Abstract
Phloem is responsible for translocation, which is the movement of the products of photosynthesis (photosynthate or sugars) from those tissues that make sugars (sources) to those tissues that use or store the sugars (sinks). Phloem also moves hormones, nutrients, and viruses. The mechanism of translocation is described by the Münch pressure flow model and relies on osmotically driven hydraulics. Phloem is a tissue made of several cell types. Translocation is via the sieve tube elements (STE), which are living parenchyma cells that lack almost everything except for the cell membrane. Companion cells provide metabolic support to, and load sugars into, the STEs. The sugars drive the hydraulics of translocation. Phloem parenchymas are responsible for radial transport, and phloem fibers provide protection and support. Gymnosperms lack phloem fibers and companion cells. The angiosperm STE and CC are derived from the same mother cell. In contrast, the gymnosperm albuminous cells, which do not share a common derivation with the STE, are responsible for STE loading and unloading.
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Crang, R., Lyons-Sobaski, S., Wise, R. (2018). Phloem. In: Plant Anatomy. Springer, Cham. https://doi.org/10.1007/978-3-319-77315-5_8
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DOI: https://doi.org/10.1007/978-3-319-77315-5_8
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