Plant Iron Nutrition in Calcareous Soils: Are Arbuscular Mycorrhizas a Help or a Hindrance?

University Of Miami, Coral Gables FL

Investigators

Abstract

Plant iron nutrition in calcareous soils: are arbuscular mycorrhizas a help or a hindrance? David P. Janos University of Miami In plants, iron plays an important role in chlorophyll synthesis, and iron-deficient plants display yellow leaves, reduced photosynthesis, and limited growth. Iron deficiency is most likely to be found in plants growing upon the approximately 20% of world soils that are rich in calcium, because their high pH and presence of large amounts of calcium carbonate restrict iron availability. Although native plant species of calcareous soils may be adapted to low iron availability, crop species and non-native trees often show serious iron deficiency on calcareous soils. In natural vegetation, most plant species ensure their supply of mineral nutrients, especially those that are relatively immobile in soil such as phosphorus, zinc, and copper, by associating with generally beneficial root-inhabiting fungi. Absorptive rootlets colonized by these fungi are called "mycorrhizas", and their most prevalent type is designated "arbuscular" after fungus structures formed within root cortical cells. Although iron too is immobile in soil, the role of arbuscular mycorrhizas (AM) in plant iron nutrition is ambiguous. Prior studies have reported either negative, none, or beneficial effects of AM on iron acquisition depending upon soil pH, temperature, and nutrient availability. Resolution of the role of AM in plant iron nutrition may come from consideration of glomalin, a protein that can contain up to 8.8% iron by dry weight, and which recently has been discovered to be produced copiously by AM fungi. Whether iron can disassociate easily from the protein to become available to plants, or whether iron is sequestered unavailable to plants is not known. The general objective of this research is to determine if glomalin sequesters sufficient iron in a calcareous soil to adversely affect plant species intolerant of low iron. Specific aims of the research are: 1) to isolate AM fungus species from calcareous soils, to screen them for glomalin production, and to determine if iron availability influences glomalin production, 2) to compare high- versus low-glomalin AM fungus species effects on the growth of two iron-sensitive, fruit tree species, Lychee (Litchi chinensis) and Star fruit (Averrhoa carambola), and 3) to examine whether or not AM fungi and glomalin inhibit the establishment of native Slash pine (Pinus eliottii) - which forms a different type of mycorrhiza - in calcareous soil under native broadleaf vegetation in South Florida. The investigator will isolate species of AM fungi from local soils, will compare them for glomalin production under uniform conditions, and will examine if different concentrations of differing iron sources affect glomalin production. Two high and two low glomalin-producing AM fungus species will be tested for their ability to affect the iron nutrition of Lychee and Star fruit plants. Finally, non-mycorrhizal Slash pine seedlings will be planted in the field in containers designed to allow or to exclude AM fungus colonization to see if they are inhibited. The plant species to be investigated are economically important with Lychee and Star fruit accounting for the 3rd and 9th most extensive acreages of tropical fruit in South Florida, and Slash pine being an important fiber source. Overall, the research will resolve ambiguity concerning arbuscular mycorrhiza effects on plant iron nutrition.