Microbiology of Aerial Plant Surfaces

658 Pages·1976·12.85 MB·English

by  C.H. Dickinson (Eds.)

Microbiology of
Aerial Plant Surfaces
Edited by
C. H. DICKINSON
Department of Plant Biology
University of Newcastle Upon Τ y ne
Newcastle Upon Τ y ne. England
T. F. PREECE
Department of Plant Sciences
University of Leeds
Leeds, England
1 9 76
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PREFACE
Biologists with very different objectives are much int-
erested in the aerial surfaces of plants and the organisms
which live there. We are getting to know these micro-habitats
better, and are beginning to understand some aspects of their
natural history. Plant pathologists are, as always, concer-
ned with the control of some of the organisms on plant sur-
faces, but current concern about biological control of disea-
ses needs a deeper knowledge than we have at the present time
of the microbiology of the aerial parts of plants. This book
comprises the papers presented at a meeting held at the Uni-
versity of Leeds in September, 1975. The contents record
progress in work on the aerial surfaces of plants during the
years 1970-1975 and they extend the review provided by the
record of the proceedings of the earlier meeting at Newcastle
University in 1970*.
The Editors wish to acknowledge the support given to
this venture by Professor P.B.H. Tinker, Head of the Plant
Science Department, University of Leeds. N. Miller-Jones
and M. Milton provided willing practical assistance in the
organisation of the symposium. We would also like to thank
Miss A. Biggins, Mrs. L. Cummings, Mrs. V. Ross and Miss G.
Williams for their tireless help and enthusiastic support in
the production of this volume.
C.H. Dickinson
T.F. Preece
February, 1976
* Reference. Ecology of leaf surface micro-organisms. Ed.
T.F. Preece and C.H. Dickinson, Academic Press,
London and New York, 1971.
ASPECTS OF THE STRUCTURE AND DEVELOPMENT OF THE
AERIAL SURFACES OF HIGHER PLANTS
ELIZABETH G. CUTTER
Cryptogamio Botany Laboratories, The University,
Manchester, Ml3 9PL, U.K.
INTRODUCTION
The epidermis, and later the cork, constitute the plant!s
contact with the outside world. Perhaps in part because of
this, the epidermis, at least, is a very versatile tissue,
comprising many different types of cells in different species.
One of the main functions of the epidermis, and later of the
cork, is protection of the underlying tissues from undue loss
of water, or from injury. Yet paradoxically the epidermis and
its derivatives serve also to control the exchange of gases,
liquids and various metabolic products with the external env-
ironment. Not surprisingly, the epidermis also responds in
various ways to environmental factors. Long studied by clas-
sical anatomical methods, our view of the structure of plant
surfaces has been extended, and certainly rendered more aes-
thetic, by the development of the scanning electron micro-
scope (e.g. Amelunxen, Morgenroth and Picksak, 1967; Troughton
and Donaldson, 1972; Troughton and Sampson, 1973; Dayanandan
and Kaufman, 1973).
In the present paper, the aim will be not merely to con-
vey some idea of the complexity and versatility of the sur-
face tissues of the aerial parts of plants, so as to give
some idea of potential microenvironments for micro-organisms,
but also to concentrate on mechanisms of exchange with the
external environment. Some pathways existing for these pur-
poses may also serve as channels of entry for micro-organisms
of various kinds. This review will deal primarily with the
surfaces of higher land plants, but aquatic organisms and
lower plants should not be forgotten. For example, Sieburth
2 E.G. CUTTER
(1975) has recently shown that the surfaces of many seaweeds
provide homes or resting-places for various kinds of micro-
organisms .
In many plants which undergo secondary growth the epid-
ermis is eventually lost, and sometimes also underlying tis-
sues; in such plants the outer layer, or layers, is replaced
by the periderm. Accounts of the epidermis and periderm can
be found in anatomical texts such as Haberlandt (1914), Foster
(1949), Esau (1960, 1965), Cutter (1969) and Fahn (1974);
consequently this paper will attempt to review more recent
work, though it is not intended in any sense to be comprehen-
sive.
EPIDERMIS
Origin
In cryptogams and gymnosperms the epidermis does not
have a separate origin from other tissues. In angiosperme
with apices having the tunica-corpus type of organisation
(Fig. la), however, the epidermis of the shoot - stem and
