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Seedling Development as a Component of Increased Yield in an Improved Alfalfa Population¹

Hairy Peruvian 32 (HP32), an alfalfa (Medicago saliva L.) population developed from ‘Hairy Peruvian’ (HP) by two cycles of phenotypic recurrent selection for high dry weight and high N concentration under both N₂- and NH₄NO₃-dependent growth conditions, was studied to determine the bases of its superior C and N assimilation characteristics. Analyses during seedling development and two subsequent regrowth cycles in the presence of 0,1, 2, or 8 mM nitrate under glasshouse conditions showed that absolute rates of dry matter and N assimilation were consistently higher in HP32 than in HP. Detailed studies of seedling growth under N₂-dependent conditions demonstrated that more prolific root nodulation at the earliest stages and an increased number of stems per plant were associated with 35% more dry matter and 55% more total N in HP32 than in HP. The number of stems in HP32 continued to be significantly greater than HP during the first and second regrowth periods and may have contributed to the yield advantages measured for HP32 in those harvests. During seedling development there were no significant differences between HP32 and HP in relative growth rate, relative N assimilation rate, or ¹⁴CO₂ partitioning patterns. These results do not imply that selection for prolific early nodulation and an increase in stem number in HP would result in more forage production, but they do indicate that a more vigorous seedling development is one result of the complex selection protocol used to produce HP32.

Key Words: Medicago sativa L. • Lucerne • Nitrogen fixation • Rhizobium • Stem number • Seed size

² Visiting scientist, associate professor, and professor, respectively; Dep. of Agronomy and Range Science, Univ. of California, Davis, CA 95616.

Received for publication November 4, 1986.