Plant stress and Light

Question 1

Drought

Answer 1

This causes a reduced water potential outside the cell due to the lack of water in the environment. Leading to a loss of Tugor, membrane damage, protein denaturation, and increased ABA synthesis to inhibit growth.

Question 2

Salinity

Answer 2

This causes a reduced water potential outside the cell due to increased solute concentration in the apoplastic environment. This leads to loss of tugor, membrane damage, protein denaturation, and increased ABA synthesis which inhibits growth.

Question 3

Freezing

Answer 3

This causes reduced water potential outside the cell due to ice crystals forming in the apoplast. This leads to a loss of tugor, membrane damage, protein degradation and increased ABA synthesis which inhibits growth.

Question 4

Constitutive stress tolerance abilities

Answer 4

Traits developed during development. Hairy leaves, waxy cuticle helps prevent water loss. Thickening of stems and leaves creates a smaller surface area to volume ratio. Folded leaves prevent more transpiration. Photosynthetic stems with a loss of leaves. C4 and/or CAM photosynthesis.

Question 5

Adaptive stress tolerance abilities

Answer 5

Traits that are usually developed by genes being induced. Heat shock proteins, help stabilise proteins, preventing denaturation. C4 and/or CAM photosynthesis. Detoxification. Pigment production. Solute production.

Question 6

Detection of low water potential

Answer 6

The xylem is able to conduct water due to the low water potential in the leaves (transpiration) pulling the water up through the vasculature. If this water potential is very low and there is not enough water, air bubbles can form. This hinders water movement and leads to cavitation of the xylem. This can trigger a guard cell response, stromatal closure.

Question 7

Compatible osmolytes/solutes

Answer 7

Solutes such as proline, sugars and quaternary ammonium are produced in cells to balance the osmotic pressure between the cytosol and surroundings. These compounds have hydroxyl groups that can substitute for water.

Question 8

Kranz anatomy

Answer 8

Bundle sheath cells surround the vascular bundles. These Bundle sheath cells only carry out the light independent reactions (calvin cycle) due to lacking a PSII. Mesophyll cells surround the bundle sheath cells and are only capable of the light dependent reaction.

Question 9

C4 photosynthesis

Answer 9

Used in maize, sorgum and sugarcane. Seperates Rubisco from O2, but at the same time seperates CO2 from Rubisco. This system allows for the plant to have CO2 levels 25-50x higher than the atmosphere. Even if the solubility goes down, the plant gets enough CO2. It works well in areas with sufficient water and temperatures over 20. At 25C C4 plants produce 2x the biomass of C3.

Question 10

CAM photosynthesis mechanism

Answer 10

During night stomata opens, letting in CO2. CO2 is used to convert PEP (phophoenolpyruvate) into Malate by PEPC. This malate is stored. During the day, the stomata shuts. The malate is converted back into PEP by NAD-ME, releasing the CO2, allowing it to be fixed in the calvin cycle, generating ATP/NADPH.

Question 11

C4 photosynthesis mechanism

Answer 11

The mesophyll contains all components of the light dependent reaction, including PSII which splits H2O into protons and O2. But the mesophyll has little Rubisco, so it is insensitive to O2. The bundle sheath cells lack PSII meaning H2O is not split to form O2, making them a low O2 environment. This allows the present Rubisco to efficiently fix CO2.
CO2 first enters the mesophyl cells and is fixed to pyruvate by PEPC, producing malate. Malate can then be moved into the bundle sheath cells where CO2 is removed from malate by NADP malic enzymes, producing pyruvate which is sent back to the mesophyll cells. The released CO2 can be fixed to RuBP by Rubisco as part of the calvin cycle to produce glucose.

Question 12

C4 nitrogen use efficiency

Answer 12

CO2 assimilation per leaf nitrogen or the plant dry mass per leaf nitrogen. C3 requires more Rubisco due to its innefficiency, requiring more nitrogen.

Question 13

C4 disadvantages

Answer 13

It is only of benefit when it is hot or water/CO2 is limited. Otherwise it just uses more ATP per molecule of glucose. C3 = 18 ATP per glucose, C4 = 30 ATP per glucose. Needs more light per molecule of glucose fixed. At 10C the yield of C3 is higher no matter the levels of light. At 20C, C4 only yields more in very high levels of light. At 35C there is a much higher yeild in C4. AS temperature increases, C4 yields more. At about 40C, there is a decrease in yield due to protein denaturation.