Root exudation and physiological status of a root-colonizing fluorescent pseudomonad in mycorrhizal and non-mycorrhizal 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 days in split-root microcosms with one side mycorrhizal with Glomus deserticola (GD) or Glomus intraradices (GI) while the other side was non-mycorrhizal. Plants with both sides non-mycorrhizal served as controls. The soil was inoculated with a bioluminescent fluorescent pseudomonad (P. fluorescens 2-79RL). This strain emits light in its exponential growth phase, such that the length of the lag phase prior to bioluminescence can be used to assess the physiological status of the bacterium. Mycorrhizal infection had no significant effect on plant growth. The percent root length infected was 8% for GD and 34% for GI. After pulse-labeling of the shoots with ¹⁴CO₂, quartz filter strips were used to collect ¹⁴C labeled root exudates at specific locations on the roots. Compared with the non-mycorrhizal roots, GI decreased ¹⁴C labeled root exudation by 78% at the root tip and by 50% at the older root parts. GD had no effect on 1¹⁴C labeled root exudation. Rhizosphere soil solutions collected with quartz filter strips were analyzed for amino acids and organic acids by GC-MS. The overall pattern of the chromatograms of the rhizosphere soil solution was similar in the non-mycorrhizal and the mycorrhizal roots. The number of peaks detected was higher in the non-mycorrhizal roots than in the mycorrhizal roots. Compared with the non-mycorrhizal plants, GI decreased the population density of P. fluorescens 2-79RL on the roots by one order of magnitude, both on the mycorrhizal and the non-mycorrhizal side. GD decreased the population 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 bioluminescence, decreased significantly from day 3 to day 6 and remained at a similar level thereafter. Mycorrhizal infection had little effect on the physiological status. Compared to the non-mycorrhizal plants, GI increased the physiological status of P fluorescens 2-79RL only during 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 population density of certain bacterial groups in the rhizosphere.