photo assimilates

Question 1

What is partitioning of photoassimilates and what determines it?

Answer 1

Partitioning: Differential distribution of photoassimilates (like sugars) to different organs.
Determined by sink strength, which depends on:
Sink size (how large the sink is).
Sink activity (how active the sink is in consuming sugars).
Factors affecting partitioning:
Proximity to source organs.
Developmental stage of sink organs.

Question 2

How is sink strength calculated, and what happens when sinks compete?

Answer 2

Sink strength = sink size × sink activity.
Sinks compete for photoassimilates, and stronger sinks (e.g., growing seeds, fruits, or roots) attract more sugars.
Partitioning is dynamic and shifts based on developmental stages and organ needs.

Question 3

What happens during carbon fixation in photosynthesis?

Answer 3

1) Carbon fixation occurs in the Calvin-Benson-Bassham cycle.
2) The net product is triose phosphate (TP).
3) TP is exported from the chloroplast via the TPT (triose phosphate transporter) in exchange for inorganic phosphate (Pi).
4) TP is used for sucrose synthesis in the cytoplasm or for starch synthesis in the chloroplast.

Question 4

How is sucrose synthesized from triose phosphate?

Answer 4

1️⃣ TP exits the chloroplast via the TPT (triose phosphate transporter).
2️⃣ In the cytoplasm, TP is converted into sucrose.
3️⃣ Sucrose synthesis is tightly regulated to balance between sucrose and starch production.

Question 5

How does sucrose synthesis regulate carbon fixation?

Answer 5

TPT controls TP export to the cytoplasm for sucrose synthesis.
High TP export: Less TP remains in the Calvin cycle, so carbon fixation slows down.
Low TP export: Less inorganic phosphate (Pi) enters the chloroplast, reducing ATP production and limiting the Calvin cycle.
This tight regulation ensures a balance between sucrose synthesis and continued carbon fixation.

Question 6

How is starch synthesized, and what is its role in plants?

Answer 6

• Starch is made in the chloroplasts during the day from substrates produced in the Calvin cycle.
• Starch consists of amylose and amylopectin, forming large, insoluble, semicrystalline granules.
• Some plants store up to 50% of their fixed carbon as starch, while others rely more on sucrose synthesis.
• When sucrose synthesis is low, plants store more starch as an alternative.

Question 7

How are sucrose and starch synthesis linked?

Answer 7

• When sucrose synthesis is low, starch synthesis increases as an alternative storage method.
• When sucrose synthesis is high, starch synthesis decreases.
• This balance ensures that excess fixed carbon is stored as starch, especially when immediate sucrose export is limited.

Question 8

How do plants grow in the dark, and what happens when there is a shortage of photoassimilates?

Answer 8

• Plants grow in both light and dark using photoassimilates (sugars) produced during the day.
• At night, growth is fueled by starch degradation to maintain sucrose supply.

Carbon starvation occurs if photoassimilates are insufficient.
- Sucrose export from leaves stops.
- Sink organs (e.g., growing roots, fruits) are starved of sugars.
- Growth of sinks and sieve elements is inhibited.

Question 9

What happens during starch degradation at night, and why is it important?

Answer 9

• During the night, plants break down starch to release sugars for growth.
• Without this process, there is no sucrose supply for sink organs, causing carbon starvation.
• This process ensures growth can continue at night when photosynthesis is inactive.
• If starch is not properly degraded, growth (like silique development) is reduced.

Question 10

How does abiotic stress affect seed yield?

Answer 10

• Abiotic stress (like drought or heat) reduces carbohydrate availability.
• Less photoassimilate is produced, resulting in fewer sugars to be exported.
• This causes carbon starvation in sink organs like seeds, reducing seed yield.
• Carbohydrate availability is critical for seed development and yield under stress.