Transport and partitioning of photosynthetic products

Question 1

Translocation:

Answer 1

plants need a flexible system for supplying various sinks (new leaves, roots etc.) from sources (mature leaves)

Sources and sinks can interchange e.g. a potato tuber is a storage organ on a full grown plant and therefore a sink. Whereas a seed potato acts as a nutrient source to grow a plant from.

Question 2

Translocation pathways: phloem

Answer 2

function of bundle sheath cells – where CO2 sequestering in C4 photosynth occurs
Companion cells connected to sieve element (companion and sieve start off as one cell then divide into two and specialise as a pair)

No lignin in phloem vessels as positive cytoplasmic tension no need to resist collapse as in xylem

(see diagrams)
Under a microscope sieve cells look almost empty, companion cells are packed with mitochodria (for energy needed for active transport against conc. Gradient in and out of the sieve tube elements)

P-proteins and callose are used to seal damaged regions in the phloem to prevent loss of photosynthetic products, also produced when plant is preparing to be dormant e.g. deciduous trees will introduce callose blocks overwinter and remove them by enzyme dissolution in spring.

Question 3

Translocation pathways: callose deposits as defence

Answer 3

insects can introduce viruses and drain photosynthetic products by puncturing the leaves with feeding tubes, the plant responds by forming callose plugs, some insects secrete enzymes that break these down to allow for continued feeding

Question 4

Translocation pathways: ordinary companion cells

Answer 4

Companion cells tend to be isolated from other cells but have many plasmodesmata connecting to their partner sieve element

Question 5

Translocation pathways: transfer cells in peas

Answer 5

In peas Pisum sativum plasmodesmata can be apoplast (crossing membranes via transport molecule) to symplast (inside cell) by transport cell

Question 6

Translocation pathways: intermediary cells

Answer 6

no apoplastic step, just symplastic, straight from bundle to intermediary to sieve cell – very little control

Question 7

Translocation pathways: phloem composition

Answer 7

It is hard to sample phloem so it is often sampled from aphids that have been feeding on it

Question 8

Phloem saps we use:

Answer 8

Maple and birch syrup

Palm sugar (dried)
^ sugar palm translocates solutes at a rate 100,000 times faster than diffusion

Agave syrup -> tequila (fermented)

Aloe vera gel

Question 9

The pressure flow model (Munch, 1930)

Answer 9

(see diagram)
Sucrose is actively loaded, water joins by osmosis, sugars are actively or inactively offloaded
Sieve plates maintain the gradient (acting as a series of valves)

The pressure-flow model – predictions and assumptions:

1: Sieve plates must be unobstructed (Knoblauch &van Bel, 1998 – next slide)

2: Energy requirement is small (not prevented by chilling)

3: Should be a pressure gradient along the sieve tubes (0.03 – 0.05 MPa m-1 determined by Wright &Fisher, 1980)

4: Water and solutes should move together in the same direction and at the same rate in sieve tubes –no bidirectional transport within a sieve tube element. Shown using radioactive tracers to tag water and carbon

Question 10

The pressure flow model: open sieve plate pores

Answer 10

experimental method in which: two windows cut in main vein of the leaf and dye placed in apical window microscope observing at the next window it is possible to view movement

Question 11

sugar: getting it in and out

Answer 11

1- sugar travels from palisade cells through to a phloem parachyma cell (symplastic)

2- sugar is then apoplastically loading into the sieve element and symplastically transferred into sieve element
- companion cell produces energy that it uses to push H+ ions into the space to symport sucrose into the companion from sieve element

Question 12

Alternatively some plants function without an apoplastic step using polymer trapping

Answer 12

In this system sugars must be prevented from backflowing by maintaining conc. Gradient
– this is done via polymer trapping:
*sucrose diffuses from mesophyll to bundle sheath cells via plasmodesmata
*raffinose and stachyose are synthesised to maintain diffusion gradient for sucrose
*raffinose and stachyose diffuse into sieve element and are exported

It is thought that the fully symplastic format was the first to evolve and apoplastic step evolved later. Some situation such as developing seeds require the apoplastic step to be possible.

Question 13

phloem unloading: sink to source transition

Answer 13

once leaves fully mature they switch from sinks to sources

e.g. Tobacco leaves
sinks - photosynthates imported by major vein
source - photosynthates loaded into matured minor veins

Question 14

phloem unloading: symplastic and apoplastic

Answer 14

see diagram:
A)unloading can be entirely symplastic
B) or can include an apoplastic step in different places (B1, B2A, B2B)

Plant can control how much it provides

Sugars are used up by sinks or stored as starches maintaining conc. Gradient

Apoplastic steps are essential in seed development and symbiotic relationships

Question 15

Phloem unloading: passive and active

Answer 15

Import into sink is passive e.g. in growing leaves and storage organs which use transported sugars rapidly for:

• respiration
• metabolism into storage materials e.g. starch, proteins
• production of compounds required for growth.

Has to be active when an apoplastic step is included as at least two membranes must be crossed

sucrose transporter proteins e.g. SUT1

invertase (converts sucrose to glucose + fructose)may be involved

Question 16

Allocation and partition of photosynthates

Answer 16

Allocation = regulation of distribution of fixed carbon to various metabolic pathways within a source or sink.

Carbon fixed within a source cell is allocated for:
* Synthesis of storage compounds
* Metabolic use
* Synthesis of transport compounds.
^ Same sort of process occurs in sink tissues.

Allocation within a source cell. Source determines:
* how much triosphosphate is used to regenerate C-B intermediates
* how much is converted to transport sugars
* how much is stored as starch

See: plant physiology 5e fig 10.22

Plants store starch in their leaves to utilise at night to provide sugar for respiration through the night

Partition = distribution of photosynthates between various sinks
Material is partitioned between competing sinks on the basis of their ability to store or metabolise it, which depends on plant’s needs at given time….

Carbon is partitioned/allocated according to various factors – very important in crops – some farmers want leaves others want developed tuber roots etc. And plant will allocate different amounts of carbon according to growing conditions.

(see diagrams)