Role of Immigration and Other Processes in Determining Epiphytic Bacterial Populations

Abstract

Conceptually, there are 4 major processes that can influence the size and composition of bacterial populations on leaves. Bacterial population sizes on a leaf can increase by both multiplication on that leaf as well as by immigration of bacteria from other leaves. In contrast, both the death of bacterial cells and their migration from a leaf can contribute to decreases in population size. It appears that most workers have assumed that growth and death of bacteria are the predominant processes that determine population sizes. These processes are obviously much more easy to study than immigration and emigration since they can be done in isolation in the laboratory or greenhouse. For example, the study of the multiplication of bacteria on plants can be easily studied by inoculating plants that are isolated from other plants in incubation chambers; increases in population size are directly attributable to multiplication of the bacteria. In contrast, studies of the importance of immigration of bacteria to the population sizes of bacteria on a leaf require a source of immigrant bacteria and that conditions facilitating immigration be maintained. Since the conditions which favour the immigration of bacteria to plants are not yet well understood, such studies must be done under field conditions. They therefore face the complications of variable environmental conditions which make the experiments difficult to reproduce. For this reason, most information relevant to understanding processes that occur on plants are inferences made from simple field or laboratory observations. Most observations have not been sufficiently detailed to partition the many factors that can influence epiphytic bacterial populations. To date, there have been few studies designed to understand the processes that contribute to epiphytic bacterial populations.