Relationships between root morphology, root exudate compounds and rhizosphere microbial community in durum wheat

A multidisciplinary approach was adopted to investigate multiple relationships between root morphology, rhizosphere soil exudates and soil microbial community in eight durum wheat [Triticum turgidum ssp. durum (Desf.)] cultivars previously characterized in terms of their ability to interact with mycorrhizal fungi. Six plants from each cultivar were grown to the five-leaf stage in a growth chamber. Root system architecture was characterized by root scans, root exudates were identified and quantified by gas chromatography–mass spectrometry (GC–MS), and the rhizosphere bacterial, archaeal and fungal community structure was assessed using terminal-restriction fragment length polymorphism (T-RFLP). Wheat cultivars were very different in terms of the morphological root traits. Cappelli, Grecale, Ofanto and PR22D89 cultivars showed the most developed root system. Root exudates included sugars, amino acids, polyalcohols and organic acids, with carbohydrates accounting for 87.3% of the whole data set. All the compounds showed significant differences among cultivars, with Grecale and Claudio showing the highest exudation ability of metabolites. T-RFLP profiles showed a greater relative abundance of bacterial taxa (55.8% on average) than those of archaea and fungi (35.6% and 8.6%, on average, respectively) in all cultivars, and responded positively to root exudation. Principal component analysis identified root dry weight, surface area and volume, carbohydrate exudation, bacterial and archaeal Shannon–Weaver diversity indices and the fungal evenness index as the traits that mainly contributed to the cultivar variability. A larger root system did not always promote a higher diversity of the rhizosphere microbial biomass. This then suggests complex interactions between traits, probably due to the differences in the quality and/or composition of the root exudates. Based on our results, we propose that durum wheat cultivars could be classified according to the different plant resource-use strategies employed, such as conservative (i.e. Cappelli, with a large root system but low exudation ability) and explorative/competitive (i.e. Grecale and Claudio, with a high exudation ability) cultivars. The results are discussed with reference to the complex relationships between the plant and the rhizospheric microorganisms in order to determine the best combinations that would enhance root efficiency.