Effects of Arbuscular Mycorrhiza and Phosphorus Supply on the Growth of Perennial Ryegrass

Aim: The basic aim of this thesis was to disentangle phosphorus status-dependent and -independent effects of arbuscular mycorrhizal fungi (AMF, Glomus hoi) on the components of plant growth: morphology and assimilation rates, in perennial ryegrass (Lolium perenne L.). Materials & Methods: In a first experiment, I assessed phosphorus response functions of leaf and plant morphological components in undisturbed plants with similar size. To this end, nonmycorrhizal and mycorrhizal plants were grown in controlled conditions at four different soluble phosphorus supplies ranging from 0 mM to 0.5 mM. In a second experiment, AMF effects on carbon economy were investigated by comparing nonmycorrhizal and mycorrhizal plants of similar size, morphology and phosphorus content. CO2/CO2 steady-state labelling was used to trace all photosynthate assimilated during one photoperiod, and a respiratory CO2/CO2 exchange system to assess dark respiration rates, and the contribution of recently fixed carbon to shoot and root respiration. Results & Discussion: Relative growth rate of mycorrhizal plants was significantly higher as a result of improved phosphorus nutrition. The presence of AMF stimulated relative phosphorus uptake rate, decreased leaf mass per area (LMA) and increased shoot mass ratio at low phosphorus supply. Lower LMA was caused by both decreased tissue density and thickness. Variation in tissue density was almost entirely due to variations in soluble carbon, while that in thickness involved structural changes. Carbon economy analyses revealed that AMF enhanced relative respiration rate of the root-soil system by 16%, inducing an extra C flow amounting to 3% of daily gross photosynthesis. Total C drain for growth and respiration of the AMF was estimated at ≤ 8% of daily gross photosynthesis. This was associated with a greater amount of new C allocated below-ground and respired in mycorrhizal plants. AMF colonization affected the sources supplying below-ground respiration, revealing a greater importance of plant C stores in supplying respiration and/or the participation of storage pools within fungal tissues. Conclusions: The relationships between relative phosphorus uptake rate, leaf and plant morphology were identical in mycorrhizal and nonmycorrhizal plants. Beneficial effects of mycorrhizal symbiosis were mainly mediated by adjustments in leaf morphology, which were largely dependent on AMF effects upon phosphorus capture. When ontological and nutritional effects are accounted for, AMF increased below-ground costs, which were not compensated by increased photosynthesis rates.