Phloem is the conducting system for distribution of photosynthetic assimilates in vascular plants and also a pathway by which systemic viral infection can be spread. The movement of solutes out of phloem conducting cells is a crucial regulation point for the distribution of phloem-mobile substances in the plant. Little is known, however, about the mechanisms or controls involved. An experimental approach to understanding these mechanisms would be to introduce labelled compounds that have a range of solute sizes and chemical structures into the phloem and follow the movement of these compounds. However, this has been precluded because the phloem cells are pressurized and present means of introducing material into the phloem also produces significant artifacts. An aphid naturally removes sugars from phloem by selectively inserting a stylet into the conducting cell. The exploratory work proposed here would attempt to overcome the problem of introduced artifacts by developing a method for solute injection directly into the phloem conducting cells via severed aphid stylets, an approach suggested by earlier measurements of turgor pressure in phloem conducting cells. An injection system will be constructed that will utilize thermal expansion to generate pressure. Precise pressure control in this injection system should be possible with a precisely-regulated thermoelectric device that has a pressure transducer incorporated into the injector. Development of this system should allow injection of tracer material into phloem cells by a slight and precisely controlled over pressurization.
