Maize (Zea mays L.) growth at low soil P levels is affected both by inherent physiological factors as well as interactions with soil microbes. The objectives of this study were (i) to quantify differences among maize inbred lines for growth at low P and response to mycorrhizal fungi, and (ii) to identify quantitative trait loci (QTL) controlling these traits in a B73 × Mo17 recombinant inbred population. Shoot dry weight and root volume were measured in the greenhouse after 6 wk of growth in a factorial experiment of 28 inbred maize lines using treatments of low vs. high P and mycorrhizal vs. nonmycorrhizal treatments. Shoot dry weight for the low P treatment in the absence of mycorrhizae ranged from 0.56 to 3.15 g. Mycorrhizal responsiveness based on shoot dry weight ranged from 106 to 800%. Shoot dry weight in the low P–nonmycorrhizal treatment was highly negatively correlated with mycorrhizal responsiveness. Plants grown at high P in the presence of mycorrhizae accumulated only 88% of the biomass of plants grown at high P in the absence of mycorrhizae, indicating that mycorrhizae can reduce plant growth when not contributing to the symbiosis. Percentage of root colonization was not correlated with mycorrhizal responsiveness. B73 and Mo17 were among the extremes for growth at low P and mycorrhizal responsiveness, and a B73 × Mo17 population of 197 recombinant inbred lines was used to detect QTL for growth at low P and mycorrhizal responsiveness. Three QTL were identified which controlled growth at low P in the absence of mycorrhizae based on shoot weight and one QTL which controlled mycorrhizal responsiveness. This study indicates that there is substantial variation among maize lines for growth at low P and response to mycorrhizal fungi. This variation could be harnessed to develop cultivars for regions of the world with P deficiency and for reduced‐input production systems.

DNA‐based genetic markers are now widely used by geneticists to locate genes for quantitative traits, and may also serve as a valuable tool for dissecting complex physiological phenomena. Van den Berg et al. (1996a QTL analysis of potato tuberization. Theor Appl Gen 93: 307–316), using restriction fragment length polymorphism (RFLP)‐mapped populations of potato, detected eleven quantitative trait loci (QTLs) for tuberization. Taylor et al. (1992 Expression and sequence analysis of cDNAs induced during the early stages of tuberisation in different organs of the potato plant [Solanum tuberosum L.]. Plant Mol Biol 20: 641–651) have identified one of the genes associated with tuberization as that for the enzyme S‐adenosylmethionine decarboxylase (SAMdc), an enzyme of the polyamine biosynthetic pathway. Chromosomal loci for SAMdc and arginine decarboxylase were established on the potato and tomato chromosomal maps, respectively, by hybridizing cDNA probes for these genes to RFLP digests. The polyamine content of leaves from an RFLP‐mapped potato population was analyzed by fluorescence detection following HPLC, with quantitation using an internal standard. The data were analyzed by the ‘qGene’ statistical program, and QTLs for polyamines were detected on seven chromosomes. At least six QTLs were found for spermine, two for spermidine, and two for putrescine. A spermidine QTL was on chromosome 5 linked to marker TG441, very close to the place where SAMdc mapped. There was some congruence between QTLs for spermine and those previously detected for tuberization and dormancy, but relationships were not consistent.