Source–sink transport as a constraint on photosynthesis and a driver of ecophysiological patterns

The movement of sugars from autotrophic to heterotrophic organs is a major potential constraint on photosynthesis in vascular plants. There is now evidence suggesting that photosynthetic activity in some species often exceeds transport capacity, which we have termed the transport-fixation capacity difference (Δ transport-fixation). Δ Transport-fixation presents plants with multiple challenges related to the regulation of photosynthesis, the allocation of reserve nonstructural carbohydrates, and responses to abiotic and biotic stressors. Conifer needles and Citrus shoots photosynthesize more than they can export from leaves (long leaf conifers) or shoots (Citrus) at a time. In both cases, Δ transport-fixation leads to competition for loading based on source position relative to sinks, resulting in greater storage of nonstructural carbohydrates in some leaves or portions of leaves until conditions reduce this intrasource competition. Additionally, while photosynthesis is upregulated in response to source:sink manipulation, stem transport does not appear to vary, suggesting that transport operates near its maximum or is restrictively buffered. In evergreen fruit trees, the result is a high degree of storage near the site of future sinks: shoot apices, where new shoots grow rapidly for short periods. It is not clear how common Δ transport-fixation is in woody species, an important carbon sink on Earth, which could present major challenges to their response to environmental conditions. We discuss the trade-offs imposed by Δ transport-fixation, its ecophysiological implications, and important targets for future research, including understanding the connections between Δ transport-fixation and the need for stress tolerance mechanisms.