HYPOBARIC STORAGE 0F CELERY AND CAULIFLOWER
SEEDLING TRANSPLANT

Thesis for the Degree of M. S.
MICHIGAN STATE UNIVERSITY
DOUGLAS JOSEPH IARDINE
1977

 

   
 

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ABSTRACT
HYPOBARIC STORAGE OF CELERY AND CAULIFLOWER
SEEDLING TRANSPLANTS
BY

Douglas Joseph Jardine

Storage of transplants requiring vernalization at temperatures
below 10°C induces flowering. Hypobaric ventilation during storage at
0°C of celery and cauliflower seedling transplants delayed floral
initiation. Storage for 2 weeks at 0°C at 25 mm-Hg prevented bolting of
celery (Utah 52-70) upon outplanting. Bolting became prevalent as the
storage duration was increased to 6 weeks particularly in air at
atmospheric pressure. Buttoning of two cauliflower varieties (Clou and
Self-Blanche) stored for 4 weeks at 0°C at 25 mmng was also prevented.
Partial Inhibition (Clou 62% and Self-Blanche 98%) was achieved by
storage for 4 weeks at 150 mm-Hg. Hypobaric storage for 4 weeks
decreased the survival percentage of celery to 96.9% and cauliflower to
91.7%. The loss of plants due to bolting and buttoning accounted for
the majority of yield decline. There appeared to be a slight but direct
deliterious effect of hypobaric storage which contributed to decreased
yields. It was not determined if the effect of hypobaric storage on
delaying floral initiation was due to enhanced diffusion of volatiles

from the plants or to the low P (ca. 0.5%) at 25 mmng total air

02

pressure.

HYPOBARIC STORAGE OF CELERY AND CAULIFLOWER

SEEDLING TRANSPLANTS

By

Douglas Joseph Jardine

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree of

MASTER OF SCIENCE

Department of Horticulture

1977

I dedicate this work to my grandfather
Raymond "Jim" Grobbel
who sparked my interest in the field of

agriculture so many years ago

ii

ACKNOWLEDGMENTS

The advice, assistance and encouragement of Drs. David R. Dilley
and Hugh C. Price are most gratefully acknowledged. Special thanks also
go to Dr. Clifford J. Pollard who served on the examining committee and
to Dr. Robert C. Herner who substituted for Dr. Price at the exam.

Special thanks go to Dr. Mikal E. Saltveit Jr. and‘Mr. Kent J.
Bradford for their help in the field and for answering my innumerable
questions during the preparation of this thesis.

Appreciation is extended to Herman and Bruce Scheur for the use of
their farm and the care they gave to the plots. Thanks also are extended
to Bill, Jerry and Dennis Grobbel for providing the plants and land for
the cauliflower experiments. A special thanks to my grandfather,

Mr. Raymond "Jim" Grobbel for taking such good care of the plots.

Finally, the author wishes to thank the Agricultural Systems
Division of the Union Carbide Corporation for supplying the celery plants

and for providing the grant under which this work was accomplished.

iii

TABLE OF CONTENTS
Page
LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . v
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . vi
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
LITERATURE REVIEW . . . . . . . . . . . . . . . . . . . . . . . . 2
CELERX . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
CAULIFLOWER . . . . . . . . . . . . . . . . . . . . . . . . . 7
HYPOBARIC STORAGE CONCEPTS . . . . . . . . . . . . . . . . . 11
MATERIALS AND METHODS . . . . . . . . . . . . . . . . . . . . . . 14

Celery . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Cauliflower . . . . . . . . . . . . . . . . . . . . . . . . . 18

 

RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . . . . . . 19

C618]?! 0 o o o o o o o o o o o o o o o o o o o o o o o o o o 19
Cauliflower . . . . . . . . . . . . . . . . . . . . . . . . . 28

 

SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . 36

LITERATURE CITED . . . . . . . . . . . . . . . . . . . . . . . . . 38

iv

LIST OF TABLES
Table Page

1 Effect of Storage Pressure and Duration at 0°C on Recovery
Ratings for Celery Transplants at Various Intervals After
Outplanting O O O O O O O 0 O O O O O O O O O O O O O O O O 20

2 Percentage Survival of Celery Transplants at Two Dates
After Outplanting as Influenced by Storage Pressure and
Duration at 0°C. . . . . . . . . . . . . . . . . . . . . . . 22

3 Effect of Storage Pressure and Duration at 0°C on Floral
Initiation in Celery . . . . . . . . . . . . . . . . . . . . 24

4 Celery Stalk Size Distribution and Trimming Waste 33
Influenced by Storage Pressure and Duration at 0 C . . . . . 29

5 Effect of Storage Pressure and Duration at 0°C on Percentage
Survival for Clou and Self-Blanche Cauliflower After
outplanting O O O O O O 0 O O O O O O O O O O O O O O O O O 31

6 Percentage of Cauliflower Plants Per Hectare Which Buttoned
as Influenced by Storage Pressure and Duration at 0 C . . . 33

7 The Effect of Storage Pressure on the Fresh Weight of
Cauliflower Plants Per Hectare . . . . . . . . . . . . . . . 35

8 The Effect of Storage Pressure on the Average Plant Weight
of Clou Cauliflower Plants . . . . . . . . . . . . . . . . . 35

LIST OF FIGURES
Figure Page

1 Schematic diagram of the hypobaric system used for the
storage of transplants . . . . . . . . . . . . . . . . . . l6

2 Effect of storage pressure and duration at 0°C on floral
initiation in celery . . . . . . . . . . . . . . . . . . . 23

3 Effect of storage pressure and duration at 0°C on total
yield of marketable celery. The yield does not include
the weight of bolters nor trimmings . . . . . . . . . . . 26

4 Effect of storage pressure and duration at 0°C on the
total number of marketable plants per hectare . . . . . . 27

5 Effect of storage pressure and duration at 0°C on
meristematic development of marketable celery plants . . . 30

vi

INTRODUCTION

The cost of raising celery transplants in greenhouses is becoming
increasingly expensive with energy and labor costs continuing to rise.
Growers may save both time and money by using plants grown out-of-doors
in California if seedlings could be raised and shipped to Michigan at a
reasonable cost. This would also free greenhouse space for other
valuable commodities such as bedding plants. Air freight is expensive
and shipping by land has its problems. Temperatures during transport
and storage warm enough to prevent seedstalk formation (e.g. 10°C) cause
excessive deterioration of the tissue during extended shipment periods.
Temperatures low enough to maintain the plants in good condition
(e.g. 0°C) cause vernalization and thus the plants form seedstalks
during the first season which makes them unmarketable.