leaves - originates from the outer tunica layer. That this
enjoys an independent existence has been shown by the work of
Satina, Blakeslee and Avery (1940) and many others on peri-
clinal chimeras. In Datura, the outermost 3 or so layers of
the apical meristem were caused to differ from each other
cytologically, usually by irradiation; such apices were in-
duced periclinal chimeras. The products of any one layer of
the meristem could thus be followed. Leaf primordia, for
example, in most dicotyledons originate as protuberances on
the flank of the apical meristem (Fig. la); the outermost
cell layer remains continuous and autonomous and gives rise
to the leaf epidermis. Enclosed as they are by young leaf
primordia, the shoot apices of some species are naturally
sterile, but in others a microflora is already present on
the surface.
In some species, the developing surface layer of the
leaf may undergo periclinal division and give rise to a mul-
tiple epidermis of 2-16 layers (Esau, 1965).
STRUCTURE OF AERIAL SURFACES 3
Figure 1. a, Longitudinal section of the apex of an
axillary shoot of potato, Solanum tuberosum cv. King
Edward, showing a 2-layered tunica (t). A young leaf
primordium is on the right. The epidermis of stem and
leaf originates from the outer tunica layer, χ ZOO.
b, Scanning electron micrograph (SEM) of the seed coat
of Petrorhagia velutina showing the tuberculate epid-
ermal cells with sinuous anticlinal walls, χ 400. c,
SEM of seed coat of P.nanteuillii, also with tubercu-
late epidermal cells having sinuous walls, χ 520.
4 E.G. CUTTER
Figure 2. a, SEM of adaxial surface of the leaf of
Gibasis schiedeana showing papillose projections of
3
the cell wall, χ 450. b, SEM of upper epidermis of
leaf of Zea mays,, showing the pattern of wax deposit-
ion, χ 3600.
STRUCTURE OF AERIAL SURFACES 5
Cell wall
Most ordinary epidermal cells have a cellulosae cell
wall, often of sinuous outline. This is well seen in the
epidermis of the testa of some seeds (Fig. lb,c). The cells
shown in Fig. lb and c also have tuberculate outgrowths, so
that the centre of the cell is raised. At least in leaves,
the sinuous outline of the cells is sometimes attributed to
stresses resulting from differential growth of the underlying
tissues. In monocotyledons leaf epidermal cells are usually
elongated in the plane of the long axis of the leaf. Scler-
eids may be present in the epidermis, or may constitute the
whole epidermis, as in the testa of many seeds. In these and
other instances the cell wall may be lignified. On petals
and less frequently on leaves the whole epidermal cell wall
sometimes extends as a papilla. In some species the cell
wall itself may be sculptured and possess regularly arranged
protuberances (Fig. 2a; Stant, 1973).
Cuticle
The cuticle is a layer of fatty material or cutin which
is deposited on the outer side of the epidermal cell wall.
The wall itself may be cutinized, i.e. impregnated with cut-
in. The cuticle is apparently protective, and is usually
believed to be impervious to water. It may act as a barrier
to pathogens and it is resistant to breakdown by other micro-
organisms. The cuticle may be ridged or striated, giving
various patterns (Fig. 2b, 3). A layer of pectin may lie
between the cell wall and the cuticle (Eglinton and Hamilton,
1967).
Although xerophytic plants are usually characterized by
a thick cuticle, experiments showed that there was no direct
relationship between thickness of cuticle and amount of water
loss; the structure and chemical composition of the cuticle
were also important (Martin and Juniper, 1970). Water loss
can occur through the cuticle, since Martin and Stott (1957)
have shown that when grapes are dried to produce sultanas
water passes from the cells to inside the cuticle, and thence
by diffusion through the cuticle until it evaporates on rea-
ching the exterior. The rate of drying of the fruit was in-
versely proportional to the amount of cuticle present.
6 E.G. CUTTER
Figure 3. SEM of leaf of Haworthia cymbiformis
angustata_, shewing ridged pattern of cuticle and wax.
In the centre a stoma lies below a raised cuticular
rim. χ 1230.
STRUCTURE OF AERIAL SURFACES 7
Figure 4. Permeable cuticles. A, Transmission electron mi-
crograph (TEM) through part of the outer wall of a secretory
cell of a sessile gland of Pinguicula grandiflora supplied
with colloidal lanthanum nitrate during the phase of absorp-
tion. This material (dark) has penetrated the cuticle (c)
and cell wall (cw) and is accumulating in the spongy wall
Continued over.