Translocation in the Phloem

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Sieve Elements-Cells of the Phloem

1. Sieve tube elements – Angiosperm2. Sieve cells – Gymnosperms

Sieve Tube Elements Sieve cells( Angiosperms) ( Gymnosperms)

Sieve tube elements and Sieve cellsSieve Tube Elements Sieve Cells

Type of Plants Angiosperms Gymnopserms

Junction Open channels, sieve plates

Blocked with membranes

P-protein present absent

Associated cells Companion cells Albuminous cells

P-protein – in all dicots, many monocots, absent in gymnosperms; - seal off damaged sieve elementsCallose – longer solution to damaged sieve tubeWound callose – efficient seal of damaged sieve plates Companion cells – transport of photosynthate from producing cells to sieve elements; take over some metabolic function during differentiation of sieve elements; supply energy

Specialized Companion cells

1. Ordinary companion cells – have chloroplasts2. Transfer cells – have wall ingrowths which

increase the surface area of the plasma membrane and enhance solute transfer

3. Intermediary cells- with numerous plasmodesmata which connect them to sorrounding cells

Patterns of Translocation

Sources –areas of supply; include photosynthetic parts or exporting storage organSink-areas of metabolism or storage ; nonphotosynthetic organs; ex. Roots, tubers, developing fruits, immature leaves

Factors which affect Translocation:1. Proximity2. Development3. Vascular connections orthostichy- vertical row of leaves

Anastomoses – vascular interconnections, alternative pathway in cases of interferences in the translocation path

Translocated Materials1. Water – solvent of carbohydrates2. Nitrogen- in the form of amino acids and amides3. Plant hormones – auxin, gibberellins, cytokinins,

abscisic acid4. Proteins- P-proteins ( wound sealing); protein

kinases( phosphorylation); thioredoxin(disulfide reduction); ubiquitin( protein turnover); chaperones(protein folding); protease inhibitors ( against insects)

5. Inorganic solutes – K, P, Mg, PO4, Cl

Reducing Sugars – not generally translocated in the phloem : aldehyde and ketoneNonreducing sugars - sucrose

Rates of Movement

1. Velocity – linera distance travelled per unit time

2. Mass transfer rate- quantity of material passing through a given cross sectionof phloem per unit time

* Transport velocities in the phloem exceed rate of diffusion over long distances

Mechanism of Translocation

phloem loading – movement of photosynthate from producing cells into sieve elements phloem unloading – from sieve elements to sink cellsPressure-Flow Model: by Ernst Munch A flow of solution in the sieve elements is driven by an osmotically generated pressure gradient between source and sink.