Pollination biology of common beans (Phaseolus vulgaris L.)

It is assumed that common bean (Phaseolus vulgaris L.) is largely self-pollinated, but its breeding system has yet to be examined in detail. The levels of natural outcrossing were estimated for six green-hypocotyled, white-seeded, common bean cultivars grown during 1989 and 1991 at two locations, Irvine and Riverside, CA. A bulk of nine purple-hypocotyled, black-seeded, cultivars acted as male parents and the green-hypocotyled, white-seeded, cultivars were female parents. Genetic variation for outcrossing rates among these six white-seeded cultivars was suggested by significant differences between the outcrossing rates for cv. Ferry Morse 53 (9.4%) and cv. PI 345576 (3.5%) with intermediate rates for cv. 'Comtesse de Chamborde' (6.8%). Strong genotype x environment interactions were present for location and year but not for cultivar response to sowing date. Consequently, common bean may not be predominantly self-pollinated all of the time, instead, the breeding system of this species may be viewed as a mixed-mating system in which selfing and cross-pollination is possible.; Natural hybrid seeds are produced as a result of outcrossing and since the common bean fruit is a multi-seeded legume, these hybrid seeds may be related through shared paternity. Hypocotyl color and shikimate dehydrogenase (SKDH) enzyme loci were used as genetic markers to assess multiple paternity in field-grown common beans. Multiple paternity within pods was identified in all the pods classified as hybrids, at a frequency of 5.8% and 8.1% in 1991 and 1992, respectively. All multiply-sired pods produced nonhybrid and hybrid seeds. As many as three successful fathers per pod were identified, but the methods precluded measuring higher levels of multiple paternity. The contention that seed set and seed yield do not respond to visits by insect pollinators was examined in four common bean cultivars.; The effect of mechanical tripping by bees (mainly bumblebees) on yield components of common bean in two field experiments was studied in 1991 and 1992. Bean plants of each of four cultivars were grown either in insect-proof cages or under open conditions in 1991. An extra treatment was added in 1992; bumblebees were captured and placed in cages with plants growing in them. The results from 1991 indicated significantly more pods (41%), slightly more seeds per pod and greater seed yield (30%) for all cultivars under open conditions. However, results from the experiment in 1992 indicated less seed yield for plants in the open conditions and no effects of putting bumblebees in cages. Manually tripping the flowers in glasshouse-grown plants to simulate the insect tripping of the same cultivars used in the field experiments did not show any effect on seed yield and its components. Only one of the cultivars (PI 164778) had 23% more seeds per pod when its flowers were manually tripped in comparison to the undisturbed flowers.