.1 to 0.2 MPa during the vegetative period, 0.3 to 0.4 MPa during the reproductive period and exceeded 0.5 MPa during grain filling. Stored soil water was steadily depleted in the unirrigated treatment. Water stress reduced grain and dry matter yields 36 and 37% respectively. Photosynthesis per unit leaf area was not decreased by water stress during the reproductive and grain filling periods. The rate of canopy apparent photosynthesis was reduced by 14 to 26% but this was solely a result of less leaf area in the stressed treatment. Canopy evapotranspiration was greatly reduced in the water stressed treatment but evapotranspiration per unit leaf area was apparently unaffected by water stress during all or a large portion of the post‐vegetative period. Reduction in leaf area was an important mechanism for transpiration control under drought stress during the entire reproductive and grain filling periods. Stomatal resistance was not a major transpiration control mechanism. In the absence of increased stomatal resistance both transpiration and photosynthesis per unit leaf area were not reduced during the continually intensifying seasonal drought.