Hypobaric storage is a relatively new process which has increasingly
diverse applications. The equipment is in development for transporting
perishable commodities over long distances under hypobaric conditions.
The purpose of this experiment was to determine if hypobaric ventilation
during storage would inhibit floral initiation of celery transplants
held at sufficiently low temperatures to cause vernalization for up to
6 weeks. Cauliflower, a crOp which responds to cold in a manner similar
to celery, was used to determine if the principle might be generally

applicable to other crops.

LITERATURE REVIEW

CELERY

Celery, Apium graveolens L., is second only to lettuce in importance
as a salad crop in value and popularity. A large part of the celery crop
grown in the United States is consumed raw, but considerable quantities
are processed in vegetable juices, soups, stews and other cooked products.

Celery is native to marshy lowlands and, according to Sturtevant
(63), its habitat extends from.Sweden southward to Egypt, and in Asia
even to the mountains of India. It grows wild in Tierra del Fuego,
California and New Zealand. The wild plant was probably first used for
medicinal purposes hundreds of years before it was used for food.

Although celery is a biennial plant, it is grown and cropped as an
annual. Celery plants, for an early crop in regions of short growing
seasons, are started in greenhouses, hotbeds or coldframes. The late
crop in such regions, and most of the celery grown in mild climates as
in California and Florida, is started in outdoor seedbeds. However,
direct seeding in the field is practiced in the coastal region of
California where the crop is grown to maturity under ideal climatic
conditions.

When seedlings are raised in greenhouses, two methods for starting
them may be used. The time of sowing is determined largely by the time

the crop is desired for use. The first method requires that seed be

drilled in rows or broadcast into flats or seedbeds. After 4 or 5 weeks,
they are set into the soil of the greenhouse or flats, spacing them

40 X_40 or 50 X 50 mm. Ten to 12 weeks usually elapse between sowing
and field transplanting. In the second method, plants are transplanted
directly from the seedbed to the field at 8 to 10 weeks.

Storage of seedlings most often consists of leaving them in the
seedbed or flats and retarding the growth rate by withholding water.
Pulled plants are generally not stored for more than a day or two and
this usually occurs when bad weather halts planting. Storage temperatures
of 0 to 10°C can be used for l to 2 days. Periods longer than 2 days
may cause the plants to bolt (66) while temperatures above 10°C will
cause excessive deterioration.

Bolting, or premature seeding as it is sometimes known, is the
development of flowers and seeds during the first year of growth by a
biennial plant. This is the result of vernalization during the early
stages of plant growth and development. Vernalization is the promotion
of flower formation by a period of low temperature and was observed and
described by several authors as early as the middle of the 19th century.
Lang (40) in a review in the Encyclopedia of Plant Physiology, pays
great tribute to the work of Gassner (29) and compares it to the
pioneering work of Garner and Allard (28) on photoperiodism. Vernal-
ization is unusual among biological phenomena in that its relation to
temperature is the reverse of that which holds for chemical reactions
in general (48).

Plants other than biennials often require a period of cold treat-
ment during their development before they can flower. These include

so-called winter annuals such as winter cereals and a certain number of

perennials. Many biennials, (e.g. cabbage), will remain in the
vegetative state for a period of years if protected from cold in winter
(40). Chailakhyan (15) points out that only temperate plants that
undergo winter conditions can be expected to have a vernalization
requirement, and these are likely to be long day plants.

Gregory and Purvis (32) demonstrated that vernalization is an
aerobic process and does not occur when plants or seeds are cooled in
an atmosphere of nitrogen. In the same work, they reported data
indicating that vernalization is a cumulative process. Sarkar (55) as
reported by Purvis (48), indicates that plants with an obligate cold
requirement are not as a rule sensitive to thermoinduction before they
have made some minimum.amount of growth. Purvis (48) reports that just
as photoperiodic reactions begin from the transformation of photo-
synthetic products i.e., sugars and other reduced products, thermo-
periodic reactions also begin with sugars stored in seeds or plant tissue.
It has been suggested that the "cold requirement" represents a deficit
of some precursor which is requisite for the photoperiodic response. In
plants with little or no cold requirement, this precursor is already
present, or is readily formed at normal temperatures (48). Denffer (21),
as reported by Lang (40), proposed that the role of temperature was to
prevent the formation of an inhibitor of flowering. Melchers (44), as
reported by Chailakhyan (15), offers the possibility of a chemical
known as vernalin which is produced under conditions of low temperature
and high sugar. It should be noted that none of these precursors,
inhibitors or chemicals have ever been isolated.

The bolting of celery is a problem which has continually plagued

growers and, in some years, caused the loss of entire crops. Bailey

(5) was one of the first to make mention of this problem. Walker (69)
offered the suggestion that perhaps bolting resulted when plants
remained in the seedbed too long. Reid (49), Whipple (70) and Abell (2)
also addressed themselves to the problem but were unable to identify the
cause of this phenomenon. Some of their suggestions included checking
growth of plants due to freezing, drying and other conditions retarding
growth development, poor seed and starting plants too early in the winter.

Thompson (64) was the first to make a systematic study of the
problem of bolting. The conclusion of his first experiments was that
the length of day controlled seedstalk development since the earlier he
planted his seeds, the more susceptible they were to bolting. However,
his results may have been confounded since cooler temperatures were also
associated with earlier seeding. Starring (59) suggested that cool
temperatures seemed to cause a fairly uniform premature seedstalk
formation. After further study, Thompson (65) reported that relatively
low temperatures (4-100C) for two weeks or longer would likely result in
premature seeding. He also found that a temperature of 10°C would not
induce seeding after two weeks but would likely induce it after a month
or more. In further studies, Thompson (66) observed that a period as
short as 2 days at 4°C appreciably promoted seedstalk develOpment and
that alternating between low, medium or high temperature did not nullify
the effect of the low temperature treatment.

