Lecture 13

Phloem

I.  Introduction

A. Phloem is a complex tissue! Primary phloem and secondary phloem (Figure). Primary derived from the procambium, secondary from the vascular cambium.

B. Main function is the transport of photosynthate (sucrose) – via Bulk or Mass Flow (image)
1. movement down from leaves (the source, where photosynthesis occurs) to other parts of the plant (sinks, e.g. roots, fruits, growing meristems)
2. movement also up the stem, during spring; photosynthates are moved upwards towards the young growing part of the plant.

C. Girdling (image). Refers to removing a ring of bark all the way around the circumference of the tree. Result: tree dies.

II. Cell types - Figure 11.1

A. Sieve elements

1. General features of conducting cells
a.  many contiguous sieve elements connected together via sieve areas
b.  sieve elements have highly modified protoplast, i.e. no nucleus. Because of this, their cellular functions are controlled by another parenchymatous cell that is adjacent to them. In angiosperms, sieve tube member accompanied by a companion cell. In gymnosperms, sieve cell accompanied by an albuminous cell.
c. Various plants showing phloem:
• image of the grass Botelua).
• image of clover (Trifolium)
• image of parsnip (Pastinaca)
• image of milkweed (Asclepias) and image of companion cells, image of sieve plate,
• image of cosmos (Cosmos)
• image and another image of cucumber (Cucumis) - some of the largest sieve plates of all plants! To handle these, they must have multiple companion cells as shown in this image.
d. the wall is usually primary; if secondary wall is laid down, it is not lignified
e. Phloem sap is under pressure.  Radiactively labeled translocators have been used to study movement, as have aphids.  Aphids inject their stylet into the phloem sieve tube.  They feed on nitrogenous compounds in sap, excrete the sugar as a droplet called honeydew.  Sap moves 10-100 cm / hour, 30 atmospheres of pressure.

2. Cell wall and sieve area.
a. The wall of the sieve element is thicker than surrounding parenchyma.  Figure 11.11A. Sometimes wall is very thick, almost occluding the cell lumen.  Figure 11.19D. Nacreous wall. Not lignified. Inner wall electron dense with radial striations, as shown in this TEM of Nicotiana Figure 11.11A.
b. Sieve areas.  Walls with pores that are actually modified primary pit fields.  May be joined vertically or laterally.  Pores similar in size (ca. size of plasmodesmata) in gymnosperms. Variable in size in angiosperms.  Areas with large pores = sieve plates.  Figure 11.2.  Sieve plates in Dillenia metaphloem.
c. Pores are lined with callose – a carbohydrate composed of glucose monomers.  In general, callose is deposited around structures that need to be separated, e.g. egg cells, sperm cells, pollen tubes. In phloem, callose accumulates with age or dormancy, eventually forming a pad around the pore (then the sieve element is no longer functional).  Callose is mostly absent when sieve element is functional, synthesized rapidly upon injury. See callose deposition around seive pore in bamboo after injury.  “Normal” callose deposition is from internal stimuli, not external influence (damage).

3. Sieve cells and sieve tube members
a. Sieve areas not restricted to sieve plates, not as specialized. Sieve tube members have sieve plates, usually on ends, specialized.  Diagram of types in Figure 11.4
b. This trend parallels the comparative conditions of tracheids (connected by pits) and vessels (connected by perforation plates).  Sieve tubes composed of sieve tube members.
c. Gymnosperms and ferns – have sieve cells.  Angiosperms – sieve tube members.
d. Compound sieve plate – composed of many sieve areas on oblique face of wedge.  Figure 11.3D of Pyrus. Tilia radial section showing sieve areas with compound sieve plates. This photo of Vitis shows both lateral sieve areas and compound sieve plates.
e. The most specialized angiosperm cell: simple sieve plate with large pores on transverse end walls.  Least specialized are sieve areas on lateral walls.

Sieve cells Sieve tube members
sieve areas sieve areas
elongate and narrow shorter and wider
sieve areas are scattered along the walls   sieve areas are only on the lateral walls
end walls are sieve areas end wall is a sieve plate - very specialized area, one or more modifications


4. Protoplast.  Many changes occur during ontogeny.  Figure 11.5.
a. Nucleus may be present as collapsed body or commonly absent.  Immature sieve element of Phaseolus (Figure 11.6) shows plastids with starch, stacked ER, Golgi (which produce vessicles during formation of thickened inner wall).
b. Golgi, ER, ribosomes, tonoplast disappear in mature sieve elements. 
c. Only plastids and mitochondria remain.  Plastids of two types: starch storing and protein as fibrils or crystals. 
d. P-protein as tubules in groups (paracrystaline) in cytoplasm. In Phaseolus root tip Figure 11.7. Double helical structure as shown in the sieve element pore of Nicotiana (Figure 11.8A). At first P-protein is aggregated, later spread out in cytoplasm. Forms stands, network, especially in pores with disturbance or end of cell functioning.

5. Sieve plate differentiation. Figure 11.5
a. Plasmodesmata mark sites for future pores, with desmotubule. 
b. Callose coats pore, except at middle lamella. Paired callose masses on each side of the two sieve elements  Figure 11.9 – appear as platelets, where they meet is the site of the pore. 
c. ER and desmotubule disappears from the middle of the pore. Figure 11.8.
d. P-protein accumulates around pores, called “slime-plugs”.  Figure 11.10 of Tetragonia (A) and Beta (B). Callose formation enhances blockage upon damage.
e.  sieve plates can be simple (one modified area, advanced, a better conductor, end wall is transverse), or compound (more primitive, end wall is slanted)
f. they do not function very long, the sieve cells and sieve tube members are ephemeral,  last only ca. 5 months. They plug up with P-protein and callose to form a slime plug (or P-protein plug) when they are finished functioning or when damaged, as illustrated HERE in this Ambrosia (ragweed) stem or HERE in this Cucumis stem.

