In many fruit plants the time of flower opening depends on the environmental conditions. The length of flowering period is also dependent upon the temperature and atmospheric humidity. The type and morphological features of flower are very important from horticultural point of view. It is very important that one should be thoroughly familiar about the bearing habit of the fruit trees.
The position of the flower on the inflorescence is a criterion determining fruit set in many species. Mode of pollination and agencies of pollination are important aspects of flower that helps the breeder in making desired crosses.
Pollen viability particularly when, immediate pollination is not possible, in storage for short time or long duration is also an important consideration in fruit breeding. In fruits, there are three sex types of flowers. All the three sex type of flowers is found in different fruits.
Hence, this article includes brief information about:
1. Bearing habit of fruits,
2. Type of inflorescence,
3. Blooming period,
4. Forms of flower,
5. Mode of pollination,
6. Agencies of pollination and
7. Pollen viability.
1. Bearing Habit of Fruits:
In every kind of plant, most of the flower buds are found in a certain definite position. In the developing buds, a stage is finally reached when such a differentiation cannot take place except when bud unfolds into a leafy structure and the subsequent formation of a fruit bud at a new growing point. The position of flower or inflorescence on the shoot, relative to the growth of current season, is characteristics of the species or the variety and is subject to little change.
Flower bearing shoots may arise from either terminal or lateral bud, either on long or short growths (spurs) or they may arise from adventitious buds. The position of the flower bud is different in different varieties. Buds having the potential to flower are formed terminally, laterally or adventitiously. The terminal buds may be formed on long or short growth, laterally on current or past season growth and adventitiously from any point on the trunk or root.
Classification of Plants According to Bearing Habit:
There are six main types of fruiting and nine distinct bearing habits. The classification being based upon the location of the fruit buds and the type of flower bearing structure to which, they give rise.
Group I:
Fruit buds borne terminally, containing flower parts only and giving rise to inflorescence with leaves e.g., Mango, litchi, loquat.
Group II:
Fruit buds borne terminally, unfolding to produce leafy shoots that terminate to flower clusters as in apple and pear. The fruit buds are mixed and invariably give rise to very short growth and a few short internodes, leaves of ordinary size and a lateral branch arising in the axil of one of the leaves. This branch may bear fruit in the following season e.g., citrus.
Group III:
Fruit buds borne terminally, unfolding to produce leafy shoots with flowers or flower clusters in the leaf axils.
This might be called as an incomplete ‘terminal bearing habit’ for the fruit bud itself is not borne terminally, but it is lateral to the growth upon which it appears. However, the flower buds are terminal. The terminal buds of the flowering shoots may differentiate flower parts for the following year’s production or new buds may develop from lateral leaf buds. It is found in pomegranate, guava etc.
Group IV:
Fruit buds borne laterally, containing flower parts only and giving rise to inflorescences without leaves. If leaves are present, they are much reduced in size e.g., peach, plum, almond, citrus, date-palm.
The sweet cherries and the domestica and institia groups of plum form their buds for the most part laterally on spurs. These come from lateral buds on the shoots of the preceding season, and their new shoots form both terminal and lateral buds on shoot or on older wood.
Group V:
Fruit buds borne laterally, unfolding to produce leafy shoots that terminate in flower clusters e.g., blackberry, raspberry, grapes.
Group VI:
Fruit buds adventitious since these are necessarily lateral, the plants included here might readily be classed with those of Group IV and V. However, this bearing habit is more or less distinct and these fruits may well be placed in a separate class e.g., the Cacao bears in the same way. The flower buds appear first on the trunk as the trees grow and then on the branches.
Group VII:
This is an another group of plants which have fruit buds in the axils of the leaves and in which these buds unfold and develop their flowers and fruits very soon after the flower parts are differentiated. However, it is not possible to draw clear line between their fruiting habit.
In the unopened flower bud of the grape, the inflorescence is terminal to a leafy shoot also within the bud. As the bud opens, however, the bud in the axil of the top most leaf of this developing shoot unfolds and continues the growth of the shoot. This results in pushing the flower cluster to one side so that inflorescence is lateral and opposite a leaf.
Group VIII:
Fruit buds borne laterally (or pseudo-terminally), unfold to produce leafy shoots with flower clusters in the leaf axils e.g., fig, mulberry, ber, avocado, persimmon.
Group IX:
Fruit buds borne both terminally and laterally, inflorescences generally terminal e.g., Walnut, pecan etc. The terminal bud may give rise to a short leafy shoot ending in a female inflorescence. The male flowers are borne on leafless inflorescence arising from lateral buds not far below the terminal.
2. Types of Inflorescence:
The knowledge of different types of inflorescence is very essential for successful orcharding. Flowers may occur singly or in closely associated clusters. The arrangement of flowers on stem or axil is known as inflorescence.
Morphologically the inflorescence is a flower bearing shoot, either determinate or indeterminate in growth. The inflorescence may terminate in a shoot or it may be borne in the axil of the leaf. The branched inflorescence usually, but not always bears bracts or modified leaves.
