Specific Leaf Weight and Photosynthesis in Alfalfa 1

Specific leaf weight (SLW = leaf dry weight per unit leaf area) and net photosynthesis (Pn) of 13 alfalfa (Medicago sativa L.) clones were positively correlated (r = 0.790). Pn increased from 20 ± 7 to 50 ± 8 mg CO2, dm−2 hr−1 when SLW increased from 1.9 to 5.3 mg cm−2. The SLW-Pn relationship was the same whether SLW was influenced by genetics or environment. Selections from four alfalfa populations (MSA-C4, MSB-11, ‘Vernal,’ and ‘DuPuits’) were grown in high (37.7 klux) and low (12.9 klux) light growth chambers. Except for DuPuits, they showed a SLW-Pn relationship similar to the 13 clones mentioned above. At high light, the Pn of DuPuits did not exceed 33 ± 6 mg CO2 dm−2 hr−1 even when SLW was greater than 4 mg cm−2. SLW-Pn relationships were similar for attached and detached leaves. Leaves selected for high SLW had the highest Pn at several light intensities between 6.3 and 64.0 klux. The possibility of increasing Pn by selecting for SLW is discusse