Plant Conditions and Photosynthetic Adaptations

Hot and Bright Conditions (>30°C, High Light)

• C3 plants: Severely disadvantaged. Photorespiration increases (higher $O_{2}$:C$O_{2}$ ratio, increased RuBisCO oxygenase activity). Net photosynthesis may fall by 25–40%.
• C4 plants: Thrive. PEP carboxylase's high C$O_{2}$ affinity and Kranz anatomy maintain high [C$O_{2}$] around RuBisCO, suppressing oxygenase activity. Light saturation point = full sunlight.
• Response: C4 plants dominate tropical grasslands (maize, sugarcane, sorghum); C3 plants are outcompeted in these conditions.

Arid / Water-Deficit Conditions

• C3 plants: Stomata close to prevent water loss → C$O_{2}$ supply decreases → photorespiration may increase.
• C4 plants: Higher water use efficiency (WUE) — more C$O_{2}$ fixed per water molecule transpired.
• CAM plants: Best adaptation — stomata closed during hot daytime (minimising transpiration); C$O_{2}$ collected at night as malic acid. Highest WUE of all photosynthesis types. Examples: cacti, Bryophyllum, Opuntia, pineapple.

Low C$O_{2}$ Conditions

• C3 plants: RuBisCO activity drops; photorespiration increases (oxygenase dominates).
• C4 plants: PEP carboxylase's high C$O_{2}$ affinity allows continued C$O_{2}$ fixation even at very low [C$O_{2}$]. C$O_{2}$ is concentrated in bundle sheath.

Deep Water (Low Light, Green Wavelengths)

• Red algae use phycoerythrin (absorbs green ~490–570 nm, the dominant wavelength at depth) to perform photosynthesis where Chl a/b would be inefficient.

Cool and Temperate Conditions

• C3 plants: Photorespiration is low at 20–25°C (higher C$O_{2}$ solubility, lower RuBisCO oxygenase activity). C3's lower ATP cost per C$O_{2}$ makes it efficient.
• C4 plants: Extra ATP cost of PEP regeneration is not offset sufficiently → C3 plants are more competitive.
• Most temperate crops are C3 (wheat, rice, potato, soybean).