A NOVEL TERRESTRIAL HALOPHILIC ENVIRONMENT - THE PHYLLOPLANE OF ATRIPLEX HALIMUS, A SALT-EXCRETING PLANT

This paper describes the microbial ecosystem found on the leaves of At riplex halimus, a salt-excreting plant in the central Negev highlands of Israel. Because of the regular nightly occurrence of dew at this lo cation, these leaves undergo a diurnal wetting so that phylloplane mic roorganisms experience large fluctuations in salinity and water activi ty, as well as tolerate repeated desiccation. During the dry season, i n the late spring and summer, a significant amount of salts and organi c material coats the leaf surface. During dew events the salt concentr ation at the leaf surface was calculated to be > 0.4 M. Direct counts of the respiring bacteria on the leaf surface ranged from 1.06 x 10(4) to 5.06 x 10(5) per cm(2). Using a variety of media it was shown that there was limited bacterial diversity which could be cultured, with g reater than 90% of the isolates being orange colored Gram-negative rod s. Viable counts ranged from 0.32 to 2.32 x 10(4) bacteria per cm(2) o f A. halimus leaf surface. No bacteria capable of nucleating ice were recovered in these studies. The dominant orange pigmented bacterium, i dentified as a halotolerant Pseudomonas sp., grew optimally at 30 degr ees C and at 5% NaCl and was capable of growth in media containing up to 20% NaCl. This bacterium could grow on a variety of organic compoun ds, including some associated with plant materials. The leaf bacteria were desiccation-tolerant when on the leaf surface or when directly wa shed off the leaves, but much less so when in isolated culture. A majo r component of the tolerance to desiccation is probably related to the compounds on the leaf surface.