Environmental and Experimental Botany xxx (2004) xxx–xxx Bacterial populations on the leaves of Mediterranean plants: quantitative features and testing of distribution models

We studied the bacterial colonization of the phyllosphere of eight perennial species occurring in a Mediterranean ecosystem in Sithonia (Halkidiki), northern Greece, over a period of 2 years. The plant species were Arbutus unedo, Quercus coccifera, Pistacia lentiscus, Myrtus communis, Lavandula stoechas, Calamintha nepeta, Melissa officinalis and Cistus incanus. They differ in a number of morphological features, mainly in habit and leaf trichome. The bacterial colonization of their leaves is highly variable. Over all species and sampling dates, observed values ranged from non-detectable to a maximum of 1.4 × 107 CFU g−1 in C. nepeta. The average size of the microbial community varied among species by a factor of about 10, from 1.3×104 CFU g−1 in A. unedo and L. stoechas to 1.3 × 105 CFU g−1 in M. officinalis and C. nepeta. Within species variability was far larger than that among species or among seasons. Apart from the fact that low values were recorded in summer, the marked seasonality of the Mediterranean climate was not reflected in the phyllosphere bacterial populations. Essential-oil producing species were not less colonized than the others. The hemicryptrophytes, M. officinalis and C. nepeta, shorter than all other species and equipped with both glandular and non-glandular trichome, consistently sustained high bacterial populations on their leaves. Ice-nucleation active (INA) bacteria were absent from all species, except for C. nepeta, on which a very low population was recorded in winter. Given these results and additional literature information, we argue that perennial species of Mediterranean-climate areas are not systematic hosts of INA bacteria. We tested the statistical distribution of the phyllosphere bacterial populations of these specie