Collaborative Research: Testing the Munch Hypothesis: Hydraulics of Phloem Transport in Vines and Trees

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Project Title: Collaborative Research: Testing the Munch Hypothesis: Hydraulics of Phloem Transport in Vines and TreesPrincipal Investigators: Holbrook, Noel M., and Michael KnoblauchNSF 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.