PHOTOSYNTHETIC CHARACTERISTICS OF C3‐C4 INTERMEDIATE FLAVERIA FLORIDANA (ASTERACEAE) IN NATURAL HABITATS: EVIDENCE OF ADVANTAGES TO C3‐C4 PHOTOSYNTHESIS AT HIGH LEAF TEMPERATURES

Abstract

Measurements of leaf gas exchange were conducted in situ for the C3‐C4 intermediate plant Flaveria floridana. Leaves exhibited measurable CO2 assimilation at atmospheric CO2 concentrations as low as 20 μmol/mol. This result demonstrates that the low CO2 compensation points observed in past studies of greenhouse‐grown C3‐C4 intermediate plants also exist in plants growing in their natural habitat. Photosynthesis rates in F. floridana were near their maximum at intercellular CO2 concentrations as low as 112 μmol/mol. The existence of near‐maximum photosynthesis rates at such low intercellular CO2 concentrations is interpreted as evidence for the existence of a CO2‐concentrating mechanism in F. floridana. Such a mechanism would also explain the observed lack of response in photosynthesis rates to reductions in stomatal conductance and intercellular CO2 concentration as the leaf‐to‐air water vapor concentration gradient is increased. Photosynthetic rates were relatively high at leaf temperatures between 35 and 40 C, compared to most C3 plants. At midday during May, when leaf temperatures were between 35 and 42 C, F. floridana leaves exhibited photosynthesis rates that were four times higher than a sympatric C3 species (Eustoma exaltatum) of similar growth form and ecological habit. The high photosynthesis rates at high leaf temperatures in F. floridana were not due to higher leaf nitrogen contents, but rather to its reduced rate of photorespiration. These results confirm that C3‐C4 intermediate photosynthesis can provide plants with an advantage at high leaf temperatures, compared to C3 plants.