INFLUENCE OF IMMIGRATION ON EPIPHYTIC BACTERIAL-POPULATIONS ON NAVEL ORANGE LEAVES

Factors that influenced the increase in epiphytic bacterial population size on navel orange leaves during winter months were investigated to test the assumption that such populations were the result of multipli cation on orange leaves, The population sizes of bacteria of different kinds, including ice nucleation-active (Ice(+)) bacteria, were from 6 - to 30-fold larger on leaves of navel orange trees adjacent to other plant species than on trees growing near other citrus species, Total a nd Ice(+) bacterial population sizes on other plant species growing ne ar navel orange trees were from 18- to 60-fold and 2- to 18,000-fold l arger, respectively, than on navel orange trees. About mice the number of bacterial cells of a given type were deposited onto petri dishes o pened simultaneously in navel orange orchards with other plant species nearby as in orchards surrounded by citrus trees. Epiphytic bacteria and airborne bacteria mere more numerous near the upwind edge of orcha rds bordering on other plant species, but not in orchards adjacent to other citrus trees, and decreased with distance from other plant speci es, Navel orange leaves also exhibited progressive increases in the ab ility to supercool as a function of increasing distance from the upwin d edge of orchards adjacent to other plant species but not in orchards adjacent to other citrus trees, While the population size of three di fferent bacterial strains remained nearly constant for 60 days after i noculation, total bacterial populations increased more than 50-fold du ring this period. These results suggest that immigration of bacteria f rom plants having high epiphytic bacterial populations could account f or most, if not all, of the seasonal increase in bacterial populations on navel orange leaves and have important implications for procedures to modify bacterial communities on leaves.