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The Mechanism of Sugar Export from Long Conifer Needles.

The green leaves of plants are optimized for carbon fixation and the production of sugars, which are used as central units of carbon and energy throughout the plant. However, there are physical limits to this optimization that remain insufficiently understood. Here, quantitative anatomical analysis combined with mathematical modeling and sugar transport rate measurements were employed to determine how effectively sugars are exported from the needle-shaped leaves of conifers in relation to leaf length. Mathematical modeling indicated that phloem anatomy constrains sugar export in long needles. However, we identified two mechanisms by which this constraint is overcome, even in needles longer than 20 cm: i) the grouping of transport conduits, and ii) a shift in the diurnal rhythm of sugar metabolism and export in needle tips. The efficiency of sugar transport in the phloem can have a significant influence on leaf function. The constraints on sugar export described here for conifer needles are likely to also be relevant in other groups of plants, such as grasses and angiosperm trees.