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Mutualistic relationships in rocky patches
Flowering plants grow in a wide range of environments, some of which are very challenging for plants. One such environment is described by Pimienta-Barrios et al. from Jalisco, Mexico and University of California, Los Angeles (pp. 239-245). This is the inselbergs, which are rocky ‘islands’ that protrude from the surrounding land, at the edge of the Chihuahuan desert in a semi-arid region of Mexico. In general, inselbergs may be colonized by plants that gain a root-hold in crevices in which some organic matter has accumulated and which have the potential to collect moisture. It is a precarious habitat and in the study area is made more difficult by the intermittent pattern of rainfall. Although the mean annual rainfall is approx. 500 mm, it nearly all occurs between May and September, the other months being dry. However, in the 6 years prior to the work described here, there had been a major drought, eventually broken in the ‘wet’ season of 2000. Needless to say, one common adaptation to life in the Mexican inselbergs is Crassulacean Acid Metabolism (CAM), in which CO2 is initially fixed at night. Another adaptation to nutrient-poor environments is the establishment of mutualistic relationships with fungi to form mycorrhizae. The plant studied by the authors, Opuntia robusta, is a CAM plant that forms an arbuscular mycorrhizal symbiosis. They worked with plants in the field and treated some with fungicide to prevent the formation of mycorrhizae. They then measured photosynthesis, water uptake and phosphorus uptake in mycorrhizal and non-mycorrhizal plants. Surprisingly, apart from a slight reduction in photosynthesis in non-mycorrhizal plants at the end of the wet season, there were no differences between mycorrhizal and non-mycorrhizal plants. These results thus challenge the accepted view that mycorrhizal symbiosis increases water and nutrient uptake leading to increased photosynthesis. There is clearly much to discover about life in a Mexican inselberg.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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