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Relationship Between Physiological and Morphological Characteristics and Yield of Nondormant Alfalfa Clones¹

Alfalfa (Medicago sativa L.) breeders are constantly striving to improve the productivity of alfalfa. Yields have been increased primarily through the selection of plant materials resistant to insects and diseases. The objective of this study was to evaluate physiological characteristics which might be used as selection criteria in alfalfa improvement. Alfalfa clones were grown under field conditions on a Mojave clay loam soil. Carbon dioxide flux was evaluated in a closed system using an infrared gas analyzer. The physiological variables measured in these studies did not account for the variation in yield among the clones. Apparent photosynthetic rates, dark respiration rates, and postillumination CO₂ burst rates expressed as mg CO₂ dm^–2 hour^–1 were not correlated with dry matter production. However, when these physiological factors were multiplied by leaf area and expressed as total CO₂ exchange per plant per hour, there was a significant relationship between yield and these calculated variables, in both studies. Regression analyses of more than 30 factors indicated that leaf area, leaf to stem-petiole ratio, and leaf weight per plant accounted for more than 95% of the variation in yield among alfalfa clones. The data from these studies suggest that morphological factors were more reliable indicators of alfalfa productivity than physiological factors.

Key Words: Photosynthesis • Respiration • Leaf Area • Leaflet to stem-petioles ratio • Leaflet weight

² Former graduate research associate (now assistant professor of crop science, California Polytechnic State Univ., San Luis Obispo, CA 93407); former graduate research assistant (now graduate assistant, Dep. of Agronomy, Univ. of Missouri, Columbia, MO 65201), and professor, Dep. of Agronomy and Plant Genetics, Univ. of Arizona, Tucson, AZ 85721.

Received for publication July 7, 1975.