Experiments on osmotically driven flow in idealized elastic membranes
The phloem provides a pathway for products of photosynthesis to be transported to different parts of the plant for consumption or storage. The Munch pressure flow hypothesis (PFH) is considered the leading framework to mathematically represent this transport. It assumes that osmosis provides the necessary pressure differences to drive the fluid flow and sucrose within the phloem. Mathematical models utilizing the PFH approximate the phloem by a relatively rigid semi-permeable tube. However, the phloem consists of living cells that contract and expand in response to pressure fluctuations. The effect of membrane elasticity on osmotically driven sucrose front speed has rarely been considered and frames the scope here. Laboratory experiments were conducted to elucidate the elastic-to-plastic pressure-deformation relation in membranes and their effect on sucrose front speeds. It is demonstrated that membrane elasticity acts to retard the sucrose front speed. The retardation emerges because some of the osmotic pressure performs mechanical work to expand the membrane instead of pressurizing water. These results offer a novel perspective about the much discussed presence of sieve plates through-out the phloem acting as structural dampers.