Role of pit membranes in macromolecule-induced wilt of plants.

Macromolecules present in low concentrations in xylem fluid of Medicago sativa L. var DuPuits will increase the resistance to xylem liquid flow. This increase in resistance was found to be reversible by backflushing the xylem. Autoradiography showed that very large molecules do not pass through pit membrane pores. A comparison of pit membrane pore sizes to molecule sizes suggests that increased resistance to xylem flow is a result of plugging pit membrane pores. It was also found that pit membranes located in two parts of the plant differ in the apparent diameter of their pores and, thus, in their susceptibility to plugging by macromolecules. Macromolecules in xylem fluid may result from hostparasite interactions and may play a significant role in the outcome of the interaction.