Pollination, Fruit set, Fruit drops, Causes of fruit drops, parthenocarpy and Parthenogenesis.pdf

Pollination is the transfer of pollen from the anther to the stigma of a flower, enabling fertilization. It is a critical process for fruit set, which is the transformation of flowers into fruits following successful pollination and fertilization.

Fruit drops refer to the shedding of fruits before maturity, which can occur due to physiological, environmental, or biotic factors. Common causes include hormonal imbalances, water stress, nutrient deficiencies, pest infestations, or poor pollination.

Parthenocarpy is the development of fruits without fertilization, leading to seedless fruits, as seen in bananas and seedless grapes. It is a desirable trait in horticultural crops for market preference.

Parthenogenesis involves the development of an embryo from an unfertilized egg, primarily observed in some plants. It contributes to seed production in certain crops and aids in breeding programs.

1. Pollination, Fruit set, Fruit drops: Causes of fruit
drops, parthenocarpy and Parthenogenesis
22CHOR11 - Fundamentals of Horticulture 2(1+1)
Dr. M. Kumaresan (Hort.)
Department of Horticulture
Vels Institute of Science, Technology & Advanced
Studies (VISTAS)
Pallavaram, Chennai, Tamil Nadu -600117

2. Pollination
Pollination: Transfer of pollen grains from anther to the stigmatic surface
Two types viz., self-pollination and cross-pollination
Self-pollination: Pollen transfer is from the stamen to the stigma of the same
flower or to the stigma of another flower on the same plant or to the stigma of a
flower on any plant of the same clonal varieties. If it is affected without the aid of
any outside agency, such as wind or insect, then this process is known as
‘autogamy’.
Cross-pollination: Pollen is to be transferred to the flower of another individual
or in the case of pomological varieties to the flower of another variety

4. Self - Pollination

5. Cross - Pollination

6. Different type of flowers
(i) Staminate flowers: Flowers in which only male flowers are found.
For example: papaya, mango, date palm, aonla, grape, coconut, cashewnut,
pomegranate, litchi, walnut etc.
(ii) Pistillate flowers: Flowers in which only female flowers are found.
For example: date palm, papaya, walnut, grape, aonla, coconut etc.
(iii) Hermaphrodite flowers: Flowers in which both male and female organs are
found. For example: mango, papaya, ber, citrus, guava

7. Pollination
Cross-pollination may be required for a number of reasons in horticultural crops
Due to the dioecious nature (e.g. Papaya, datepalm, nutmeg) or monoecious nature
(cucurbits)
Due to the peculiar flower structure, (e.g. brinjal, delicious apples, vanilla).
Due to the dichogamy nature (onion, carrot, sapota)
Due to the behaviour of bisexual flowers as functionally unisexual (e.g. avocado and
allspice)
Due to self-incompatibility or self-sterility factors (e.g. apple, buckwheat)

8. Artificial Pollination
Self-incompatibility is common in apple and pear. This has been recently overcome
with the ‘recognition’ or ‘mentor pollen’ technique. In this technique pollination is
affected with a 2:1 mixture of Methanol killed or irradiated compatible pollen and
self-pollens which results in seed set.
Another technique ‘pioneer pollen’ is also reported to increase the seed set in fruit
crops. In this method, pollination twice with compatible pollens at an interval of
1-2 days is done.
It appears that the first applied pollens promote the activities of the pollens in the
second application; hence the first applied pollen is called ‘pioneer pollen’.

9. Artificial Pollination
In emergent situations, when adverse weather conditions prevail or the orchard has
inadequate pollinizers, hand pollination can be resorted to. This is much helpful in
temperate fruit orchards and is being practiced in European countries and not yet
in India.
The followings are some of the methods by which artificial pollination is done:
1. Artificially collecting the pollens, mixing it with the spores, of Lycopodium
(fern) and applying to the flowers with a soft brush.
2. Placement of bouquets: in this method, branches of flowers of pollinizers are
hung in the trees to be pollinated.

10. Fruit set
In an orchard, all the fruit trees do not bear equally or regularly, sometimes one
fails to bear at the same time another tree of the kind under similar conditions
produces a heavy crop.
This problem may be due to failure to set the fruits, unfruitfulness and sterility.
Fruit setting: initial setting of fruit at or just after the time of blossoming and to its
remaining on the plant until maturity.
Fruitful: not only blossoms and sets fruit but carries it through to maturity.
Plants which are unable to do this are also as ‘unfruitful or barren’.

