PHOSPHONATE INHIBITION AS A FUNCTION OF PHOSPHATE CONCENTRATION IN ISOLATES OF PHYTOPHTHORA-PALMIVORA

Three isolates of Phytophthora palmivora showing differing sensitiviti es to phosphonate were grown in the presence of a uniform concentratio n of the anion (1 mM), together with concentrations of phosphate which varied from zero to 3 mM. The phosphonate-sensitive isolate was inhib ited by phosphonate at all levels of phosphate, but the resistant isol ates were inhibited by phosphonate only when phosphate was limiting to growth and were able to exclude phosphonate more effectively than the sensitive isolate at higher concentrations of phosphate. The percenta ge inhibition in each strain was proportional to the internal concentr ation of phosphonate, but the slope of the plot of inhibition against internal phosphonate varied between strains. Differences in the capaci ty to discriminate between phosphate and phosphonate therefore provide part of, but not the whole explanation for differences in sensitivity between isolates. There was a significant increase in the internal co ncentration of orthophosphate in the mycelia which were inhibited by p hosphonate.