Pawar and Thompson (45) demonstrated that plants of ages 14 days
and 160 days subsequently flowered when subjected to the same treatment.
Plants of different size, but of the same age, were equally affected by
low temperature exposure.

Platenius (46) was the first to work on the physiology of seedstalk

development in celery. He observed that the percentage of sugar

increased in plants at 4 to 10°C as compared to those kept at 20 to

25°C and that these levels could be reduced by placing the plants at a
high temperature. "Bolters" could be produced from young plants

provided they received a low temperature treatment. High sugar containing
plants grown at medium temperatures did not bolt indicating that low
temperature is more important than carbohydrate accumulation. He
concluded that the ultimate cause of seedstalk formation may be the
localized accumulation of certain chemical compounds in the meristematic
tissues, possibly not involving more than a few cells.

Curtis and Chang (19), in a clever experiment, placed rubber coils
around the meristematic tissue of young celery plants. When the plants
were placed in a warm environment with cold water passing through the
coils, floral initiation occurred. When the plants were placed in a
cold environment with warm water moving through the coils, initiation of
flowering did not occur.

Wittwer, Coulter and Carolus (73), Coulter (l8) and Clark and
Wittwer (16) studied the effects of growth regulators on seedstalk
development in lettuce and celery. 2,4-dichlorophenoxyacetic acid,
triiodobenzoic acid and napthalene acetic acid treatments all hastened
seedstalk development. However, treatment of celery withtx-o-chloro-
phenoxypropionic acid at 100 ppm prevented seedstalk development.

Emsweller (26) demonstrated that it is possible to secure strains
of celery that will not bolt to seed under conditions that cause a very
high degree of bolting in other strains. He also demonstrated that it
is possible to isolate lines so strongly annual that they produce some

plants with seedstalks even when grown under the most favorable

conditions for vegetative growth. A general statement on varieties
would indicate that most self-blanching or yellow varieties bolt readily
while the green varieties, which are most commonly grown, bolt less
readily. Bouwkamp and Honma (7) using the reciprocal cross of inbred
yellow and green lines, demonstrated that the easy bolting response in
celery is controlled by a dominant major gene (Vr). Their test for
homogeneity indicated that this character was genetically independent
from other varietal characteristics.

Though the vernalization and flowering of celery in the first
season is a problem for many growers, it is of great benefit to producers
of seed. Hanisova and Krekule (33) have described methods whereby low
temperatures are used to induce bolting in order to increase the speed
of production. Their methods allow the production of seed on a yearly
basis for all but the most bolt resistant varieties. It also gives a

more uniform seedstalk develOpment.
CAUL IFLOWER

Cauliflower, (Brassica oleracea var. botrytis L.), is grown for its
white, tender head or curd, formed by shortened flower parts. Cauli-
flower was first cultivated in EurOpe but was in common use over 2000
years ago. Like celery, the cultivated cauliflower is a biennial which
is cropped as an annual. In the production of the early or spring crop,
considerable loss occasionally results from the occurence of buttoned,
or "prematurely headed" plants. These plants are characterized by small
exposed heads appearing shortly after setting plants out in the field

while normal plants of the same age appear to be entirely vegetative.

Carew and Thompson (14) determined that these ”vegetative" plants
actually form curds at the same time but this is obscured by heavy
vegetative cover. They claim that it is rather misleading to use the
term premature heading. Buttoned plants have no market value, and in
years when the condition is severe, may cause considerable economic loss.

Early references to the problem of buttoning of cauliflower consist
mostly of statements based on observations rather than on research.
Bailey (4) and Brill (8) attributed buttoning to factors which slowed
the growth rate. Judson (38) attributed buttoning to stunting of plants
by keeping them in the seedbed too long. Jones and Ernst (37) agreed
with Judson and also suggested that extremely low temperatures while the
plant is young may cause it to button.

In the past 50 years, researchers have attributed buttoning to a
number of factors including soil nitrogen (50) and the C:N ratio of the
plant (31), transplant age and size (41), temperature (1, 57), variety
and light (57, 71).

The first investigation devoted solely to the problem of buttoning
was conducted by Robbins, Nightingale and Schermerhorn in 1930 (50).
They observed that plants with a limited supply of nitrogen in the
nutrient solution buttoned while those with adequate nitrogen did not.
Further investigation showed that this condition was associated with a
high percentage of carbohydrates and a low percentage of assimilated
nitrogen in the plant. They observed 37% more carbohydrate on a dry
weight basis in stems and petioles, 188% more in blades and 16% more in
the roots of the buttoned plant as opposed to the vigorously growing
vegetative plant. At the same time, the concentration of assimilated

nitrogen was much higher in the vigorous vegetative plant with 5% more

in roots, 104% more in stems and petioles and 163% more in blades.

Carew and Thompson (14) also observed that plants grown in a soil with a
low nitrogen content and poor early season growing conditions buttoned
more than plants with an adequate nitrogen supply.

Fontes and Ozbun (27) reported that treatments promoting carbohydrate
accumulation i.e. low temperature, promoted flowering in broccoli whereas
prevention of carbohydrate accumulation by SADH, high temperature,
juvenile plants or leaf pruning reduced flowering. Grainger (31)
suggested that a high C:N ratio must be attained to initiate flowering.
Sadik and Ozbun (52) demonstrated that flowering of cauliflower would
be attenuated by reducing carbohydrate synthesis during cold treatment
or by depleting carbohydrates after a cold treatment. They suggested,
that ignoring other factors, high carbdhydrate levels accompany or
precede flowering.

