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PubMed2005

Root exudation, phosphorus acquisition, and microbial diversity in the rhizosphere of white lupine as affected by phosphorus supply and atmospheric carbon dioxide concentration.

White lupine (Lupinus albus L.) was used as a phosphorus (P)-efficient model plant to study the effects of elevated atmospheric CO(2) concentrations on (i) P acquisition, (ii) the related alterations in root development and rhizosphere chemistry, and (iii) the functional and structural diversity of rhizosphere microbial communities, on a P-deficient calcareous subsoil with and without soluble P fertilization. In both +P (80 mg P kg(-1)) and -P treatments (no added P), elevated CO(2) (800 micromol mol(-1)) increased shoot biomass production by 20 to 35% and accelerated the development of cluster roots, which exhibit important functions in chemical mobilization of sparingly soluble soil P sources. Accordingly, cluster root formation was stimulated in plants without P application by 140 and 60% for ambient and elevated CO(2) treatments, respectively. Intense accumulation of citrate and increased activities of acid and alkaline phosphatases, but also of chitinase, in the rhizosphere were mainly confined to later stages of cluster root development in -P treatments. Regardless of atmospheric CO(2) concentrations, there was no significant effect on accumulation of citrate or on selected enzyme activities of C, N, and P cycles in the rhizosphere of individual root clusters. Discriminant analysis of selected enzyme activities revealed that mainly phosphatase and chitinase contributed to the experimental variance (81.3%) of the data. Phosphatase and chitinase activities in the rhizosphere might be dominated by the secretion from cluster roots rather than by microbial activity. Alterations in rhizosphere bacterial communities analyzed by denaturing gradient gel electrophoresis (DGGE) were related with the intense changes in root secretory activity observed during cluster root development but not with elevated CO(2) concentrations.

Atmosphere↗
PubMed1968

Determination of root exudates in a steril continuous flow culture. I. The culture method.

A sterile plant culture consisting of culture vessels, culture solution container, collecting flasks for percolating nutrient solution, illumination and aeration systems and a suitable pump is described. Its difference with other culture methods is a very slow, continuous percolation of the nutrient solution through the rooting medium. Well defined and controllable conditions can thus be established in the rhizosphere over long culture periods. Samples can be collected at short intervals without disturbing the rhizosphere in any way nor endangering the sterility of the culture. One of the fundamental factors determining the special ecological characteristics of the plant rhizosphere is the liberation of organic and inorganic substances by the plant root. During the study of this phenomenon it became evident that the amount of substances liberated varies within wide limits (factors 100 to 1000) according to the conditions in which the root is developing.

Journal Article↗
PubMed1968

Determination of Root Exudates in a Steril Continuous Flow Culture. II. Short-Term and Long-Term Variations of Exudation Intensity.

The exudate production of alfalfa under the conditions of the sterile flow culture was quantitatively measured. In the first 40 days 3.10(-3) mumoles amino-N, 2.5 muequivalents of organic acids and approximately 10(-4) mumoles of reducing sugars were liberated per plant and per day into the percolating nutrient solution. The amino acid concentration in the outflow varies according to a daily periodicity. The exudation of a colored substance also shows daily periodical variations. This pattern is different from the pattern of the amino acid exudation, however, and directly coupled to shoot illumination. Short-term 2,4-dinitrophenol additions to the nutrient lower the liberation of amino acids into the percolating solution.

Journal Article↗
PubMed1991

Mechanism of aluminum tolerance in snapbeans : root exudation of citric Acid.

One proposed mechanism of aluminum (Al) tolerance in plants is the release of an Al-chelating compound into the rhizosphere. In this experiment, two cultivars of snapbeans (Phaseolus vulgaris L. "Romano" and "Dade") that differ in Al tolerance were grown hydroponically with and without Al under aseptic conditions. After growth in nutrient solutions for 8 days, aliphatic and phenolic organic acids were analyzed in the culture solutions with an ion chromatograph and a high pressure liquid chromatograph. The tolerant snapbean, "Dade", when exposed to Al, exuded citric acid into the rhizosphere in a concentration that was 70 times as great as that of "Dade" grown without Al, and 10 times as great as that of "Romano" grown with or without Al. The sensitive cultivar, "Romano", exuded only slightly more citric acid into the growing medium under Al-stress, compared to nonstressed conditions. Citric acid is known to chelate Al strongly and to reverse its phytotoxic effects. Also, citric acid has been shown previously to enhance the availability of phosphorus (P) from insoluble Al phosphates. Thus, one mechanism of Al-tolerance in snapbeans appears to be the exudation of citric acid into the rhizosphere, induced either by toxic levels of Al or by low P due to the precipitation of insoluble Al phosphates. Our experiment was not able to distinguish between these two factors; however, tolerance to both primary and secondary Al-stress injuries are important for plants growing in Al-toxic soils.

Journal Article↗