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Heterosis and Ploidy Effects on the Growth of Alfalfa Callus¹

The influence of heterosis and ploidy level on callus fresh weight production in alfalfa (Medicago sativa L.) was studied using diploids (2x), their isogenic tetraploid (4xCD) counterparts derived by chromosome doubling, hybrids at 2x and 4x levels, and 4x and isogenlc 8xCD materials Chromosome doubling has an inbreeding fect equivalento sewing (2 to 3 generations in 4x), thus 4xCD and 8xCD materials were partial inbreds. Both the 4xCD and 8xCD materials produced less callus than their non-inbred 2x and 4x counterparts, respectively. A set of 4x partial inbreds obtained by two generations of conventional self-pollination (S₂) also produced less callus than the non-inbred (S₀). Among hybrids evaluated, there was no heterosis for callus production among 2x hybrids which were derived from non-inbred parents, but there was appreciable heterosis among most 4x hybrids which were derived from partially inbred 4xCD parents. Midparent heterosis for callus production was highly significant (p = 0.01) in three of four hybrids on one medium and in fottr of six 4x hybrids on the other medium. Ploidy effects which were not confounded with the inbreeding associated with chromosome doubling were evaluated using three sets of equivalent 2x and 4x hybrids. Two of the three 4x hybrids produced several fold more callus than equivalent 2x ybrids and the differences were highly significant (p= 0.01). Heterosis and ploidy effects were found to be more pronounced on one of two media used in several experiments. The potential use of heterosis as a selective mechanism in tissue culture experiments was discussed.

Key Words: Medicago sativa L. • Tissue culture • Inbreeding

² Research assistant (Present address: Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40506) and professor of agronomy, Univ. of Wisconsin, Madison.

Received for publication July 13, 1978.