Abstract:
Young soybean plants (Glycine ~. L. cultivar
Harosoy '63), grown under controlled conditions, were
exposed to gamma radiation on a single occasion. One hour
following exposure to 3,750 rads, the mature trifoliate
leaf of the soybean plant was isolated in a closed system
and permitted to photoassimilate approximately 1-5 pCi of
14C02 for 15 minutes. After an additional 45 minute-period,
the plant was sacrificed and the magnitude of translocation
and distribution pattern of 14C determined.
In the non-irradiated plants 18~ of the total 14C
recovered was outside the fed leaf blades and of this
translocated 14c, 28~ was above the node of the fed leaf,
38~ in the stem below the node, 28~ in the roots and 7~ in
the petiole. As well, in the irradiated plants, a smaller
per cent (6~) of the total 14 C recovered was exported out of
the source leaf blades. Of this translocated 14c , a smaller
per cent (20~) was found in the apical region above the node
of the source leaf and a higher per cent (45~) was recovered
from the stem below the node and in the petiole (11~). The
per cent of exported 14 C recovered from the root was unaffected
by the radiation. Replacement of the shoot apex
with 20 ppm IAA immediately following irradiation, only
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partially increased the magnitude of translocation but did
completely restore the pattern of distribution to that
observed in the non-irradiated plants.
From supplementary studies showing a radiationinduced
reduction of photosynthetic rates in the source leaf
and a reduction of the cumulative stem and leaf lengths in
the apical sink region, the observed effects of radiation on
the translocation process have been correlated to damage
incurred by the source and sink regions.
These data suggest that the reduction in the
magnitude of translocation is the result of damage to both
the source and sink regions rather than the phloem conducting
tissue itself, whereas the change in the pattern of translocation
is probably the result of a reduced rate of
14C-assimilate movement caused by a radiation-induced
decrease of sink metabolism, especially the decrease in the
metabolism of the apical sink.
