The responses of root morphology and phosphorus-mobilizing exudations in wheat to increasing shoot phosphorus concentration

The adaptations of root growth and rhizosphere processes for soil phosphorus (P) acquisition have been investigated intensively in wheat (Triticum aestivum). However, only a few studies paid attention to these responses to shoot P status. This study aimed at investigating the responses of root morphology and P-mobilizing exudation to increasing shoot P concentration. A broad range of wheat shoot P concentrations (1.0-7.1 mg per g dry weight) was set up with 11 rates of P supply: 0-1200 mg P per kg soil. Root morphology and exudation parameters were measured after 37 days of plant growth. Shoot dry biomass reached a maximum when shoot P concentration was 4.63 mg per g dry weight. The maximum shoot P concentration for total root length, specific root length and the proportion of fine root (diameter ≤ 0.2 mm) length to total root length was 3 mg per g dry weight. Rhizosphere acidification was positively correlated with shoot P concentration when this was < 5 mg per g dry weight. Shoot P concentration did not change acid phosphatase activity in the rhizosphere. Citrate concentration in the rhizosphere was suppressed by increasing shoot P concentration. In contrast, malate concentration in the rhizosphere showed a positive correlation with shoot P concentration. In conclusion, wheat root morphological and P-mobilizing exudation traits showed different behaviours with increasing P deficiency stress. Maintaining root biomass and length is the major strategy rather than root exudation for wheat to cope with extreme P deficiency.