Manipulation of Flowering. Proceedings of Previous Easter Schools in Agricultural Science

411 Pages·1987·9.72 MB·English

by  J. G. Atherton (Auth.)

Proceedings of Previous Easter Schools in Agricultural Science, published by Butterworths, London
•SOIL ZOOLOGY Edited by D.K.McE. Kevan (1955)
•THE GROWTH OF LEAVES Edited by F.L. Milthorpe (1956)
•CONTROL OF THE PLANT ENVIRONMENT Edited by J.P. Hudson (1957)
•NUTRITION OF THE LEGUMES Edited by E.G. Hallsworth (1958)
•THE MEASUREMENT OF GRASSLAND PRODUCTIVITY Edited by J.D. Ivins (1959)
•DIGESTIVE PHYSIOLOGY AND NUTRITION OF THE RUMINANT Edited by D. Lewis
(1960)
•NUTRITION OF PIGS AND POULTRY Edited by J.T. Morgan and D. Lewis (1961)
•ANTIBIOTICS IN AGRICULTURE Edited by M. Woodbine (1962)
•THE GROWTH OF THE POTATO Edited by J.D. Ivins and F.L. Milthorpe (1963)
•EXPERIMENTAL PEDOLOGY Edited by E.G. Hallsworth and D.V. Crawford (1964)
•THE GROWTH OF CEREALS AND GRASSES Edited by F.L. Milthorpe and J.D. Ivins (1965)
•REPRODUCTION IN THE FEMALE MAMMAL Edited by G.E. Lamming and E.C. Amoroso
(1967)
•GROWTH AND DEVELOPMENT OF MAMMALS Edited by G.A. Lodge and G.E. Lamming
(1968)
•ROOT GROWTH Edited by W.J. Whittington (1968)
•PROTEINS AS HUMAN FOOD Edited by R.A. Lawrie (1970)
•LACTATION Edited by LR. Falconer (1971)
•PIG PRODUCTION Edited by D.J.A. Cole (1972)
•SEED ECOLOGY Edited by W. Heydecker (1973)
HEAT LOSS FROM ANIMALS AND MAN: ASSESSMENT AND CONTROL Edited by J.L.
Monteith and L.E. Mount (1974)
•MEAT Edited by D.J.A. Cole and R.A. Lawrie (1975)
•PRINCIPLES OF CATTLE PRODUCTION Edited by Henry Swan and W.H. Broster (1976)
•LIGHT AND PLANT DEVELOPMENT Edited by H. Smith (1976)
PLANT PROTEINS Edited by G. Norton (1977)
ANTIBIOTICS AND ANTIBIOSIS IN AGRICULTURE Edited by M. Woodbine (1977)
CONTROL OF OVULATION Edited by D.B. Crighton, N.B. Haynes, G.R. Foxcroft and G.E.
Lamming (1978)
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SEED PRODUCTION Edited by P.D. Hebblethwaite (1980)
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PHYSIOLOGICAL PROCESSES LIMITING PLANT PRODUCTIVITY Edited by C. Johnson
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Edited by M.H. Unsworth and D.P. Ormrod (1982)
CHEMICAL MANIPULATION OF CROP GROWTH AND DEVELOPMENT Edited by J.S.
McLaren (1982)
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SHEEP PRODUCTION Edited by W. Haresign (1983)
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ETHYLENE AND PLANT DEVELOPMENT Edited by J.A. Roberts and G.A. Tucker (1985)
THE PEA CROP Edited by P.D. Hebblethwaite, M.C. Heath and T.C.K. Dawkins (1985)
PLANT TISSUE CULTURE AND ITS AGRICULTURAL APPLICATIONS Edited by Lyndsey
A. Withers and P. G. Alderson (1986)
CONTROL AND MANIPULATION OF ANIMAL GROWTH Edited by P.J. Buttery, D.B.
Lindsay and N.N. Haynes (1986)
COMPUTER APPLICATIONS IN AGRICULTURAL ENVIRONMENTS Edited by J.A. Clark.
K. Gregson and R.A. Saffell (1986)
• These titles are now out of print but are available in microfiche editions
Manipulation of Flowering
J. G. ATHERTON
University of Nottingham School of Agriculture
Butterworths
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First published 1987
© The several contributors named in the list of contents, 1987
British Library Cataloguing in Publication Data
Manipulation of flowering.—(Easter
schools of agricultural science)
1. Plants, Flowering of 2. Plant
regulators 3. Crops—Growth
I. Atherton, J.G. (Jeffrey Gordon)
II. Series
631.5'4 SB126.8
ISBN 0-407-00570-6
Library of Congress Cataloging in Publication Data
Manipulation of flowering.