Transplant age and size is also an important consideration. Loomis
(41) observed that the larger the plant at transplanting, the greater
the severity of the growth check, and this resulted in more buttoning
in cauliflower. Gilbert (30) and Jensma (36) pointed out the need for
young plants and good growing conditions. Sadik (51) and Aamlid (1)
determined that l6~20 leaves must be formed before the curd is initiated.
Wiebe (71) on the other hand, claims the juvenile stage ends at 4-8
leaves. In work with Chinese cabbage, Eguchi, Matsamura and Koyama (25)
reported that old seedlings were more sensitive to cold temperature than
young seedlings in relation to seedstalk formation. Skapski and Oyer
(58) actually defined cold sensitivity in terms of plant size; plants
‘with stem diameters of approximately 5 mm and a fresh weight greater

than 5 grams had a greater tendency to button than smaller ones.

lO

Cobbet (17) suggested and Aamlid (1) demonstrated that a high
temperature would inhibit curd formation. Skapski (57) reported that
low temperature initiated curds while high temperature kept plants in
their vegetative state. Wiebe (72) reported that the highest temperature
which would induce curd formation was 23°C for the variety 'Aristokrat'.
Sadik (51), using two varieties of cauliflower, demonstrated that one
variety, February-Early March, required a cold treatment for curd
initiation while the other, Snowball M, required no cold treatment for
flowering. In further testing the vernalization requirement, Wiebe (71)
found that subjecting seeds to low temperatures had no affect on the
curd initiation of cold requiring varieties. Heide, Junttilla and
Samuelson (34) reported that low temperature during the seed development
phase of red beet plants increased the bolting susceptibility of the
progeny. Salter and Ward (53) concur with Skapski (57) that cold
temperature treatments may be applied to fall varieties to yield a more
uniform maturity.

Skapski and Oyer (58) and Skapski (57) have observed that buttoning
is a varietal characteristic with early season varieties being much more
susceptible than later types. Wiebe (71) also suggests that light has
no affect on the plant before vernalization and only a slight affect
afterwards, namely that lowlight intensity causes a slight delay in
maturity.

Low temperature appears to be the key cause of both bolting in
celery and buttoning in cauliflower. Low temperatures however, are
required to maintain the young plants in a viable condition until
outplanting. Hypobaric storage (12) is a process under development for

the handling and storage of various types of perishable commodities

11

including rooted and unrooted cuttings (9). The idea was conceived that
hypobaric storage of celery and cauliflower plants may attenuate the
effect of low temperature on floral initiation and became the subject of
this thesis. Since hypobaric storage is a new concept, certain

principles that require elaboration follow.

HYPOBARIC STORAGE CONCEPTS

In principle, a commodity is maintained at its normal cold storage
temperature in a chamber kept at some desired subatmospheric pressure
while it is continuously ventilated with fresh humid air by using a
vacuum pump to keep the storage pressure constant. The first recorded
use of reduced atmospheric pressure for the purpose of prolonging the
storage life of vegetative material was that of Stoddard and Hummel in
1957 (61). Using a modified kitchen refrigerator, they demonstrated
that various products, e.g. lettuce, celery and green beans, could be
kept from 3 to 6 days longer when the pressure was reduced from ambient
atmospheric pressure to approximately 660 mm-Hg. Burg and Burg (10)
observed that fruit kept better in a partial vacuum than at normal

atmospheric pressure at an equivalent P because ethylene was kept at

02
hyponormal levels since its diffusivity was increased as the atmospheric
pressure was decreased. The principle involved was shown (11) to be an
application of Ficks law of diffusion:
-x A' ”(Cm - c...)
T

 

ds/dt a

where ds/dt is the rate of gas transport, x is the fraction of the

surface area through which gas exchange occurs, A' is the surface area

12

of the fruit, Gin and Cout are the concentrations of gas within and
outside the fruit, D is the diffusion coefficient and T is the effective
thickness of the barrier to diffusion.

If the commodity is in equilibrium with its environment, the rate
of gas evolution equals the rate of synthesis in the tissue. Accordingly,
Ficks law states that the outward diffusion rate is determined by the
product of the surface area perforated by air-filled spaces times the
gas concentration gradient, times the diffusion coefficient, divided by
the thickness of the barrier limiting gas exchange. The diffusion
coefficient D is inversely related to the absolute air pressure provided

the rate limiting step for gas exchange occurs in an air phase:

m
D I Do(T/T°) (Po/P)

where D is the diffusivity of the gas in air at absolute temperature T
and pressure P, Do is the diffusivity at 273°K and 760 mm-Hg and (m) is
a constant with a value between 1.5 and 2 depending on the gas mixture.
Throughout the temperature range used for storage, the temperature
correction is insignificant, and therefore the diffusivity is inversely
related to absolute pressure (11).

The increase in the diffusivity of gases allows volatiles such as
ethylene, acetaldehyde, carbon dioxide and farnesene to escape more
easily while at the same time it allows oxygen to diffuse in more readily
(11). Thus, oxygen concentrations which might ordinarily cause fermen-
tation reactions to occur under normal atmospheric pressure, are often
enough to sustain aerobic metabolism at greatly reduced pressures.

A low partial pressure of O2 delays senescence and prolongs storage

life by reducing the rate of metabolism as indicated by the low

13

respiration rate (6). Ethylene synthesis (42, 13, 20) and action (10,
56) are also diminished at low 02 tensions. Ethylene above a certain
threshold level accelerates the respiration rate and senescence of many
fruits and vegetative tissue (39, 47, 10) and therefore shortens the
storage life.

This system of storage, variously termed hypobaric storage,
subatmospheric pressure storage, low pressure storage(LPS) or vacuum
storage, can be used to extend the useful life of fresh fruits,
vegetables, cut flowers, cuttings, potted plants, meat, poultry, fish,
shrimp and other metabolically active matter. Burg and Burg (12)
demonstrated that lowering the total pressure to 150 mm-Hg could extend
the storage life of bananas by more than four times over those stored at
normal pressure. Dilley (22) reported that hypobaric storage increased
the retention of flesh firmness, reduced scald and increased the shelf
life of several apple varieties. The loss of flesh firmness, pigmen-
tation and total titratable acidity of apricots, peaches and pears was
extended over a longer period of time by reducing atmospheric pressure to
about 100 mm-Hg (54). It has been reported that the ripening of tomatoes
‘may be delayed by atmospheric pressures in the 100 mm-Hg range (6, 67,60,
62, 74). The report by Stenvers and Bruinsma (60) indicates that the
effect of LPS is not on the reduction of the ethylene concentration but
rather on the low partial oxygen pressure. Bangerth (6) however, showed
that ethylene removal was a key factor.

