Legume responses to low phosphorus fertility T.K. Raab University of Colorado Mountain Research Station, Campus Box 334, Boulder, Colorado 80309-0334 INTRODUCTION A variety of low-molecular weight organic compounds inhabit the spaces with soils and bathe the roots of plants. These chemicals serve in communication in plant-animal relations, and solubilize slowly-available nutrients from soils. There is much interest in their distribution and transport across soil-water-root interfaces. We exposed several mung bean (Vigna mungo L.) plants over 1 –15 weeks to either nutrient-replete conditions or low-phosphorus. Plants are sown directly into “rhizoboxes” of dimensions 20 cm x 10.5 cm x 1 cm thick, and a portion of the roots exposed to the mid-IR synchrotron light of ALS Beamline 1.4.3. The IR spectra are acquired from the root/soil interface through a 50 mm x 20 mm x 1 mm thick zinc selenide trapezoidal window (Spectral Systems Inc.; Hopewell Junction NY). Chemically detailed and fine-spatial scale IR reflection spectra are compared to discern root responses to abiotic stressors in wild and cultivated members of the legume plant family. Low-phosphorus availability was the first challenge of our model edible legume, Vigna mungo L. In other experiments, we collected spectra from the wild desert legume Lupinus arizonicus L. to compare responses between species. A second, parallel goal of the research is to identify spectral features useful in measuring diffusion coefficients with IR microscopy. Amino acids are important soil resources of N and C to plants and soil microorganisms [1,2], and their availability is likely to increase under projected future atmospheric CO 2 levels. Thus it is necessary to estimate their diffusion coefficients in realistic substrates. In the last two years of experiments we grew plants in acid- washed silica/feldspar sand of particle diameter > 250 μm. In preliminary experiments in 1998 to screen suitable solid media for root studies at Beamline 1.4.3, we discovered that high organic matter soils from Colorado and Hawai’i have too little reflected IR signal for spectromicroscopy. RESULTS FOR THIS YEAR In Figure 1, you see typical IR reflection spectra acquired from the root zone of mung bean (Vigna mungo L.) seedlings exposed to either low-phosphorus(A) or nutrient-complete (B) conditions. The plants were cultured on ¼-strength modified Hoagland’s Solution, with micronutrients and NH 4 + :NO 3 - ratio appropriate for legumes. Seeds were sown directly in the root boxes, and the mung bean plants subsequently expressed large branching root systems, a portion of which was coincident with the ZnSe IR-transmitting windows. Spectra are acquired from 4500 – 650 cm -1 , since the low energy cut-off for this window material is 650 cm -1 .