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173

POLLINATION OF COMMON BEANS

J. G. Waines Department of and Sciences, University of , Riverside, CA 92521-0124, USA

Cross-Pollination of Maie-Slerile Common Bean A segregating population of a genetic male-sterile common bean in the background of 5- 593 was obtained from Dr. Mark J. Bassett at the University of Florida. From this, a few homozygous recessive male sterile were selected and they were crossed to homozygous dominant male fertile plants, 5-593, to produce an F1 generation. This was selfed and the F2 generation was planted out on the Agricultural Experiment Station at the University of California, Riverside. A single row hand planter, with a bean plate, was used that dropped every 4 inches on average. 'Hiere were two planting dates. The first, sown May 5th. 2003, flowered plants in mid June and July, when day temperatures ranged between 30 and 40 C. The 5-593 genotype is not well adapted to these conditions, and the plants grew poorly and even the male-fertile plants matured few pods. Very few pods were observed on the male-sterile plants. The second planting sown August 8^*" 2003, flowered in mid September when day-time maximum temperatures were less. Even so, the male-fertile plants tended to grow faster and smothered the male-sterile plants, which were smaller and weaker. The male-sterile plants are homozygous for the morphological marker, spindly branch, sb/sb, which is recognizable in the field and contrasts with Sb/Sb and Sb/sb phenotypes that have normal branches, and fertility. Two honeybee hives were placed in the field in mid September, when bean plants began to . were seen to occasionally visit bean , but they preferred to elsewhere. and flower thrips also lived among plants in the plot. Twelve, 30-feet rows were planted on each planting date, and the rows were 30 inches apart. Irrigation was by forrow, every other row, two days a week for eight hours. The mean number of plants-per-row was 98. The mean number of male-sterile, spindly branch plants-per-row was 23, and the mean number of male-sterile plants that set at least one pod was 13 (56.5%) Therefore 43.5% of the male-sterile plants were not pollinated by any . For the male-sterile plants with at least one pod, the number of pods ranged from 1 to 12, with a total number of 415 and a mean of 2.78 pods per plant. per pod ranged fi-om 1 to 7. A total of 1615 seeds were obtained with a mean of 3.89 seeds per pod. These field experiments demonstrated that an August planting was better than a May planting for pod and seed production with this Florida adapted, male-sterile material in southern California. Only about half of the male sterile plants produced a pod. Male-fertile plants were not rogued, but formed a source of fertile for transfer to male-sterile plants by . Over 50 % of the male-sterile plants were pollinated by insects, possibly honeybees. Alternating rows of homozygous male-fertile 5-593 plants with rows of segregating F2 plants, where the normal branched male-fertile plants are rogued to prevent the spindly-branched plants fi^om being smothered by normal plants, might be a more efficient way to encourage pollen transfer to male-sterile plants. It should be possible to identify the spindly branch plants before flowering, and remove the normal branched plants. The male-sterile flowers form parthenocarpic pods, which confirm the male-sterile phenotype. 174

A search should be made for a more efficient pollen vector than the honeybee. In our previous field experiments we used and carpenter bees. These were encouraged to visit common bean plants by locating the plots near Mimulus plots. Salvia gregiU or another plant attractive to bumble bees and carpenter bees might also encourage visits to common bean flowers. A third way to increase the efficiency of production might be to transfer the spindly branch alíele into a locally adapted that is more attractive to insect .

A Self-Pollinating White Kidney Line Research by Tucker and Harding (1975) reported that common beans of various gene pools and classes were largely self-pollinated at UC Davis, CA, over two years. These results contrast with reports fi-om northern California by Barrons (1939), and southern California (Wells et al. 1988; Ibarra-Perez et al. 1997) where outcrossing rates of 0.0 to 85.0% were recorded over one to two years. One explanation of these conflicting results may lie in the different methods used to sample segregating seed populations used by the various workers. Another may be the kind of pollinating insect present, honey bees are less attracted to common bean flowers than bumblebees or carpenter bees. Rarely in outcrossing experiments were the of potential pollinators reported. All of the research on outcrossing rate in common bean implies that there should be cultivar or genotypic differences among accessions. Some may be almost 100% selflng in the presence of a specific , while others may show high rates of outcrossing. Moreover these differences should be inherited in inbred populations. One culitvar that we have not been able to outcross over several years in the field in the presence of honeybees and other insects at Riverside, CA, is a 'White Kidney' line released by Smith in the 1940s. Morphological and genetic examination of the flower structure of this line may indicate why ít is completely inbreeding under field conditions in southern California.

References Barrons, K.C. 1939. Natural outcrossing in beans at different degrees of isolation. Proc. Am. Soc. Hortic. Sei. 36:637-640. Ibarra-Perez, F.J., B. Ehdaie and J.G. Waines. 1997. Estimation of outcrossing rate in common bean. Sei. 37:60-65. Tucker, C.L. and J. Harding. 1975. Outcrossing in common bean Phaseolus vulgaris L. J. Am, Soc. Hortic Sei. 199:283-285. Wells, W.C, W.H. Isom, and J.G. Waines. 1988. Outcrossing rates of six bean lines. Crop Sei. 28:177-178.