Maize responds to low shoot P concentration by altering root morphology rather than increasing root exudation.

Background and aims: Alterations in root growth and rhizosphere processes in maize ( Zea mays L.) occur under phosphorus (P) deficiency, but the dynamics of root morphological and physiological modifications with increasing shoot P concentration remain unclear. This study investigated root responses to a wide gradient in shoot P status. Methods: A range of maize shoot P concentrations (1.0-4.0 mg g) was established using controlled pot experiment with eleven rates of P supply from 0 to 1200 mg P kg soil. Root morphology and rhizosphere processes were characterized 28 days after planting. Results: Maize reached maximum biomass at shoot P concentration of 2.7 mg g. Root morphological responses (i.e. total root length, specific root length and proportion of fine roots) showed a strong increasing trend with decreasing shoot P concentration (1.1-1.3 mg g), but they decreased when shoot P concentration was extremely low (below 1.1 mg g). In contrast, with increasing shoot P concentration, root morphological responses decreased, but root physiological responses (rhizosphere acidification, acid phosphatase activity and carboxylate exudation in the rhizosphere) were enhanced, and no decrease was noted even at high shoot P concentration (4.0 mg g) corresponding to excess P supply. Conclusions: Increasing maize shoot P concentration induced a decrease in root morphological responses and an enhancement in root exudation, with maize response to P deficiency being dependent on root morphological rather than physiological traits. [ABSTRACT FROM AUTHOR]