Kinds of Inflorescence:
There are several kinds of inflorescence or arrangements of flowers in a cluster:
1. Solitary Flower:
A flower may be borne singly in the axil of a foliage at the terminus shoot or on a specialized stalk e.g., trifoliate orange, quince, guava, peach, apricot, almond etc.
2. The Racemose Inflorescence:
In this case, the peduncle continues to grow and new flower buds with their subtending brackets continue to form near the growing points, thus, producing a succession of new flowers. The lowest flowers, being the oldest are first to open and blossoming proceeds upward. This indeterminate type of inflorescence is by far the most common.
There are various types of inflorescence in this class:
(a) Raceme:
Flowers are borne on the pedicels which are about equal in length. This type is found in the wild cherry, currents, gooseberry, blackberry, blueberry, Raspberry, etc.
(b) Catkin:
A spike or raceme with a slender rachis, bearing many unisexual epipetalous flowers, which falls as a whole, when the fruit is mature e.g., mulberry, chestnut, walnut, peanut etc.
(c) Corymb:
The main axis is elongated and the pedicels are of unequal length in such a way that the lower ones are the longest and the upper ones or central ones are shortest resulting in flowers lying in a horizontal plane e.g., Pear.
3. The Cymose Inflorescence:
In this type at the growing point, a flower bud is produced so that the floral axis cannot elongate further. The other flower buds are produced below this point on the axis towards the base. Thus this becomes determinate growth and the opening of the flower is basipetal or centrifugal. The solitary flower is in a sense, a cymose type e.g., citrus, phalsa, sapota, strawberry, persimmon, etc.
The cymose inflorescence is further of two types and these are:
(a) Fasicle:
A cyme with flowers closely crowded on a very shorter peduncle and with pedicels of about equal length e.g., cherry (sweet and sour), plum, ber.
(b) Panicles:
In this case main axis is indeterminate but the secondary and ultimate axis is a cymose e.g., grapes, mango, litchi, loquat, pistachio, etc.
4. Anomalous Types of Inflorescence:
Some types of inflorescence are also observed which do not fit into the conventional types e.g., in apple. Terminal flowers open first which suggest a cyme but then the basal ones open next and then the anthesis of individual flowers proceed more or less upward to the apex which suggests a racemose type.
5. Special Types of Inflorescence:
a. Spadix:
It is a large, boat shape structure (spathe) enclosing an inflorescence e.g., date-palm, banana.
b. Hypanthodium:
It is the modification of capitulum in which the receptacle is hollowed into a flask. It opens to the outside by the apical pore which is borne on the inner-side of the receptacle in which the male flowers lie towards the apical pore and female flowers towards the base e.g., Fig.
3. Blooming Period:
The blooming period of different fruit plants varies due to prevailing agro-climatic conditions in different regions of India. Broadly, blooming time of various fruit plants grown in North and South India is divided into three periods.
(a) February to May:
Apple, pear, peach, plum, apricot, almond, walnut, mango, citrus, grape, litchi pomegranate and datepalm flowers in North India.
Banana, papaya, pineapple, sapota and fig flowers during this period in South India.
(b) June to September:
Guava, ber and papaya bloom during this time in North India. Pomegranate and citrus gives flower in South India at this time.
(c) October to January:
Guava, sapota, ber and loquat flowers in North India. Sapota, guava, mango, ber flowers during this time in South India.
Flowering Period in Different Fruits:
The mango flower’s late in January or in the beginning of February or even as late as March in many parts of North India. The flowering in citrus takes place mostly in spring. In lime and lemons, flowering takes place almost throughout the year. In kinnow, flowering takes place during end February to early March in North India. Flowers in grapes mostly develop in spring. Guava tree normally produces as many as three crops in a year. In northern India, three distinct flowering seasons i.e., summers, rainy and autumn have been observed. The flowering in litchi takes place in February. The flowering period in ber lasts for about two and a half months from September to November.
The blooming period of loquat is very long, lasting from mid July to January or sometimes even up to May. The flowering in date palm takes place during end February to early March. In evergreen pomegranate, the flower buds of the spring flush are borne on mature wood of one year old shoot, where as the flowers which appear during July-August are borne on current year’s growth. In deciduous cultivars, the flowers are borne on current year’s growth between July and August.
The flowering in Jamun starts in the first week of March and continues upto middle of April. In phalsa, flowering starts from February-March and continues till May. The papaya is found to flower throughout the year. Sapota flowers throughout the year with main two flowering seasons i.e., July to November and again in February-March. Normally, there are three growth flushes in cashew and in South India, the flower bud emergence commences by the middle of September and continues till the end of February, the main season being October-November. The flowering period of custard apple is very long commencing from March-April, continues upto July-August.
The peak flowering is observed in April and May. In the climate of Poona, the avocado produces flowers from November to January in three main flushes. In apple, flowering takes place in April in Himachal Pradesh. The flowering in pear starts in last week of February and continues upto third week of March. In peach, flowering starts in the first week of February and continues till end of February. Flowering in Japanese plum starts in the second fortnight of February and last up to first week of March.
