Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2008
Publication Date: 6/28/2008
Citation: Resendes, M.L., Bryla, D.R., Eissenstat, D.M. 2008. Early events in the life of apple roots: variation in root growth rate is linked to mycorrhizal and nonmycorrhizal fungal colonization. Plant and Soil Journal. Available: http://www.springerlink.com/content/275k6l4m56234623/
Interpretive Summary: When new roots emerge into the soil, they encounter a myriad of soil organisms, some beneficial, others not. One such class of these organisms, very common in both natural and agricultural soils, are mycorrhizal fungi. Like many pathogenic soil fungi, mycorrhizal fungi penetrate living roots of plants to acquire energy-rich carbohydrates needed for their growth and function. However, unlike pathogens, external hyphae produced by mycorrhizas absorb soil nutrients and translocate them to the fungal-plant interface where they are transferred to the roots. In effect, these fungi act as extensions of the plant's root system. Few studies have examined the daily process of mycorrhizal colonization at the individual root level under controlled conditions, and none that we are aware of examined this in the field where a complex of soil microbes interacts. Consequently, the events that occur in the first days of a root’s life under field conditions are poorly understood. In this study, we examined daily patterns of mycorrhizal colonization of individual new roots in a well-established apple orchard and determined if their presence was positively or negatively associated with the occurrence of other fungi of a nonmycorrhizal and possibly pathogenic origin. Our observations provide evidence that: (1) roots are almost never colonized by both mycorrhizal and nonmycorrhizal fungi in the first weeks of life, perhaps due to a combination of causes including direct interference by the mycorrhizal fungi and healthy roots being less susceptible to nonmycorrhizal fungi; (2) mycorrhizal fungi selectively colonize faster growing roots, thus, providing greater opportunity for occupancy of a longer-living host that receives a greater fraction of carbohydrates from the shoot; and (3) mycorrhizal fungi may increase the length of time and rate of growth of roots that are colonized, which would increase the benefits the fungus may gain from the host. Such information has important implications for nutrient and root disease management in apple and other perennial crops.
Technical Abstract: A study was conducted to characterize early events of mycorrhizal and nonmycorrhizal fungal colonization in newly-emerging roots of mature apple (Malus domestica) trees and to determine the relationship to fine root growth rate and development. New roots were traced on root windows to measure growth and then collected and stained to quantify microscopically the presence of mycorrhizal and nonmycorrhizal fungal structures. Most new roots were colonized by either mycorrhizal or nonmycorrhizal fungi but none less 25 d old were ever internally colonized by both. Compared to nonmycorrhizal colonization, mycorrhizal colonization was associated with faster growing roots and roots that grew for a longer duration, leading to longer roots. While both types of fungi were observed in roots as quickly as 3 d, intraradical colonization by mycorrhizal fungi was generally faster (peaking at 7–15 d) than that by nonmycorrhizal fungi and often occurred more frequently in younger roots. Few roots (15–35%) had no fungal colonization by 30 d after emergence. We observed marked discrimination of roots between mycorrhizal and nonmycorrhizal fungi and provide evidence that mycorrhizal fungi may select for faster growing roots and possibly influence the duration of root growth by non-nutritional means.