Reproductive biomass in Holcus lanatus clones that differ in their phosphate uptake kinetics and mycorrhizal colonization

In normal populations of the common grass Holcus lanatus there is a polymor phism for arsenate resistance, manifested as suppressed phosphate uptake (S PU), and controlled by a major gene with dominant expression. A natural pop ulation of SPU plants had greater arbuscular-mycorrhizal colonization than wild type, nonSPU plants. It was hypothesized that, in order to survive alo ngside plants with a normal rate of phosphate (P) uptake, SPU plants would be more dependent on mycorrhizal associations. We performed an experiment u sing plants with SPU phenotypes from both arsenate mine spoils and uncontam inated soils, as well as plants with a nonSPU phenotype. They were grown wi th and without a mycorrhizal inoculum and added N, which altered plant P re quirements. We showed that grasses with SPU phenotypes accumulated more sho ot P than nonSPU plants, the opposite of the expected result. SPY plants al so produced considerably more flower panicles, and had greater shoot and ro ot biomass. The persistence of SPU phenotypes in normal populations is not necessarily related to mycorrhizal colonization as there were no difference s in percentage AM colonization between the phenotypes. Being mycorrhizal r educed flower biomass production, as mycorrhizal SPU plants had lower shoot P concentrations and produced fewer flower panicles than non-mycorrhizal, nonSPU plants. We now hypothesize that the SPU phenotype is brought about b y a genotype that results in increased accumulation of P in shoots, and tha t suppression of the rate of uptake is a consequence of this high shoot P c oncentration, operating by means of a homeostatic feedback mechanism. We al so postulate that increased flower production is linked to a high shoot P c oncentration. SPU plants thus allocate more resources into seed production, leading to a higher frequency of SPU genes. Increased reproductive allocat ion reduces vegetative allocation and may affect competitive ability and he nce survival, explaining the maintenance of the polymorphism. As mycorrhiza l SPU plants behave more like nonSPU plants, AM colonization itself could p lay a major part in the maintenance of the SPU polymorphism.