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EXPERIMENT STATION LIBRARY
station IJuUetiii 274
Jimc, 1933
POLLINATION AND FRUIT
SETTING IN THE APPLE
By L. P. LATIMER
Cheesecloth cages used to keep insects from trees
during pollination experiments
New Hampshire Aaricultiiral Experiment Station
University of New Hampshire
Dui'ham, X. H.
TABLE OF CONTENTS
PAGE
Development of Orcharding and the Pollination Problem ... 3
Flower Bud Formation and Development 5
Mechanics of Apple Fertilization
Pollination 8
The Flower 8
Fertilization of the Egg Cells 10
Pollination of the Apple Under New Hampshire Conditions
The Need for Pollenizers 13
Effect of Location 15
Tests for Self-unfruitful Orchards 16
Pollination Experiments in New Hampshire, 1927-32
Pollination of Mcintosh 17
Pollination of Cortland 25
Pollination of Delicious and Gravenstein 25
Relation of Pollen Viability to Set of Fruit in Cross Pollination . . 26
Effect of Position of the Flower in the Cluster on Set of Fruit . . 26
Pollination and Orchard Planting and Management
Coincidence of Blooming Periods of Varieties 27
Bees in the Orchard 30
Distribution of Pollenizers in the Orchard 30
Reworking Orchards to Pollenizers Zl
Temporary Relief from Lack of Pollenizers 38
Summary 38
Pollination and Fruit Setting
in the Apple
By L. p. LATIMER
Pollination has come to be a general problem in our New England
orchards in recent years due to the fact that fewer varieties of apples are
now planted in a given orchard than was the case a quarter of a century or
more ago.
I. Development of Orcharding and the
Pollination Problem
The earliest settlers in America brought apple seeds from Europe and
planted them here. As time went on more seedlings were developed and
occasionally plants of a named variety were imported from Europe. Later
the special merits of certain seedlings were appreciated and grafts made
from them to perpetuate the type. Thus different settlers and their descend-
ants in the new world developed fruit gardens containing several to many
trees of quite different types.
In those days wild bees, including bumble bees, were also plentiful and
well able to carry all the pollen needed for pollination of self-sterile varie-
ties which, then as today, would not set fruit when pollenized with their
own pollen.
The result was that in those early times the large mixture of varieties and
plentifulness of wild bees in proportion to the size of planting led to no
difficulties of set as are experienced today. Hence no problem of pollination
became apparent.
Up to the middle of the 18th century, commercial orchards as we now
know them had not been definitely established in America. Grafted trees
were planted before this time, but plantings of "common" or "cider" apples
still predominated.
The rapid growth of New York City, and its importance as a seaport,
resulted in a large market and outlet for American apples. Boston was a
similar center. During the last quarter of the 18th century, considerable
trans-Atlantic shipments of apples were made.
The Newton Pippins (green and yellow) had l)y this time come to be
known as America's highest-quality apple. Its popularity on the continent
followed the receipt of a box by Benjamin Franklin in London in 1758. The
increasing tendency since then has been to plant only those varieties which
seemed most profitable in large centers of population and in foreign trade.
This resulted in the development of the truly commercial orchard where
only grafted and budded varieties of proven merit were planted.
At first the list of such varieties was rather large, because of the various
tastes and demands of consumers who were still used to the products of the
4 N. H. Agricultural Experiment Station [Bulletin 274
older fruit gardens. As the years passed, fewer and fewer varieties were
planted. Local demands and the profit from the different varieties in the
market governed the planter's choice. Varieties tender to cold or unpro-
ductive of large and regular crops, were culled from the orchard. Those
especially subject to disease and insect attacks and those not universally
popular because of color, flavor, or keeping qualities, came to be placed in
the catalog of obsolete varieties.
New varieties were eagerly sought in order to obtain apples better in
some respect than those already established. These changes finally led to
conditions occasionally found in the last of the 19th century and often dur-
ing the present century, where apple plantings that bloomed heavily failed
to produce fruit.
One of the earliest examples of this condition appeared in the pear
industry. Many plantings of Bartlett pears in large blocks were made in
the 70's and 80's. When they reached bearing age they produced little or
no fruit. The trouble was traced to the fact that under most conditions the
Bartlett is partially or wholly self-sterile. The remedy was to plant some
other variety with the Bartlett to provide pollen for its flowers, thus insur-
ing a crop of fruit.