Proceedings of the 45th University of Nottingham
Easter School in Agricultural Science held at Sutton
Bonington, April 7-10, 1986.
Bibliography: p.
Includes index.
1. Plants, Flowering of—Congresses. I. Atherton,
J.G., 1948- . II. Easter School in Agricultural
Science (45th: 1986: Sutton Bonington, Nottinghamshire)
SB126.8.M36 1987 635.9Ί544 86-23301
ISBN 0-407-00570-6
Photoset by Latimer Trend & Company Ltd, Plymouth
Printed and bound by Robert Hartnoll Ltd, Bodmin, Cornwall
PREFACE
Genetic, environmental and chemical means of manipulating flowering are used
extensively in agriculture and horticulture. Promotion of flowering is required by
growers of early flower, seed and fruit crops and by plant breeders, whilst suppression
of flowering is particularly important to sugar producers and growers of vegetables
and late crops. Considerable research effort has been expended on the elucidation of
processes that regulate flowering. The 45th University of Nottingham Easter School
in Agricultural Science held at Sutton Bonington from 7-10 April 1986 brought
together crop researchers, plant physiologists, geneticists and other interested parties
from industry and academia to consider the results from this research that were most
relevant to agriculture and horticulture. Considerable mutual benefits were derived
from this interaction and consequently more effective manipulation of flowering can
now be expected.
This book presents the edited proceedings of the Easter School. The first main
section examines measurement and prediction of flowering. It addresses problems of
how best to measure flowering when the aim is either to construct predictive models
or to assist physiological interpretations. This integrated assessment of flowering
provides a useful perspective for the detailed analyses of particular stages in flowering
which follow. Juvenility and the nature of determination in meristems are examined,
followed by aspects of vernalization, photoperiodic induction and flower evocation,
initiation and development to anthesis. Each section opens with an extensive review
and continues with a number of research orientated chapters. The main sections are
preceded by an analysis of the flowering problems and followed by a critical view of
how to achieve a better understanding and use of the physiology of flowering.
The success of the Easter School was a credit to the contributors and delegates
alike, and I thank them all most sincerely. The splendid organization of the School
was largely due to the skill and hard work of Mrs Marion Wilton and I am
particularly grateful to her. I would like also to thank Charles Wright, David Hand,
Trevor Lord, Carol Williams and other members of my department for their capable
management of the visual aids and visits. Finally I wish to thank the Session
Chairmen—John Monteith, Alan Longman, Ian Sussex, Dick Whittington, Lloyd
Evans, Ken Cockshull and Peter Harris.
The Easter School could not have taken place without the generous financial
support kindly donated by many commercial organizations. They are listed separa-
tely in the Acknowledgements. I would also like to acknowledge here the grant aid
received from the Royal Society and the British Council.
Jeff Atherton
ν
ACKNOWLEDGEMENTS
Donations are gratefully acknowledged from the following:
Asmer Seeds Limited
BASF United Kingdom Limited
Bayer UK Limited
British Petroleum Company pic
Ciba-Geigy Agrochemicals
Dow Chemical Company Limited
FBC Limited
Imperial Chemical Industries pic
Marks and Spencer pic
May and Baker Limited
Miln Marsters Group Limited
Monsanto Europe SA
Nickerson RPB Limited
Sandoz Products Limited
Sharpes (Charles Sharp and Company, pic)
Shell Research
Tozer Limited
Unilever Research
Yoder Toddington Limited
vi
1
THE FLOWERING PROBLEM
W.W. SCHWABE
Department of Horticulture, Wye College,
University of London, Nr. Ashford, Kent
In introducing the topic of flowering for this book, it would not be appropriate merely
to review the latest literature nor is it the place to present fresh results on some narrow
aspect of the wide field, which anticipate the detailed research reports that follow. An
attempt is made here to pinpoint some crucial areas which would justify inclusion
under the heading of The flowering problem'. These are not new, but are discussed in
order to focus attention on areas where progress has been made, or would be useful.
Of these topics the one referred to as 'evocation' is morphogenetically perhaps the
most striking aspect and may deserve rather more attention.
Apical change
Basically, the problem of flowering is to discover the underlying causes for the
relatively sudden transition of a vegetative growing point from the production of
leafy organs to the formation of floral organs. The floral organs themselves almost
certainly represent homologies with leaves, e.g. sepals, petals, stamens and carpels.
