Respiration and Growth in Germinating Seeds
141
of irradiation damage by high dosages is via the respiratory system, or
that respiration reflects metabolic injury at a deeper level. A large body of
data indicates that the chromosome may be the primary site. Nevertheless,
damage at the chromosomal level must express itself at the metabolic level
before it can affect morphology or growth. The respiratory system may
be a key mechanism in this process. SKOOG [13], and GORDON and WEBER [4],
and others have shown that ionizing radiation affects auxin metabolism.
Auxin concentration, in turn, exerts a marked influence on the respiratory
rate. Our data do not indicate whether the effects of gamma-irradiation on
respiration reflect possible auxin destruction during or after irradiation or
are due to some more direct effect of irradiation on respiratory metabolism.
The fact of greatest interest here is that gamma-irradiation injury affects
respiration during imbibition in the same way as heat treatments and adverse storage conditions: It lowers respiratory rates and increases respiratory quotients.
The stimulation of plant growth by exposing seeds to low doses of
ionizing radiation has been reported by many investigators and was reviewed
recently [12]. Although in most cases the reported stimulations were small
and not always reproducible, SAX [12] concludes that the documentation
is sufficient to demonstrate growth stimulation by small doses of irradiation.
An interesting aspect of the present studies is the apparent stimulation in
seedling growth produced by the 5-krad treatments. Although the stimulations were small and not always statistically significant, they were observed
in all four species tested, and were apparent regardless of whether seedling
growth was measured as stem axis length or as seedling fresh weight.
In corn, we found that stimulation of seedling growth was preceded
by a stimulation in respiration (Table 1). In corn and sorghum, the 20-krad
irradiated seeds grew at about the same rate as the non-irradiated controls.
The respiration of these seeds, however, was similar to that of seeds given
the growth-promoting 5-krad treatments. Conversely, in radish, the growthpromoting 5-krad treatment had no apparent effect on respiration. Since
the effect of the irradiation treatments on respiration can be separated from
that on seedling growth, the growth- and respiration-stimulating mechanisms
are not identical.
Desiccation-induced dormanry of sorghum: Seeds of sorghum (Sorghum
vulgare Pers.) dried in a forced-air dryer from an initial moisture content of
12 % to either 10 or 7 % exhibited physiological dormancy [9]. Dormancy
was more marked in seeds dried to 7 % than to 10 % moisture, and was
more pronounced in germination at 15° or 20° than at 25°C. That drying
to a moisture content of 7 % did not cause permanent injury is evidenced
by normal germination at high temperatures and by reversal of dormancy.
Cutting through the integumentary membrane and slightly into the endosperm with a dissecting needle was highly effective in promoting the ger-
141
of irradiation damage by high dosages is via the respiratory system, or
that respiration reflects metabolic injury at a deeper level. A large body of
data indicates that the chromosome may be the primary site. Nevertheless,
damage at the chromosomal level must express itself at the metabolic level
before it can affect morphology or growth. The respiratory system may
be a key mechanism in this process. SKOOG [13], and GORDON and WEBER [4],
and others have shown that ionizing radiation affects auxin metabolism.
Auxin concentration, in turn, exerts a marked influence on the respiratory
rate. Our data do not indicate whether the effects of gamma-irradiation on
respiration reflect possible auxin destruction during or after irradiation or
are due to some more direct effect of irradiation on respiratory metabolism.
The fact of greatest interest here is that gamma-irradiation injury affects
respiration during imbibition in the same way as heat treatments and adverse storage conditions: It lowers respiratory rates and increases respiratory quotients.
The stimulation of plant growth by exposing seeds to low doses of
ionizing radiation has been reported by many investigators and was reviewed
recently [12]. Although in most cases the reported stimulations were small
and not always reproducible, SAX [12] concludes that the documentation
is sufficient to demonstrate growth stimulation by small doses of irradiation.
An interesting aspect of the present studies is the apparent stimulation in
seedling growth produced by the 5-krad treatments. Although the stimulations were small and not always statistically significant, they were observed
in all four species tested, and were apparent regardless of whether seedling
growth was measured as stem axis length or as seedling fresh weight.
In corn, we found that stimulation of seedling growth was preceded
by a stimulation in respiration (Table 1). In corn and sorghum, the 20-krad
irradiated seeds grew at about the same rate as the non-irradiated controls.
The respiration of these seeds, however, was similar to that of seeds given
the growth-promoting 5-krad treatments. Conversely, in radish, the growthpromoting 5-krad treatment had no apparent effect on respiration. Since
the effect of the irradiation treatments on respiration can be separated from
that on seedling growth, the growth- and respiration-stimulating mechanisms
are not identical.
Desiccation-induced dormanry of sorghum: Seeds of sorghum (Sorghum
vulgare Pers.) dried in a forced-air dryer from an initial moisture content of
12 % to either 10 or 7 % exhibited physiological dormancy [9]. Dormancy
was more marked in seeds dried to 7 % than to 10 % moisture, and was
more pronounced in germination at 15° or 20° than at 25°C. That drying
to a moisture content of 7 % did not cause permanent injury is evidenced
by normal germination at high temperatures and by reversal of dormancy.
Cutting through the integumentary membrane and slightly into the endosperm with a dissecting needle was highly effective in promoting the ger-