Bangerth (6) has reported that hypobaric storage delays the
degradation of vitamin C in tomatoes, parsley, spinach, cress and radish.
Bangerth (6) and Wu et al. (74) observed that low atmospheric pressures

delay chlorophyll degradation, inhibit lycopene synthesis and prevent

14

the conversion of starch to sugar in tomato fruits. Wu et a1. (74)
observed an absence of aroma and flavor when tomatoes were initially
removed from hypobaric conditions but after several hours the flavor
and aroma returned to levels similar to fresh tomatoes. Burg (9) and
Dilley, Carpenter and Burg (23) reported that the storage life of cut
flowers, potted plants and cuttings could all be extended far beyond
that observed with conventional storage. This extension of life has
been attributed to the removal of ethylene by the system. Spore
germination, mycelial growth and sporulation of tested fungi were
inhibited by hypobaric conditions (3). An increase in inhibition was
observed with a decrease in pressure demonstrating the same type of
additive effect observed by Dilley et al. (23). Finally, Jadhau, Patil
and Salunkhe (35) have observed that greening of Russet Burbank potato
tubers can be completely inhibited at 126 mm-Hg and 80-85% relative

humidity.

MATERIALS AND METHODS

Celery. Celery plants (Utah 52-70) were grown by.Agricultural
Systems of Union Carbide Corporation at Watsonville, California and
shipped by air freight in polystyrene boxes or wooden crates to Lansing,
Michigan on April 27, May 11, May 25 and June 8. The plants arrived
within 36 hours and were received with wet ice still on the tops. The
plants were stored overnight at 5°C before being placed under treatment.
Plants in shipping containers which had 2 cm or more of roots in water
from the melting ice were discarded. Similar procedures were followed

for the remaining shipments with the exception of the May 25 shipment

15

in which case the roots were dry and were dipped briefly in water.

Each shipment consisted of 4 containers with approximately 1000
plants per container. The lots were divided in half and placed in 3
gallon plastic pails to accommodate the storage facilities. Storage was
for 0, 2, 4 or 6 weeks in air under the following conditions: 0°C at
25 mm-Hg, 0° at 760 mm-Hg, 10° at 25 tum-Hg and 10° at 760 mm-Hg. At

25 mm-Hg the O is 0.57 and 0.44% v/v respectively, at 00 and 10°C after

2
correcting for the presence of water vapor.

The storage chambers consisted of two, 3 compartment retorts, each
measuring 30 X 60 X 60 cm. The chambers were maintained in a temperature
controlled room and hence were "jacket cooled”. The chamber doors
consisted of plexiglass plates fit against a silastic rubber gasket. A
vacuum outlet from the chamber was connected to a vacuum pump. Fresh,
ethylene-free air was admitted through a vacuum regulator (Matheson
Model 49) at the desired flow rate and pressure. The air was bubbled
through water at the operational pressure prior to entering the chamber.
Humidification was accomplished in a 4 liter flask maintained 2 to 5°C
higher than the storage chamber temperature by placing the flask on a
thermostatically controlled heating mat. A schematic of the system is
shown in Figure 1. This system is similar to that described by Burg
and Burg (12). A second set of chambers was also used with the only
difference being an increased chamber volume.

Outplanting was made on June 11 at Hamilton, Michigan. The soil was
a deep muck and weather conditions were sunny with temperatures of 22 to
27°C. When removed from storage, all plants held at 10°C at 760 mm-Hg
for 2, 4 and 6 weeks were badly deteriorated and decayed. Plants held

at 10°C at 25 mm-Hg for 6 weeks were also unplantable. The remaining

l6

VACUUM MANOME'ER I
REGULATOR 1
E'HYlENE
SCRUBBER
.§%2 %

 

‘ )
"'W‘ * .
s :1 ":1
\X l'.:' J
“AW '3'
if 1.41“
, sad u
. J 5

i

I‘ >‘-.
.J; '25}
. ‘s. .44 -3

 

I
" VACUUM
nowunn HUMIDIFIER rum

 

Figure 1. Schematic diagram of the hypobaric system used for the
storage of transplants.

l7

10°C treatments, 25 and 760 mm-Hg stored for 2 and 4 weeks, though in
poor condition, were planted but not included in any further experimental
evaluation.

A two-row, self-propelled, Holland mechanical transplanter was
used for outplanting. Each treatment had 4 replicates with all but 3
replicates containing 50 plants each. A shortage of plants required
2 replicates to contain 35 plants and one 40 plants. A completely
randomized design was used. The cultivation, fertilization and spray
program was carried out according to the standard commercial practices
of Michigan. Growth ratings were taken on June 17, June 23, July 8 and
August 12; a survival count was also taken on June 23. Subjective
ratings were made on a scale of 0-9 where 0 was a dead plant and 9 was
equal to the best of the cooperating growers plants already in the

field. On August 8, an infestation of bacterial blight, Pseudomonas

 

apii, occurred and an application of copper fungicide was made to correct
the problem. \

The plants were harvested on September 2, 81 days after outplanting.
All harvesting was done by hand. Plants were divided into two groups;
marketable and unmarketable. A plant was considered unmarketable if it
had bolted or was suffering from aster yellow virus. The number of A
bolters and diseased plants were recorded and the plants discarded.
The remaining plants were sorted into size classes of large, medium and
small corresponding roughly to a 24, 30 or 36 count market pack. After
sorting, the stalks were trimmed, counted and weighed. The trimmings
for each plot were collected and weighed separately. In addition, 5
plants were selected at random from each plot for measurement of

meristem length to assess incipient bolting.

18

Cauliflower. On July 26, cauliflower plants (40 days old) were
harvested from a field seedbed in Richmond, Michigan. Two varieties,
Self-Blanche from the Harris Seed Co. and Clou from the Stokes Seed Co.,
were used. A portion of the plants harvested the same day were planted
immediately and served as controls. Each variety was considered a
separate experiment and a split-split-plot design was employed with 3
replications per treatment and 20 plants per replicate. The soil was a
Boyer sandy loam. On the day the controls were planted, soil moisture
was low and the temperature was near 30°C. Planting was done with a
two-row, Holland mechanical transplanter. Irrigation was applied
immediately afterwards but some of the plants had already wilted beyond
recovery.

