Variation in morphological and physiological root traits and organic acid exudation of three sweet potato (Ipomoea batatas) cultivars under seven phosphorus levels

Plants invest their carbon resource in various strategies to enhance phosphorus (P) uptake and use when growing in low P soils. Sweet potato has evolved and persisted in P-impoverished soils, but exhibits highly variable growth and yield. Variation among sweet potato cultivars in ability to access and use P may affect growth and yield: the influence of root system traits in this regard has been little researched. In this study we explored the root traits and rhizosphere chemistry of three sweet potato cultivars under P deficient and sufficient conditions. Three sweet potato cultivars, two native to Papua New Guinea (Tambul Mai and Whagi Besta) and one internationally grown commercial cultivar (Beauregard), were grown with seven levels of P (0-360 mg P kg(-1) soil added as KH2 PO4) for 84 days in a glasshouse in low P soil. Dry mass yield, tissue P concentration, root exudation of organic acids, colonisation by arbuscular mycorrhizal fungi (AMF) and root morphological and physiological traits were measured. All cultivars maintained a constant tissue P concentration below 1.0 mg P g(-1) DM when P supply was low. There were few distinct differences among the cultivars in the response to P addition for growth, total root length, specific root length, root tissue density, average root diameter, root mass fraction and colonisation by AMF. Four organic acids were exuded by roots. Malate and fumarate were abundant only in cultivars from Papua New Guinea, while citrate and oxalate were common across all cultivars. Under P deficiency, all sweet potato cultivars used internal P efficiently, preferentially allocating carbon to shoots and to supporting AMF and organic acid release, rather than to increasing the root mass or forming specialised root structures. Arnong the root factors studied, the type of organic acid exuded was noteworthy, with citrate and malate being reported for the first time.