Phloem Translocation Notes

Translocation in the Phloem

Survival on land poses challenges for terrestrial plants, mainly the need to acquire and retain water. Plants evolved roots (anchor, absorb water/nutrients) and leaves (absorb light, exchange gases). Long-distance transport systems evolved for efficient exchange between shoot and root.

  • Xylem: transports water and minerals from root to aerial parts.
  • Phloem: translocates photosynthesis products from mature leaves to growth/storage areas (including roots), and redistributes water/compounds throughout the plant.

Pathways of Translocation

Phloem and xylem extend throughout the plant. Phloem is usually on the outer side of vascular tissues. In secondary growth, it forms the inner bark.

  • Sieve elements: conduct sugars/organic materials.
    • Sieve tube elements (angiosperms): highly differentiated.
    • Sieve cells (gymnosperms): less specialized.
  • Other phloem tissues:
    • Companion cells.
    • Parenchyma cells (store/release food).
    • Fibers/sclereids (protection/strengthening).
    • Laticifers (latex).

Only sieve elements are directly involved in translocation.

Bundle sheath surrounds small veins/vascular bundles, isolating veins from intercellular spaces.

Sugar Translocation in Sieve Elements

Removal of bark ring (including phloem) stops sugar transport to roots without affecting xylem water transport. Radiolabeled 14CO2^{14}CO_2 showed sugars from photosynthesis are translocated via phloem sieve elements.

Mature Sieve Elements

Mature sieve elements are living cells specialized for translocation. They lack nuclei, tonoplasts, microfilaments, microtubules, Golgi bodies, and ribosomes. Retained organelles include modified mitochondria, plastids, and smooth endoplasmic reticulum. Walls are non-lignified, sometimes thickened.

Sieve Areas

Sieve elements have sieve areas with pores interconnecting cells. Pores range from <1 µm to ≈15 µm diameter.

Sieve Plates (Angiosperms)

Sieve areas can differentiate into sieve plates with larger pores, usually on end walls of sieve tube elements forming sieve tubes. Pores are open channels.

Gymnosperms

All sieve areas are similar, with pores meeting in median cavities. Smooth endoplasmic reticulum (SER) covers sieve areas and is continuous through pores/cavities.

P-Protein and Callose

P-protein (phloem protein, formerly