This discovery was made and published by M. B. Waite (1) in 1892-3
and the knowledge has since been invaluable to fruit growers. Even now
self-unfruitful varieties are occasionally planted in solid blocks. In New
England the Mcintosh falls in this class. History will without doubt repeat
itself in this respect whenever new varieties are introduced that are more
profitable than existing ones.
When the error of planting a self-unfruitful variety in a solid block is
discovered perhaps 10 or 15 years later, the remedy is to graft pollenizers
into certain trees in the orchard and to include pollenizers in future plant-
ings. The mistake is costly because five or six years pass before the pollen-
izers grafted into the orchard bloom sufficiently to be effective in cross
pollination. This would make the tree needing pollination at least twenty
years old before a full crop could be produced.
Not so well known is the fact that good pollenizers themselves may not
be readily pollenized by the main crop variety which has shown need of
pollination. Although Delicious wall pollenize Mcintosh, for example, this
is no indication that Mcintosh will in turn pollenize Delicious. Further-
more, inter-sterility may exist between two varieties.
Several varieties of apples in New England produce paying crops, even
when i)lanted in large blocks without pollenizers. They may therefore be
considered self-fruitful sorts. Baldwin is outstanding in this class.
The fact that a variety may be self-fruitful does not signify that it is also
a good pollenizer. The self-fruitful Baldwin is practically useless as a pol-
lenizer for any other important commercial variety grown in New
Hampshire.
Darwin emphasized in 1859 that "Nature . . . abhors perpetual self-
fertilization." He indicated in his work that cross- fertilization is of value
in the plant kingdom. Half a century earlier investigators were aware that
bees carried pollen from one flower to another but were unaware of the
exact nature of the benefit.
(1) M. a. Waite. The pollination of pear flowers. U.S.D.A. Bur. Veg. and Plant Path.
Bui. 5 (1893)
June. 1933] Pollination and Fruit Setting in the Apple 5
The need of cross-pollination is thus a relatively new discovery following
in the footsteps of commercialization of the apple industry. Since the time
Waite attempted to discover what effects the self and cross-transfer of pol-
len would have on the resulting fruit, it has heen determined that the
amount of fruit setting on the various varieties can be increased through
cross-pollination by many other sorts and that, in most cases, this is a very
necessary consideration.
To determine the list of suitable pollenizers for the various cultivated
sorts, certain experiment stations have conducted pollination tests. The re-
quirements of some of the older sorts have been pretty well worked out, but
there will always be the necessity for determining the requirements of new
varieties as they are developed, and likewise the efTectivenss of new varie-
ties as pollenizers for the older ones.
The experiment station may save growers much time by recommending
quality orchard varieties that will poUenize one another satisfactorily. With
a list of such varieties and some idea of the degree of coincidence in their
blooming dates, the grower should be able to pick desirable combinations.
It is to be assumed that he will exercise some judgment in choosing varie-
ties with relation to market demands, climatic and regional adaption and
other points related to profitableness.
II. Flower Bud Formation and Development
Inasmuch as the set of fruit is intimately connected with the blossom
(the apple fruit being formed from the basal portion of the flower), it is
very essential to keep the tree in such a state of vigor that it will produce a
plentiful supply of strong flower buds.
The first essential is that the tree be grown properly. A strong frame-
work, proper fertilization of the soil and good management of water-
supply are necessary. When the tree has reached the age to produce flower
buds, attention must be paid to this process. Without these, or with the
formation of only weak ones, no fruit will result.
The flower-cluster buds on the apple tree are produced principally on
short growths or shoots called spurs. With some varieties flower-bud clus-
ters are also formed at the tips of new shoots or in the axils of the leaves
of new growth. In any event this is a summer development preceding
blossoming.
Microscopic examination of buds on spurs of the Baldwin and Mcintosh
has shown that in New Hampshire the first stages of flower-bud differenti-
ation can be detected from the middle to the last of July.* At this time
with the aid of the microscope, buds that are going to form only leafy
shoots or spurs may be readily told from those that have the possibility of
ultimately forming fruits. It must be remembered that for the buds to ma-
ture into good blossoms and fruit, favorable conditions must be maintained
for their continued development.