They are of bilateral symmetry, and this is also borne out by their vascular
anatomy—cf. the 'gonophyll theory' of Melville (1962, 1963). Thus the production of
lateral organs still continues the foliar pattern on a cauline structure, which itself
continues to exhibit radial symmetry. The reasons why lateral organs should be
formed at all on such cauline structures clearly goes back to the Psilotalean
organization pattern and a consideration of possible mechanisms is beyond the scope
of this discussion on flowering.
The switch to the formation of a new type of lateral organ is nearly always
associated with general changes at the growing point itself, often in both the so-called
'bare apex' and the primordial initiation zone, the 'anneau initial'. This could be via
the operation of a series of quite specific morphogens, but the kind of lateral organ
being formed on the cauline structure may itself be a consequence of the size of the
cauline axis (e.g. diameter, or surface area)—cf. Bernier and Nougarède (e.g. in
Bernier, 1979)—and, for instance, a smaller meristem than the normal vegetative apex
might entail ipso facto the development of sepals or anthers rather than leaves, though
this is still in the realm of speculation. Moreover, in many instances there is an
associated change in apical dominance, nearly always a weakening. This is usually
temporary, and when the new type of organ, homologous with leaves, is formed,
dominance is often re-established—in as much as this is still possible. There are
3
4 The Flowering Problem
several distinct types of development: apices ending in a single flower (e.g. Viscaria,
apices ending in an inflorescence terminated by a single inflorescence (e.g. the terminal
spikelet in the wheat ear) or with a compound inflorescence (e.g. in the compositae).
There are only relatively few examples where the terminal apex itself normally
reverts again to the formation of leaves, in a continuation of vegetative growth (e.g.
Ananas comosa). As a teratological phenomenon, however, this is not uncommon.
Another factor that is of significance in this respect is the apparent antagonism (or
competition) between foliar growth and cauline development. While a discussion of
the possible mechanism of such antagonism and the reasons for the development of
axillary buds, i.e. cauline structures in the axils of leaves, would again take this review
beyond its confines, there is little doubt about the existence of this competition. It is
seen very clearly in the Hop apex (Humulus lupulus), where at the onset of flowering
cauline development gradually predominates (Thomas and Schwabe, 1970). Another
excellent example is afforded by the development of the cereal ear, where the shoot
apex, as seen soon after seed germination, is a relatively short cylindrical, cauline,
structure with single ridges (leaf initials). The latter weaken and their developmental
period becomes progressively curtailed, while the development of the axillary buds in
their axils (cauline structures) becomes pari passu greater, until only the upper ridge
(spikelet) develops at all, the cauline development having totally suppressed foliar 4
development {Figure 1.1) (Hutley-Bull and Schwabe, 1980). Subsequently the
development of the spikelet axis itself, with the formation of glume initials (i.e. foliar
type structures) repeats this pattern of development.
Similarly, in the development of the composite flower, foliar development is
increasingly reduced until the first inflorescence bracts are formed and then it is
suppressed either completely, or reduced to minute scales when floret initiation occurs
on the much enlarged receptacle. In those members of the Compositae family where
total suppression is the rule (Chrysantheminae—according to Rendle, 1925) a partial
reversal from the flowering condition can involve their reappearance (Schwabe,
1951).
The temporary lapse of apical dominance referred to above would appear to
coincide with this temporary supremacy of cauline development, but this dominance
is re-established as soon as foliar structures arise on the new axillary cauline
meristem; the new 'foliar' type of development being represented by the floral
organs themselves. Although the underlying mechanism is, obviously, not yet
understood, even in part, it would seem likely that auxin production and transport
may be causally involved. If the young, bilaterally symmetrical, organs (usually
leaves) are the source of the apical auxin production, perhaps especially in their
earlier stages of development (e.g. Snow, 1937), a lapse in their formation would
explain a 'wave trough' of reduced auxin flow, and consequently reduced apical
dominance, and this may then be followed by a new surge when floral organs are
being formed in large numbers, such as in the composite inflorescence. A closely
similar situation might obtain in the cereal ear between the stages of spikelet initiation
and the formation of the glumes. A resulting expression of this behaviour may also be
found in the relative internode lengths below an inflorescence in the Compositae.
Thus in the Chrysanthemum one, or more commonly several, relatively short
internodes are formed below the inflorescences, followed again by very long ones. The
length of these latter internodes, however, depends on the presence of the developing
florets, and when these are removed the internodes fail to elongate, unless an
exogenous auxin supply (IAA in lanolin) replaces them, in which case normal
elongation is restored (Schwabe, 1968).
5
Figure 1.1 Growth rates of successive leaves of wheat cv Timmo on terminal apex in
relation to spikelet development in (a) long (16 h) and (b) short (8 h) days at 15 °C.
