Abstract:
Since previous investigations have shown that low levels of ionizing
radiation can induce a reduction in the rates of apparent photosynthesis
and in the magnitude of photoassimilated l4C exported out of a leaf, the
present studies were designed and conducted to determine the relationship,
if any, between the radiation effects on these two physiological
processes. The experiments were particularly designed to determine if the
radiation-induced reduction in export is the result of the reduction in
photosynthesis and hence availability of materials for translocation or
the result of a reduction in the amount of energy available for the vein
loading process.
This study has shown that the radiation-induced reduction in l4C
export out of a leaf is likely related to a loss of energy available for
the vein loading process rather than a reduction in the supply of materials
available for export due to reduced C02 uptake. The process of photophosphorylation
was shown to be reduced by exposure to radiation to an extent
similar to the reduction in the export of l4C which was also observed.
Both of these processes returned to their pre-irradiation rates 120
minutes following radiatruon exposure. The rate of photosynthetic C02 uptake
was also reduced by radiation exposur~ howeve~ this process did not
return to the control level nor was the extent of reduction as large as
observed for photophosphorylation and photoassimilate export. The observed
relationship between the reductions of export and photoph~sphorylation
pointed to the utilization of photosynthetically produced ATP in the vein
loading process.
The radiation-induced reduction in the export of l4C was observed at
the highest light intensity used in this study which would also imply the
involvement of the photophosphorylation process as an energy seurce for vein
loading. The lack of radiation-induced reduction in export at low light intensities was interpreted as being due to the utilization of respiratory
derived ATP, a process known to be insensitive to radiation at the levels
used in this study, as the energy source for the vein loading process.
Studies using plants not stressed by radiation showed that there was an
increase in export of 14C with higher light intensities.
In summary, the data has been interpreted as showing that at high
light intensities the ATP, produced by photophosphorylation, is available
for use in the vein loading process. The site of ATP utilization could
not be determined from the data obtained in this study but possible sites
have been indicated from the work done by other physiologists and are discussed
in the thesis.
