RESPONSES BY IRON-EFFICIENT AND INEFFICIENT OAT CULTIVARS TO INOCULATION WITH SIDEROPHORE-PRODUCING BACTERIA IN A CALCAREOUS SOIL

Rhizosphere bacteria may enhance plant uptake of Fe by producing sider ophores that chelate sparingly soluble Fe3+ in calcareous soils. To ev aluate the extent to which plants benefit from colonization of the roo ts by prolific siderophore-producing bacteria, we inoculated two oat c ultivars with six strains of bacteria that produced high concentration s of siderophores under Fe-limiting conditions in vitro. Oat cv Coker 227, an Fe-efficient cultivar, which produces the phytosiderophore ave nic acid, and cv TAM 0-312, an Fe-inefficient cultivar, which does not produce the phytosiderophore, were grown in a calcareous soil (Weswoo d silt loam) on a light bench in the laboratory. Half of the plants we re fertilized with a nutrient solution containing 5 mM Fe and half wit h a nutrient solution containing no Fe. After 6 weeks of growth, we co mpared colonization of the roots by the inoculant bacteria and the dry weight and Fe content of roots and shoots. Three species of Pseudomon as colonized the roots of both oat cultivars in high numbers (greater- than-or-equal-to 10(6) cells g-1 root dry weight), whereas the remaini ng bacteria colonized the roots in substantially lower numbers (less-t han-or-equal-to 10(4) cells g-1 root dry weight). Plants fertilized wi th 5 mM Fe were larger and supported greater numbers of rhizosphere ba cteria per gram of root than plants not supplied with Fe. Comparisons of the Fe content and dry weight of roots and shoots revealed few sign ificant differences between inoculated and uninoculated plants, or amo ng the plants inoculated with the different strains of siderophore-pro ducing bacteria. The differences that were observed revealed no consis tent response to inoculation. We conclude that inoculation of the root s of the two oat cultivars with bacteria that produce high concentrati ons of siderophores in response to an Fe deficiency had little or no e ffect on Fe acquisition by the plants.