Photosynthesis and photophosphorylation in radiation-stressed soybean plants and the relation of these processes to photoassimilate export

Abstract

Soybean plants 18–21 days post initiation of germination received acute doses ranging from 0.06 to 5kR of gamma radiation from a 60Co source. Rates of photosynthetic CO 2 uptake at 1600 and 2800 ft-c were measured by infrared gas analysis before, and 15–75 min after irradiation. The magnitude of translocation of photoassimilated 14CO 2 and the rates of 32P i incorporation into ATP by chloroplasts isolated from the leaves was determined from control and irradiated plants. Fifteen minutes following a dose of 4.1 kR, plants illuminated at 2800 ft-c showed a 37% reduction in the magnitude of 14C export and a 47% reduction in the rate of photophosphorylation. At 2 hr post-irradiation, neither process was significantly different in magnitude from the non-irradiated controls. When rates of photosynthetic CO 2 uptake and 14C export were measured in irradiated plants illuminated at 800, 1600 or 2800 ft-c, significant reductions were only observed in the irradiated plants illuminated at 2800 ft-c. In these plants, the extent of export was reduced by 27%, a value considerably higher than the observed 5% reduction in CO 2 uptake. In addition to providing further information regarding the radiosensitivities of the processes of photosynthesis and translocation, the data have been interpreted as evidence that ATP, produced by photophosphorylation, can contribute to the vein loading component of translocation.