Interspecific Cross Involving the Lima Bean Phaseolus lunatus L.
Albert P. Lorz
|Fig. 1. Emergence behavior of P. lunatus x P. polystachyus
F1 hybrid (left) compared to that of P. lunatus. Soil removed to show hypogaeal position of the cotyledons of the hybrid. (Line drawings traced from a photographic negative.)
The presentation of this brief account of preliminary success in the crossing of the Lima bean Phaseolus lunatus L. with the native wild thicket bean P. polystachyus (L.) B.S.P. may provide bean breeders with an incentive to investigate further the possible contributions of this native source of germ plasm to any Lima-bean breeding program.
The cross P. polystachyus ♀ x P. lunatus ♂ was attempted an estimated 50 times with no success, but an estimated 100 attempts to effect the reciprocal cross resulted in the production of seven F1 plants for which there is objective evidence that these are bona fide hybrids, although none of the plants has yet matured to the point of flower and seed production.
All the hybrids expressed completely or nearly completely the hypogaeal germination of the staminate, P. polystachyus parent. In two out of the seven a tendency toward the epigaeal habit of the pistillate parent was expressed by the slight elevation of the cotyledons to a level barely above the survace of the soil. With deeper planting this might even have escaped observation. In a Lima control the cotyledons were elevated to a height of 3 in. (Fig. 1).
One of the hybrids has its pistilate parent a Lima selection resembling the commercial variety Fordhook. The Lima parent of the other six F1 hybrids is iteslf an F1 hybrid from a cross between a Fordhook-type selection and line M-270 from O. W. Norvell. M-270 is a small-seeded, colored-flowered, short-day, wild, viny type with a strong perennial tendency.
The principal objective of the P. lunatus x P. polystachyus cross has been to incorporate into the former species the hypogaeal germination of the latter in an attempt to solve the emergence problem in Limas, where a high mortality among seedlings is likely to result from the breaking of the hypocotyl ("neck-breaking") as it elongates in an often unsuccessful effort to push the extremely large Lima cotyledons through an encrusted or impacted soil. In P. polystachyus and several other bean species elongation of the stem of the plant involves only the development of the epicotyl and the plumule, and the cotyledons remain at planting depth because of the absence of elongation of the hypocotyl.
Apart from consideration of this one obviously valuable germination character is the possibility that a native species, such as P. polystachyus, having a comparatively wide geographic distribution, may possess yet unrecognized resistances to diseases and pests and may also possess physiological attributes of survival value. According to Small (1), P. polystachyus ranges from Florida as far north as Minnesota, Ontario, and Maine and as far west as Texas and Nebraska, so it seems apparent that the perennial rootstock at least has some degree of cold-hardiness even though the plant usually occurs in protected locations.
In view of the consideration that P. polystachyus may be a valuable source of germ plasm to bean breeders, that the realization that such a cross is possible may be of interest to taxonomists, and that the writer has encountered in the literature no other instance of the involvement of P. lunatus in any interspecific hybrid combination, it was deemed advisable to publish now this preliminary report of progress even though the subsequent breeding behavior of the F1 hybrids is yet to be established.
Manuscript received January 23, 1952.