ROOT EXUDATION AND PHYSIOLOGICAL STATUS OF A ROOT-COLONIZING FLUORESCENT PSEUDOMONAD IN MYCORRHIZAL AND NONMYCORRHIZAL PEPPER (CAPSICUM-ANNUUM-L)

The effect of mycorrhizal infection on root exudation and the survival and physiological status of a bioluminescent fluorescent pseudomonad on the roots of pepper was examined. Pepper plants were grown for 27 d ays in split-root microcosms with one side mycorrhizal with Glomus des erticola (GD) or Glomus intraradices (GI) while the other side was non -mycorrhizal. Plants with both sides non-mycorrhizal served as control s. The soil was inoculated with a bioluminescent fluorescent pseudomon ad (P. fluorescens 2-79RL). This strain emits light in its exponential growth phase, such that the length of the lag phase prior to biolumin escence can be used to assess the physiological status of the bacteriu m. Mycorrhizal infection had no significant effect on plant growth. Th e percent root length infected was 8% for GD and 34% for GI. After pul se-labeling of the shoots with (CO2)-C-14, quartz filter strips were u sed to collect C-14 labeled root exudates at specific locations on the roots. Compared with the non-mycorrhizal roots, GI decreased C-14 lab eled root exudation by 78% at the root tip and by 50% at the older roo t parts. GD had no effect on C-14 labeled root exudation. Rhizosphere soil solutions collected with quartz filter strips were analyzed for a mino acids and organic acids by GC-MS. The overall pattern of the chro matograms of the rhizosphere soil solution was similar in the non-myco rrhizal and the mycorrhizal roots. The number of peaks detected was hi gher in the non-mycorrhizal roots than in the mycorrhizal roots. Compa red with the non-mycorrhizal plants, GI decreased the population densi ty of P. fluorescens 2-79RL on the roots by one order of magnitude, bo th on the mycorrhizal and the non-mycorrhizal side. GD decreased the p opulation density by one order of magnitude only on the side where the fungus was present. The physiological status of P. fluorescens 2-79RL on the roots, as measured by the length of the lag phase prior to bio luminescence, decreased significantly from day 3 to day 6 and remained at a similar level thereafter. Mycorrhizal infection had little effec t on the physiological status. Compared to the non-mycorrhizal plants, GI increased the physiological status of P. fluorescens 2-79RL only d uring the first 6 days, while GD had no effect at all. It is concluded that mycorrhizal infection may decrease root exudation and alter the composition of the rhizosphere soil solution, thereby reducing the pop ulation density of certain bacterial groups in the rhizosphere.