PHOTOSYNTHETIC RATE RESPONSES OF GINKGO BILOBA (VAR.: MAGYAR) TREES TO COMBINED ATMOSPHERIC ENRICHMENT OF CO2, O2 AND INCREASED ATMOSPHERIC PRESSURE

Geologic and paleontologic evidence suggests that the Earth’s atmosphere may have been enriched in oxygen and carbon dioxide during the Cretaceous Period. The O₂ enrichment may have elevated atmospheric pressure and, combined with CO₂ enrichment, greatly affected terrestrial ecosystems. A response toward higher primary productivity would constitute a hypothetical solution to the paradox of unusually large herbivorous dinosaurs. By measuring instantaneous net photosynthetic rates, we tested the hypothesis that Ginkgo biloba trees grown under Cretaceous atmospheric conditions (2000 ppm CO₂ and 30% O₂) had higher productivity than those grown under modern atmospheric conditions (370 ppm CO₂ and 21% O₂). Five different atmospheric compositions were created within a hyperbaric chamber:

1. a control (modern conditions with no atmospheric alteration)

2. increase in air pressure to 1.25 ATA only

3. 1.25 ATA and CO₂ to 2000 ppm

4. 1.25 ATA and O₂ to 30%

5. 1.25 ATA and enrichment of both CO₂ and O₂ to 2000 ppm and 30%, respectively.

Six 70-100 cm tall ginkgoes were acclimated to each composition for twenty-four hours. Following the acclimation period, we assessed net photosynthetic rates using the Li-Cor 6400 Portable Photosynthesis System along a CO₂ concentration series (370 ppm to 2000 ppm) under 450 µmol/m²/s PAR light intensity. Our results show that photosynthetic rates increased linearly in response to enriched CO₂ up to 1650 ppm for all acclimation conditions. Elevated O₂ significantly reduced CO₂ assimilation rates at all CO₂ concentrations relative to the control curve. However, even with the oxygen inhibition, at highest CO₂ concentrations photosynthesis was significantly higher than modern rates. Hence, the data show a 200- 250% increase in net photosynthesis between modern and enriched Cretaceous conditions. These results are consistent with the hypothesis that Cretaceous atmospheres stimulated higher plant productivity through elevated photosynthetic rates.