The rest of the plants were transported to East Lansing and
stored for 2 or 4 weeks at 0°C and either 25, 150 or 760 mm-Hg
atmospheric pressure. The hypobaric system was that described previously
but the containers employed were 46 liter milk cans modified for
hypobaric use. At 2 and 4 weeks, plants were removed and outplanted at
Richmond. The second and third plantings were made by hand because
plants from the earlier setting made it difficult to get the planter
into the plots. The plants were then grown utilizing standard
commercial cultural practices.

Due to the prediction of severe frost, all plants were harvested on
October 28. Buttoning was determined subjectively using descriptions
from the literature (14) and the experience of the grower. Buttoned
plants were counted and discarded. Curds, when present, were
separated from the rest of the plant and weighed. Only an average

'weight of curd per plant was determined. After removal of the curd,

19

the fresh weight of the remaining above ground portion of the plants
was taken. Since most of the non-buttoned plants had not yet formed a
curd, this fresh weight was used as a basis for plant yield data. A

survival count was also taken at this time.
RESULTS AND DISCUSSION

.921EEX' Recovery ratings were taken at l, 2, 4 and 9 weeks
(Table 1). After 1 week in the field, all plants were in fair condition
but none were as good as those of the grower also planted by him on the
same day which were used for reference in the ratings. In all cases,
comparisons were made with plants which received no storage. At least
two explanations are possible; first, the California plants were grown
in sand and were transplanted into a muck soil. It is conceivable that
such plants may require an adaptation period to adjust to a different
soil pH, nutrient balance or water availability. Alternatively, and
perhaps more likely, the California-grown plants, being considerabley
larger than those normally transplanted in Michigan may have experienced
more stress at outplanting. Dry soil conditions and warm, sunny
weather likely put an increased stress on a root system already damaged
by the original harvesting operation. Plants pulled from the seedbed
have many of the fine root hairs responsible for the majority of water
uptake by the plant damaged or removed. Since the California-grown
plants had a larger above ground surface area, their water requirements
would be greater than smaller plants. At later evaluations (4 and 9
weeks), the California-grown plants were similar to the Michigan-grown

plants used for comparison. The capability of plants to reach normal

20

Table 1. Effect of Storage Pressure and Duration at 0°C on Recovery
Ratings2 for Celery Transplants at Various Intervals After Outplanting.

 

 

 

 

Pressure Duration Date
(mm-Hg) (weeks) 6/17 6/23 7/8 8/12
25 0 7.25 7.50 8.50 8.75
25 2 7.75 7.75 8.50 9.00
25 4 6.25 6.25 7.50 9.00
25 6 4.75 5.50 6.75 8.50
760 O 7.00 7.50 8.25 9.00
760 2 6.50 \ 7.25 7.50 8.75
760 4 7.00 \ 7.00 8.50 9.00
760 6 5.00 6.50 7.75 9.00

 

z1 a dead; 9 a best

21

maturity was apparently not hampered by either low temperature (0°C) or
low pressure (25 mm-Hg) storage for at least 6 weeks.

Table 2 shows the percentage survival 2 weeks after planting and at
harvest. Some plants originally counted as survivors on June 23
apparently did not survive. Analysis of the data revealed no real
difference between any of the treatments however, there is a trend that
may indicate that the percentage survival decreased as the duration of
storage increased.

The crucial point of the experiment was to determine whether
bolting could be inhibited by using hypobaric storage. The results are
given in Figure 2 and Table 3. The controls (non-stored plants), as
expected, had no bolters. After 2 weeks there were 1415 bolters per
hectare from the 0°C at 760 mmAHg treatment but no bolters were found
among plants stored at 25 mm-Hg at the same temperature. The difference
was not statistically significant but 1415 bolters per hectare is an
unacceptable number with regards to grower profit. At 4 and 6 weeks
there was a significant interaction between storage duration and
pressure with regard to the number of bolters. The response to pressure
therefore is dependent on the length of storage and vice versa.

Storage at 25 mmng did not eliminate bolting over long storage
periods at 0°C, but it did severely attenuate it. In looking for a
plausible explanation for this phenomenon we must go back to the
literature. As indicated in the review, Platenius (46) observed an
increase in sugar accumulation in meristematic tissue of celery prior
to seedstalk formation. Sadik (51) and Grainger (31) have indicated
that this accumulation is necessary before floral initiation can occur.

Wu et al. (74) have demonstrated that hypobaric storage of tomatoes

22

Table 2. Percentage Survival of Celery Transplants at Two Dates After
Outplanting as Influenced by Storage Pressure and Duration at 0° C.

 

 

 

 

Pressure Duration Date
(mm-Hg) (weeks) 6/23 9/2
25 o 99.5y 95.5abz
25 2 100 98.0ab
25 4 100 96.9a
25 6 97.5 93.7b
760 0 100 99.5a
760 2 99.5 98.4a
760 4 100 95.9ab
760 6 100 97.7ab

 

zMean separation within columns by Duncan's multiple range test. Means
followed by the same letter are not significantly different at the
0.05 level.

XAbsence of letters indicates nonsignificance.

23

 

 

Bolton/Hectare X 10"3

 

 

 

Weeks in Storage at 00

Figure 2. Effect of storage pressure and duration at 0°C on floral
initiation in celery.

 

I! r Ill-ll I.‘ ‘1' ulllll: A
'l INIIIIII I

24

Table 3. Effect of Storage Pressure and Duration at 0°C on Floral
Initiation in Celery.

 

 

 

Pressure Duration Bolters/Ha.
(mm-Hg) (weeks)
25 0 0e."
25 2 0a
25 4 7314a
25 6 16663b
760 0 0a
760 2 1415a
760 4 44981c
760 6 63846d

 

zMean separation by Duncan's multiple range test. Means followed by
the same letter are not significantly different at the 0.05 level.

