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Studies on Asparagus
The rate of respiration and the concentration of sucrose,
glucose, and fructose were measured along the length of intact asparagus (Asparagus
officinalis L. cv. Giant Jersey) spears during a 23 d of storage at 0 oC.
Carbon dioxide production at each of five positions along the spear was initially high,
but underwent a rapid and extensive decline within the first 24 h after harvest, with the
rate of decline proceeding more slowly thereafter. The respiration rate was highest at the
tip (section 1), decreasing as the distance from the tip increased (section 2 through 5,
with the section # 5 being more basal). Initially the respiration rate of the tip was
approximately four times that of the base, but after 23 d at 0 oC, the
respiration rate of the tip was only twice that of the base. Sugar levels were measured in
sections 1 through 4. Sugar levels declined with time, but increased, unlike respiration,
with distance from the tip. Sucrose underwent a rapid decline within the first 24 h of
storage in the tip, and in sections 3 and 4. Sucrose depletion was most extensive in the
tip, reaching more than 95% by day 23. Glucose underwent the most rapid decline initial in
section 2. The relative higher rate of glucose depletion in section 2 may have being to
support respiration and cell wall biosynthesis. The sucrose/fructose and sucrose/glucose
ratios decreased for the tip and increased for the section 2 with storage duration,
suggesting that sugars were translocated from lower sections as sugars in the spear tip
were depleted. After 7 d storage, the rate of carbon used for respiration in the tip
outstripped carbon loss due to sugars depletion, suggesting that hexoses were imported
from more basipetal locations and used to support respiration. RESPIRATORY AND FERMENTATIVE METABOLISMS IN ASPARAGUS TIPS UNDER LOW O2/HIGH CO2 ATMOSPHERES Asparagus (Asparagus officinalis L.) spears were
placed in modified atmosphere packages contained in ventilated glass jars at 1 °C. This
system was used to generate a range of O2 levels (0.16 to 16 kPa) in
combination with four CO2 levels (0, 5, 10 and 20 kPa). CO2
production, O2 uptake, RQ, soluble sugars, visual quality, ethanol,
acetaldehyde, lactate, and lactic dehydrogenase (LDH), pyruvate decarboxylase (PDC), and
alcohol dehydrogenase (ADH) activities were measured. The respiration rate dropped as O2
declined. The fermentation threshold was approximately 1 kPa O2 for the 0 kPa
CO2 treatment and increased as CO2 levels increased. The RQ was
dependent on both O2 and CO2 levels. The RQ increased as O2
declined below 2 kPa; this response was enhanced by CO2 partial pressure.
Ethanol and acetaldehyde accumulated in the packages at low O2 partial
pressures, increasing in concentration as RQ increased. The level of sucrose declined
markedly, relative to harvest, in the 6 days required for the packages to reach steady
state O2. Sucrose, glucose and fructose contents were highest for spears
exposed to 5 and 10 kPa CO2 at O2 partial pressures greater than 2
kPa. Lactate tended to accumulate at O2 levels below between 2 kPa relative to
higher O2 levels. A CO2 partial pressure of 20 kPa brought about a
marked elevation in lactate levels at O2 partialpressures greater than 4 kPa.
Alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC) activities were increased as
the level of CO2 increased for O2 levels above 4 kPa. ADH activity
increased at O2 levels below 1 kPa. Lactate dehydrogenase (LDH) activity was
lower than that of ADH. At O2 levels greater than 4 kPa, LDH activity increased
as CO2 partial pressure increased. ADH activity was greatest for all CO2
treatments at O2 partial pressures between 0.5 and 2 kPa. Visual quality tended
to increase as O2 level increased. Visual quality tended to be reduced by CO2
for any given O2 level. The visual quality of spears was maintained best in
response to at 5 kPa CO2 followed, in order, by 0, 10 and 20 kPa CO2.
