Influence of Carbon Source, Oxygen Concentration, Light Intensity, and Temperature on¹³C/¹²C Ratios in Plant Tissues

Isotopic differences in source carbon are reflected in the 13C/12C ratios of plant tissues; thus, greenhouse- or growth-chamber-grown plants usually have less 13C than field-grown plants of the same species due to recycling of respired CO2. Sea grasses grown in artificial seawater under greenhouse conditions reflected a 5-9???? depletion in 13C relative to the same species from Redfish Bay. Changes in the temperature or light intensity at which plants grew also resulted in changes in isotopic ratios. In general, isotopic fractionation was less at optimal growth conditions for a given ecotype. As deviation from optimal conditions became great, the isotopic fractionation increased with a limit of 4 or 5 per mil. Environmental parameters did not seem to transform C4 plants into C3 plants, and vice versa. Xanthium populations from various worldwide localities and hybrids between the populations, compared for differences in response to variations in growth conditions, presented an isotopic response to temperature that was not linear. Genetic control of photosynthetic physiology seemed precise enough to be detected by the isotopic technique.