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Phloem transport: a review of mechanisms and controls

Veerle De Schepper (UGent) , Tom De Swaef (UGent) , Ingvar Bauweraerts (UGent) and Kathy Steppe (UGent)
(2013) JOURNAL OF EXPERIMENTAL BOTANY. 64(1). p.4839-4850
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Abstract
It is generally believed that an osmotically generated pressure gradient drives the phloem mass flow. So far, this widely accepted Münch theory has required remarkably few adaptations, but the debate on alternative and additional hypotheses is still ongoing. Recently, a possible shortcoming of the Münch theory has been pointed out, suggesting that the Münch pressure flow is more suitable for herbs than for trees. Estimation of the phloem resistance indicates that a point might be reached in long sieve tubes where the pressure required to drive the Münch flow cannot be generated. Therefore, the relay hypothesis regained belief as it implies that the sieve tubes are shorter then the plant’s axial axis. In the source phloem, three different loading strategies exist which probably result from evolutionary advantages. Passive diffusion seems to be the most primitive one, whereas active loading strategies substantially increase the growth potential. Along the transport phloem, a leakage-retrieval mechanism is observed. Appreciable amounts of carbohydrates are lost from the sieve tubes to feed the lateral sinks, while a part of these lost carbohydrates is subsequently reloaded into the sieve tubes. This mechanism is probably involved to buffer short-term irregularities in phloem turgor and gradient. In the long term, the mechanism controls the replenishment and remobilization of lateral stem storage tissues. As phloem of higher plants has multiple functions in plant development, reproduction, signalling, and growth, the fundamental understanding of the mechanisms behind phloem transport should be elucidated to increase our ability to influence plant growth and development.
Keywords
loading, Carbon transport, Münch theory, phloem, plant defence, relay hypothesis, signalling, sink, sources, unloading, sugar transport, leakage-retrieval mechanism, SEVERED APHID STYLETS, ABIES L. KARST., SIEVE-ELEMENT, ASSIMILATE TRANSPORT, TURGOR PRESSURE, MUNCH HYPOTHESIS, WATER TRANSPORT, COMPANION CELL, SOURCE LEAVES, MASS-FLOW

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Citation

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Chicago
De Schepper, Veerle, Tom De Swaef, Ingvar Bauweraerts, and Kathy Steppe. 2013. “Phloem Transport: a Review of Mechanisms and Controls.” Journal of Experimental Botany 64 (1): 4839–4850.
APA
De Schepper, V., De Swaef, T., Bauweraerts, I., & Steppe, K. (2013). Phloem transport: a review of mechanisms and controls. JOURNAL OF EXPERIMENTAL BOTANY, 64(1), 4839–4850.
Vancouver
1.
De Schepper V, De Swaef T, Bauweraerts I, Steppe K. Phloem transport: a review of mechanisms and controls. JOURNAL OF EXPERIMENTAL BOTANY. 2013;64(1):4839–50.
MLA
De Schepper, Veerle, Tom De Swaef, Ingvar Bauweraerts, et al. “Phloem Transport: a Review of Mechanisms and Controls.” JOURNAL OF EXPERIMENTAL BOTANY 64.1 (2013): 4839–4850. Print.
@article{4293477,
  abstract     = {It is generally believed that an osmotically generated pressure gradient drives the phloem mass flow. So far, this widely accepted M{\"u}nch theory has required remarkably few adaptations, but the debate on alternative and additional hypotheses is still ongoing. Recently, a possible shortcoming of the M{\"u}nch theory has been pointed out, suggesting that the M{\"u}nch pressure flow is more suitable for herbs than for trees. Estimation of the phloem resistance indicates that a point might be reached in long sieve tubes where the pressure required to drive the M{\"u}nch flow cannot be generated. Therefore, the relay hypothesis regained belief as it implies that the sieve tubes are shorter then the plant{\textquoteright}s axial axis. In the source phloem, three different loading strategies exist which probably result from evolutionary advantages. Passive diffusion seems to be the most primitive one, whereas active loading strategies substantially increase the growth potential. Along the transport phloem, a leakage-retrieval mechanism is observed. Appreciable amounts of carbohydrates are lost from the sieve tubes to feed the lateral sinks, while a part of these lost carbohydrates is subsequently reloaded into the sieve tubes. This mechanism is probably involved to buffer short-term irregularities in phloem turgor and gradient. In the long term, the mechanism controls the replenishment and remobilization of lateral stem storage tissues. As phloem of higher plants has multiple functions in plant development, reproduction, signalling, and growth, the fundamental understanding of the mechanisms behind phloem transport should be elucidated to increase our ability to influence plant growth and development.},
  author       = {De Schepper, Veerle and De Swaef, Tom and Bauweraerts, Ingvar and Steppe, Kathy},
  issn         = {0022-0957},
  journal      = {JOURNAL OF EXPERIMENTAL BOTANY},
  keyword      = {loading,Carbon transport,M{\"u}nch theory,phloem,plant defence,relay hypothesis,signalling,sink,sources,unloading,sugar transport,leakage-retrieval mechanism,SEVERED APHID STYLETS,ABIES L. KARST.,SIEVE-ELEMENT,ASSIMILATE TRANSPORT,TURGOR PRESSURE,MUNCH HYPOTHESIS,WATER TRANSPORT,COMPANION CELL,SOURCE LEAVES,MASS-FLOW},
  language     = {eng},
  number       = {1},
  pages        = {4839--4850},
  title        = {Phloem transport: a review of mechanisms and controls},
  url          = {http://dx.doi.org/10.1093/jxb/ert302},
  volume       = {64},
  year         = {2013},
}

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