The upcoming discussion will update you about the comparison among C3, C4 and CAM.
Comparison # C3:
1. Plants operate Calvin Cycle only in all green cells.
2. There is only one CO2 acceptor, i.e., RuBP.
3. The first stable product of photosynthesis is PGA (a C3 acid).
4. “Kranz anatomy” is not found. There is no chloroplast dimorphism. They have well defined grana with both PS-I and PS-II.
5. There is no CO2 concentrating device. Fixation and assimilation of C takes place only through Calvin cycle in the day. So, there is no decarboxylation mechanism.
6. Photorespiratory toss of photo- synthates is very prominent due to dual action of rubisco and lack of PEPcase. Up to 40% of photosynthates may be lost.
7. CO2 compensation point is 40-100 µ||-1.
8. Intracellular CO2 concentration in light is 200 µ||-1.
9. Stomatal frequency is 2000 – 31000.
10. Water use efficiency is 1-3 gCO2 fixed/kg water transpired.
11. Maximum growth rate is 5-20g m-2 d-1.
12. Maximum productivity 10-30 t ha-1y-1.
13. Typical species of economic importance are wheat, barley, rice, potato.
14. 89% world flora (in species number).
15. Widely distributed and dominant in forests.
Comparison # C4:
1. Plants operate C4 cycle in MC in addition to C3 cycle operating in BSC.
2. There are two C02 acceptors — PEP and RuBP.
3. The first stable product is malate or aspartate (aC4acid).
4. The leaves show “Kranz anatomy”. The chloroplasts are dimorphic. The MC chloroplasts are granal whereas the BSC chloroplasts are agranal lacking PS-II.
5. Plants are specially characterized by CO2 concentrating mechanism. So, there is initial carboxylation in MC followed by decarboxylation in BSC. Both are occurring in same time (day) but separated in space.
6. Photorespiration cannot be detected due to the high activity of PEP case in MC. The C4 cycle gears the C3 cycle by pumping C02 in BSC. Rubisco cannot behave as oxygenase.
7. 0-10 µ||-1.
8. 100 µ||-1.
9. 10000-16000.
10. 2 – 5 g of CO2 fixed/kg of water transpired.
11. 40-50g m-2d-1.
12. 60 – 80 t ha -1 y-1.
13. Maize, millet, sugarcane, sorghum.
14. < 1%.
15. Warm to hot open sites (grassland).
Comparison # CAM:
1. Plants operate only C3 cycle in MC for carbon assimilation.
2. Same as C4.
3. The initial fixation product is malate in dark, which remains stored in vacuole.
4. No “Kranz anatomy” is found. The chloroplasts are not dimorphic.
5. Plants show CO2 accumulating device as malate during night as they are adapted to arid zone. So, acidification and de-acidification occur in the same space (MC) but separated in time. The former takes place in dark while the latter takes place in light.
6. Photorespiration cannot be detected as the stomata remain closed during day. The photo-respiratory CO2 cannot escape instead is re-fixed by Rubisco.
7. 0-10 µ||-1 (in dark).
8. 10000 µ||-1 (in dark).
9. 100-800.
10. 10-40g of CO2 fixed/kg of water-transpired.
11. 0.2g m-2d-1.
12. Generally <10t ha-1y-1.
13. Pineapple.
14. 10 %.
15. Xeric sites (includes epiphytes).