* E. J. Rasmussen. The period of blo-ssom-bud differentiation in the Baldwin and Mcintosh
apples. Amer. Soc. Hort. Rci. Report 26-255-260 (1929).
N. H. Agricultural Experiment Station [Bulletin 274
Fig. 1. Bud of Baldwin. July 29, 1928 (after Rasmussen).
Typical of leaf and shoot buds.
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Fig. 2. An earlv stage in tlie development of a flower bud cluster.
Ikddw'.n, August 20. 1928 (after Rasmussen).
June, 1933] Pollination and Fruit Setting in the Apple 7
Fig. 1 shows just how a leaf or shoot bud looks when cross-sectioned and
observed under the microscope. The hard outer scales enclose embryonic
hairs, leaves and leaf primordia. The growing tip located in the center has
a smooth, rounded surface. All leaf and flower buds in the first stage of
development show this appearance.
After the first of July in New Hampshire, some of the buds on trees of
bearing age may show a change in the character of this central growing
point. Instead of remaining nicely rounded, the outline of the growing tip
becomes marked by angular projections. These elongate, so that after a
period of some three weeks, the buds have changed in appearance to that
noted in Fig. 2. Here the rudiments of the various flowers of the cluster
have been formed. Fig. 3 explains the parts and their regular succession.
The first prominences to appear are those of the outermost part or calyx ;
those of the pistil and ovules are last.
Under favorable conditions the flower buds continue to develop slowly
Fig. 3. Illustrating the development of the flower bud cluster. Bald-
win, March 9, 1933 (25x). A. Terminal bud of the cluster. B. Lateral
buds of the cluster, a. Primordia of calyx or sepals, b. Primordia of
petals, c. Primordia of stamens, d. Primordia of pistil, e. Cavity
which eventuallj' becomes the ovary with its contents — ovules, etc.
through fall and winter. During April the primordia rapidly complete their
development into calyx, petals, stamens and ovary. Then in May warm
weather brings rapid growth and swelling of the flower buds. They subse-
quently burst into bloom, with matin-e pollen and ovules.
Much can happen to prevent many or all of these embryonic flower buds
from reaching the mature stage. Factors that strongly inhibit fruit or
flower-bud formation are: 1. A heavy set' of fruit such as may occur espe-
cially in biennial bearers like Baldwin, Wagener, and Wealthy. 2. Heavy
shading, as from foliage of an extra-vigorous tree. 3. Cool w^eather or
8 N. H. Agricultural Experiment Station [Bulletin 274
dark days about the time fruit-bud formation usually takes place. 4. Im-
proper balance between food and nutrients, which may be caused by lack of
leaf surface due to insect attacks or lack of minerals in the soil, especially
nitrogen. Once flower buds have been initiated, conditions for carbohy-
drate, moisture, and mineral supplies must be kept as near the optimum as
possible to insure continued development of the buds.
The tree must also be brought safely into the rest period by prevention
of growth late in the season. Late growth may result from injudicious
applications of nitrogen or excess moisture in the soil. In the cultivated
orchard a cover crop planted about July 1 will alleviate the latter condition.
In the winter there is danger with the more tender varieties that 25° to
30° F., below zero will freeze some or all of the flower buds. Once killed by
freezing, no other flower buds will form in time to open in the spring. The
leaf buds are more hardy and do not freeze at these temperatures. Provided
the hazards of winter have been avoided, there is still danger from spring
frost where a temperature of 25° or 26° F. may totally ruin the fruit crop
through freezing of the unfolding buds or open blossoms. Under such con-
ditions the pistils are usually injured. The pollen may also be injured and
often the ovules.
To insure the proper nutrient and moisture conditions, an appHcation of
nitrogen is generally made in the sod or sod-mulch orchard a short time
before the trees come into bloom in May and a mulch applied a little before
July 1 to conserve moisture. This early nitrogen application is beneficial
to the setting of fruit, to fruit-bud formation, and to the further develop-
ment of the fruit. Light frost after the buds have appeared may not inter-
fere at times with the set or pollen formation but may cause injury to the
outer layer or epidermis of the flower parts. The result will be a russeted
fruit or one with russet rings, cracks, or malformations.
III. Mechanics of Apple Fertilization
Pollination:
Pollination means the transfer of pollen from the stamens to the pistils
of the flowers. With apples this is mostly eft'ected by insects, principally the
various kinds of bees. The pollination of apples cannot be assured through
the agency of the wind.
