PEP carboxylase shows very little discrimination so C 3 and C 4 plants have different carbon isotope ratios. The ratio can be used to determine when C 4 plants became abundant on the planet. The C 4 pathway and plants with it were favored, especially in grasslands. Fig 9.13 and 9.14. Sunlight imposes a large heat load on leaves, which they must dissipate.
Sensible heat is lost by air circulation around leaf.
Evaporative heat loss occurs using heat energy to turn aqueous water to vapor. Where water is scarce and stomates are closed, sensible heat loss is the major method. Some plants emit the hydrocarbon isoprene from their leaves when T and light are high. It has been hypothesized that isoprene stabilizes photosynthetic membranes and reduces high T damage. Quantum Yield and Temperature.