Impact of sucrose sinks on phloem transport

The movement of photosynthates within plants is a focus in plant physiology, ecohydrology, and earth systems modeling. The phloem, one of the plant’s hydraulic systems, facilitates this transport. It is believed to be optimized for efficient photosynthates transport, notably sucrose. This has implications ranging from local impacts on plant survival during drought to ecosystem-scale effects on carbon and water cycling. Most models for phloem transport rely on the pressure-flow hypothesis, where sucrose is loaded in leaves, drawing water from the xylem through osmosis, generating pressure gradients for transport. Experimental challenges in measuring sugar fluxes have led to reliance on theoretical models, though discrepancies exist, especially for long-distance transport. Criticism of the pressure-flow hypothesis notes low hydraulic conductance in sieve tubes, possibly hindering sucrose transport in taller plants. This research explores osmotically driven flows through the development of a new one-dimensional numerical model that includes sources from photosynthesis and sinks towards the stem and roots. The model also incorporates a concentration-dependent viscosity and the xylem water potential. It shows that different allocation schemes of sucrose sinks towards the stem of the plant influence the speed at which sucrose is transported. These findings provide insight into how carbon allocation along the phloem may have evolved to enhance the efficiency of transporting soluble compounds in the phloem.