cranfield university gemma amy chope understanding the mechanisms behind onion bulb ...

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CRANFIELD UNIVERSITY

GEMMA AMY CHOPE

UNDERSTANDING THE MECHANISMS BEHIND ONION
BULB DORMANCY IN RELATION TO THE POTENTIAL
FOR IMPROVED ONION STORAGE

CRANFIELD HEALTH
Plant Science Laboratory

Ph.D THESIS

2006
CRANFIELD UNIVERSITY
Cranfield Health
Plant Science Laboratory

Ph.D THESIS
Academic Years 2003-2006

GEMMA AMY CHOPE

UNDERSTANDING THE MECHANISMS BEHIND ONION BULB
DORMANCY IN RELATION TO THE POTENTIAL FOR IMPROVED
ONION STORAGE

Supervisors: Dr. Leon A Terry & Prof. Philip J White

December 2006

This thesis is submitted in partial fulfilment of the requirements
for the degree of Doctor of Philosophy

© Cranfield University, 2006. All rights reserved. No part of this publication may be reproduced without
the written permission of the copyright holder.
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ABSTRACT

Onion (Allium cepa L.) is the most economically important Allium crop. Onion
cultivars adapted for growth in temperate regions require long days for bulb initiation, so
the summer crop is stored over-winter. Deterioration in store is largely due to resumption
of growth. Extended storage of onions is currently dependent on the synthetic sprout
suppressant maleic hydrazide, whose future use is uncertain due to pressure to reduce
residues in foods.
Abscisic acid (ABA) has previously been linked with dormancy in a relative of
onion (A. wakegi). Bulb ABA concentration in three onion cultivars (viz. Renate, Ailsa
Craig and SS1) with contrasting storage lives declined exponentially during controlled
atmosphere (CA – 3% CO , 5% O ; 2°C) storage at the same rate in each cultivar.
2 2
Sprouting occurred at a minimal ABA concentration (ca. 50-120 ng g-1 DW). It was
proposed that extended periods of high concentrations of ABA may delay sprouting.
An ABA analogue (PBI-365) and exogenous ABA, were applied as preharvest
foliar sprays (cvs. Renate, Carlos, Dinaro, Hysam, Red Baron and SS1), or as postharvest
bulb soaks (cv. Hysam) with the aim of increasing endogenous ABA concentration and,
thereby, extending the storage period. Endogenous bulb ABA concentration was not
affected. Bulb ABA concentration again decreased during storage at a range of
temperatures (4, 12 and 20°C) and sprouting occurred at minimal ABA concentration (ca.
75-150 ng g-1 DW). After the onset of sprouting, ABA concentration increased again,
probably due to synthesis by the sprout. The concentration of certain carbohydrates has
been linked with storage potential. No straightforward relationship between ABA and
non-structural carbohydrate (NSC; fructose, glucose, sucrose and fructans) metabolism
could be determined. It was therefore postulated that ABA is more likely to play a role in
mediating cell elongation rather than cell division, or that minimal ABA concentration
could be a trigger for remobilisation of carbohydrates.
Controlled atmosphere (CA) is used to extend storage life of onions; however,
shelf-life can be compromised. The effects of the transition between CA (3 % CO , 5 %
2
O ; 2°C) and air storage on ABA concentration, quality characteristics, respiration rate
2
Gemma A. Chope Cranfield University Ph.D Thesis 2006
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(RR) and NSCs in three onion cultivars (viz. Renate, Carlos and SS1) were investigated.
The RR of the short storing cultivar, SS1, was greatest by ca. 0.5-fold. The RR increased
on removal from CA storage, with no accompanying decrease in carbohydrate
concentration, indicating that the increased RR may have been a transient stress response.
Storage of onions cv. SS1 for three weeks in air, followed by three weeks CA was as
effective in suppressing sprout growth as six weeks continuous CA storage. Bulb ABA
concentration decreased significantly between the time of harvest and after curing.
Therefore the current practice of curing onions for extended periods at high temperatures
could be reducing bulb ABA concentration, and therefore storage life.
The literature concerning the role of ethylene in onion storage is limited and
conflicting. The effect of 1-methylcyclopropene (1-MCP; an ethylene perception
inhibitor) on sprout growth in onions cv. SS1 stored at a range of temperatures (4, 12 and
20°C) was investigated, along with quality characteristics, NSCs and ABA. Sprout
growth was reduced in onions treated with 1-MCP and stored at 4 or 12°C, but not at
20°C. Approximately 2-fold greater concentrations of sucrose, glucose and fructose were
maintained in 1-MCP-treated bulbs stored at 12°C as compared with untreated bulbs. It
appeared that 1-MCP reduced carbohydrate metabolism.
Both ethylene and ABA are highly likely to impact significantly on onion storage
life. The results are discussed in relation to the potential to influence storage life by
changes in horticultural practices, including the recommendation to re-evaluate the curing
and drying protocol, and to investigate the possibility of delaying the start of CA storage.
Gemma A. Chope Cranfield University Ph.D Thesis 2006
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ACKNOWLEDGEMENTS

