|Abstract or Summary
- Phaseolus vulgaris, the common bean, is susceptible to many
bacterial, fungal and viral diseases. However, resistance to several
diseases has been identified in related Phaseolus species, P.
coccineus, P. acutifolius and P. lunatus. Thus, interspecific
hybridization could result in transfer of beneficial characteristics.
One of the problems encountered in interspecific hybridization of
Phaseolus resides in the failure of the hybrid embryos to reach
maturity. Embryo rescue techniques are required to recover
interspecific hybrid plants. In vitro culturing of immature pods
represents an alternative to embryo rescue.
In order to develop appropriate pod culture methodology, immature
selfed pods of P. vulgaris cv. Great Northern were cultured at three
stages of development (containing embryos of 0.5, 0.6-1.0 and 1.5-2.5 mm
in length respectively). Seed weights and embryo lengths increased
significantly for all three classes of pods when cultured upright,
supported by glass wool, in tubes containing a liquid medium. One or
two embryos per pod, usually located at the middle positions, could
develop further. The same pod culture procedure promoted precocious
germination of cotyledonary embryos of P. vulgaris Great Northern x P.
acutifolius P.I.321637, without advancing the embryos to further
developmental stages. These findings suggest that the developmental
arrest of the interspecific hybrid embryos in vivo is due to intrinsic
Since growth regulators may be involved in the retardation of
interspecific hybrid embryo growth, seed and embryo growth were measured
after a pod culture period in the presence of 10⁻⁶M of N⁶-benzyladenine
(BAP), gibberellic acid (GA₃) or abscisic acid. The growth regulators
did not affect seed weight and embryo length in cultured P. vulgaris
pods. However, the length of embryos in the largest class of P.
vulgaris Great Northern x P. acutifolius P.I.321637 pods were
significantly lower in the presence of BAP and GA₃ as compared to the
control without growth regulators. Thus, at the concentration tested,
growth regulators were not beneficial for further embryo development but
BAP and GA₃ seemed to depress root elongation of the germinating embryos
of the interspecific hybrid.
Although mature hybrid seed can not be generated in pod culture,
hybrid plantlets can be obtained after a brief phase of culturing the
germinating embryos. The germinating embryos from pod culture offer
some advantage over embryos dissected at an earlier stage, since the
presence of roots enhances nutrient uptake and hardening. Further
studies leading to identification of the factors that cause
developmental arrest of interspecific hybrid embryos may aid in devising
the optimal pod culture medium for further growth of hybrid seeds and