|M.Sc Student||Rachel Weissman|
|Subject||An Investigation of the Physical and Physiological Effects|
of Xylem Sap Proteins on Plants
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Emeritus Neumann Peter|
|Full Thesis text|
Water transports passively through plant xylem from the root to the leaves. In angiosperm trees the xylem is based on tubular vessels with pits in the walls that allow flow between vessels. Gymnosperm trees have a tracheid based xylem system with anatomically different torus-margo pits. Decreases in the axial hydraulic conductance of the xylem can have harmful and even lethal effects on plants. Proteomic investigations have shown that the xylem sap in angiosperms contains a wide range of proteins. This research aims to find possible physical and physiological effects of xylem sap protein on plant hydraulic function in both angiosperms and gymnosperms and to investigate the nature and location of any inhibition of xylem hydraulic conductivity by protein. Research was focused on angiosperm Olive (Olea europaea L.) trees and gymnosperm stone pine (Pinus pinea L.) trees. The xylem sap of both tree species was analyzed for protein content. A pressure chamber was used to assay the flow rate through olive and pine twig sections and the xylem retention of artificial xylem sap containing ovalbumin as a model protein. A confocal laser scanning microscope was used to find where infused fluorescent ovalbumin accumulated in pine twig xylem. Finally, effects of xylem protein on transpiration were assayed using sunflower (Helianthus annuus L.) seedling explants. Both the protein concentration in olive xylem sap and the xylem retention of ovalbumin in olive twigs were found to be higher than those in pine. In the flow assays, a progressive decrease in flow rate was seen in both olive and pine twigs treated with perfusion solution containing different ovalbumin concentrations. CLSM images showed a build-up of ovalbumin conjugate on pine tracheid walls and on the margo and torus of tracheid pits. Feeding ovalbumin solution decreased transpiration rate in sunflower explants. In olive xylem, a blockage mechanism is suggested involving the accumulation of ovalbumin on the pit membrane, decreasing the hydraulic conductance between vessels. In pine, ovalbumin may accumulate on the tracheid walls, causing a decrease in inner tracheid width, thus causing a decrease in the flow rate through the tracheids. It may also accumulate on the margo membrane or torus, causing a narrowing of the water passages. In addition, protein layers may cause adherence of the torus to the pit aperture. Each of these interactions with protein could cause decreases in the flow rate between tracheids and the observed decreases in xylem hydraulic conductivity.