I would like to express my gratitude to Dr. Leon Terry and Prof. Philip White for
their tireless support, guidance and patience throughout this project. I am also grateful to
Allen Hilton, Balwinder Cook and Frank Davis at Cranfield University for technical
support, and to the staff at Warwick HRI including Andrew Mead for statistical advice,
and Sylvie Fairless and Peter Brooks for sourcing journals. This studentship was funded
by the HDC, and I would like to thank them for the financial support.
This experience has been enhanced by all the friends I made at Warwick HRI,
especially the occupants of office 131, who made me laugh, the Plant Mineral Nutrition
group, including Helen, John, Rory, Will and Mark who made me very welcome and
taught me valuable skills, not least of which was the art of white van driving.
Last, but not least, I would like to thank my parents and sister for their support,
Adam for being there for me, and of course Kitty for keeping me company during long
days in front of the computer.

Gemma A. Chope Cranfield University Ph.D Thesis 2006
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LIST OF CONTENTS

ABSTRACT ……………………………………………………………………………..i
ACKNOWLEDGEMENTS …………………………………………………………....iii
LIST OF CONTENTS …………………………………………………………....iv
LIST OF FIGURES ………………………………………………………...…..x
LIST OF TABLES …………………………………………...…………….xviii
NOTATION …………………………………………………………....xx

1.0 CHAPTER ONE: INTRODUCTION ……………………………………..1
1.1 Project background ……………………………………………………………..1
1.2 Aim and objectives .…………………………………………………………….1
1.2.1 Aim ……………………………………………………………..1
1.2.2 Objectives ..……………………………………………………………2
1.3 Thesis structure ..……………………………………………………………2

2.0 CHAPTER TWO: LITERATURE REVIEW ……………………………..6
2.1 Allium cepa L. ……………………………………………………………..6
2.2 Economic importance of the onion crop in the UK ……………………………..6
2.3 The onion lifecycle ……………………………………………………………..8
2.3.1 Bulb initiation and formation ……………………………………………..9
2.3.2 Onion bulb dormancy ……………………………………………………10
2.4 Quality attributes of marketable onions ……………………………………11
2.5 Strategies to delay sprouting ……………………………………………………12
2.6 Pre-harvest factors that affect storage life ……………………………………12
2.6.1 Pre-harvest nutrition ……………………………………………………12
2.6.2 Growing season temperature ……………………………………………13
2.6.3 Maleic hydrazide ……………………………………………………14
2.6.4 Crop maturity at harvest ……………………………………………15
2.6.5 Harvesting process ……………………………………………………15
Gemma A. Chope Cranfield University Ph.D Thesis 2006
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2.7 Postharvest factors that affect storage life ……………………………………16
2.7.1 Curing and drying ……………………………………………………16
2.7.2 Irradiation ……………………………………………………………17
2.7.3 Nitrous oxide ……………………………………………………………17
2.7.4 Storage environment ……………………………………………………18
2.7.4.1 Storage temperature ……………………………………………18
2.7.4.2 Controlled atmosphere storage ……………………………19
2.7.4.3 Humidity ……………………………………………………20
2.8 Alternative strategies to delay sprouting ……………………………………20
2.9 Changes occurring in onion bulbs during storage and sprouting ……………21
2.9.1 Mitotic activity ……………………………………………………21
2.9.2 Flavour precursors and pungency ……………………………………22
2.9.3 Carbohydrates ……………………………………………………………24
2.9.4 Fresh weight and water loss ……………………………………………26
2.9.5 Respiration ……………………………………………………………26
2.9.6 Plant growth regulators ……………………………………………27
2.10 Conclusions ……………………………………………………………………32

3.0 CHAPTER THREE: TEMPORAL CHANGES IN ABSCISIC ACID
CONCENTRATION AND QUALITY CHARACTERISTICS DURING
CONTROLLED ATMOSPHERE STORAGE OF ONION BULBS OF
THREE CULTIVARS WITH CONTRASTING STORAGE
POTENTIAL ……………………………………………………………………33
3.1 Abstract ……………………………………………………………………33
3.2 Introduction ……………………………………………………………………34
3.3 Materials and methods ……………………………………………………37
3.3.1 Plant material and storage regime ……………………………………37
3.3.2 Experimental design ……………………………………………………37
3.3.3 Sample preparation ……………………………………………………38
3.3.4 Quantification of total fructans ……………………………………38
Gemma A. Chope Cranfield University Ph.D Thesis 2006
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3.3.5 Pyruvate assay for assessment of pungency ……………………………39
3.3.6 Bulb firmness and total soluble solids assessments ……………………40
3.3.7 Mineral analysis ……………………………………………………40
3.3.8 Abscisic acid (ABA) quantification ……………………………………40
3.3.9 Statistical analysis ……………………………………………………42
3.4 Results ……………………………………………………………………42
3.4.1 Mineral concentrations ……………………………………………42
3.4.2 Sprout growth ……………………………………………………………44
3.4.3 Dry weight ……………………………………………………………46
3.4.4 Total soluble solids ……………………………………………………46
3.4.5 Firmness ……………………………………………………………46
3.4.6 Pyruvate ……………………………………………………………47
3.4.7 Total fructans ……………………………………………………………49
3.4.8 Abscisic acid ……………………………………………………………49
3.5 Discussion ……………………………………………………………………53
3.6 Conclusion ……………………………………………………………………58

