Canopy Structure and Photosynthesis of Alfalfa Genotypes Differing in Nodule Effectiveness1

Abstract

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.