Principal Investigators: Holbrook, Noel M., and Michael Knoblauch
NSF Project Numbers: 1021779 and 1022106
The phloem, a network of cellular conduits that allows plants to transport the products of photosynthesis from sites of synthesis or storage to sites of utilization in respiration and/or growth, is the major pathway through which plants integrate photosynthesis and growth. Phloem transport is hypothesized to be driven osmotically, but whether this occurs through a continuous cytoplasmic pathway is unknown. In this project, measurements of phloem transport rates, turgor pressure and hydraulic conductivity based on anatomical data will be used to investigate the hydraulic functioning of phloem in vines and trees. A major goal is to determine if the gradients in turgor pressure sufficient to drive transport at observed rates actually exist, thus supporting the hypothesis of a continuous intracellular pathway; alternatively the phloem could be segmented into discrete units into which solutes are actively re-loaded. Resolution of this issue is critical for understanding the hydraulic constraints underlying the movement of carbohydrates within plants. Given the central role of the phloem in linking photosynthesis and growth, a comprehensive understanding of phloem transport is essential for predicting and potentially modifying how plants respond to altered resource availability and/or stressful climatic conditions.
This collaborative project will support both a graduate student and a postdoctoral fellow, and will provide research opportunities for undergraduates. Women and underrepresented minorities will be actively recruited for these positions. Contributions to K-12 and public education include an interactive exhibit for the Arnold Arboretum's visitor center, which receives over 250,000 visitors per year, including many school groups. In addition, an on-line version will be posted on the Arboretum's web site as part of the "Tree Basics" series. A second exhibit of large-format images, accompanied by educational materials, will be displayed in a variety of public spaces, beginning in Pullman, WA and subsequently throughout the state.
Phloem is the name for the vascular tissue that transports carbohydrates from sites of synthesis or storage to wherever they are needed by a plant. A major component of phloem transport is from the leaves, where carbohydrates are produced by photosynthesis, to the roots, where they provide substrates for respiration and growth. The phloem forms part of a plant's circulatory system and the impact of the phloem on plant growth and function is similar to that of our own circulatory system on our health and physiology. Yet surprisingly our understanding of what drives movement through the phloem is limited, especially in large plants. The goal of this study was to examine the mechanism of phloem transport in tall trees and long vines. In the 1930's, Munch proposed what is now known as the pressure flow hypothesis to explain fluid transport through the phloem. According to this hypothesis, a sugar-rich sap is pushed through the phloem, similar to how blood flows through our arteries although with the positive pressures being generated by an osmotic, rather than a mechanical, pump. What remains uncertain, however, is whether the osmotically generated pressures are sufficient to account for phloem transport across the entire length of the plant. The reason for this gap in knowledge is that the phloem is difficult to study, as the tubes are themselves living cells that cease transport if they are disturbed. While this has utility for the plant, in the same way that blood clotting plays an important role in our own bodies, it has made it impossible to obtain accurate measurements of the basic parameters that determine phloem transport. Information needed to assess the sufficiency of the Munch pressure flow hypothesis include measurements of the hydrostatic pressure, flow rate, and viscosity of the transported sap, as well information on the structure of the phloem conduits so that their hydraulic resistance could be estimated. A major component of this project was to develop new measurement tools and protocols for phloem cells. We have been successful in developing methods for measuring all needed parameters and became, in the final year of the project, able to test in large plants whether the pressures generated osmotically were sufficient to account for the movement of carbohydrates from leaves to roots. However, our methods require that we work on intact plants and thus to accomplish our goals required that we take a fluorescent microscope up into the canopy of a large tree (using an mechanical lift). Our findings provide strong evidence for the pressure flow hypothesis. Specifically, as plants grow in size and length they both increase the hydrostatic pressures that drive the sap through the phloem and they alter the structure of their phloem conduits so that the hydraulic resistance to flow is decreased. The importance of our work relates to the central role played by the phloem in the plants that make up our food supply. Over 90% of the calories that we eat has, at some time or other, been transported through the phloem. Thus, understanding the basic transport mechanism of phloem may help in breeding more crops with higher productivity. The phloem is also a major way in which viruses move through plants. Better understanding of the phloem may help in making crop plants more resistant to disease.