B. Companion Cells
1. Phloem translocation depends upon companion cells, or in gymnosperms, albuminous cells.
2.  They are a form of parenchyma.  Sugar is both loaded at sources and unloaded at sinks.  The companion cells are connected to the sieve elements by plasmodesmata, branched at the companion cell side.  Figure 11.11 of Nicotiana (A) and Mimosa (B).
3. Dense with cytoplasm, especially ribosomes. 
4. Some resemble secretory cells, with wall ingrowths (called then transfer cells). 
5. Companion cells are ontogenetically related to sieve elements, derived from the same precursor cell.  Can be one or more companion cells on one or more sides of sieve element. In gymnosperms, the functional counterpart of companion cells are called albuminous cells (= Strasburger cells).  These are not derived from the same precursor as sieve elements but originate from adjacent cells

C. Parenchyma cells
1. Regular components of phloem.  Contain starch, tannins, crystals. 
2. In secondary phloem, present as axial parenchyma and ray parenchyma. 
3. Parenchyma cells may be divided into smaller cells, each with crystals (Figure 11.1D). 
4. Often associated with fibers or sclereids, they may have lignified walls. 
5. Can also function in loading and unloading photosynthate, thus parenchyma can range from storage to functioning almost like a companion cell in relation to sieve element.

D. Sclerenchyma
1. Fibers are common in primary and secondary phloem. Extraxylary fibers, specifically phloem fibers, occur in both 1˚ and 2˚ phloem (Tilia).
2. Occupies various positions in stem, may be living or not living, septate or not.  Sclereids in axial and radial system in secondary phloem.  Also fiber-sclereids.

III. Primary phloem Figure 11.12 of Avena
1. Occurs, as with xylem, as protophloem and metaphloem.  Protophloem occurs in actively growing parts. It is stretched and becomes nonfunctional later, and even obliterated completely. The bicolateral vascular bundle of Cucurbita nicely shows primary phloem.
2. Only metaphloem is functional in vegetative plants. 
3. Metaphloem is associated with elongating cells – fiber primordia. These mature into fibers at time sieve elements cease to function.  Often found at the periphery of the phloem region – called pericyclic fibers. Figure 11.13 of Phaseolus vulgaris. Protophloem may not have associated companion cells, but metaphloem does.

IV. Secondary phloem

A. General features
1. Less abundant than secondary xylem.
2. Older phloem is crushed, separated from axis by periderm. Thus, it does not accumulate like secondary xylem.  The functional phloem is on the inner bark of woody stems and roots.

B. Conifer phloem Figure 11.14, diagram of Thuja
1. Simple, less variable than dicot secondary phloem. Transverse section of Pinus cambium and secondary phloem.
2. Contains a variety of cells:
• Sieve cells of Pinus Figure 11.4, with associated rays. Sieve cells with many sieve areas, commonly on radial faces. Image and another image of sieve areas.
• Parenchyma cells occur singly or in strands (resemble sausages in links!).
• Albuminous cells occur at ends of rays.
• Fibers & sclereids. Fibers absent in Pinaceae but present in Taxaceae and Cupressaceae.  When present, in uniseriate bands (Thuja, Figure 11.15A), alternating with parenchyma and sieve cells. Only a narrow band of phloem is active.  The rest is crushed near the periderm.  In Pinus (Figure 11.16), the crushed sieve cells appear wavy, as do the rays and other parenchyma.

C. Dicot phloem Figure 11.17 diagram of Liriodendron
1. Secondary phloem composed of sieve tubes, companion cells, parenchyma, fibers. 
2. Fibers may be present or absent (e.g. Aristolochia).  When present, scattered, in tangential bands (Robinia), or parallel (Fraxinus, Figure 11.18, or Tilia)  In this cross section of Carya you see the cambium, secondary phloem, secondary xylem, fibers, uniseriate and multiseriate rays. In some phloem sclereids are absent but in others fibers, sclereids or both fibers and sclereids may be present (ResearchGate page). The septate fibers of Vitis store starch.
3. Secondary phloem can be storied or non-storied. 
4. End walls of the sieve elements may be inclined, with large, compound sieve plates as seen in Liriodendron, Figure 11.17
5. Rays can resemble xylem rays, e.g. uni- multiseriate. Composed of parenchyma, sclereids, or sclerified parenchyma cells with crystals.  Or the rays may become dilated – their axis diameters increased (Figure 11.19; Tilia, Liriodendron).  This is accomplished by radial anticlinal cell divisions. Sometimes divisions are in the median part of the ray, which then looks like a meristem.
6. In old phloem, sieve tubes are crushed, or if open, they fill with gases.  Parenchyma can compress the sieve tubes or invade their lumen (thylosoids).  They then store starch. Image of Vitis (grape) with collapsed sieve tube members.
7. Phloem produced in the spring may die in the fall, but exceptions include Vitis where sieve elements resume growth in spring after dormancy.
Last updated: 14-Oct-22 / dln