Morphological and Anatomical Features of Alfalfa Leaves as Related to CO2 Exchange1

Cuttings from alfalfa (Medicago sativa L. cv. ‘Mesa‐Sirsa’) genotypes which exhibited a wide range in leaflet size were grown under spaced‐planted and nonspaced‐planted conditions and used to determine the relation. ships between leaflet anatomy, leaflet morphology, and CO2 exchange. These same relationships were evaluated under controlled environmental conditions using alfalfa population MSE‐6 which has a broad germplasm base. Area per leaflet and leaflet width were negatively associated with photosynthetic rate per unit leaf area. Specific leaf weight (SLW) was positively correlated with photosynthesis; however, the correlation coefficients were consistently lower than the r values for leaflet size vs photosynthesis. Photosynthesis expressed as total CO2 uptake per plant was not significantly associated with SLW and leaflet size. Plants with small leaves had the greatest SLW, palisade tissue thickness, and leaf thickness. A positive association was observed between palisade tissue thickness and photosynthesis expressed on a leaf area basis. The significance of leaflet size as a selection tool for photosynthetic rate in alfalfa is discussed.