Orchid speciation and coexistence in the context of above and belowground mutualists

SAAB Annual Meeting Abstracts

Foliar phosphorus accumulation in relation to leaf traits: an example in a tropical wet forest in Hawaii R. Ostertag Department of Biology, University of Hawaii at Hilo, 200 W. Kawili Street, Hilo, HI 96720, United States The ability of plants to develop efficient uptake and storage mechanisms when P becomes available may have strong selective advantages. Hawaiian wet forests have previously been shown to contain examples of both N- and P-limitation, as demonstrated by fertilization experiments at a 300-yr-old Nlimited site and a 4.1-myr-old P-limited site. Using fertilization plots that exist within these two soil fertility extremes, I show that after fertilization, plants have the ability to increase both inorganic and organic foliar phosphorus to a much greater degree than foliar nitrogen. This foliar P accumulation, or luxury consumption, transcends soil nutrient availability, species differences, and life forms. Although foliar P accumulation was consistent across species, its magnitude varied by species, ranging from 73-498% increases relative to unfertilized plants. To determine if this P accumulation may be related to other leaflevel traits, I sampled eight common species at the P-limited site. Photosynthesis, leaf mass per area (LMA), and N and P concentrations were sampled from individuals growing in longterm fertilization plots (control, +N, +P). For species in +P plots, P accumulation was significantly negatively correlated to photosynthesis on a mass and area basis and photosynthetic Puse efficiency, and positively correlated to LMA. In contrast, there were no significant correlations for plants in +N plots; in addition, foliar N accumulation was unrelated to any of the leaf traits measured. These results suggest the species that are best at storing P are those with characteristics related to slower growth. The ability of species to accumulate P in leaves after pulses of P availability may be part of the suite of co-evolved traits related to efficient resource use in low fertility environments. doi:10.1016/j.sajb.2009.02.089

Orchid speciation and coexistence in the context of above and belowground mutualists A. Pauwa , R.J. Watermanb , c , T.G. Barracloughb ,c , V. Savolainenb , c , M.I. Bidartondob , c a Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa b Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, United Kingdom c Division of Biology, Imperial College London, Silwood Park Campus, Ascot, Berkshire SL5 7PY, United Kingdom Mutualists can potentially play a central role in the evolution of biodiversity by facilitating the speciation and coexistence of their associates. Here we examine the interplay between the evolution of pollinator preference and evolution of

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mycorrhizal fungal preference in group of related oil-secreting orchids. We find that closely related species tend to be pollinated by different species of oil-collecting Rediviva bees, and experimentally demonstrate that this results in reproductive isolation. In contrast, we find that closely related species tend to use the same mycorrhizal fungi, potentially preventing their co-occurrence through competition. We show how the interaction between the rapid evolution of pollinator preference and slow evolution of fungal preference potentially generates a series of orchid communities in which closely related species occur in different communities, while distantly related species co-occur. doi:10.1016/j.sajb.2009.02.090

Carboxylates in the rhizosphere of canola, wheat, lupins and pulses: Their role in P acquisition from sparingly soluble forms S.J. Pearsea, E.J. Veneklaasa, G.R. Cawthraya, M.D.A. Bollanda,b, H. Lambersa a School of Plant Biology (M084), The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia b The Department of Agriculture and Food Western Australia, PO Box 1231, Bunbury WA 6231, Australia ★ Awarded prize for best paper presented by Young Scientist

Native Australian soils contain very low amounts of phosphorus. The soils of south-western Australia are ancient and highly weathered. Consequently, the availability of phosphorus in these soils is too low for cropping purposes, so the application of P is necessary to maintain productivity. When P is applied to soil, typically as soluble superphosphate, it tends to be transformed to increasingly less soluble forms over time. Sparingly soluble forms of soil P are relatively inaccessible to Triticum aestivum; however, many grain legumes have a higher P-acquisition efficiency, allowing them to access pools of soil P that T. aestivum cannot. The P-acquisition efficiency of some grain legumes has been attributed in part to their ability to release large quantities of carboxylates, coupled with the development of cluster roots for species such as Lupinus albus. We conducted a series of experiments studying carboxylate release from a range of crop species, and investigating its role in variation among species for acquisition of phosphorus from sparingly soluble forms. We conclude that studies of carboxylate exudation and sparingly soluble forms should never use a single form if the aim is to draw generalised conclusions on P-uptake efficiency from sparingly soluble forms. While the benefit of carboxylates for providing access to poorly soluble P has been demonstrated, questions remain as to other potential roles for carboxylates, particularly in species that do not form cluster roots. doi:10.1016/j.sajb.2009.02.091