Münch, morphology, microfluidics - our structural problem with the phloem

Abstract

The sieve tubes of the phloem are enigmatic structures.Their role as channels for the distribution of assimilates was established in the 19th century, but their sensitivity to disturbations has hampered the elucidation of their transport mechanisms and its regulation ever since. Ernst Münch's classical monograph of 1930 is generally regarded as the first coherent theory of phloem transport, but the 'Münchian' pressure flow mechanism had been discussed already before the turn of the century. Münch's impact rather rested on his simple physical models of the phloem that visualized pressure flow in an intuitive way, and we argue that the downscaling of such models to realistic, low-Reynolds-number sizes will boost our understanding of phloem transport in this century just as Münch's models did in the previous one. However, biologically meaningful physical models that could be used to test predictions of the many existing mathematical models would have to be designed in analogy with natural phloem structures. Unfortunately, the study of phloem anatomy seems in decline, and we still lack basic quantitative data required for evaluating the plausibility of our theoretical deductions. In this review, we provide a subjective overview of unresolved problems in angiosperm phloem structure research within a functional context.

Phloem Transport Velocity Varies over Time and among Vascular Bundles during Early Cucumber Seedling Development

“…Small disturbance can cause phloem transport to stop or slow down temporarily, and larger disturbances can cause more severe wounding responses, including the occlusion of sieve tubes (Knoblauch and Peters, 2010). To examine whether disrupting transport in one vascular bundle effects transport in nonconnected bundles, three disturbance tests were conducted: (1) removal of the cotyledon attached to the bundle being measured, (2) removal of the cotyledon opposite to the bundle being measured, and (3) agitation of the seedling by gently shaking it.…”

“…Carbohydrate transport occurs in the phloem and can be regulated by sink and source activity depending on carbon availability and environmental conditions (Marcelis, 1996;Minchin and Thorpe, 1996;Paul and Foyer, 2001). Although Münch (1930) first proposed a mechanism for phloem transport over 80 years ago, methodological limitations associated with studying phloem transport in vivo (Knoblauch and Peters, 2010) have prevented many facets of phloem physiology and anatomy from being understood. Recent work has clarified aspects of phloem loading and examined the implications of sap sugar concentration and phloem ultrastructure on phloem transport (Rennie and Turgeon, 2009;Froelich et al, 2011;Jensen et al, 2011), but what remains unknown is whether phloem transport is stable over space and time and how the structure of the phloem network impacts carbon movement in plants.…”

Phloem Loading through Plasmodesmata: A Biophysical Analysis

“…This transport process drives photosynthetic products to remote tissues (sinks) for growth and storage, coupling synthesis, and intercellular transport processes in the leaves and sink tissues to global, hydraulic transport through the phloem sieve tubes and xylem vessels. Significant uncertainties remain regarding the structure, chemistry, and transport phenomena governing these processes (Knoblauch and Peters, 2010;Turgeon, 2010aTurgeon, , 2010b. Improved models of export will inform our understanding of whole-plant physiology and open opportunities to engineer sugar concentrations and transport processes to improve growth and yield (Schroeder et al, 2013;Giraldo et al, 2014).…”

Physical Limits to Leaf Size in Tall Trees

“…The total solute concentration is ∼ 25 % wt/wt, and sugars, of which sucrose is the most abundant type, constitute 80% -90% of this [14]. Although sugar transport is the primary role of the phloem it also plays a major role in the response to wounding and environmental stimuli [15]. The flow in the phloem is driven by differences in hydrostatic pressure between source (leaves) and sink (e.g., roots, fruits or other place of growth and storage) tissues, believed to be generated by gradients in osmotic potential between distal parts of the plant according to the Münch pressure flow hypothesis [16].…”