Pollination is only one step toward insuring a set of fruit. FertiHzation
of the egg cells in the ovules is necessary with most apples to cause fruit to
set. This fertilization process results in the formation of seed. Further
growth of the tissue of the fruit is directly correlated with seed develop-
ment. The pollen of only certain varieties can under certain conditions
effect satisfactory fertilization and set, hence the need to study the value of
difl"erent varieties as pollenizers for the different apple sorts.
To show the relation of various factors to the fertilization of flowers and
set of fruit, a short description of the structure of the flower and the pro-
cess of egg fertilization is presented.
The Flower:
Fig. 4 shows the cross section of an apple flower. Figs. 5, 6 and 7 show
and describe the essential parts in detail. The important parts of the flower
with relation to pollination are the stamens and pistils. Each stamen con-
June, 1933] Pollination and Fruit Setting in the Apple
Fig. 4. Cross section of apple flower, a. Sepal or calyx, b. Petal,
c. Filament of stamen, d. Anther of stamen, containing pollen, e. Pis-
til, f. Stigma of pistil, g. Ovary containing ovules. It is the ovary and
the adjacent fleshy parts of the flower that ultimately become the fruit.
• /
Fig. 5. Upper portion of stamen (Mcintosh), a. Anther (40x).
b. Filament.
10 N. H. Agricultural Experiment Station [Bulletin 274
sists of an anther, a small yellowish sac attached to the end of a slender
\vhite filament or stalk. The anthers contain thousands of minute rounded
cells known as pollen grains which contain sperms, the male elements of the
flower.
The cultivated varieties of apples ordinarily have 20 stamens, although
occasionally small petal-like organs may be found to have replaced some of
the stamens, and sometimes the filaments of the stamens may appear petal-
like.
Each pistil in the apple consists of five slender green stalks which are
coalesced in the lower part to form a single cylinder of tissue, usually more
or less covered with fine hairs. The five individual parts of the pistil are
called styles. Each style is capped with a stigma. The stigma has a papil-
lated surface (see Fig. 7.). It is on this surface that the pollen grains are
left by the insects and this is the place where the germination of the pollen
grains takes place.
The female portion of the flower consists of three parts : ovary, style
and stigma. The ovary is the basal portion of this apparatus. It lies im-
Fig. 6. Cross section of mature anther of Mcintosh (35x). a. Fila-
ment attachment, b. Cavity containing pollen grains, c. Integuments
which open out as the anthers dry, exposing the pollen.
bedded in the tissues beneath the calyz and contains the ovules, occasionally
20 in number ])ut in most varieties 10. Each ovule contains a female element
or egg cell, so that with perfect fertilization it is possible in some varieties
to develop 20 seeds in the apple. Fertilization and complete development
of all the ovules into seeds seldom occurs in any apple, five to 10 seeds
being the usual number.
Fertilization of the Egg Cells:
To fertilize the 20 ovules of a Mcintosh apple ovary, at least 20 dififer-
ent pollen grains are needed. As it happens only a few of the thousands of
pollen grains ])roduce(l ultimately reach an ovule to effect fertilization.
Individual pollen grains when mature are crowded closely together in
the anther. When the flower bud opens and the anther is exposed to the
sun and dry air, it splits open leaving the pollen exposed.
This pollen looks to the unaided eye like a fine, powdery-yellow dust.
Each grain is too small to be seen as an individual by the unaided eye. The
pollen grain (Fig. 8A) when saturated with water is slightl}- triangular in
June, 1933] Pollination and Fruit Setting in the Apple
11
uJ-^^J^O^X
A
FiQ. 8
B
-a
— a
Fiq.9
Fig. 7. End of pistil of Mcintosh (45x). a. Papilla-like terminal cells
of stigma, b. Pollen grain caught in terminal cells.
Fig. 8. A. Pollen grains after absorbing water (270x). B. Pollen
grains before absorbing water (270x).
Fig. 9. Pollen grains after three hours in germinating medium of 15
per cent sucrose (270x). a. Pollen tubes shortly after germination of
the pollen.
shape but with a well-rounded surface which is somewhat roughened, enab-
ling it to adhere to the bodies of bees and other insects that visit the flowers.
