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Seasonal Variation in Photosynthesis, Respiration, and Growth Components of Nondormant Alfalfa (Medicago sativa L.)¹

Alfalfa (Medicago sativa L.) plants were grown under field conditions to determine causes of yield fluctuation throughout the growing season. Photosynthesis, dark respiration, forage yield, specific leaf weight (SLW), leaf size, leaf:stem ratio, and leaf area index (LAI) were measured for six harvests.

High atmospheric temperatures during periods of reduced forage yield resulted in a maximum decrease of 38% in apparent photosynthesis and a 19% decrease in dark respiration. This reduction in photosynthesis may contribute to the "summer slump" in yield of alfalfa grown in warm climates. Specific leaf weight decreased with increased temperatures and followed a similar seasonal pattern under two plant densities.

Leaf area index continued to decrease throughout the season in a pattern similar to forage yield. The data suggest that the decline in leaf area index was not a result of a decrease in leaflet size.

Key Words: Specific leaf weight • Leaf area index • Leaf size • Forage yield

² Assistant Professor of Crop Physiology, University of Wyoming, Laramie, WY 82070; Professor, Department of Agronomy and Plant Geneitcs, University of Arizona, Tucson, AZ 85721; and Instructor, Big Bend Community College, Moses Lake, WA 98837.

Received for publication April 16, 1973.