Phosphorus starvation boosts carboxylate secretion in P-deficient genotypes of Lupinus angustifolius with contrasting root structure

Lupinus angustifolius L. (narrow-leafed lupin) is an important grain legume crop for the stockfeed industry in Australia. This species does not form cluster roots regardless of phosphorus (P) nutrition. We hypothesise that this species may have adaptive strategies for achieving critical P uptake in low-P environments by altering shoot growth and root architecture and secreting carboxylates from roots. Three wild genotypes of L. angustifolius with contrasting root architecture were selected to investigate the influence of P starvation on root growth and rhizosphere carboxylate exudation and their relationship with P acquisition. Plants were grown in sterilised loamy soil supplied with zero, low (50 µm) or optimal (400 µm) P for 6 weeks. All genotypes showed a significant response in shoot and root development to varying P supply. At P deficit (zero and low P), root systems were smaller and had fewer branches than did roots at optimal P. The amount of total carboxylates in the rhizosphere extracts ranged from 3.4 to 17.3 µmol g–1 dry root. The total carboxylates comprised primarily citrate (61–78% in various P treatments), followed by malate and acetate. Genotype #085 (large root system with deep lateral roots) exuded the greatest amount of total carboxylates to the rhizosphere for each P treatment, followed by #016 (medium root system with good branched lateral roots) and #044 (small root system with short and sparse lateral roots). All genotypes in the low-P treatment significantly enhanced exudation of carboxylates, whereas no significant increase in carboxylate exudation was observed in the zero-P treatment. Small-rooted genotypes had higher P concentration than the medium- and large-rooted genotypes, although larger plants accumulated higher total P content. Large-rooted genotypes increased shoot P utilisation efficiency in response to P starvation. This study showed that narrow-leafed lupin genotypes differing in root architecture differed in carboxylate exudation and P uptake. Our finding suggested that for L. angustifolius there is a minimum plant P concentration below which carboxylate exudation is not enhanced despite severe P deficiency. The outcomes of this study enhance our understanding of P acquisition strategies in L. angustifolius genotypes, which can be used for the selection of P-efficient genotypes for cropping systems.