Mycorrhizal colonization and drought interactions of Mediterranean shrubs under greenhouse conditions

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Abstract
Juvenile plants of Thymus mastichina, Thymus zygis, Lavandula pedunculata, Genista hirsuta, and Cistus ladanifer were transplanted from the field to the greenhouse; the soil used was the same in which they had grown at their natural site. The plants were grown to maturity to determine comparative growth, water status, and mycorrhizal colonization under greenhouse conditions and to determine the influence of drought on the symbiosis. After 24 months, L. pedunculata showed the most developed root system; G. hirsuta showed the lowest root/shoot ratio and the lowest percentage of mycorrhizal colonization. This percentage ranged from 32.0% to 82.4% for G. hirsuta and L. pedunculata, respectively. There was a positive relationship between root biomass and root/shoot ratio with mycorrhization percentage, and a positive response of water potential to that percentage also occurred. After a moderate drought treatment over 4 weeks, root and shoot biomass and root/shoot ratio as well as mycorrhizal colonization decreased in stressed plants; the decrease in mycorrhizal colonization may have caused plant biomass to decline. The ectomycorrhizal symbiosis between C. ladanifer and Laccaria laccata appeared to be more severely affected by hydric stress than the endomycorrhizal symbiosis of the rest of the shrubs. This should be considered when this symbiosis is to be established in Mediterranean regions.