Assimilate transport in phloem sets conditions for leaf gas exchange

ABSTRACT Carbon uptake and transpiration in plant leaves occurs through stomata that open and close. Stomatal action is usually considered a response to environmental driving factors. Here we show that leaf gas exchange is more strongly related to whole tree level transport of assimilates than previously thought, and that transport of assimilates is a restriction of stomatal opening comparable with hydraulic limitation. Assimilate transport in the phloem requires that osmotic pressure at phloem loading sites in leaves exceeds the drop in hydrostatic pressure that is due to transpiration. Assimilate transport thus competes with transpiration for water. Excess sugar loading, however, may block the assimilate transport because of viscosity build‐up in phloem sap. Therefore, for given conditions, there is a stomatal opening that maximizes phloem transport if we assume that sugar loading is proportional to photosynthetic rate. Here we show that such opening produces the observed behaviour of leaf gas exchange. Our approach connects stomatal regulation directly with sink activity, plant structure and soil water availability as they all influence assimilate transport. It produces similar behaviour as the optimal stomatal control approach, but does not require determination of marginal cost of water parameter. Requiring stomata to open such that assimilate transport from leaves is maximized reproduces known plant responses of stomatal conductance to various environmental conditions, above and below ground, in a single theoretical framework. The model agrees with field observations and produces similar behavior than the optimal stomatal control approach. However, here no marginal cost of water is needed but the stomatal behavior results from axial and radial water flows and the osmotic competition for water between phloem and the transpiration stream.