The five major gene loci, Le/le, La/la, Cry/cryc/cry1, Na/na and Lm/lm are known to govern internode length in peas. Combinations of the different alleles at these loci determine the phenotypes tall, dwarf, cryptodwarf, slender, nana and micro. In this paper a new phenotype cryptotall (Le la cryc Na Lm) is described and the na mutant is shown to be completely epistatic to the Le/le gene pair. In addition, an extremely short mutant with a phenotype known as compactum is shown to be the result of a mutation at the na locus. The mutant allele in compactum has a similar, perhaps identical, effect to allele na. The Na and Lm loci are shown to be genetically and physiologically distinct although both loci are reported to occur on chromosome 6. Within the dwarf class plants possessing genotype la Cry are shown to have longer internodes than plants possessing La Cry or La cry5.

The effect of the loci Le, La and Na on cell length and number of cells per internode was examined in progenies segregating for the phenotypes tall, dwarf, cryptodwarf, cryptotall and nana. All three loci appeared to alter internode length in a similar anatomical fashion. In the epidermal cells, cell length appeared to be principally affected, with cell numbers influenced to a lesser extent while the reverse situation occurred in the cells of the outer layer of the cortex. These results are similar to the effects of applied gibberellic acid and raise the question of the relationship of these loci to gibberellin metabolism and/or sensitivity.

Grafting studies between the phenotypes tall, dwarf, cryptodwarf, nana and compactum indicated that the effect of the Na locus is graft transmissible while the effects of the Le, La and Cry loci are not. This would suggest that the Le, La and Cry loci are influencing some process(es) occurring directly within the dividing and/or elongating region of the apex. The Na gene, on the other hand, appears to produce some substance(s) in the mature leaf and stem tissue, and possibly also to a small extent in the roots, which is capable of being transported across a graft union to the growing region in the apex. The Na gene is both genetically and physiologically complementary to the Le gene since dwarf stocks (le Na) are just as effective as tall stocks (Le Na) in promoting growth of nana scions (Le na).

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