Research Plant Physiologist USDA ARS Raleigh, North Carolina
Plants export much of the sugar produced by photosynthesis from leaves to carbon sink organs, where they fuel growth and development. The sugar export rate is influenced by various factors, including intracellular sugar metabolism and transport, intercellular water flow, inter-organ sugar gradients and cell anatomies. Understanding how these factors combine to adjust sugar export according to growth strategy and environmental conditions requires quantitative data.
The relatively simple structure of conifer needles enabled us to generate or model such data. Theoretical modeling of sugar and water flows in conifer needles based on anatomical data from a range of species indicated a strongly reduced sugar export efficiency in tips of needles longer than 5-10 cm. A surprising result, considering that many productive conifers have longer needles. However, the result was corroborated by the discovery of mechanisms that could overcome the physical limitation in these species.
One mechanism is the channeling of sugar and water, achieved by a layered arrangement of transport conduits in the phloem vascular system. Another is the decoupling of the diurnal rhythms of carbon storage and remobilization at the tip and base of long needles. Moreover, we found evidence for sucrose transporters facilitating sugar import into phloem cells that could spatially and temporally control sugar export rates.
Besides the single-veined needles, the parallel-veined leaves of grasses might face similar structural limitations to phloem transport. Results will be discussed with respect to factors that shape phloem anatomy and ecophysiological consequences of utilizing specialized mechanisms to maintain efficient sugar export from leaves.