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Canopy Structure and Photosynthesis of Alfalfa Genotypes Differing in Nodule Effectiveness¹

Photosynthesis is linked to N₂ fixation and to other plant metabolic processes, but little is known of how N₂-fixation capability affects the development of the leaf canopy and leaf photosynthetic rates. Field experiments were undertaken to determine the effect of differences in N₂-fixation capability mediated by plant genes on canopy structure and apparent photosynthesis of alfalfa (Medicago saliva L.). An alfalfa genotype lacking the capability of establishing an effective symbiosis with Rhizobium was compared to normal alfalfa. Entries were grown at two rates of supplemental N (0 and 100 kg ha^–1 as NH₄NO₅) to provide genetic- and culturally-controlled differences in N₂ fixation. Hostmediated genetic differences in N₂-fixation capability (ineffective vs. effective nodules) elicited pronounced changes in rates of apparent photosynthesis, canopy structure, and specific, leaf weight. Compared with alfalfa having effective nodules (Eff), alfalfa with ineffective nodules (In) averaged 17% greater specific leaf weight, but 73% less leaf area over both N regimes and three growth-harvest cycles. For unshaded leaves at the top of the canopies, rate of apparent photosynthesis per leaf area of In was similar to that of Eff on vegetative plants 2 weeks before harvest and in canopies at 5% flower. Leaf canopies of In contained a more vertically uniform leaf area than Eff canopies unless they were supplied with 100 kg ha^–1 combined N. In vegetative In plants 2 weeks before harvest, supplemental N at a rate of 100 kg ha^–1 enhanced leaf area by 50%, but tended to reduce rate of apparent photosynthesis per leaf area. In Eff plants at this same growth stage, supplemental N also reduced apparent photosynthesis, but leaf area was not affected. In canopies at 5% flower, supplemental N did not change rate of apparent photosynthesis of either In or Eff. These results show that N stress induced by host-mediated ineffective nodulation greatly reduced leaf area and photosynthetic rate per ground area, but that effective nodules were not a prerequisite for maintaining normal photosynthetic rates per leaf area even under severely limiting soil N supply.

Key Words: Symbiotic N₂ fixation • Nitrogen utilization • Leaf area distribution • Specific leaf weight • Nodule function • ¹⁴CO₂ labeling • Medicago sativa L.

² Former research specialist, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota (Present address: Institut fur Pflanzenbau, ETH Zentrum, Universitatstrasse 2, 8092 Zurich, Switzerland) and Plant Physiologist, USDA-ARS in the Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 1509 Gortner Av., St. Paul, MN 55108.

Received for publication August 27, 1982.