 

II {Illa-AN

25

reduced the rate of degradation of starches to sugar. If this
inhibition of starch degradation occurs in celery also, it may offer an
explanation for the reduced number of bolters at low pressure. Melchers
(44) and others have postulated the presence of a substance known as
vernalin which is produced under conditions of low temperature and high
sugar. The chemical has never been identified but its action or
synthesis, if it indeed exists, could be inhibited by low pressure
either by reducing sugar accumulation or inhibiting some other metabolic
process. It may also be possible that some volatile such as ethylene
is removed which is necessary for vernalization to occur. The low
oxygen tensions present under hypobaric conditions may offer the best
explanation as to the nature of the inhibition of bolting under low
pressure conditions. Gregory and Purvis (32) demonstrated that
vernalization is an aerobic process. Under the low oxygen tension
(0.57% v/v), the vernalization response may occur but at a highly reduced
rate. This may explain the observation that bolting at 25 mm-Hg, though
greatly attenuated, increases with time.

In constructing yield data, only marketable plants were used. Figure
3 indicates total marketable yield. After 2 weeks there was no signif-
icant decrease in yield between plants stored at 25 or 760 mmng. At
4 and 6 weeks there was a drastic drop in yield per hectare at both
pressures but the plants stored at the ambient pressure yielded
significantly less than those stored at 25 mmifig. This drop in yield
'was due mostly to an increased number of bolters and fewer marketable
plants per hectare (See Figure 4).

Maturity for market was based on stalk size. Consumer preference

studies, based on sales, have indicated that the medium size stalk

26

 

70 I’m

 

60-
c?—
2
)g £5C"'
o
I
T:
35 40-
2
.2
> 30-
2
a
3
32%-
a
E

10f-

oL.L k 4
o 2 4 6

 

 

Weeks in Storage at 00

Figure 3. Effect of storage pressure and duration at 0°C on total
yield of marketable celery. The yield does not include the weight
of bolters nor trimmings.

27

 

 

Plants/Hectare X 10'3

 

0 2 4 5

Week. In Storage at 06

Figure 4. Effect of storage pressure and duration at 0°C on the total
number of marketable plants per hectare.

28

(2% dozen/crate) is most preferred followed by small and then large.
Table 4 indicates that as storage time increased, the plants produced
smaller stalks at both pressures. This may relate to the time of
recovery in the field after storage; with longer recovery times
delaying the time to reach market maturity. The danger of delaying
harvest until the plants attain their maximum size is that seedstalks
already initiated but not yet emerged, may elongate, making the plant
unmarketable.

Figure 5 indicates the induction of floral initiation as measured
by the length of the meristem when the plants were cut in half
longitudinally. The only significant difference occurred at 4 weeks.
The general trend however, was that those at the ambient pressure were
more differentiated than those stored at low pressure and as such,
would tend to bolt sooner. The drop in meristem.length from 4 to 6
weeks at 760 mm-Hg may indicate genetic variability in the variety.
With plants stored for 6 weeks, those most susceptible to bolting had
already bolted by harvest time while those with some bolting resistance
were in various stages of seedstalk elongation. In this treatment,
the bolt resistant plants probably made up the majority of the sample
and thus the sample was biased, accounting for the apparent reduction
in floral initiation between 4 and 6 weeks of storage at 0°C and
760 mm-Hg.

Cauliflower. Clou is considered an early season variety and Self-
Blanche a fall variety. As such, Self-Blanche does not tolerate the
heat of summer nearly as well as Clou. Table 5 indicates the percentage
survival for both varieties after outplanting. Data for the controls

demonstrate that Clou was more resistant to the adverse planting

 

. flit [I III Ill. [ [II III I

29

Table 4. Celery Stalk Sizez Distribution and Trimming Waste as Influenced
by Storage Pressure and Duration at 0°C.

 

 

 

 

Pressure Duration Percentage of Weight
(mmng) (weeks) Small Medium Large Trimmings
25 o 66y x
. a 43.5a 18.0cd 31.9
25 2 10.5a 44.6a 11.8bc 33.1
25 4 8.6a 50.2a 6.2ab 35.0
25 6 23.3a 35.9a 6.3ab 34.5
760 O 6.5a 48.8a 12.3bc 32.4
760 2 9.1a 37.7a 21.9d 31.3
760 4 19.6a 47.5a 0.0a 32.9
760 6 47.0b 21.1b 0.0a 31.9

 

szall, medium and large correspond to a 24, 30 and 36 count per crate.

yMean separation within columns by Duncan's multiple range test. Means
followed by the same letter are not significantly different at the
0.05 level.

xAbsence of letters indicates nonsignificance.

 

' .Illllllul ill I Ill'll III {I If.‘
1" IN! A III! I (III-II l

30

 

551-

760 nun-Hg

55?-

45)-

 

Merletem Length Imml

 

 

 

 

4 6

Weeks In Storeoe e! 00

Figure 5. Effect of storage pressure and duration at 0°C on meristematic
development of marketable celery plants.

 

 

{{IAI

31

Table 5. Effect of Storage Pressure and Duration at 0°C on Percentage
Survival for Clou and Self-Blanche Cauliflower After Outplanting.

 

 

 

Variety Pressure Duration % Survival
(mm-Hg) (weeks)
Self-Blanche control 0 79.4abz
" 25 2 93.3ab
" 150 2 98.3a
" 760 2 96.7ab
" 25 4 30.0a
n 150 4 93.3ab
" 760 4 93.3ab
Clou control 0 96.7y
" 25 2 93.3
" 150 2 91.7
" 760 2 95.0
" 25 4 91.7
" 150 4 95.0
" 760 4 85.0

 

zMean separation by Duncan's multiple range test. Means followed by
the same letter are not significantly different at the 0.05 level.

XAbsence of letters indicates nonsignificance.

32

conditions than was Self-Blanche. This is consistent with the acclaimed
heat tolerance difference of these varieties. All other treatments
showed fair to good recovery with the exception of the Self-Blanche
plants held at 25 mm-Hg for 4 weeks which gave only a 30% recovery.
There is no apparent explanation for this high mortality. It appears to
be a variety characteristic. Further testing is necessary to determine
the underlying physiological basis. In general, the rate of survival
decreased as the pressure decreased indicating a stress created by the
low pressure.

