Applied Photosynthesis — Agricultural and Biotechnological Context

1. C4 Crops in Tropical Agriculture C4 crops (maize, sugarcane, sorghum) dominate tropical agriculture because their $CO_{2}$ concentrating mechanism eliminates photorespiration, giving them 30–50% higher net photosynthesis efficiency than C3 crops (wheat, rice) at temperatures above 30°C. They also have higher water use efficiency (WUE), making them more drought-resistant. C4 plants require more ATP per $CO_{2}$ (~5 vs. ~3 for C3), but this is offset by eliminated photorespiration losses.

2. $CO_{2}$ Fertilisation in Greenhouses Based on Blackman's law, $CO_{2}$ concentration is often the limiting factor in greenhouses with high light and controlled temperature. Enriching greenhouse $CO_{2}$ to 0.1–0.15% (from atmospheric 0.042%) increases C3 crop yields by 20–40%. This principle does not work as effectively for C4 crops because their $CO_{2}$ concentrating mechanism already saturates $CO_{2}$ supply to RuBisCO.

3. C4 Rice Project (IRRI) Rice (Oryza sativa, a C3 plant) is the world's most important food crop. Scientists at IRRI are attempting to engineer C4 photosynthesis into rice by introducing C4 genes (PEP carboxylase, PPDK, malic enzyme) and Kranz-like anatomy. Expected benefits: ~50% increase in yield, 30–50% improvement in water use efficiency, reduced nitrogen fertiliser requirements (RuBisCO is the largest nitrogen sink in leaves).

4. Herbicide Action via Photosynthesis Inhibition Several herbicides target the photosynthetic electron transport chain:

• DCMU (Diuron): Blocks PQ binding site at PS II → stops non-cyclic electron flow → kills plants
• Paraquat (methyl viologen): Diverts electrons from ferredoxin, generating superoxide radicals → oxidative damage Understanding photosynthesis biochemistry enables rational herbicide design.

5. Biofuel Production Microalgae with high photosynthetic efficiency (e.g., Chlamydomonas, Chlorella) are being engineered for lipid production for biodiesel. Improving photosynthetic efficiency (reducing photorespiration, improving light utilization) is a key strategy for biofuel cost reduction.

6. Climate Change and Photosynthesis

• Rising $CO_{2}$ (420 → projected 560 ppm): Benefits C3 plants more than C4 ($CO_{2}$ fertilisation effect)
• Rising temperatures: Increases photorespiration in C3 plants; benefits C4 plants
• Shifting climate zones may alter crop distribution, favouring C4 expansion into higher latitudes
• CAM plants (succulents) will likely expand as droughts become more frequent