Nodulation promotes cluster-root formation in Lupinus albus under low phosphorus conditions

Abstract

Phosphorus deficiency often limits crop productivity, while phosphate rock, which is used to produce phosphorus fertilisers, is a non-renewable resource. Formation of cluster roots is an adaptation to nutrient-poor soils in Lupinus species, including L. albus. Lupinus species also produce nodules, which require a large investment of phosphorus. Our aim is to test whether nodulation promotes cluster-root formation in L. albus. Seedlings of L. albus, either inoculated with rhizobia or non-inoculated, were grown in nutrient solution with a low phosphorus supply. Non-inoculated plants were provided with the same amount of nitrogen in the form of nitrate as the nodulated ones acquired, from both air and nutrient solution, based on preliminary experiments. We measured biomass, phosphorus and nitrogen concentrations as well as photosynthesis just prior to each harvest. Nodulated plants and non-nodulated control plants produced the same amount of biomass. Nodulated plants had, on average, 86% more cluster roots than non-nodulated ones at the four harvests. As hypothesised, nodulation significantly promoted cluster-root formation; it also enhanced rates of photosynthesis. Nodulation promoted cluster-root formation and photosynthesis, presumably because nodules are significant sinks for phosphorus and photosynthates. Our results do not provide evidence for a trade-off between investment of resources in nodules and cluster roots.