Above- and below-ground durum wheat responses to polyphosphate application: Effects on root activities and bacterial phosphate solubilization in the rhizosphere

Polyphosphates (PolyP) are one of the promising mineral phosphorus (P) fertilizer types exhibiting positive effects on crop yields and plant P nutrition owing to their progressive hydrolysis, and low reactivity with soil matrix. Up to now, only a few studies reported that PolyP hydrolysis and use efficiency is conditioned by several soil- and root/microbes-factors. However, compared to conventional P inputs, very little is known on the wheat response to PolyP application, the contribution of P solubilizing bacteria (PSB) to enhance PolyP use efficiency, and the key belowground mechanisms responsible for enhanced P availability and acquisition from PolyP.
Through greenhouse-experiments, agro-physiological responses of durum wheat were assessed under three PolyP fertilizers (PolyA, PolyB, and PolyC) versus one orthophosphate (OrthoP) applications at three doses; 30 (D30), 60 (D60), and 90 (D90) kg P/ha under controlled conditions. In addition, four PSB (Bacillus siamensis, Rahnella aceris, Pantoea hericii, Bacillus paramycoides) and their consortium (PSBCs) were tested for their capacity to solubilize the PolyP (“PolyB” and “PolyC”) along with their growth promoting effects on growth, P use efficiency (PUE), and yield parameters of durum wheat under PolyB application. Through rhizobox- and hydroponic-based experiments, we studied the root growth dynamics and the role of root-induced rhizosphere processes in P availability and acquisition from PolyB and PolyC.
The application of PolyP (especially B and C) at D60, compared to unfertilized plants, significantly increased shoot dry weight, root traits and root P acquisition efficiency (RPAE) as well as aboveground physiological traits (e.g., nutrient (N, P, and K) acquisition and chlorophyll fluorescence parameters). Similarly, PSB inoculation, notably PSBCs significantly increased available P from PolyP (B and C), which is strongly attributed to the production of P-hydrolyzing enzymes (acid phosphatases (APase) pyrophosphatase (PPase)) and organic acids as key mechanisms governing PolyP hydrolysis. At 10 days after sowing (DAS), the co-application of PolyP-PSBCs (PolyB and PolyC) enhanced root hairs length, root length and root inorganic P (Pi) content, compared to uninoculated plants. The co-application of PolyB-PSBCs significantly increased plant growth and wheat P nutrition, nutrient contents, and yield parameters along with contrasting effects on belowground traits depending on the considered growth stage. Hence, PolyB-PSBCs co-application promoted morphological root traits at 30 DAS, while root/rhizosphere APase activities, RPAE, and PUE were significantly enhanced at 75 DAS. Rhizobox- and hydroponic-experiments confirmed that PolyP (especially PolyB) application increased P uptake and plant growth performance through modulating root growth dynamics and inducing significant rhizosphere acidification and APase activities in roots and in the rhizosphere soil.
Under our experimental conditions, PolyP application promoted interactions between above-below-ground traits at 60 DAS. These interactions were reflected by increased wheat growth performance, P acquisition, PUE and other aboveground traits. Therefore, exploring the biological potential of root systems, rhizosphere microbial activities and associated rhizosphere processes can ensure a slow P release from PolyP in line with crop demand, hence making these fertilizers an efficient source of P.