4.0 CHAPTER FOUR: NEITHER PRE- NOR POSTHARVEST
APPLICATION OF EXOGENOUS ABSCISIC ACID (ABA) OR AN ABA
ANALOGUE AFFECTS ENDOGENOUS ABA CONCENTRATION OF
ONION BULBS…………………………………………………………………60
4.1 Abstract ……………………………………………………………............60
4.2 Introduction ……………………………………………………………………61
4.3 Materials and methods ……………………………………………………63
4.3.1 Plant material ……………………………………………………………63
4.3.2 Experiment 1 – Pre-harvest foliar spray method ……………………63
4.3.3 Harvest procedure ……………………………………………………65
4.3.4 Experiment 2 - Postharvest bulb dip method ……………………………65
4.3.5 Storage regime and experimental design ……………………………65
4.3.6 Sample preparation ……………………………………………………66
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4.3.7 Physical assessments ……………………………………………………66
4.3.8 Biochemical assessments ……………………………………………66
4.3.8.1 NSC extraction ……………………………………………67
4.3.8.2 NSC quantification ……………………………………………67
4.3.9 Statistical analysis ……………………………………………………68
4.4 Results ……………………………………………………………………………68
4.4.1 Experiment 1 - Consequences of a pre-harvest foliar spray of ABA
or PBI-365 ……………………………………………………………68
4.4.1.1 Pre-storage cultivar characteristics ……………………………68
4.4.1.2 Sprout growth ……………………………………………………72
4.4.1.3 Firmness ……………………………………………………74
4.4.1.4 Dry weight ……………………………………………………74
4.4.1.5 Total soluble solids ……………………………………………77
4.4.1.6 Pyruvate concentration ……………………………………77
4.4.1.7 Non-structural carbohydrates ……………………………………80
4.4.1.8 Abscisic acid ……………………………………………………89
4.4.2 Experiment 2- Consequences of postharvest bulb dip into solutions of
ABA or PBI-365 ……………………………………………………91
4.4.2.1 Sprout growth ……………………………………………………91
4.4.2.2 Firmness and dry weight ……………………………………92
4.4.2.3 Total soluble solids and pyruvate concentration ……………93
4.4.2.4 Abscisic acid ……………………………………………………94
4.5 Discussion ……………………………………………………………………95
4.6 Conclusion ………………………………………………………………..…100

5.0 CHAPTER FIVE: THE EFFECT OF 1-METHYLCYCLOPROPENE
(1-MCP) ON THE PHYSICAL AND BIOCHEMICAL
CHARACTERISTICS OF ONION CV. SS1 BULBS DURING
STORAGE ......................................................................................…..……..102
5.1 Abstract ………………………………………………………..…….…...102
Gemma A. Chope Cranfield University Ph.D Thesis 2006
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5.2 Introduction ………………………………………………………..…….…...102
5.3 Materials and methods ……………………………………….…….……104
5.3.1 Plant material and storage regime ……………………….………….104
5.3.2 Application of 1-MCP…………………………………………………..104
5.3.3 1-MCP quantification ………………………………………………..…105
5.3.4 Storage conditions and sampling regime ………………….……….105
5.3.5 Sample preparation …………………………………………………..106
5.3.6 Physical assessments …………………………………………………..106
5.3.7 Biochemical assessments …………………………………………..106
5.3.8 Statistical analyses …………………………………………………..107
5.4 Results …………………………………………………………………………..107
5.4.1 Effect of 1-MCP on physical properties of onion bulbs …………..107
5.4.1.1 Sprout growth …………………………………………………..107
5.4.1.2 Firmness and dry weight …………………………………..108
5.4.2 Effect of 1-MCP on biochemical properties of onion bulbs …………..110
5.4.2.1 Total soluble solids and pungency …………………………..110
5.4.2.2 Abscisic acid …………………………………………………..111
5.4.2.3 Non-structural carbohydrates …………………………………..112
5.5 Discussion …………………………………………………………………..117
5.6 Conclusion …………………………………………………………………..122

6.0 CHAPTER SIX: THE EFFECT OF THE TRANSITION BETWEEN
CONTROLLED ATMOSPHERE AND AIR STORAGE ON BULBS OF
ONION CULTIVARS SS1, CARLOS AND RENATE ..............…..……..123
6.1 Abstract ……………………………………………………..……...…….123
6.2 Introduction ……………………………………………………..……...…….123
6.3 Materials and methods ……………………………………….…….……129
6.3.1 Plant material ……………………………………………….………….129
6.3.2 Controlled atmosphere transition treatment …………………………..129
6.3.3 Experimental design …………………………………………………..130
Gemma A. Chope Cranfield University Ph.D Thesis 2006