1988. Mycorrhizal dependence and growth habit of warn-season and cool-season tallgrass prairie plants. Can. J. Bot. 66: 1376-1380. Warn-season (C,) and cool-season (C,) mycorrhizal grasses were 63-215 and 0.12-4.1 times larger in dry weight than noninoculated controls, respectively. Nonmycorrhizal warm-season plants did not grow and frequently died, while cool-season plants grew moderately well in the absence of mycorrhizal symbiosis. Like warm-season grasses, tallgrass prairie forbs were highly dependent on mycorrhizal symbiosis, even though they are not known to employ the C, photosynthetic pathway. Thus, phenology may be more critical than photosynthetic pathway in determining mycorrhizal dependence. Warm-season grasses and forbs had coarser, less frequently branched root systems than cool-season grasses, supporting the hypothesis that mycorrhizal dependence is related to root morphology. Cool-season grasses may have developed more fibrous root systems because mycorrhizal nutrient uptake was not effective in the colder temperate environment in which they evolved. In contrast, warmseason plants and dependence on mycorrhizal fungi may have coevolved, because both symbionts are of tropical origin. HETRICK, B. A. D., KITT, D. G., et WILSON, G. T. 1988. Mycorrhizal dependence and growth habit of warn-season and cool-season tallgrass prairie plants. Can. J. Bot. 66 : 1376-1380. Des herbes mycorhiziennes de climats chaud (C,) et de climats frais (C,) montrent respectivement des accroissements de 63-215 et de 0,12-4,l fois supCrieurs B ceux des tCmoins non inoculCs. Les herbes de climats chauds non mycorhizCcs ne poussent pas et m&me meurent souvent, alors que celles de climats frais montrent une croissance modCree en absence de sym-biotes mycorhiziens. Tout comme les herbes de climats chauds, les arbustes des prairies B hautes herbes sont fortement debendants de la symbiose mycorhizienne, bien qu'on ne leur reconnaisse pas la capacitC d'utiliser le sentier mCtabolique de la photosynthkse en C,. I1 semble donc que la phinologie jouerait un r6le plus dkterminant que le senlier photosynthCtique, dans la ditermination de la dCpendance mycorhizienne. Les herbes et les arbustes de climats chauds posskdent des systkmes racinaires plus robustes et moins ramifiCes que les herbes de climats frais, ce qui supporte l'hypothkse que la dipendance mycorhizienne serait like B la morphologie racinaire. Les herbes de climats frais pourrait avoir dCveloppC des systkmes racinaires plus fibreux parce que l'accumulation mycorhizienne des nutriments serait moins efficace dans l'environnement plus froid des rCgions tempCrCes oh elles ont CvoluC. Par contre, les plantes de climats chauds ainsi que les champignons mycorhiziens dont elles dependent pourraient avoir co-CvoluC, puisque les deux symbiotes sont d'origine tropicale.[Traduit par la revue]

In a series of experiments, factors effecting mycorrhizal growth response in prairie plants were examined. Two prairie grasses (Andropogon gerardii Vitman and Sorghastrum nutans (L.) Nash) and two forb species (Petalostemum purpureum (Venten) Rydb. and Liatris punctata Hook) benefited significantly from Glomus etunicatum Becker & Gerd. inoculation in fumigated or steamed soil. This benefit from mycorrhizae is probably related to phorphorus availability, since addition of 100 ppm phosphorus overcame the mycorrhizal growth response. No stimulation of growth occurred from additional vesicular–arbuscular, mycorrhizal inoculum to nonsterile soil. Neither was growth in nonsterile soil comparable to that achieved in inoculated, sterilized soil, suggesting a suppression of mycorrhizal growth response in nonsterile soil. In a related experiment addition of 1 or 10% nonsterile soil to sterilized, inoculated soil resulted in significantly reduced plant growth and greatly diminished mycorrhizal root colonization. Thus, soil microorganisms may suppress mycorrhizal growth responses, explaining the lack of mycorrhizal growth response observed in nonsterile soil. Of the four mycorrhizal fungi compared (Gigaspora rosea Nicol. & Schenk, Glomus deserticolum Trappe, Bloss & Menge, Glomus etunicatum Becker & Gerd., and Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe), the two indigenous species most stimulated plant growth. Under severe moisture stress (−4500 kPa) a significant mycorrhizal growth response was evident for G. etunicatum, but not for G. deserticolum inoculated plants. Growth response does not necessarily suggest drought tolerance, since the magnitude of response was similar in adequately watered and severely draughted plants.

SUMMARYThe influence of a vesicular-arbuscular (VA) mycorrhizal fungus, phosphorus (P) fertilization, and soil microorganisms on growth and root architecture of big bluestem {Andropogon gerardii Vitman) was investigated, hi pasteurized soil, mycorrhizal inoculation significantly improved plant growth and increased root length and the number and the diameter of the primary, secondary and tertiary roots. These differences between mycorrhizal and non-mycorrhizal plants diminished with added P. In pasteurized soil amended with non-sterile soil sievate, differences between mycorrhizal and non-mycorrhizal plants were still obvious, but in many treatments these plants grew more poorly (had less dry weight, root length, number or diameter) than their counterparts in unamended pasteurized soil. Growth in non-sterile soil was also suppressed, and mycorrhizal responses were not detected since all of the plants in non-sterile soil became mycorrhizal whether or not they were inoculated. Two analyses of calculated parameters which describe root-system architecture were conducted. The first, specific root length (SRL), revealed that mycorrhizal symbiosis dramatically alters root morphology in soils of low fertility. These changes were similar to the changes evoked by added P. The second, path length (P,,) revealed that mycorrhizal fungi (and to some degree other soil microbes) significantly alter root architecture by reducing the relative amount of root branching. Apparently, mycorrhizal plants develop a more elongate, exploratory growth pattern which permits the funga) hyphae to extract nutrients from a larger volume of soil. In contrast roots of nonmycorrhizal plants maintain a more highly branched pattern of root growth, and the roots themselves play a more critical role in the direct extraction of nutrients from the soil. These differences in root topology were not directly associated with the concentration of exogenous P, but instead appeared to be controlled by the mycorrhizal fungi themselves. Thus, while internal P content of plants mediates the establishment of the mycorrhizal symbiosis, the fungi can alter the root architecture of the plant to a form which best accommodates the symbiosis under the prevailing fertility and rhizosphere conditions in the soil. By altering root-system architecture in this manner, the mycorrhizal fungi can control, at least to some degree, the dependence of the host on the symbiosis. Thus, the topology of the root system is contingent upon the microflora in the rhizosphere. The topological analysis revealed differences in root architecture not detected by any of the other rneasures of root morphology. These differences suggest that mycorrhizal fungi affect root architecture and plant growth in ways not directly associated with phosphorus uptake.

No abstract