The proposed work seeks to improve the currently incomplete understanding of factors that determine, and may limit, plants' maximal rates of photosynthesis and productivity. By combining physiological and anatomical assessments, it will be tested whether there is a fundamental relationship between the maximum rate of plant photosynthesis and specific anatomical features that may be determinants of the capacity of leaves' sugar-exporting (phloem) vessels. Sugar export may sometimes present a bottleneck to carbon distribution throughout the plant and thus to plant productivity. In particular, leaf loading vein density and the ratio of loading cells per exporting phloem vessel will be assessed in species with different mechanisms of phloem loading. If a relationship between photosynthetic capacity and phloem features can be established, these features will be available to: (i) identify plant species and varieties with superior photosynthetic performance and productivity; (ii) inform future engineering efforts directed at manipulating these anatomical features to enhance plant productivity. It will furthermore be established whether features such as high loading vein density are associated with enhanced productivity under dry conditions and/or cool temperatures. If such an advantage can be established, this will aid in the selection of plants with superior productivity in warm/arid regions that are expanding under global climate change and at higher latitudes and altitudes into which crop cultivation is expanding under the present global warming trend and where plants will experience seasonal exposure to colder than usual temperatures. The knowledge gained from this work will aid in the effort to maintain, and improve, crop productivity as well as carbon sequestration, which are urgent goals given the burgeoning human population as well as the current and projected impact of climate change. Furthermore, the PIs will utilize their existing connections to recruit students from diverse backgrounds into the proposed research and resulting publication/dissemination efforts as well as for outreach efforts. Lastly, the PIs are planning to continue their publication activity in venues reaching a broad audience including non-specialists.
Plant leaves posses a network of veins as conduits for distributing water throughout the leaf as well as for collecting and transporting the sugars produced in the leaf’s photosynthesis to the rest of the plant. This project has identified specific augmentations in features of the plant’s sugar-transporting conduits (the phloem) that are associated with, and may be prerequisite for, increases in the rate of photosynthesis. Increased rates of photosynthesis may, in turn, contribute to an increased productivity of plants (and, thereby, the production of food, fiber, building material, biofuels). For some plant species, this augmentation involved an increase in the total number of phloem cells per vein in response to growth under different environmental conditions. In others, a greater number of phloem cells as well as greater vein density was associated with higher photosynthesis rates among different species. Lastly, yet other species did not vary the number of phloem cells per vein, but did feature larger-sized phloem cells as well as, again, a greater vein density, associated with higher rates of photosynthesis, either in response to different environmental conditions during growth or for the comparison among different species. In all cases, these differences in phloem features are related to the capacity for 1) loading the sugars produced in photosynthesis from the leaf into the veins and 2) transporting those sugars from the leaves to the rest of the plant. Greater numbers of phloem cells per vein are associated with 1) increased cell membrane area for actively loading the sugars produced in the photosynthetic cells of the leaf into the phloem cells of the veins and 2) a greater cross-sectional area ("diameter") of the phloem conduits to facilitate a greater flux of sugar-laden phloem sap from leaves with higher rates of photosynthesis. On the other hand, larger phloem cells (found only in species employing a mechanism for moving the sugars into the phloem that does not involve transport across the cell membrane) are likely associated with greater numbers of the proteins within those cells that provide the active, energy-driven step for concentrating sugars in the phloem. Lastly, for all cases where leaf vein density was found to vary, this greater vein density presumably permits a greater overall collection and movement of the sugars from the leaf to the rest of the plant. These findings suggest that active loading of sugars produced in the leaf into the leaf veins, and the physical route for sugar export from those leaves, constitutes a potential limitation to leaf photosynthesis – and that a greater capacity for sugar export and transport allows for a greater leaf photosynthetic capacity. In the economic analog presented in Figure 1, increased investment in industrial production would only be sustainable (profitable) if sufficient infrastructure were available for distribution of more products to the consumer. Likewise, increased investment by the plant in expensive-to-maintain photosynthetic machinery should only be expected if sufficient sugar export infrastructure is available to distribute the resulting increased sugar production to the plants’ sugar-consuming parts (stems, roots, flowers, fruits, seeds, and developing leaves). Recognition of the key role that these leaf vein features play in setting the ceiling for photosynthesis will be important in identification, breeding, or engineering of plants for increased productivity under various climatic conditions, since productivity is dependent on the maximal photosynthesis rates achieved by the leaves of a plant. Given the continued burgeoning of the human population, this may be critical in the efforts to increase the production of food, fuel, fiber, and materials that are necessary to support our needs.