6 The Flowering Problem
The change to floral organ production is nearly always preceded by shape changes
of the meristem. This may, at its simplest, be assessed by a ratio of two measurements
(e.g. as in the hop plant: Figure 1.2) or in terms of size as in the Chrysanthemum,
where receptacle formation causes an enlargement of some 400 times the area of the
vegetative meristem, and this breaks up into hundreds of small floret apices at that
stage.
The transition to floral development thus seems to involve a series of correlative
events and changes that occur before, during and after evocation is initiated—
generally as a consequence of a signal from the leaves.
12 April 3 May 24 May 14 June
Date
Figure 1.2 Changes in apical diameter (μπι) and the ratio of diameter: height of apical
dome with time in the hop, Humulus lupulus cv Fuggle.
The interaction of the meristem and the signal received
The initial stimulus causing this morphological switch appears to originate generally
in the leaves, at least where photoperiodic control is involved, although in such plants
as Rafflesia there must be other sources.
There have been many attempts during the long search for the flowering mechan-
ism to identify the first site of response in the meristem. It may not have seemed
unreasonable to regard the meristem in the vegetative state as relatively undifferen-
tiated biochemically, as an agglomeration of more or less identical cells metabolically,
with perhaps differing degrees of activity. The stimulation of some target cells, or
tissues, by a fairly non-specific trigger could then set off the chain reaction (cascade).
However, more probably there are metabolically different tissues in the shoot
meristem even though less obvious than the 'quiescent centre' which is often obvious
in the root. These may be stimulated into 'action' (possibly faster rates of cell
W.W.Schwabe 7
division, or changed planes of division and/or cell expansion). This would suggest
that the stimulus could be non-specific, but with a specific target. Equally, it may be
that only a specific 'flowering stimulus' could pass along this route and reach the
tissue concerned.
The work of Bernier and his group in determining what these changes are and
equally Lyndon and co-workers' studies are clearly of the greatest significance here
(Bernier et al, 1981; Lyndon and Francis, 1984). As they contribute themselves to
this book (Chapters 21 and 24), only a brief reference to their work must suffice. They
have shown that there are changes that occur very rapidly in response to the leaf
stimulus being received, but do not necessarily lead to the irreversible shift to a new
morphogenetic pathway. However, the hope that they could be simply equated with a
synchronization of cell cycles has been proved wrong by Lyndon and Francis (1984).
While this may not be generally true, it does appear that there are target tissues in the
meristem, i.e. groups of cells that become differentially activated and are often
referred to as the 'prefloral meristem', which may then be responsible for the shape
and/or size changes, temporary loss of apical dominance and for the changed type of
organ produced, i.e. floral differentiation. Much effort has gone into discovering the
possible changes initiated in terms of nuclear activation and hormonal (cytokinin)
effects and also soluble sugar levels, and, if nothing else, they have demonstrated the
complexity of the sequence of responses at that stage. As always in these situations, it
is far from easy to discern the true sequence of events and to distinguish between
cause and effect.
The signal reaching the meristem and its pathway
In all those many species where flowering is triggered, and thereby synchronized, by
the external environment, it is an inevitable hypothesis that some stimulus passes
from the area, or site of perception, to the growing point where the changes are
initiated; hence the search for the very first effects. What kinds of stimulus could reach
the meristem? There is the possibility of a physical stimulus, as suggested by Milburn
(1979), e.g. hydraulic pressure changes, perhaps with rhythmic fluctuations, in the
phloem which could be rapidly transmitted over long distances. The probability of
this is, however, slight, firstly because of the rather non-specific nature of such a
signal, and secondly because numerous disparate environmental effects may have the
same action. It would also be difficult to envisage the operation of such a mechanism
when a prolonged period of favourable cycles is needed to reach a threshold. The
possibility of electrical charges or potential differences is equally unsatisfactory for
similar reasons. This brings one back to the old hypothesis that the signal is a
chemical one, and there are then numerous possibilities. It could be a quantitative
change in any of several major compounds, e.g. carbohydrate (soluble sugars),
amino-acids, or lipids or other components, or combinations of any of them. Once
again the non-specificity of such signals argues against their having a primary role,
though they may be involved perhaps as a secondary message. The transport of more
specific, hormone-like substances still seems more probable. Here, the choice has
hovered for many years between the possibility of quantitative changes in one or more
of the five known hormones, or by the intervention of an unknown flowering
hormone, long-since named 'florigen' by Chailakhyan (1936). There is no need to
reiterate the mass of evidence for a graft-transmissible stimulus. The latter suggestion
may still be said to have general acceptance. If we put the blame not on hormonal