Mobilization and Utilization of Seed Reserves

Once seeds have imbibed water and are hydrated, metabolic functions begin to accelerate if dormancy was previously broken. Many of the first enzymes to become active upon hydration are involved in mobilization of seed storage reserves.

Stored sources of energy and nutrients include:

Starch

Starch is found in amyloplasts.
Starch is a polymer of glucose bound by alpha-1,4-glucose bonds. Starch molecules may be branched.
Starch serves as a source of reduced carbon for respiration and metabolism.

Enzymes in involved in starch degradation:

b-amylase: cleaves every other bond, produces maltose. Works only from the ends of starch molecules.

a-amylase: cleaves interior bonds, which provides more ends for enzyme access.

Starch phosphorylase: uses phosphate to remove one glucose at a time, yielding glucose-1-phosphate.

Sucrose

 High energy content
Water soluble - stored in vacuole
Chemically stable - the bond between glucose and fructose eliminates reactive groups

Fats (oils)

 Stored in fat bodies
High energy content Broken down by b-oxidation in glyoxysome for respiration and metabolism

 

Storage proteins

 Source of nitrogen, sulfur and free amino acids
Stored in protein bodies
Storage proteins are rich in amino acids with abundant nitrogen and sulfur. Amino acids rich in N are asparagine, glutamine and lysine. Sulfur-containing amino acids are methionine and cysteine.
Storage proteins are broken down by aminopeptidases, carboxypeptidases, and endopeptidases.

Phytin

 Found in protein bodies
Compact source of phosphate and mono (K+) and divalent cations (Ca++ and Mg++).

Once storage reserves are breaking down, it is necessary to move the small molecules from the source to those cells or organs that have a need for the product. In general meristematic and actively growing cells are energy and nutrient sinks. In germinating seeds, cotyledons and endosperm are sources.

Phloem serves as the transport pathway for solutes.

Characteristics of solute transport causes:

  1. when phloem is cut, contents move out. Thus, phloem is under pressure.
  2. Phloem sap is rich in solutes (sucrose and other sugars, amino acids)
  3. There is a gradient in solute transport - high at source, low at sink
  4. Movement of solute is more rapid than diffusion
  5. Transport requires energy (blocked if respiration is inhibited)

The Münch hypothesis for phloem transport

 Solutes like sucrose are "pumped" into phloem at the source.
The increase solute decreases the phloem water potential and water enters by osmosis
The increased water movement increases the pressure of the system.
At the sink, solutes are removed from the phloem.
Removal of solutes increases the water potential of the phloem cells relative to surrounding cells in the sink tissue and water exits the phloem by osmosis

This inward flux of water at the source increases pressure enough that the phloem sap is pushed along the phloem towards the sink where the pressure is being reduced by removal of solutes and loss of water. The pressure gradient results in "bulk flow" of the solution so that water and solutes can move at rates substantially higher than by diffusion alone.