Phloem loading: Sink/source vs. Productivity Symplast vs. apoplast pathways: distinguish at near the sieve element-companion cell complex apoplast: suc major selective transporter energy required: respiratory inhibitors, active loading Organic acids, hormones  passive  s -1.3 MPa  s -3.0 MPa symplast: suc and others, such as raffinose and stachyose or transfer cells

Sucrose-H + symporter SE CC  in the apoplast of phloem loading  in the membrane of company cells or sieve elements  dissipate proton energy to couple the uptake of sucrose

Regulating sucrose loading by the sucrose-H + symporter — are not completely clear  The turgor pressure of the sieve elements below a threshold level, a compensatory increase loading?  Sucrose concentration in the apoplast [Suc] apoplast ?  The available No. of symporter molecules feed Suc  SUT1 transcription , SUT1 and itsmRNA rapidly degrade, protein phosphorylation involved a diurnal regulation  Nutrient supply— potassium availability enhance sucrose efflux into apoplast  sink growth

Intermediary cells in the symplast of phloem loading ¤ diffusion ¤ raffinose, stachyose, in addition to sucrose ¤ a polymer-trapping model

The type of phloem loading is corrected with plant family and with climate herbaceous trees, shrubs, vines temperate and arid tropic and subtropic exceptions Coexist New loading ? Symplastic loading advantages?

Phloem unloading and short-distance transport — via symplast or apoplast PCMB: inhibit sucrose across membrane

The transition of a leaf from sink to source sink  source depend on species, 25% → 40-50% The cessation of import and the initiation of export are independent events. The extent of maturation and sampling position Feed 14 C on source leaf for 2 h Squash The base still a sink sink leaf source leaf