Perhaps 75,000 pollen grains are produced by each vigorous, healthy
apple flower and all these could easily be carried on the body of a single
bee. Should each pollen grain from a single flower ultimately fertilize an
ovule, then the pollen from this one flower would be capable of causing the
setting of from 7,500 to 15,000 fruits, or the crop of 10 trees. What actu-
ally happens is that not more than one pollen grain in 10.000 ever reaches
12
N. H. Agricultural Experiment Station [Bulletin 274
and fertilizes an ovule ; so it takes the blossoms on one whole tree of a
pollenizer to insure the production of a good crop on ten other trees.
Under natural conditions in the orchard many factors prevent the effi-
cient utilization of pollen. Weather and the presence or lack of insects are
most important in this respect. The honey and bumble bees are the principal
insects involved in the transfer of apple pollen.
The mature pollen grain is a specialized cell containing a great amount of
stored food. A large nucleus occupies a portion of it, or in the case of apple
pollen, two nuclei, one of which is known as the "tube" nucleus because it
directs the course of the pollen tube growth dow-n the style into the ovary.
The other is known as the generative nucleus because it contains the male
elements that function later in the fertilization process.
Fig. 10. Terminal and undcTlying cells of stigma (OOOx).
The nuclei and the stored food are enclosed by two layers of material.
The outer one is composed chiefly of cutin, a substance very resistant to
moisture and to the penetration of chemical substances.
The bee visiting the opened flower in search of either nectar or pollen
brushes against the opened anthers and becomes covered with pollen grains
whicli stick to the hairs covering the body of the insect. Some of these
grains mav be deposited on the surface of the stigma of other flowers or
possibly the same flower.
On reaching the stigma the normal pollen grain germinates by sending
out a so-called "tube" (Fig. 9.). This penetrates the tissue of the stigma
and grows downward through the central portion of the style in the direc-
tion of the ovary. This i)rogress is accompanied by the movement of the
tube nucleus down through the stylar tissue, with the generative nucleus
traveling liehind or l)eside it. During this period the latter divides into two
sperm nuclei.
June, 1933] Pollination and Fruit Setting in the Apple
13
As the nuclei move farther and farther down the style, the older part of
the tube becomes plugged with callose, which separates the growing part
forever from the original pollen grain.
The pollen tube finally reaches the ovary, enters an ovule and discharges
the male nuclei or sperms. One of these fuses with the egg nucleus and
an embryo is formed. The ovule then develops into a structure known as
the seed. Without fertilization the seed would not develop. If reports of
their occurrence can be considered authentic, cases of seed development
without fertilization of the ovules rarely occur in the apple.
It is the development of viable seed that is so important to the set and
further development of the fruit. A tree soon sheds flowers in which a
sufficient number of ovules are not fertilized. Other flowers in which ovules
become aborted are also generally dropped. Flowers may set fruit heavily,
apparently with plenty of seeds, yet the June drop may be heavy because of
the competition for food and water. To insure the sticking of fruit that has
been fertilized, it is important, therefore, that the tree be properly supplied
with moisture and nitrogen.
__b
Fig. 11. a. Pollen grain after alighting on stigmatic surface b.
IV. Pollination of the Apple Under
Neyv Hampshire Conditions
The Need fcr Po'Jenizers:
Self-unfruitfulness. A large majority of apple varieties and nearly all
of the important commercial sorts are unable to develop seed or set a crop
of fruit when pollenized with their own pollen. These are considered self-
unfruitful, because of the general inability of the pollen to efifect self-
fertilization.
In orchard observations the self-unfruitfulness of certain varieties has
often been strongly suspected, but carefully controlled hand-pollination
experiments have been necessary to prove the fact. The supposition has
also been advanced that certain varieties were especially good poUenizers
for these self-unfruitful sorts. This has often been indicated in orchards,
one section of which has a given variety in the vicinity of certain other
14 N. H. Agricultural Experiment Station [Bulletin 274
varieties. Heavier sets of fruit have consistently been obtained in such
areas as compared with other parts remote from these pollenizer trees.
This can be checked through hand-pollination experiments.
Even though liberally supplied with bees, plantings of large blocks of a
single variety of apple such as Delicious, for instance, have often failed to
set fruit. This has frequently led to the supposition that under existing
environmental conditions the lack of set was due to self-unfruitfulness.