11. Fruit set
Fertility: Ability of the plant not only to set and mature fruits but to develop viable
seeds.
Inability of a plant to do this is known as infertility or sterility.
Fertile plants are necessarily being fruitful and all the plants need not be fertile.
Self-fruitfulness: Ability of the plants to mature fruit without the aid of pollen from
some other flower, plant and those plants are known as self-fruitful plants.
Self-fertility: Ability of plant to mature viable seeds without the aid of pollen from
some other flower. Such plants are known as ‘self-fertile.

12. Causes of fertility unfruitfulness
The causes of fertility unfruitfulness and failure to set the fruits be grouped into
two main categories as those ‘internal factors and those ‘external or environmental
factors.
1. Unfruitfulness Associated with Internal Factors
These factors may be grouped into three as:
• Those due to evolutionary tendencies
• Those due to genetic influences and
• Those due to physiological factors

13. I - Unfruitfulness Associated with Internal Factors
A) EVOLUTIONARY TENDENCIES
Production of imperfect flowers as a cause of evolution may lead to unfruitfulness.
Dioecious and monoecious nature: Monoecious forms like melons are self-fruitful
but in dioecious forms like papaya pollinizers have to be provided.
It is necessary to retain few of the staminate trees to ensure a good fruit set.
Nutmeg which is a dioecious tree also requires pollinizers for good fruit set.
The ornamental type of pomegranate flowers fails to set fruit. This is because they
are unisexual.

14. I - Unfruitfulness Associated with Internal Factors
A) EVOLUTIONARY TENDENCIES
Heterostyly: In certain cases, the structure and form of the flowers are peculiar in
nature to prevent self-pollination and make cross pollination more certain. If cross
pollination does not take place, the plant is likely to remain unfruitful even though
perfect sex organs have been developed. One such form is heterostyly; the style and
stamens are at different height that self-pollination is prevented. For example, in
brinjal, four types of flowers are present depending upon the length of style viz.,
long; medium, pseudo short and true short style types. The last two do not produce
any fruits at all.
In Delicious group of apples, upright position of the stamens combined with the
spreading the petals permit the bees to collect the nectar without touching the pistil
or anther.

16. I - Unfruitfulness Associated with Internal Factors
A) EVOLUTIONARY TENDENCIES
Dichogamy: when the receptivity of the stigma and viability of pollen occur in
different periods, it is known as dichogamy. This results in low production of fruits.
If stamens ripen before the pistil, it is known as protandry (e.g. Annona) and if pistil
ripens before the stamens, it is known as protogyny (e.g. Sapota).
In Avocado, each flower opens twice and is
closed in between. It behaves as a female flower
during first opening time and during the second
time, it behaves like male flower thus preventing
self-pollination.

17. I - Unfruitfulness Associated with Internal Factors
A) EVOLUTIONARY TENDENCIES
Abortive flowers or aborted pistils or ovules: In Pecan nut the terminal portion of
the pistillate clusters is almost defective and is shed at pollination time.
The late flowers of strawberry are always abortive.
Impotence of pollen grains: Many varieties of grapes produce impotent or non-viable
pollens though they appear to have perfect flower type.

18. Unfruitfulness Associated with Internal Factors
B) GENETIC INFLUENCES
These types of causes are more directly due to genetic factors and are inheritable.
1. Sterility and unfruitfulness due to hybridity: More widely if the parents for
crossing are related, greater is the degree of self-sterility. A hybrid between pear
and quince viz., ‘Purina’ flowers and sets fruits freely but the fruits are seedless.
Seedlessness in most of the commercial varieties of banana and pineapple are due
to hybridity.
2. Incompatibility: It refers to the incompatibility between pollen and the ovule of
the same plant or of the same variety. Pollen and ovule are fertile but they fail to
effect conjugation. In apple, pear and plum incompatible varieties require another
pollinizer for fruit setting.

19. I - Unfruitfulness Associated with Internal Factors
C) PHYSIOLOGICAL INFLUENCES
1. Due to slow growth of the pollen tube. e.g. Cole crops.
2. Premature or delayed pollination: In tobacco flowers, if matured pollens are
applied to the immature pistils, they germinate and enter the ovules but the flowers
soon fall.
3. Nutritional conditions within the plant: Nutritive conditions of plant just before
or at or and just after the time of blossoming is important in determining the
percentage of flowers carrying for setting and for maturity. Its effect may be on
pollen viability or on fertility of the pistils. In apple, pollen from older trees is
always less fertile than the pollens from young vigorous trees. In plums, exhaustion
or weakening of the trees in one season by over bearing, drought and poverty of soil
is associated with the production of many defective pistils in the following year.

