Determining the impact of both environmental variation and developmental stage on plant-mycorrhizal associations is important, as both can shift the association along the mutualism-parasitism continuum. This study examines the effect of phosphorus level on the response of Allium vineale to mycorrhizae across all plant life stages, including plant fecundity and the relative allocation of resources to three different reproductive modes (flowers, asexual underground offsets, and asexual aerial bulbils) . For A. vineale, the impact of mycorrhizae varies significantly with life stage, as an early growth depression at 1 mo was reversed by 15 mo, resulting in mycorrhizal plants having larger bulbs over all P levels and producing more bulbils and larger offsets than nonmycorrhizal plants at lower P levels. However the presence of mycorrhizae did not affect the relative allocation of resources among the three reproductive modes. These results emphasize the importance of long-term studies of plant-mycorrhizal interactions that include fecundity estimates. In addition, they indicate that spatial variation in nutrient availability in the field has the potential to shift the overall effect of mycorrhizae from beneficial to neutral, with greater benefits found in sites with lower phosphorus levels.
The association between mycorrhizal fungi and plant roots was important in the evolution of land plants (Brundrett, 2002) and currently occurs in at least 80% of all plant species (Smith and Read, 1997). Colonization by arbuscular mycorrhizal (AM) fungi can provide multiple functions, such as increased nutrient uptake, drought tolerance, and resistance to pathogens (Newsham et al, 1995). Whereas many studies have shown that AM fungi can increase plant growth rates, it is widely recognized that there is considerable variation in response to colonization among plant species (Hart and Klironomos, 2002; Jones and Smith, 2004). This variation in response has been ascribed to a range of causes, including abiotic and biotic environmental factors, differential effects of colonization over the life cycle of the plant, and specificity in the association between the fungal-plant partners (Johnson et al, 1997; Jones and Smith, 2004).
Environmental factors such as light and nutrient availability may shift the plantmycorrhizal balance from beneficial, to neutral, or even negative (Johnson et al, 1997). For example, under high phosphorus conditions plant biomass may be reduced in the presence of mycorrhizal fungi (e.g., Buwalda and Goh, 1982; Peng et al, 1993; Olsen et al, 1999; see Smith and Smith, 1996 for a review) as the fungus may continue to draw carbohydrates from the plant, despite the fact that the plant can obtain P directly from the soil. In addition, whereas plants often show the greatest positive growth response at low phosphorus conditions, some studies have found the opposite (e.g., Li et al, 2005).
The plant-mycorrhizal interaction may also shift between a mutualistic and parasitic interaction depending on the life stage of the plant. When plants are young, the cost of the carbohydrate drain by the fungus may be greater than the benefit received by the plant from increased phosphorus availability, resulting in growth depression (e.g., Bethlenfalvay et al, 1982; Koide, 1985). Age of the plant was also important for Hyaanthoides non-scripta, as the mycorrhizal associated shifted from facultative to obligate with plant age (Merryweather and Fitter, 1995). The relative cost-benefit of the association may also shift seasonally, as the mycorrhizae are cosdy to Erythronium americanum in the fall but beneficial in the spring (Lapointe and Molard, 1997). These shifts among parasitic, neutral, and mutualistic interactions over time indicate the importance of longer term experiments diat incorporate all stages of the plant life cycle. Incorporating reproductive output and fitness estimates is of particular importance for understanding the ecological and evolutionary implications of plant-mycorrhizal associations. …