Abstract:
Low levels of ionizing radiation induce two
translocation responses in soybean: a reduction in
photoassimilate export from leaves and a change in the
distribution pattern of exported photoassimilate within
the plant. In this investigation these responses have
been further studied specifically to ascertain the site
of radiation damage and to better understand the
physiological responses observed.
Experimentally the primary data was obtained from
studies in which a mature trifoliate leaf of a young soybean
plant (Glycine ~ L. cultivar Harosoy '63) is
isolated in a closed transparent chamber and allowed to
photoassimilate 14C02 for 15 minutes. This is followed
by an additional 45 ~_il'1;ute period before the plant is
sectl.o ne d an d 14 C-ra dl' oactl.v.l ty d eterml. ne d'l n a 11 parts.
Such 14c data provides one with the magnitude and distribution
pattern of translocation. Further analyses were
conducted to determine the relative levels of the major
photosynthetic products using the techniques of paper
chromatography and autoradiography. Since differences between control and irradiated
P 1 ants were not 0 b serve d l' n t h e par tl't"lo nlng 0 f 14 C
between the 80% ethanol-soluble and -insoluble fractions
14 or in the relative amounts of C-products of photosynthesis,
the reduction in export in irradiated plants
is not likely due to reduced availability of translocatable
materials.
Data presented in this thesis shows that photoassimilate
export was not affected by gamma radiation
until a threshold dose between 2.0 and 3.0 krads was
reached. It was also observed that radiation-induced
damage to the export process was capable of recovery in
a period of 1 to 2 hours provided high light intensity
was supplied.
In contrast, the distribution pattern was shown
to be extremely radiosensitive with a low threshold dose
between .25 and .49 krads. Although this process was also
capable of recovery,lt" occurred much earlier and was
followed by a secondary effect which lasted at least for
the duration of the experiments.
The data presented in this thesis is interpreted
to suggest that the sites of radiation action for the two
translocation responses are different. In regards to
photoassimilate export, the site of action of ionizing
radiation is the leaf, quite possibly the process of photophosphorylation which may provide energy directly
for phloem loading and for membrane integrity of the
phloem tissue* In regards to the pattern of distribution
of exported photoassimilate, the site is likely the apical
sink, possibly the result of changes of levels of endogenous
hormones. By the selection of radiation exposure dose and
time post-irradiation, it is possible to affect independently
these two processes suggesting that each may be
regulated independent of the other and involves a distinct
site.
