Factors influencing plant response during mycorrhizal establisment and formation: The cost-benefits in a a symbiotic continuum

Abstract

The goal of this thesis was to understand: i) What factors determine the variance in plant response to mycorrhization, and how; ii) How does this variance relate to the balance between mycorrhizal cost and benefit. Two main and different proxies were used: i) The effects of mycorrhization on plant productivity and vitality; ii) C and N metabolism readjustments in response to mycorrhizal formation. The mycorrhizal partners chosen were Pinus pinaster L. and Pisolithus tinctorius (Pers.) Coker & Couch. The effects of mycorrhization over the plants productivity were tested in conditions that went from severely N limited to near optimum N supply. In these conditions the plants response to mycorrhization was consistently found to be determined by its effect on the plants’ N uptake. It is concluded that the cost efficiency of N acquisition is not important in the outcome of the symbiosis, but only the balance between the N supply by the fungus and the N demand by the plant. A new index is proposed, the mycorrhizal N demand/supply ratio (MDSN), which was found to accurately explain how this balance changes. The plant’s photosynthetic performance (PIabs) was used as an indicator of plant vitality/stress, and was not reliably correlated with mycorrhizal effects on growth or N status, confirming that these two parameters are not good indicators of stress or plant fitness. It was also demonstrated that investing more in fungal growth at one particular moment is not necessarily a disadvantage in the long term. The validity of the proposal of parasitism in ECM interactions is discussed. It is concluded that the hypothesis that the benefits exchanged by the partners are byproducts instead of costly benefits, explains the reported results.

Alternate abstract:

A micorrização é tradicionalmente considerada benéfica para a planta hospedeira, uma vez que, de modo geral, aumenta a tomada de nutrientes, e subsequente aumento do crescimento da planta. Nomeadamente as interacções ectomicorrízicas (ECM) são particularmente importantes em ecossistemas limitados pelo N. O melhoramento da tomada e concentrações na planta deste nutriente como resultado da formação de ECM têm sido frequentemente observados. No entanto, o número de publicações relatando respostas negativas ou nulas à micorrização, em termos de produtividade, tem vindo a aumentar. Estas observações têm sustentado a hipótese de que esta associação possa apresentar um contínuo de respostas, variando de mutualista a antagonista. Pensa-se ainda que em qualquer combinação planta-fungo, a resposta à micorrização possa deslocar-se ao longo deste contínuo. É ainda incerto quais os factores que determinam o tipo de resposta. Presume-se no entanto que o resultado de uma relação micorrízica dependa do balanço entre a necessidade de energia pelo fungo e a de nutrientes pela planta. O objectivo da presente tese foi compreender: i) Quais os factores que determinam a variação da resposta das plantas à formação de ECM; ii) Como se relaciona esta variação com o balanço custo:benefício associado à interacção micorrízica.