No abstract

Design II matings were made among randomly selected clones of 'Arlington' red clover (Trifolium pratense L.). Progeny were evaluated in vitro on two regeneration media for callus growth and differentiation. Additive genetic variance was a significant source of variability for nearly all traits evaluated, including somatic embryogenesis. In vitro traits, such as rapid callus growth, colony vascularization, root initiation, chlorophyll production and embryogenesis were highly heritable and should respond to breeding and selection. Dominance genetic variance was significant for only a few in vitro characters. Maternal and cytoplasmic factors were significant primarily in the early subcultures. Highly significant additive genetic correlation of performance on two regeneration media was found. A population selected on one of the regeneration media for such characteristics as improved plantlet regeneration, rapid callus growth, long term colony viability or the frequency of root initiation should show correlated improvement on the other medium. No significant differences for embryogenesis were attributable to differences in the regeneration media used. Furthermore, no interaction of additive genetic effects with regeneration media were observed. These data indicate that improvement in the frequency of plantlet regeneration from callus of red clover could effectively be achieved by breeding and selection for embryogenic types.

Mutations (Rht genes) blocking sensitivity to gibberellic acid (GA) were used to examine phytohormone mediated cell wall expansion in wheat (Triticum aestivum L.). Irreversible extensibility of immature leaf segments, as determined by stress/strain (instron) measurements, declined with Rht gene dose. Exogenous GA3 significantly increased wall extensibility in the nonmutant controls but had no effect on the near-isogenic GA-insensitive genotypes. Furthermore, ancymidol, an inhibitor of gibberellin biosynthesis, diminished wall extensibility in the nonmutant control. Extensibility of immature segments was highly correlated with mature leaf sheath length (R = +0.95). The results indicate that wall yielding properties of expanding wheat leaves are associated with leaf cell expansion potential and that GA is involved in the determination of those properties.The plane of cell division together with the rate and orientation of irreversible expansion determine cell size and shape. The rate of plant cell expansion may be further reduced to a few physical components including: (a) cell wall extensibility, which is the capacity of the wall to undergo irreversible (plastic) extension in response to a load; (b) the yield threshold of the wall, which is the minimum stress required to initiate extension; (c) hydraulic conductance, the capacity ofthe tissue to transport water to the expanding cell; and (d) the difference in water potential between the cell and extracellular space (16). Lockhart's (16) equations show that a change in the growth rate can take place with changes in wall structural/ mechanical properties which determine extensibility and/or changes in hydraulic processes and/or turgor changes.Heyn (9)

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.