This study was performed to determine the effects of low O2
and high CO2 atmospheres on sucrose-metabolizing enzymes and glycolytic
metabolism in asparagus (Asparagus officinalis L.) spear tips. Spear tips were
stored at 1 °C with 12 different O2 partial pressures of starting from
approximately 0.16 to16 kPa in combination with CO2 treatments of 0, 5, 10, and
20 kPa using a system combining modified atmosphere packages in flow-through containers.
Glycolytic intermediate levels and enzyme activities were examined in spear tips at
harvest time and once steady state atmospheres were reached (approximately 6 days). From
the sugar-metabolizing enzymes, bound acid invertase (BAI) followed by sucrose synthase
(SS) had higher activity after 6 days of storage indicating that sucrose is degraded
mostly in the apoplast. With the exception of PYR and PEP, which sequentially increased
for O2 levels below 2 kPa, O2-deficient range caused a marked
reduction in sugars and glycolytic intermediates relative to spears at harvest, a trend
that seemed to be independent of CO2 partial pressures. After 6 days storage at
O2 levels above 2 kPa, G6P and F6P contents increased relative to levels
present in the tissues at harvest. The effects of CO2 were variable, but the
same level of O2 combined with 20 kPa CO2 caused a 50% reduction in
G6P relative to the harvest and a 3-fold decrease when compared to spears placed at 16 kPa
O2 in the absence of CO2. Low O2 apparently enhanced the
interconversion of PEP to PYR and F6P to F1,6P2 relative to high O2.
Higher CO2 enhanced the interconversion of PEP to PYR, but only at lower O2.
Asparagus spears tips seems to be differently affected by O2 and CO2.
Low O2 seemed to have a more marked effect on increasing the activity of
pyruvate kinase as compared to phosphofructokinases. On the other hand, high CO2,
besides having an effect on PK, seems to have a larger effect on the conversion of F6P to
F1,6P2., at lower O2-deficient range, which may account for the
acceleration of glycolysis observed under those conditions. This study was performed to determine the effects of low O2
and high CO2 atmospheres on phosphate and adenylate metabolism in asparagus (Asparagus
officinalis L.) spear tips. Spears were stored at 1 °C with 12 different O2
partial pressure treatments starting from approximately 0.16 to 16 kPa in combination with
CO2 partial pressures of 0, 5, 10, and 20 kPa using modified atmosphere
packaging combined with a flow-through system. Pyrophosphate (PPi), inorganic phosphate
(Pi), ATP, ADP, and AMP contents were measured at harvest and after steady state
atmospheres were reached (approximately 6 days). When spears in packages at 16 kPa O2
were compared to spears at harvest, relatively little change occurred in adenylate or
phosphate pools. PPi and ATP contents decreased as the O2 partial pressure
declined below 16 kPa O2. The most rapid rate of decline occurred as the O2
partial pressure declined below the fermentation threshold (approximately 1 kPa O2).
The decrease in Ppi and ATP with declining O2 was paralleled by a dramatic
increase in Pi, ADP, and AMP. After 6 d storage at O2 levels below 2 kPa, Pi,
ADP, and AMP contents increased about five-fold relative to levels present at harvest. CO2
treatments had a large influence on metabolite content. In general, as CO2
increased, PPi and ATP decreased, while Pi, ADP and AMP increased. Within the range of gas
combinations tested, O2 had a relatively greater effect than CO2.
The adenylate energy charge (AEC) declined with a decline in the O2 partial
pressure, declining most rapidly below 2 kPa O2. Low O2 reduced AEC
relative to high O2. Increasing CO2 partial pressure reduced AEC.
However, the effect of CO2 on AEC was not evident at lower O2.
Decreases in PPi, ATP, and AEC, and parallel increases in Pi, ADP, and AMP with decreasing
O2 and increasing CO2 suggest impairment of oxidative
phosphorylation and a non-sustaining carbon metabolism, which may limit asparagus spear
survival under O2-deficient conditions. Randolph M. Beaudry |
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