Testing the Münch hypothesis of long distance phloem transport in plants

Knoblauch, Michael; Knoblauch, Jan; Mullendore, Daniel L.; Savage, Jessica; Babst, Benjamin A.; Beecher, Sierra; Dodgen, Adam C.; Jensen, Kaare H.; Holbrook, N. Michele

doi:10.7554/elife.15341

Cited by 150 publications

(141 citation statements)

References 23 publications

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“…According to Münch's pressure flow hypothesis, the osmotic potential difference between source and sink tissues generates a pressure driven flow of photosynthates from sources to sinks and thus out of the leaf (Münch, ). Although not tested empirically for leaves (however, for long‐distance transport in stems, see Knoblauch et al ., ), three of our observations support Münch's hypothesis in the context of leaves. First, the total conductive area of 7 th vein orders is c .…”

Section: Discussionsupporting

confidence: 89%

The scaling of the hydraulic architecture in poplar leaves

et al. 2017

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Although much is known about the hydraulics of xylem, the hydraulic interconnectivity and dimensional scaling of phloem with respect to xylem in leaves has not been adequately studied to test alternative hydraulic architectural rules such as da Vinci's rule or Murray's rule, or physiological models such as Münch's Pressure Flow hypothesis. Using confocal and electron microscopy as well as mathematical analyses, we examined the hydraulic architecture of the mature leaves of the model species Populus tremula × alba across all seven hierarchical orders of the vascular branching. We show that: phloem and xylem conductive areas increase from minor to major veins; the sum of the conductive areas for each vein order increases exponentially from major to minor veins; the volume of individual sieve tube and vessel members increases from minor to major veins; and phloem conductive area scales isometrically with respect to xylem area across all vein orders. The application of first principles to our data shows that conductive areas scale according to da Vinci's rule and not according to Murray's rule, and that the phloem network in poplar leaves can generate the pressure gradient envisioned in Münch's hypothesis.

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Section: Discussionsupporting

confidence: 89%

The scaling of the hydraulic architecture in poplar leaves

et al. 2017

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“…Recently, direct sieve tube pressure, viscosity, flow velocity, and tube geometry measurements in morning glory showed, however, that the majority of energy provided by the pressure differential between source and sink is consumed to drive flow, and that a high pressure differential in the root system is unlikely to exist, calling the current model into question (Knoblauch et al, 2016). In principle, there are two possibilities of how the phloem sap might escape the PSEs.…”

Section: Resultsmentioning

confidence: 99%

“…In this scenario, allocation of carbon is controlled by the lateral hydraulic conductance in the unloading zone. Recent phloem turgor measurements in morning glory, however, did not support this model, as the bulk of the pressure is consumed by friction within the SEs, and only small pressure gradients are available for unloading (Knoblauch et al, 2016). The Fisher (2000) model also does not explain how both small solutes and macromolecules can leave the phloem simultaneously.…”

Section: Introductionmentioning

confidence: 99%

Phloem unloading in Arabidopsis roots is convective and regulated by the phloem-pole pericycle

Ross‐Elliott

Jensen

Haaning

et al. 2017

eLife

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214

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In plants, a complex mixture of solutes and macromolecules is transported by the phloem. Here, we examined how solutes and macromolecules are separated when they exit the phloem during the unloading process. We used a combination of approaches (non-invasive imaging, 3D-electron microscopy, and mathematical modelling) to show that phloem unloading of solutes in Arabidopsis roots occurs through plasmodesmata by a combination of mass flow and diffusion (convective phloem unloading). During unloading, solutes and proteins are diverted into the phloem-pole pericycle, a tissue connected to the protophloem by a unique class of ‘funnel plasmodesmata’. While solutes are unloaded without restriction, large proteins are released through funnel plasmodesmata in discrete pulses, a phenomenon we refer to as ‘batch unloading’. Unlike solutes, these proteins remain restricted to the phloem-pole pericycle. Our data demonstrate a major role for the phloem-pole pericycle in regulating phloem unloading in roots.DOI: http://dx.doi.org/10.7554/eLife.24125.001

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“…Data about gymnosperms or data collected in the field conditions are especially scarce (Knoblauch et al . ; Turgeon ).…”

Section: Introductionmentioning

confidence: 99%

Gradients and dynamics of inner bark and needle osmotic potentials in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst)

Paljakka

Jyske

Lintunen

et al. 2017

Plant Cell & Environment

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Preconditions of phloem transport in conifers are relatively unknown. We studied the variation of needle and inner bark axial osmotic gradients and xylem water potential in Scots pine and Norway spruce by measuring needle and inner bark osmolality in saplings and mature trees over several periods within a growing season. The needle and inner bark osmolality was strongly related to xylem water potential in all studied trees. Sugar concentrations were measured in Scots pine, and they had similar dynamics to inner bark osmolality. The sucrose quantity remained fairly constant over time and position, whereas the other sugars exhibited a larger change with time and position. A small osmotic gradient existed from branch to stem base under pre-dawn conditions, and the osmotic gradient between upper stem and stem base was close to zero. The turgor in branches was significantly driven by xylem water potential, and the turgor loss point in branches was relatively close to daily minimum needle water potentials typically reported for Scots pine. Our results imply that xylem water potential considerably impacts the turgor pressure gradient driving phloem transport and that gravitation has a relatively large role in phloem transport in the stems of mature Scots pine trees.

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Testing the Münch hypothesis of long distance phloem transport in plants

Cited by 150 publications

References 23 publications

The scaling of the hydraulic architecture in poplar leaves

The scaling of the hydraulic architecture in poplar leaves

Phloem unloading in Arabidopsis roots is convective and regulated by the phloem-pole pericycle

Gradients and dynamics of inner bark and needle osmotic potentials in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst)

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