Lesson 2.4 Crop Production (Translocation) (Lec)

Translocation

Translocation

transport of photosynthates from the source to the sink, i.e. from the region of synthesis to the region of utilization

Transpirational stream

upward movement of solution from the roots through the xylem to the uppermost leaves

Source

any exporting region that produces sugars beyond its own needs (e.g. mature leaves)

Sink

any area that does not produce enough sugar to meet its own needs (e.g. roots, fruits, growing tips)

Xylem

conducts water and inorganic elements from the ground to the aerial parts; made of tracheids and vessel elements; one-way (upward) flow; no end walls; thick walls stiffened with lignin

Phloem

conducts photosynthates/assimilates from the leaves (source) to other plant parts (sink); two-way flow; cells have end walls with perforations

Sieve tubes

join to form a continuous tube in the phloem

Companion cells

transport photosynthates from leaf cells into sieve tube elements through plasmodesmata

Phloem sap contents

mostly sugars (80-106 mg/mL), plus amino acids, organic acids, protein, K, Cl, phosphate and Mg

Passive transport (diffusion)

a solute diffuses across a membrane down a concentration gradient; no direct energy (no ATP) needed

Active transport

a solute is moved across a membrane against the concentration gradient; requires energy (ATP)

Symplast route

continuum of cytoplasm within a plant tissue, formed by plasmodesmata passing through pores in cell walls (travel through the cytoplasm)

Apoplast route

continuum between plant cells, formed by the continuous matrix of cell walls (travel through the cell wall)

Plasmodesmata

channels through cell wall pores that connect the cytoplasm of adjacent cells

Bulk flow

movement of water and solutes together due to a pressure gradient driven by the transpiration stream

Osmosis

movement of water through a semipermeable membrane into a solution of higher solute concentration

Water potential

Solute Potential + Pressure Potential

Pure water

has a water potential of 0; water moves toward the side with more negative water potential

Feedback inhibition

the sink cannot use the increased production, so sugars build up in the system and photosynthesis is reduced

Feedforward inhibition

fast utilization due to more sink availability