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A Comparison of Apparent Photosynthesis in Sericea Lespedeza and Alfalfa¹

Sericea lespedeza [Lespedeza cuneata (Dumont G. Don.)] is well adapted to acid soils of the southern USA, but has a low yield potential. A greenhouse study was conducted to compare irradiance, temperature and water-stress responses, and the photosynthetic potential of sericea and alfalfa (Medicago sativa L.). Shoot tips were enclosed in an acrylic chamber and responses of apparent photosynthesis (AP) to temperature across the range of 10 to 50°C and irradiance from 0 to 2 mmol quanta m^–2, s^–2 were determined on alfalfa and two cultivars of sericea. In a separate experiment, AP of the two species was compared across a range of tissue water potentials. In both experiments transpiration and dark respiration were also measured. Under all conditions tested, except very low irradiance and severe water stress, AP of sericea was one-half or less than that of alfalfa. Maximum AP ranged from 22 to 30 µmol C0₂ m^–2 s^–1 for alfalfa, but only from 10 to 15 µmol CO₂ m^–2 s^–1 for sericea. The two cultivars of sericea, ‘Serala’ and ‘AU-Lotan’, did not differ in AP. Temperature response of AP was similar for the two species, with the optimum being about 25 to 30°C. Saturation of AP occurred at about one-half of full sunlight in alfalfa and sericea at 30°C. Apparent photosynthesis decreased in both species with decreasing tissue water potential, and under severe water stress the rates tended to converge for sericea and alfalfa. Although responses to irradiance, temperature, and water-stress were similar in sericea lespedeza and alfalfa, photosynthetic potential appeared to be much lower in sericea lespedeza.

Key Words: Transpiration • Temperature • Light intensity • Forage legumes • Water stress • Medicago sativa L. • Lespedeza cuneata (Dumont G. Don.) • Dark respiration

² Professor and assistant professor, respectively, Agronomy Dep., Univ. of Georgia, Athens, GA 30602.

Received for publication April 7, 1986.