20. II. Unfruitfulness Associated With External Factors
Nutrient supply: Sterility normally occurs in certain families due to over feeding,
e.g. Graminae, Cruciferae and leguminaceae. It has been observed that Jonathan
apple, which is ‘self-sterile, on rich soil is becoming ‘self-fertile’ in poor soils.
Pollinizers: In temperate fruit crops like apple, adequate number of pollinizers are
to be provided, otherwise it affects the fruitset.
Pruning and training: Pruning tends to produce more true hermaphrodite condition
in variety ‘Hope’. If pruning is not done the variety tends to remain sterile and
produces aborted pistils.
Locality: Jonathan apple which is sterile in one location is reported to be ‘self-
fertile’ in another location.
Season: Ideal is a hybrid grape cultivar which is ‘self-impotent’ early in the season
but turns to be self-potent later on.

21. II. Unfruitfulness Associated With External Factors
Temperature: High temperature at flowering will dry up the stigmatic fluid and
prevent pollination. Tomato varieties do not produce any fruit when grown at high
temperature i.e. when day temperature goes beyond 37°C.
Light: The development of stamens and petals in strawberry flowers takes place
only when plants are exposed to long photoperiods.
Pests and diseases: Hopper in mangoes, mildews in mangoes and grapes etc. affect
the development of fruit leading to poor fruitset.
Spraying the trees when they are in bloom: Spraying reduces the crop set during
blossoming. Some of the fungicides have inhibiting effect on pollen grains.
Bordeaux mixture at 200 to 10,000 ppm prevent the germination of pollen grains
on the stigma.

22. Fruit Drop
Fruit trees usually bear a large number of flowers and only a small percentage of
which are enough to give a normal yield. For instance a single inflorescence of
mango contains as many as 6000 flowers but an average of 5 fruits per inflorescence
would provide a good to heavy crop, however, the actual percentage of fruitset will
be much lesser.
When the fruitset is much more than the trees can normally carry to maturity,
there will be drop of fruits at various stages of development as an adjustment of
tree to its resources.
Such a drop is natural and beneficial to the trees and it will prevent exhaustion of
the resources and breaking of branches by over bearing.

23. Fruit Drop
This drop occurs at three stages as indicated below:
First drop: It occurs shortly after flower opening. Usually, flowers with
aborted pistils drop off at this stage. Lack of pollination, low stigmatic
receptivity, defective flowers, poor pollen transference, and occurrence of
incompatibility are some of the causes attributed for this drop.
Second drop: It occurs a fortnight later than the first drop. This drop
includes unfertilized flowers and some fertilized flowers. Fertilized
flowers also drop off at this stage as a result of adjustment in the trees
between nutritional factors and fruitset.

24. Fruit Drop
Third drop: This drop occurs when the fruits are ‘marble size’ due to the formation
of abscission layers in the young fruit stalks.
This drop is generally common in most deciduous fruits and as it coincides with
the month June, this drop is also known as ‘June drop’.
This natural thinning of fruit helps the trees to produce remaining fruits with good
size.
The above drops which occur in three stages are beneficial to the fruit tree.

25. Fruit Drop
Pre-harvest Drop: Another kind of fruit drop which is a loss to the grower is a ‘pre-
harvest drop’. In this case, dropping or shedding of fruits takes place before
harvest. At this stage, half and three-fourth developed fruits are shed due to many
causes. This is a loss to the growers and is a serious problem confronted by them
especially in apples, mango and citrus fruits. In all the above cases viz., dropping of
flowers, flower clusters or fruits are preceded by the formation of a layer or several
layers of cells at the base of petiole or pedicel or peduncle and at the right angle to
this axis. These cells are more or less rectangular in shape and are not cemented
together tenaciously and not crossed by fibers or vascular tissues. Consequently,
breakage or abscission takes place at that point. The formation of this layer of cells
is a natural phenomenon taking place as natural maturity is reached. It may be
hastened by certain environmental conditions.

26. Causes of Fruit Drop
Climatic factors: Climatic factors such as high temperature, low humidity and very
low temperature hasten the formation of abscission layers, consequently the fruits
drop. It has been observed in South India that shedding of fruits in mango will be
more if the temperature is high and humidity is low.
Physiological factors: Abnormal fluctuations of soil moisture favour heavy fruit
drop.
Nutritional: Lack of available nitrogen and similar other nutritional factors may
cause fruit drop. The shed is more in weak shoots than in the strong ones and also
more in young trees than in medium or old trees.

27. Causes of Fruit Drop
Cultural practices: Deep digging or deep ploughing during the fruit development
phase will injttrt the roots and cause the fruits to be shed. Drought or lack of
irrigation especial) , in mangp during the third drop stage increases the dropping
per cent
Pathological causes: Incidence of pests and diseases will cause more shedding of
fruits. For instance, high incidence of diseases like powdery mildew and
anthracnose and pests like hopper and mealy bugs in mango favour more fruit
drops.
Varietal factor: Within a kind of fruit, the varieties differ among themselves in the
extent of fruit drop. In one study, it has been found that under similar conditions
the extent of shed varied from 0.9% in Willard variety to 32.5% in Jehangir variety
of mango.

