UTILITY OF MICROCOSM STUDIES FOR PREDICTING PHYLLOPLANE BACTERIUM POPULATION SIZES IN THE FIELD

Population sizes of two ice nucleation-active strains of Pseudomonas s yringae a ere compared on leaves in controlled environments and in the field to determine the ability of microcosm studies to predict plant habitat preferences in the field. The P. syringae strains investigated were the parental strains of recombinant deletion mutant strains defi cient in ice nucleation activity that had been field tested for their ability to control plant frost injury. The population size of the P. s yringae strains was measured after inoculation at three field location s on up to 40 of the same plant species that were studied in the growt h chamber. There was seldom a significant relationship between the mea n population size of a given P. syringae strain incubated under either wet or dry conditions in microcosms and the mean population size whic h could be recovered from the same species when inoculated in the fiel d. Specifically, on some plant species, the population size recovered from leaves in the field was substantially greater than from that spec ies in a controlled environment, while for other plant species field p opulations were significantly smaller than those observed under contro lled conditions. Population sizes of inoculated P. syringae strains, h owever, were frequently highly positively correlated with the indigeno us bacterial population size on the same plant species in the held, su ggesting that the ability of a particular plant species to support int roduced bacterial strains is correlated with its ability to support la rge bacterial populations or that indigenous bacteria enhance the surv ival of introduced strains. Microcosm studies therefore seem most effe ctive at assessing possible differences between parental and recombina nt strains under a given environmental regime but are limited in their ability to predict the specific population sizes or plant habitat pre ferences of bacteria on leaves under field conditions.