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The objectives of the present investigation were: (1) to explore an area, root exudation of peanuts, in which no previous work has been reported; (2) to study the effects of O₂ and CO₂ tensions, both individually and combined, on peanut root exudation; and (3) to determine quantitatively the sugars exuded from the roots of peanut plants grown under gnotobiotic conditions.
Seed from the peanut plant, Arachis hypogaea, L. var. NC-2 was used, Peanut embryos, with the cotyledons removed, were surface sterilized; placed aseptically into sterile plexiglass isolator chambers; and planted in vermiculite saturated with Hoagland and Arnon nutrient solution contained within transplanting tubes. Because of slow embryo and seedling growth without the cotyledons, plants were not transplanted into 500 ml flasks until after 90 days.
Three separate experiments were conducted. In all three, N₂ gas was metered to the roots of six plants in each of two isolator chambers. In addition, in the first study, roots of six plants in each of two isolator chambers received a gas mixture composed of 21% O₂, 10% CO₂, and 69% N₂; in the second study, they received a gas mixture composed of 21% O₂ and 79% N₂; and in the third study, they received a gas mixture composed of 10% CO₂ and 90% N₂.
Duration of each experiment was 6 wks. Samples consisting of nutrient solution containing peanut root exudates from three plants were collected every 2 wks. Peanut plants were harvested and fresh weights, dry weights, and lengths of both shoots and roots (separate) were obtained.
Root exudate analysis consisted of filtration to remove insoluble materials, flash evaporation to reduce the volume, salt removal by cation and anion exchange resins, freeze-drying, thin-layer chromatography to separate and identify the various sugars, and densitometry to obtain the quantity of each sugar.
Analysis of variance showed for all adjusted harvest valves no significant difference as a result of treatment or replication at the 5% level. For all measured growth parameters, Duncan's multiple range test showed no significant differences within replicates caused by treatment. Five sugars, galactose; mannose; xylose; ribose; and dihydroxyacetone, were identified in peanut root exudates. Galactose was present in all 72 root exudate samples, dihydroxyacetone in 69, ribose in 33, mannose in 8, and xylose in 5. Analysis of variance showed a significant difference at the 20% level in the total amount of galactose exuded as a result of treatment and a significant difference at the 25% level in the amount of galactose exuded as a result of plant age. These results are indicative of consistent trends which need further investigation. Duncan's multiple range test showed an interaction between treatment and time which was significant at the 5% level. Young plants (2 wks old) showed a significantly greater amount of galactose exuded under aerobic conditions. As the plants matured (4 and 6 wks old) there was a trend toward greater amounts of galactose exuded under anaerobic conditions than under aerobic. Analysis of variance showed no significant differences at the 25% level in the total amounts of dihydroxyacetone exuded as a result of treatment and no significant difference at the 25% level in the amounts of dihydroxyacetone exuded as a result of plant age. These results are indicative of consistent trends which need further investigation. Duncan's multiple range test showed in young plants (2 wks old) a Significantly greater amount of dihydroxyacetone exuded under aerobic conditions at the 5% level. As the plants matured (4 and 6 wks old) there was a trend toward greater amounts of dihydroxyacetone exuded under anaerobic than under aerobic conditions. |
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