28. Fruit Drop - Prevention
The pre-harvest drop may be reduced by controlling the causes to a certain extent
Proper and timely culture such as irrigation and manuring, plant protection,
provision of pollinizers and wind breaks will help to prevent or reduce the amount
of fruit drop. A definite relationship between the auxin content and the abscission
of fruits during various stages of development has been established in apple fruits.
In the final stages of the fruit growth, a rapid decline in auxin content is correlated
to degeneration of endosperm causing pre harvest fruit drop. This led to the
thought that high concentration of auxins supplied exogenously may inhibit fruit
drop.
The possibility of reducing the preharvest drop by means of plant regulator sprays
has been well established in many fruit crops.

29. Fruit Drop - Prevention
Naphthalene Acetic Acid (NAA) and its related compounds (10 to 20 ppm) are
very effective in reducing the drop of fruits in pome fruits such as apples and pears.
2,4-Dichloro phenoxy acetic acid at very low concentration i.e. < 20 ppm checks
preharvest drop in citrus fruits. 2.4-D and 2.4.5-T at 5 to 10 ppm when sprayed on
Kodai orange decreased fruit drop and increased fruit retention.
NAA at 10 ppm was effective in reducing the fruit drop in litchi and GA at 60 ppm
reduces the fruit drop in jamun.
Great care must be exercised in the use of hormones as it is possible cause much
damage to the foliage and new growth.

30. Parthenocarpy
The term parthenocarpy originates from the Greek words, parthenost meaning
virgin and 'karpos' meaning fruit. It refers to the production and development of
seedless fruits. In botany and horticulture, parthenocarpy literally mean virgin fruit.
It is the natural or artificially induced production of fruits without fertilization of
ovules. The fruit is therefore seedless.
Some parthenocarpic cultivars are of ancient origin. The oldest known cultivated
plant is a parthenocarpic fig, first grown at least 11,200 years ago. In the recent
years, the crazy for seedless fruits is increasing among the consumers. The
seedlessness results in certain fruits due to the phenomenon of ‘parthenocarpy’
which refers to the development of fruits without fertilization or even without the
stimulus that comes from pollination.

31. Parthenocarpy
Parthenocarpic fruits are usually seedless and need not be always. If a
parthenocarpic fruit develops even without the stimulus from the pollination, then
this phenomenon, is referred as ‘vegetative parthenocarpy’ (autonomic) e.g. Banana
and Japanese Persimmon. On the other hand, if a parthenocarpic fruit develops
from the mere stimulus of the pollination (but fertilization won’t take place), it is
known as ‘stimulative parthenocarpy’.
The female flowers of triploid watermelon require the pollen grains of diploid
varieties to develop into a seedless fruit. Diploid pollen grains give a stimulus for
the ovary to develop into a fruit without seed. When Allahabad Round variety of
guava was self-pollinated it resulted in the development of parthenocarpic fruit due
to the stimulation provided by pollen hormones.

32. Parthenocarpy
Black Corinth variety of grapes is seedless due to stimulative parthenocarpy. In grapes CV
Thompson Seedless pollination and fertilization take place but the embryo gets aborted
subsequently resulting in seedlessness. This phenomenon of development of seedlessness is referred
as ‘steno-spermocarpy’.
The seedless or parthenocarpic fruits are advantageous since there is a greater preference among
the consumers for the seedless fruits owing to its quality and hence they fetch higher prices than
the seeded fruits of the same kind (e.g. seedless grapes, guava or oranges).
Besides, the problem of unfruitfulness may not arise if a fruit develops parthenocarpically and the
grower is assured of his crop (e.g. banana). One drawback with the seedless fruits is that they are
usually small in size (e.g. grapes) and irregular in shape (guava) I Recently, in citrus particularly
mandarin oranges, less seeded varieties have been developed through mutation breeding (gamma
irradiation) where the isolated solid mutants produce sterile pollen grains which when pollinate
the flowers result in lesser seeded fruits.

33. Parthenocarpy

34. Parthenogenesis
In some plants, fruits develop parthenocarpically, still they produce viable seeds
(e.g. Mangosteen and Strawberry).
This phenomenon is referred as parthenogenesis.
The seedlings of such fruits are genetically uniform. In certain cases, seeds develop
parthenogenetically but they are non-viable.
When female flowers of jack were pollinated with pollen grains of bread fruit, seeds
formed in jack but they did not germinate as they were non-viable.