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L. W. WOODSTOCK
mination of dormant seeds at 15° and 20°C. The site of the block in the
germinative process thus appears to be the integumentary membrane.
Changes in the physical nature of the membrane caused by desiccation
which result in restriction of gas exchange may account for the dormancy.
Respiration during imbibition of seeds dried to 10 or 7 % moisture
content was inhibited [9]. The inhibition was especially evident when respiration was measured in an atmosphere of 100 % oxygen. Under such conditions, differences between 7 %-moisture and control seeds were detected
within 2 hrs after the start of imbibition. The dormant seeds also exhibited
higher RQ values than non-dormant controls.
In seeds from a given treatment, respiration increased linearly with
fresh weight during the first 6 hrs after planting [9]. This suggests that in
both 7- and 12 %-seed lots moisture was limiting for respiration during
this period. The two curves show, however, that at anyone moisture
content, the respiration of the previously desiccated seeds is higher than
that of the controls. The initially lower moisture content of the dried seeds
cannot by itself, therefore, account for inhibited respiration during imbibition. More probably, the drying treatments caused physical changes in
the seeds which induced dormancy. The dormancy, so induced, is reflected
by changes in metabolism (including respiration) upon reactivation of the
enzymes during the early stages of imbibition.
The effect of chilling treatments on the respiration and growth of lima bean seeds:
In lima beans, imbibition at 5-15 °C causes an immediate and pronounced
inhibition in respiration and a subsequent inhibition in seedling growth
[18]. Exposure to cold water after imbibition does not cause injury. Chilling
injury was roughly proportional to the inhibition of respiration and when
respiratory rates at 4 hrs after the beginning of imbibition were compared
to seedling axis lengths after 5 days' germination, positive correlations
were found. Bleached seeds were more susceptible to chilling injury than
normal green seeds. Respiration of bleached was lower than that of green
seeds only during the first 2 hrs of germination, i.e., during the temperaturesensitive period. These results, and observations by POLLOCK [11] that
anaerobic conditions augment the injury, suggest that respiratory rates
during imbibition may be a factor in determining susceptibility to chilling
injury. In lima beans, the surface area of the seed increases about 4-fold
during 2 hrs of imbibition. Subsequent rates of increase in surface area
are lower and are limited to the elongating regions of the embryonic axis.
Marked leaching of organic materials from the seed accompanies chilling
injury [11]. The data indicate the following explanation for chilling injury
in lima beans: During low-temperature imbibition, the respiratory rate is
too low to supply the energy needed for orderly physiological stretching of
the membranes. Under these conditions, membranes are physically torn
as cells expand under the force of imbibitional water uptake. Extensive
L. W. WOODSTOCK
mination of dormant seeds at 15° and 20°C. The site of the block in the
germinative process thus appears to be the integumentary membrane.
Changes in the physical nature of the membrane caused by desiccation
which result in restriction of gas exchange may account for the dormancy.
Respiration during imbibition of seeds dried to 10 or 7 % moisture
content was inhibited [9]. The inhibition was especially evident when respiration was measured in an atmosphere of 100 % oxygen. Under such conditions, differences between 7 %-moisture and control seeds were detected
within 2 hrs after the start of imbibition. The dormant seeds also exhibited
higher RQ values than non-dormant controls.
In seeds from a given treatment, respiration increased linearly with
fresh weight during the first 6 hrs after planting [9]. This suggests that in
both 7- and 12 %-seed lots moisture was limiting for respiration during
this period. The two curves show, however, that at anyone moisture
content, the respiration of the previously desiccated seeds is higher than
that of the controls. The initially lower moisture content of the dried seeds
cannot by itself, therefore, account for inhibited respiration during imbibition. More probably, the drying treatments caused physical changes in
the seeds which induced dormancy. The dormancy, so induced, is reflected
by changes in metabolism (including respiration) upon reactivation of the
enzymes during the early stages of imbibition.
The effect of chilling treatments on the respiration and growth of lima bean seeds:
In lima beans, imbibition at 5-15 °C causes an immediate and pronounced
inhibition in respiration and a subsequent inhibition in seedling growth
[18]. Exposure to cold water after imbibition does not cause injury. Chilling
injury was roughly proportional to the inhibition of respiration and when
respiratory rates at 4 hrs after the beginning of imbibition were compared
to seedling axis lengths after 5 days' germination, positive correlations
were found. Bleached seeds were more susceptible to chilling injury than
normal green seeds. Respiration of bleached was lower than that of green
seeds only during the first 2 hrs of germination, i.e., during the temperaturesensitive period. These results, and observations by POLLOCK [11] that
anaerobic conditions augment the injury, suggest that respiratory rates
during imbibition may be a factor in determining susceptibility to chilling
injury. In lima beans, the surface area of the seed increases about 4-fold
during 2 hrs of imbibition. Subsequent rates of increase in surface area
are lower and are limited to the elongating regions of the embryonic axis.
Marked leaching of organic materials from the seed accompanies chilling
injury [11]. The data indicate the following explanation for chilling injury
in lima beans: During low-temperature imbibition, the respiratory rate is
too low to supply the energy needed for orderly physiological stretching of
the membranes. Under these conditions, membranes are physically torn
as cells expand under the force of imbibitional water uptake. Extensive