A mechanism of translocation based on high proton mobility and K+ counterflux at negatively charged surfaces in sieve elements

Large pH gradients between the sources and the sinks involved in translocation of metabolites arise owing to photosynthesis and nitrate reduction in the leaves and respiration in the sinks. pH equalization between the sinks and the sources is proposed to be brought about by a rapid movement of H⁺ from sinks to sources along the negatively charged surfaces lining the translocation pathways and the ray symplast. This movement is made possible by a charge-compensating movement of K⁺ in the electrical double layer. In the sieve tubes and specially at the sieve plate pores the movement of K⁺ in the diffused layer is suggested as the driving force for translocation of metabolites. In the transport network of plants K⁺ moves in loops, acting like a conveyor belt in the phloem. The proposed mechanism explains all experimental observations related to translocation and also solves the problem of pH-stating in the source and sink cells. Its implications for shoot-root cooperation have been also indicated.