74
Ν. G. PON
seems to be generally one of an increase. Thus during the pollen germination of pine seeds, the anaerobic pathway is partly supplemented by
the aerobic hexose monophosphate pathway (486). The activity of the
TPN
+
-reducing system, linked with pentose phosphate cycle substrates,
is also highest in extracts of seedlings and young leaves (473). (At midstalk or below, in the mature plant, no such activity is present.) In the
growing pea leaves, the 6-PG DH activity increases sharply from germination to a peak value and then decreases to a low value (468, 490).
Since the system utilizing ribose-5-phosphate also rises with increasing
time of germination, the conclusion was reached that the EMP pathway
plays a major role in glucose metabolism in the embryonic tissue, but the
pentose phosphate cycle increases its contribution with increasing age
(490). More or less the same findings were obtained from the aging of
mung-bean seedlings (488); i.e., the amounts and specific activities of
both G-6-P DH and 6-PG DH rise with increasing germination time up
to 48 hours. Seventy-two hours afterward, both activities decrease until,
after 96 hours, the extract of the mung-bean seedlings is completely
devoid of the dehydrogenases. This last result is in contradiction with
the conclusions reached by other investigators (477, 490).
Dichlorophenoxyacetic acid (2,4-D) affects the metabolism of glucose differently in different species. For example, in the bean stem
tissues, treatment by 2,4-D results in an increased participation of the
EMP pathway (477). On the other hand, in root tips of pea, corn, and
oat seedlings, the effect of 2,4-D is to increase the amount of glucose
catabolized via the pentose phosphate cycle (483, 485). Sorghum seedlings also give the same response to 2,4-D treatment (487).
Studies with fluoride-sensitive species of Chenopodium and Polygonum showed that the fluoride-damaged leaves had markedly lower
C-6:C-l ratio of the respiratory C0 2 derived from glucose-6- and -1-C
14
than that of the control (undamaged) leaves (476). The authors attributed the decreased C-6:C-l to the inhibition of enolase by fluoride ion;
however, in some cases, the rates of oxygen uptake and of the oxidation
of glucose-6-C
14 actually increased in HF fumigated Chenopodium and
in fluoride treated Polygonum. Fluoride added to potatoes susceptible
to Phytophthora infestans inactivates a major portion of the total respiration (491). Nevetheless, a high level of oxidation still occurs by way
of the shunt mechanism. It appears that the respiration of hexose monophosphates participates in the protective reaction against infection. In
this last regard, Daly et al. found that higher plants infected by obligate
plant parasites have a characteristically high rate of respiration and a
low C-6:C-l ratio (492). They concluded that nearly all the increased
respiration is accommodated by the hexose monophosphate pathway.
Ν. G. PON
seems to be generally one of an increase. Thus during the pollen germination of pine seeds, the anaerobic pathway is partly supplemented by
the aerobic hexose monophosphate pathway (486). The activity of the
TPN
+
-reducing system, linked with pentose phosphate cycle substrates,
is also highest in extracts of seedlings and young leaves (473). (At midstalk or below, in the mature plant, no such activity is present.) In the
growing pea leaves, the 6-PG DH activity increases sharply from germination to a peak value and then decreases to a low value (468, 490).
Since the system utilizing ribose-5-phosphate also rises with increasing
time of germination, the conclusion was reached that the EMP pathway
plays a major role in glucose metabolism in the embryonic tissue, but the
pentose phosphate cycle increases its contribution with increasing age
(490). More or less the same findings were obtained from the aging of
mung-bean seedlings (488); i.e., the amounts and specific activities of
both G-6-P DH and 6-PG DH rise with increasing germination time up
to 48 hours. Seventy-two hours afterward, both activities decrease until,
after 96 hours, the extract of the mung-bean seedlings is completely
devoid of the dehydrogenases. This last result is in contradiction with
the conclusions reached by other investigators (477, 490).
Dichlorophenoxyacetic acid (2,4-D) affects the metabolism of glucose differently in different species. For example, in the bean stem
tissues, treatment by 2,4-D results in an increased participation of the
EMP pathway (477). On the other hand, in root tips of pea, corn, and
oat seedlings, the effect of 2,4-D is to increase the amount of glucose
catabolized via the pentose phosphate cycle (483, 485). Sorghum seedlings also give the same response to 2,4-D treatment (487).
Studies with fluoride-sensitive species of Chenopodium and Polygonum showed that the fluoride-damaged leaves had markedly lower
C-6:C-l ratio of the respiratory C0 2 derived from glucose-6- and -1-C
14
than that of the control (undamaged) leaves (476). The authors attributed the decreased C-6:C-l to the inhibition of enolase by fluoride ion;
however, in some cases, the rates of oxygen uptake and of the oxidation
of glucose-6-C
14 actually increased in HF fumigated Chenopodium and
in fluoride treated Polygonum. Fluoride added to potatoes susceptible
to Phytophthora infestans inactivates a major portion of the total respiration (491). Nevetheless, a high level of oxidation still occurs by way
of the shunt mechanism. It appears that the respiration of hexose monophosphates participates in the protective reaction against infection. In
this last regard, Daly et al. found that higher plants infected by obligate
plant parasites have a characteristically high rate of respiration and a
low C-6:C-l ratio (492). They concluded that nearly all the increased
respiration is accommodated by the hexose monophosphate pathway.