EFFECTS OF IONIZING RADIATION ON RATES OF CO2 EXCHANGE OF PINE SEEDLINGS.

Exposure to ionizing radiation is known to modify rates of metabolism of higher plants in various ways. Zill and Tolbert (1) showed that acute exposures to about 100 kR of y-radiation reduced rates of C02 fixation in wheat seedlings by 75 %. Field studies of CO2 exchange rates of Pinus rigida trees in the Brookhaven Irradiated Forest (2) showed reduction of rates of CO2 absorption under chronic irradiation at much lower total exposures than the acute exposures used by Zill and Tolbert. About 6000 R at 30 R/day reduced net photosynthesis to zero. The more detailed field studies of metabolic rates in this forest reported by Woodwell and Hadley (3) confirmed the depression of net photosynthesis by total exposures to 4000 to 7000 R at rates of less than 10 R/day and showed in addition that the patterns of C02 exchange of trees are modified by irradiation. The objective of the study reported here was to examine the effects of acute, rather than chronic, exposure to ionizing radiation on the pattern of CO2 exchange of pine seedlings under controlled conditions, to add to the previous field observations of chronically irradiated trees of Bourdeau and Woodwell (2) and Woodwell and Hadley (3). The experiments reported in this study show that acute exposures to as little as 1250 R depress rates of net photosynthesis of pine leaves without affecting rates of C02 evolution in the dark. Rates of C02 evolution by stems, however, were initially stimulated, but ultimately depressed by irradiation.