Microbial competition for nutrients on leaves of sycamore and lime
An investigation of the natural phylloplane microflora of sycamore and lime was carried out in conjunction with a study of phylloplane fungi and bacteria in vitro. The aim was to determine whether an organism's success in colonising the phylloplane reflected that organism's ability to compete for organic nutrients. Microbial populations on leaves increased from June onwards. The microflora of both plants consisted mainly of white yeasts, Aureobasidium pullulans and Cladosporium spp. Few bacteria were isolated. Unlike sycamore, newly expanded lime leaves held a substantial population which declined within a few days. Uptake of tracer amounts of 14C-glucose and 14C-amino-acids from droplets on leaves closely reflected the size of the microbial population. There was little competition for applied 14C-nutrients early in the season but competition was intense from August onwards. Aphid honeydew was an important source of nutrients from June onwards. Microbial growth was probably nitrogen-limited during this period. Availability of water was probably the major factor restricting microbial populations early in the season. In vitro, bacteria utilised sugars only very poorly while yeasts utilised them readily. The rate of nutrient uptake and utilisation depended on the previous history of the cells. Yeast cells from sugar-rich medium took up 14C-amino-acids very quickly but did not rapidly convert it into 14CO2. Uptake of 14C-amino-acids by certain bacterial isolates was inhibited on leaf surfaces compared with on glass. Yeasts and some bacteria produced extracellular polysaccharides when grown on sugar-rich medium. Experiments on uptake of 14C-nutrients by pairs of microorganisms in dual-culture or in "mixed pure culture" showed the feasibility of nutrient competition acting as a mechanism which determines the relative populations of the competing species. No evidence was found to suggest that inhibitors were produced by any of the isolates tested.