WALNUT POLLINATION DYNAMICS: POLLEN FLOW IN WALNUT ORCHARDS
We have been conducting analyses of pollination dynamics in a California 'Chandler' walnut (Jtiglans regia) orchard. Our objectives are to document effective sources of pollen during the dichogamous bloom cycle of the two cultivars present in the orchard by determining pollen parentage of nuts, to describe the bloom phenology and to develop recommendations on pollinizer requirements in walnut orchards consistent with management of blackline disease and pollen-induced pistillate flower abortion
... PFA). Trees are spaced at 28.5m. Prevailing winds are from the north and west. The orchard has 'Cisco' pollinizers every fourth row along the north and west outside rows of the orchard and there is a single 'S. Franquette' tree on the east edge. Other potential pollinizers are approximately a kilometer distant. We are analyzing pollen flow by conducting paternity analyses using microsatellite (SSR) DNA markers on nuts collected from trees at various distances from the pollinizer rows. Our results for 2001 are based on nuts harvested from flowers tagged at two bloom dates. The results show much higher than expected levels of pollen parentage originating from sources outside the orchard (i.e. neither self pollen, 'Cisco' pollen nor 'S. Franquette' pollen). During Chandler staminate lull bloom, when copious amount of self pollen is present in the orchard, high levels of non-self paternity occur at the upwind portions of the orchard. For 2002, we expanded our level of temporal resolution by tagging flowers 8 times over the bloom cycle. SSR-based paternity anal yses of approximately 900 nuts identified according to bloom date and orchard location again show high levels of paternity from sources outside the orchard. Because we cannot exclude 'Chandler' or'Cisco' pollen from sources outside the orchard, it is likel y that our estimates of pollen from distant external sources is significantly higher than the data would indicate.