Like celery, the purpose of the cauliflower experiment was to
alter a response to cold treatments. Table 6 lists the percentage of
buttoned heads per hectare for both varieties. For Self-Blanche,
though not significantly different, the number of buttoned heads tended
to decrease as the pressure was decreased. This difference was signif-
icant for Clou. At ambient pressure, over 42% of the heads buttoned while
at 25 mmng, none of the plants buttoned even after 4 weeks of storage
at 0°C. A pressure of 150 mm-Hg gave an intermediate amount of
buttoning indicating that the effects of the low pressure treatment are
additive. The data is consistent with the hypotheses put forth
previously for celery. The attenuation of vernalization by low oxygen
tensions still appears to be the most feasible. The intermediate
oxygen level yielded an intermediate number of buttoned heads. In
addition, the results appear to be in agreement with Skapski's (57)
observation that buttoning is, in part, variety dependent. The reason
the summer variety was more susceptible to buttoning than the fall
variety may be that summer varieties do not require as low a temperature

or as long a duration of cold to trigger the vernalizing mechanism.

 

.l Alf lllI 'IIII‘II .l'lll. lll‘ IIIII' I'll-ll '0'

33

Table 6. Percentage of Cauliflower Plants Per HectaredWhich Buttoned
as Influenced by Storage Pressure and Duration at 0 C.

 

 

 

Variety Pressure Duration % Buttoned
(mm-Hg) (weeks)
Self-Blanche control 0 0.0y
" 25 2 0.0
" 150 2 0.0
" 760 2 0.0
" 25 4 0.0
" 150 4 1.9
" 760 4 3.6
Clou control 0 0.0az
" 25 2 0.0a
” 150 2 12.8ab
" 760 2 18.9b
" 25 4 0.0a
" 150 4 37.8c
" 760 4 42.6c

 

zMean separation by Duncan's multiple range test. Means followed by
the same letter are not significantly different at the 0.05 level.

XAbsence of letters indicates nonsignificance.

34

The foregoing results argue against traditional beliefs concerning
the cause of buttoning. As reviewed earlier, Carew and Thompson (14),
Robbins et al. (50) and others agree that no single factor contributes
to buttoning but rather it is a combination of factors including
nitrogen supply, temperature, transplant age and transplant size. The
plants used in this experiment were of a normal size and age and they
were planted in soil with an adequate supply of nitrogen. Growing
conditions were favorable until well after the time the plants began
buttoning. Accordingly, there are two possible conclusions; first, low
temperature alone may be enough to induce buttoning in cauliflower.
Alternatively, young plants with 4-8 leaves are capable of being
vernalized. This is consistent with Wiebe (71) and at odds with Sadik
(51) and Aamlid (1) who claim that a cauliflower plant requires from
16—20 leaves before it is capable of being vernalized. However, the
age at which juvenility ends may be variety dependent. The plants used
in this experiment were young, but old enough to be vernalized according
to Wiebe's parameters.

Yield studies were confounded by the early harvest. Because
plantings were made at different times, no statements can be made
concerning differences caused by the duration of storage. The original
intent was to use curd weight as a measure of yield. Since a majority
of the marketable plants did not develop curds by harvest time, the
fresh weight of the above ground portion of the plant was substituted.
The results are presented in Table 7. The decreased yields for Self-
Blanche at low pressure are a direct result of the poor survival rate.
The increased yields at low pressure for Clou are a result of fewer

buttoned heads.

 

I'. ill I III fl. ill ('1 all I

35

Table 7. The Effect of Storage Pressure on the Fresh Weight of
Cauliflower Plants Per Hectare.

 

 

 

Variety Pressure Kg/Ha
(mm-Hg)
Self-Blanche 25 27513az
" 150 29395a
" 760 30071a
Clou 25 272583
" 150 26341a
" 760 2356lb

 

zMean separation by Duncan's multiple range test. Means followed by
the same letter are not significantly different at the 0.05 level.

When an average weight per plant was computed, plants of the Clou
variety stored at 25 mm-Hg were significantly larger than those stored

at 760 mmng (See Table 8). There were no differences between 25 and

Table 8. The Effect of Storage Pressure on the Average Plant Weight of
Clou Cauliflower Plants.

 

 

 

Variety Pressure Grams/Plant
(mm-Hg)
Clou 25 1060az
" 150 1063a
" 760 939b

 

zMean separation by Duncan's multiple range test. Means followed by
the same letter are not significantly different at the 0.05 level.

 

 

.I I I'- i,‘\ "I A III

36

150 mm-Hg. Differences for Self-Blanche were nonsignificant although
there was a tendency towards smaller plants as the pressure was
decreased. The larger average plant size for Clou may be due to a
higher retention of chlorOphyll and carbohydrate reserves during the
storage period. Tolle (68), Wu et al. (74) and Bangerth (6) have all
demonstrated that low pressure storage inhibits chlorophyll degradation
in tomatoes. Dilley and Price (24) have also demonstrated that
chlorophyll is retained in pepper and tomato transplants at low pressure.
With greater chlorophyll retention, the photosynthetic capacity of the
low pressure stored plants would be greater for the first few days

after transplanting which may be vital to reestablish plant growth.

SUMMARY AND CONCLUSIONS

Celery plants were held at 0°C for two weeks at 25 mm-Hg with no
seedstalk formation occurring after transplanting while those stored at
ambient pressure showed a tendency to bolt. At durations longer than
two weeks, some bolting occurred but considerabley less than that
observed in plants stored at ambient pressure. The 6 week storage
treatment at 0°C and 760 mm-Hg had 91% bolters while those stored under
the same conditions but at 25 mm-Hg had only 25% bolters. Cauliflower
showed a similar response with the 25 mm-Hg pressure preventing
buttoning even after 4 weeks of storage at 0°C. Those stored 4 weeks
at ambient pressure showed approximately a 40% buttoning rate for the
Clou variety. Yield differences in celery were related to both pressure
and duration with stalk size decreasing with duration but increasing

at lower pressures.

 

 

N! I Ilk,‘ Jill! ll ll I'll-l I [,IIII I

37

Prevention of floral initiation for two weeks should be enough
time to move plants overland from California to Michigan or other
production sites. A cost analysis will be necessary to determine the
cost/benefit ratio. Lougheed (43) has estimated that hypobaric storage
has a 36% higher cost than conventional controlled atmosphere storage
for apples. Hypobaric storage and transportation facilities are
expensive and the costs may not justify the benefits for seedling

transplants, but this remains to be determined.

 

 

. .llillfllll’lfVNI

LITERATURE CITED

10.

11.

12.

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