Translocation • Occurs in Phloem • Bidirectional • Composition • Mechanism of translocation • Munch Pressure flow hypothesis • Phloem loading and unloading • Source-sink relationships Phloem Structure • Sieve tube members: Primary Angiosperms; Sieve cells: phloem Gymnosperms • Companion cells • Sieve tube members/companion cells derived from same mother cell • Both living at maturity • Sieve tube members lose organelles at maturity 2ndary contain cytoplasm Phloem • Sieve plate with pores • P-protein Phloem Electron Micrograph Companion cell Sieve tube elements Composition of phloem • Girdling experiments • analysis of aphid exudate • composition • sugars • amino acids (nitrogen):ureides, glutamic acid, glutamine • organic acids • hormones Composition Sugars • Types of sugars • sucrose main transport sugar: non- reducing sugar • some species: raffinose, stachyose transported • sugar alcohols: sorbitol, mannitol Nitrogen compounds • Transport from the root • Amino acids • uriedes Mechanism of translocation • Munch Pressure flow hypothesis • Sugar loaded into phloem • increase in solute potential • leads to increase in water uptake from xylem • builds up hydrostatic pressure • unloaded at sink Source-sink relationships • Bidirectional: up and down plant • is it in same cell, not clear • rate 30-150 cm/hr, slower than water Source to Sink • Proximity of source to sink is critical • sinks may change during life cycle • young leaves net import • as mature become exporter • roots, shoots, tubers, fruits are strong sink Phloem Loading/unloading • Active process • evidence from respiratory inhibitors • Loading: apoplast or symplast • apoplast retrieve leaky sucrose? Sugar transporters •Sugar- H+ co-transport symplast via plasmodesmata:polymer trapping Summary of loading Unloading at sink • symplast young leaves • apoplast seeds/grain • acid invertase Loading/unloading.