Minna-Maarit Kytöviita

Mycorrhizal benefit in two low arctic herbs increases with increasing temperature

Climate change may influence the relationship between arctic plants and their symbiotic mycorrhizal fungi. The benefit of the symbiosis for the host plant affects vegetation succession and may be a key parameter in predicting vegetation responses to warming. We investigated the mycorrhizal benefit in the low arctic perennial herbs Potentilla crantzii and Ranunculus acris in symbiosis with the arbuscular mycorrhizal fungus Glomus claroideum. Temperature response in the mycorrhiza-mediated acquisition of nitrogen (N) and phosphorus (P), growth, and photosynthetic nutrient-use efficiency were determined. Near the average natural soil temperature (12°C), mycorrhiza did not improve plant nutrient capture but significantly enhanced plant P capture at 17°C. Photosynthetic nitrogen-use efficiency was higher at 17°C than at 12°C and was further increased by mycorrhiza at 17°C. Photosynthetic phosphorus-use efficiency was not affected by temperature or mycorrhiza. Increasing the growing temperature by 5°C increased the relative shoot growth rate by 15%. Mycorrhizal symbiosis did not enhance plant growth rate, but the plants gained between 20% and 90% more mycorrhiza-mediated P when grown at higher temperature. The results suggest that these low arctic species have good potential to respond positively to increasing temperatures.

Mycorrhiza does not alter low temperature impact on Gnaphalium norvegicum

Extreme arctic-alpine vegetation has relatively low affinity to form mycorrhizal symbiosis. We asked whether the mycorrhizal growth benefit for the host plant is lower at low temperatures. We investigated the role of two root-associated fungi and temperature in growth, carbon–nitrogen relations and germination of an arctic-alpine herb. Seeds of Gnaphalium norvegicum were germinated at 8° or 15°C with or without arbuscular mycorrhizal (AM, Glomus claroideum) and dark septate endophytic (DSE, Phialocephala fortinii) inocula in a climate chamber. We found that germination percentage, shoot and root biomass, shoot N% and root AM colonization were lower at 8°C than at 15°C. P. fortinii inoculation had a positive impact on germination at both temperatures, whereas G. claroideum produced no effect. N% was lower in AM plants at both temperatures. Plant biomass and shoot N content were higher in AM plants than in control plants at 15°C, but not at 8°C. DSE inoculation tended also to have positive effects on plant biomass and N content at 15°C. At 15°C, rate of photosynthesis, photosynthetic nutrient use efficiency and specific leaf area were positively affected by G. claroideum, which suggests that G. claroideum formed a carbon sink and possibly enhanced the seedling water economy. The positive effects of P. fortinii were probably due to its saprotrophic function in the substrate because it did not colonize the roots. These results suggest that the effects of AM and DSE on plant growth are affected by temperature and that the mycorrhizal benefit for the host plant was lower at the lower temperature. Low saprotrophic activity and decreased mycorrhiza-mediated nutrient acquisition may thus constrain plant nutrient acquisition in cold environments. Decreased mycorrhizal benefit may be related to the comparatively low mycotrophy of cold environment vegetation.

The phenolic compounds in Cladonia lichens are not antimicrobial in soils

According to classic text books on lichen biology, the phenolic secondary chemicals in lichens have antibiotic effects on soil microorganisms and mycorrhizal fungi in ecosystems. However, the experimental evidence for this under natural conditions is still relatively scarce. We examined some of the assumptions behind the concept of antimicrobial effects of lichen secondary substances: (1) the secondary substances of Cladoniastellaris, usnic and perlatolic acids, are leached out from the lichens by rainwater; (2) these substances inhibit the microbial activity of soil, and; (3) since they are extremely resistant to microbial decomposition, the soil underneath a continuous lichen mat is enriched in usnic and perlatolic acids. Our results did not support any of these assumptions. The evidence for the antimicrobial activity of lichen secondary substances seems to be weak in comparison to other suggested functions such as light filtering and herbivore protection. We suggest that it is time to re-evaluate the evidence for the antimicrobial ecological role of lichen secondary substances in natural systems.

Defoliation and the availability of currently assimilated carbon in the Phleum pratense rhizosphere

Abstract It has been hypothesised that defoliation and aboveground herbivory increase the availability of currently assimilated C to organisms living in plant rhizospheres. We established a growth chamber experiment consisting of Phleum pratense individuals growing in sand culture to examine the short- and long-term effects of defoliation on the availability of current C assimilates in the P. pratense rhizosphere. Using 14CO2 pulse labelling, we followed partitioning of currently assimilated C between shoots, roots and rhizosphere-derived organic matter (RDOM). The experiment constituted of two treatments, defoliation history and recent defoliation, in a fully factorial design. Defoliation history had three levels: (1) no past defoliation, (2) defoliation 1 week before labelling, and (3) defoliation 1, 2 and 3 weeks before labelling, while recent defoliation had two levels: (1) no defoliation and (2) defoliation immediately before labelling. Recent defoliation reduced the amount of 14C radioactivity in shoots, roots and RDOM, while defoliation history did not have a significant effect. Neither treatment affected the proportions of shoot, root and RDOM radioactivity-to-total recovered radioactivity or the ratio of RDOM radioactivity-to-root radioactivity. The results suggest that the amount of current C assimilates found in the P. pratense rhizosphere decreases shortly after defoliation, but is not affected by defoliations that have occurred at least 1 week earlier. The results further suggest that the defoliation treatments, despite affecting the quantity of assimilated current C, do not affect the allocation pattern of assimilated C within P. pratense individuals or the ratio of current assimilates found in P. pratense roots-to-those available in its rhizosphere. On the whole, our results do not support the hypothesis that defoliation increases the availability of current photosynthate to soil decomposer food webs.