The water status of the xylem and phloem are closely coupled, yet the driving force, tissue structure, and generally the direction of transport are diametrically opposed. In contrast to the numerous studies on how xylem structure influences patterns of water transport, the water relations of sieve tubes and their effect on assimilate transport have received little attention. Lack of a technique to monitor sieve-tube turgor pressures means that some of the most basic questions regarding phloem transport remain unanswered. In particular, estimates of the magnitude of turgor gradients and how sieve-tube turgor responds to changes in xylem water potential are largely unknown. This project lays the groundwork for a long-term effort in phloem water relations and phloem:xylem interactions. Dr. Holbrook will develop techniques for in-situ monitoring of sieve tube-turgor pressure through combining aphid stylets and cell-pressure probe, and will explore spatial and temporal variation in sieve-tube turgor pressure in response to changes in plant water balance. With this Research Planning Grant, she will visit other labs to learn component skills and will perform preliminary experiments in her own lab. Development of an in-situ method for monitoring sieve-tube turgor pressure in intact plants will greatly enhance understanding of phloem water relations. In particular, it would enable researchers to examine interactions of xylem tension and phloem physiology and to explore experimentally how the capacity for water transport through the xylem is balanced with the requirement of generating sufficient turgor pressures for long-distance movement through the phloem.
