Physiological Reactions to an Imposed Drought by Macroptilium atropurpureum and Cenchrus ciliaris in a Mixed Sward

Abstract

M. atropurpureum cv. Siratro and C. ciliaris cv. Biloela (buffel grass) plants in well-watered and droughted mixed swards were studied to investigate how the two species survive during periods of drought. Rates of photosynthesis and water use efficiencies were generally highest in well-watered buffel grass, whereas the conductance of the abaxial surfaces of leaves was highest in well-watered Siratro. The minimum and daily mean water potentials were consistently lower in well-watered buffel grass than in well-watered Siratro, indicating a lower hydraulic conductance in the soil-plant system of buffel grass. Gross photosynthesis and the conductance of the abaxial surfaces of leaves of both species fell with falling water potential, as did the water use efficiency of buffel grass. Droughted Siratro exhibited paraheliotropic leaf movement, and a reduction in stomatal aperture in the middle of the day. Despite its greater sensitivity to water deficits, Siratro remained physiologically active throughout at least a portion of the day even on the last day of a drying cycle when its leaf water potential ranged from -1 MPa at dawn to a minimum of -1.8 MPa, when the corresponding water potentials for buffel grass were about -4 MPa and about -5 MPa. Siratro had a higher water potential mainly because of the deeper penetration of its roots and its consequent access to water lower in the profile. However, the higher water potential was also due to stomatal opening, and therefore photosynthesis, in droughted Siratro occurring only at times of the day when evaporation, temperature and solar radiation were low. The conductance of the abaxial surfaces of droughted buffel grass leaves was generally steady, or declined gradually during the day. Buffel grass was able to survive drought because of its inherent desiccation tolerance, whereas Siratro was able to survive drought in a mixed sward because it did not compete with buffel grass for soil water.