Toward a Realistic Representation of Sucrose Transport in the Phloem of Plants

Abstract

A variety of fluid transport phenomena in plants do not require active pumping or externally imposed pressure difference to move solutes and fluids. Instead, they rely on osmotic pressure, a passive transport process, to move loaded solutes and water simultaneously. A prototypical example is sucrose transport in the phloem of plants formulated in the 1930s by Ernst M 𝐴𝐴 𝐴 u nch and is now labeled as the pressure-flow hypothesis (Münch, 1930) or pressure flow hypothesis (PFH) (van Bel, 2003). The PFH is routinely used to describe the transport of photosynthates from production sites (leaves) to areas of consumption or storage (growing tissues and roots). This mechanism has been used to explain aspects of plant mortality under extreme weather conditions (Sevanto et al., 2014) and ecosystem-scale impacts on carbon and water cycling (Nikinmaa et al., 2013), which is why it continues to be a subject of active research in eco-hydrology (

The Phloem: A Case of Theory Against Experiment

“…These insights were obtained by Nakad et al. (2023) on the basis of a lab model and theoretical considerations. The experimental design of their approach was quite simple: A simple elastic, porous tube (to simulate the plasma membrane of the sieve tube elements) is embedded within a pure water solution and connected to a pressure transducer on one end and a stopcock on the other end.…”

“…As was found by Nakad et al (2023), the absence of sieve plates might be worse for flow rate than their presence: The loading of assimilates into the phloem builds up an osmotic pressure gradient that is directed away from the loading site and causes a corresponding redistribution of the phloem sap (including its sucrose content). If the sieve tubes were rigid this sap flow would be directed toward the unloading site.…”

“…elements is important (McCubbin & Braun, 2021). As was demonstrated, however, in a recent study of Nakad et al (2023), elastic response of the flexible cell walls reduces flow velocity. Also, the sieve plates appear to be "in the way," as these represent obstacles leading to a somewhat higher flow resistance (Jensen et al, 2012).…”

“…Clearly, the unwanted consequence of the sieve tubes' elasticity can be mitigated if fairly rigid sieve plates-as transverse stiffeners-restrict the radial expansion of the sieve tubes. These insights were obtained by Nakad et al (2023) on the basis of a lab model and theoretical considerations. The experimental design of their approach was quite simple:…”

“…This is the key part—the experimental setup allows for a nondestructive means to measure turgor pressure. Then dextrose (simulating sucrose) solutions with a marker dye are injected at the stopcock end and allowed to propagate through the tube. The evolving dynamics of the system is recorded by measuring the pressure (using the transducer), the tube volume and the propagation of the dye‐labeled dextrose front by taking repeated images of the tube. A simple mathematical description of this system enables Nakad et al. (2023) to estimate from the experimental results the front propagation in a tube consisting of an inelastic but otherwise equivalent membrane. And by comparing this with the results obtained for the elastic tube, they come up with a 40% loss in front propagation speed due to the elasticity of the tube.…”