If your goal is to increase the diffusin coefficient (area per unit time that a phosphate molecule can diffuse), increasing the temperature within a tolerable range for the plant is the only way. If your goal is to increase the flux (amount of phospate per unit area per unit time), you can again increase the temperature and/or the concentration gradient of phospate between the phosphate source point and the destimation point if feasible.
If to increase P content in plant by diffusion from soil, then PSB (phosphorus solublizing bacteria) can be used which help to soubilize P in soil and further uptake by plant.