Yield of Alfalfa as Related to Carbon Exchange

Photosynthetic rate has not proved to be an effective selection tool for forage yield in alfalfa (Medicago sativa L.). A consistantly positive correlation between yield and photosynthesis, when expressed on a leaf area and leaf weight basis, of individual plants has not been established. Since the leaf area per plant often confounds yield and carbon exchange research, this study was conducted to evaluate individual genotypes for potential total CO₂ incorporation. This method of expressing photosynthesis would combine the effects of genotype variability in photosynthetic rate per unit leaf area and total leaf area per plant. Two clones which previously exhibited high photosynthetic rates and two clones with low photosynthetic rates per unit leaf area were selected from the cultivar ‘Mesa-Sirsa’ and were transplanted into an established alfalfa field in order to obtain a normal plant population in the field experiment. Data were collected on two harvest dates. In the controlled experiment 146 plants from the alfalfa population MSE-6 were grown in individual pots. Photosynthesis was measured at light intensities of 22 and 44 klux for two stages of growth. Dark respiration was also determined. Carbon exchange data were expressed on a leaf area, leaf dry weight, and per plant basis. Significant positive correlations were observed between photosynthesis expressed on a leaf weight and leaf area basis; however, these expressions of carbon exchange were not significantly related to total CO₂ uptake per plant. Forage yield was significantly correlated (r=.96"* and .93"*, the first and second harvests, respectively) with total apparent photosynthesis per plant, but not with photosynthesis, expressed on a leaf weight and leaf area basis m the field study. Photosynthesis was also significantly associated with forage yield in the controlled environment experiment. Total dark respiration per plant after 25 days of regrowth was found to be highly correlated (r=.85**) forage yield. The apparent photosynthesis to dark respiration ratio was not associated with forage yield. The data from these studies suggest that the total CO₂ uptake limits alfalfa forage production more than carbon exchange per unit leaf area.