The Effects of Low Level Moisture Stresses on the Rate of Apparent Photosynthesis in Corn 1

T HE idea that moisture stresses reduce growth rates and economic yields in crop plants is not new, but until recently little has been done to define the limits of the term "stress" or to describe in detail the environmental conditions under which this stress may occur. The objective of the experinaents reported here was to determine the point at which moisture stress begins to affect the rate of apparent photosynthesis and to examine certain hypotheses concerning the mechanism of this effect in corn (Zea mays L.), rather than to study plant reactions to a full range of possible moisture stresses. Denmeads and Brix (2) have shown that it is impossible to predict the moisture status of plants from the soil moisture tension alone, since plant water stress depends on the relative rates of water absorption and loss. Slatyer (15) suggests that the whole pathway of water movement from soil to atmosphere is best interpreted as a thermodynamic continuum. Broyer (4) defines "net influx specific free energy" as the action capacity between the algebraic sum of the specific free energies tending to cause water to move into the system and those tending to cause water to move out of the system.