Contrasting belowground strategies of maize varieties under combined water and phosphorus deficiency: the role of root exudation.

Although many temperate agricultural soils contain substantial P, most of this P is unavailable to plants, and its mobility is further restricted by drought. Root exudation contributes to drought tolerance and P mobilization and is considered to play a major role in soil resource acquisition strategies in crops. Its position, however, within the root economics space (RES) remains contradictory. We assessed differences between maize landraces and modern cultivars in root economic strategies, plasticity under combined water and phosphorus deficiency, and the consequences for plant performance. We investigated root and rhizosphere responses of six maize varieties (three landraces, three modern hybrids) grown in a controlled pot experiment under combined water and P limitation. Pots were assigned four treatments: well-watered (20% Water content (WC)) and water-stressed (8% WC) condition, combined with high (47.97 mg P kg^-1) or low (23.3 mg P kg^-1) P. After four weeks of growth, water-stressed plants underwent a two-week drought period, adjusting to 8% WC after one week, while well-watered plants continued to grow under optimally watered conditions. Root traits were assessed through root scanning and dry biomass measurements. Root exudates were collected using a soil-hydroponic hybrid method and analysed for dissolved organic carbon, sugars, organic acids, carboxylates and phenolics. Soil DNA was analysed for its bacterial and fungal composition. We found that landraces followed a “do-it-yourself” RES strategy, whereas modern varieties adopted an “outsourcing” strategy that was associated with increased root exudation. Water availability drove rapid plastic responses in root exudation, with the strongest response under combined deficiency. In contrast, morphological root traits were driven by P, rather than water. Under combined deficiency, landraces maintained higher P use efficiency while moderns exhibited greater P acquisition efficiency. These findings demonstrate that contrasting root economic strategies of different maize varieties shaped the performance under combined P and water stress.