Almond is one of the first trees to bloom in spring. Flowering takes place in very early season occurring between January to April. In Himachal Pradesh, the flowering in apricot cultivars mostly occurs in the month of March. The peak flowering period of walnut is mid-April which is prolonged till the first week of May in late blooming cultivars under Shimla conditions.
4. Forms of Flower:
There are three sex types of flowers i.e., hermaphrodite or perfect, staminate and pistillate flowers. Hermaphrodite flowers are called perfect or complete in which both the stamens and pistils are functional. Pistillate flowers are devoid of anthers. Staminate flowers are ones with non-functional pistils.
(i) Perfect Flower:
Apple, peach, plum, grape, mango, avocado, cashew-nut, papaya, citrus, sapota, guava, etc.
(ii) Staminate Flowers:
Papaya, mango, litchi, grape, date, cashew-nut, pomegranate, walnut, etc.
(iii) Pistillate Flowers:
Papaya, walnut, grape, pecan, etc.
(iv) Teratological Flowers:
Papaya, mango, grape, pomegranate, etc.
5. Mode of Pollination:
Pollination means the transference of the pollen from the anther to the receptive stigma whether of the same flower or of a different flower. As the pollen is not capable of locomotion, this act either involves some agent for this transference or the anther must be placed in the flower right above the stigma so that pollens may drop directly on the stigma by sheer gravity.
I. Autogamy or Self Pollination:
If a stigma be pollinated by the pollen of the same flower.
(i) Cleistogamy:
In these cases the flowers never open as opposed to most flowers which show chasmogamy (i.e., flowers open normally during anthesis). In cleistogamous flowers the pollens are shed within the closed flowers so that self-pollination is obligatory e.g., Grape, sapota and papaya.
(ii) Homogamy:
The stamens and carpels of a flower mature at the same time. So, there is a greater chance of self-pollination although that is not obligatory. Some homogenous flowers, however, show special mechanism for self-pollination e.g., citrus, peach, apricot, etc.
II. Allogamy or Cross Pollination:
When the pollen of a flower pollinates the stigma of another flower located on a different plant, whether of the same kind or not, it is called allogamy. Special contrivances ensuring cross-pollination as noted below are very conspicuous.
(i) Dicliny or Unisexuality:
Cross pollination is the rule among diclinous plants i.e., those bearing unisexual flowers.
(a) Monoecious:
Occurrence of male and female flower’s on different locations on a single tree e.g. walnut, pecan, and chestnut.
(b) Dioecious:
Male and female flowers borne on difficult plants of same species e.g. papaya, date palm, pistachio, fig, strawberry
(ii) Self Incompatibility:
This is the condition when a flower cannot be fertilized by the pollen of the same flower or sometimes, from any flower of the same strain of plants. Cross pollination is obligatory in such plants, e.g., almond, apple, mango, pear, pineapple, cherry, apricot.
(iii) Dichogamy:
When stamens and carpels of a bisexual flower mature at different times, pollination between them becomes ineffective. Sometimes, however, it is found that self-pollination may take place at a later stage if cross-pollination fails.
Dichogamy may be of two types:
(a) Protandry:
When the anthers ripen first, as a result when the anthers bursts, it pollinates stigma of other flowers but not its own stigma which is not yet ripe e.g., walnut, coconut.
(b) Protogyny:
The carpel matures first as in many members of Annonaceae. When the stigma is receptive, its own pollen is not ripe so that it has to depend on foreign pollens e.g., fig, annona, banana, plum, pomegranate and avocado.
(iv) Heterostyly:
In certain plants there are flowers of two (dimorphic) or three (trimorphic) different forms with anthers and stigmas at different levels. This dimorphism or trimorphism usually involves heterostyly (Styles of different lengths). In dimorphic heterostyly, the flowers have short styles and long filaments and other flowers on the same tree have long style and short filaments/stamens. In trimorphic heterostyly, stamens and styles are of three different length e.g. Pomegranate, litchi, sapota and almond.
(a) Pin type- Sapota, pomegranate
(b) Thrum type- Almond
6. Agencies of Pollination:
When the anthers are mature, the pollen grains are shed from the pollen sacs and are disseminated by various agencies like insects, wind, birds and human being. Some of the pollen grains may, by one agency or another finally reaches the stigma of the flower. Depending upon, the type of agency which is instrumental for, the transference of the pollen from the anthers to the stigma.
Various terms are used to indicate the type of the pollination using a particular type of agency e.g.:
(i) Pollination by wind or anemophily
(ii) Pollination by animals or zoophily
The zoophily may include:
(a) Pollination by insects or entomophily
(b) Pollination by birds or ornithophily
Given below is the information about different fruit plants pollinated through insect, wind and bird and human being:
(a) Insect Pollinated Fruits- Apple, pear, peach, plum, almond, walnut, mango, guava, ber, litchi, citrus and annona.
(b) Wind Pollinated Fruits- Papaya, date-palm, cashew-nut, pomegranate and coconut.
(c) Bird Pollinated Fruits– Banana and pineapple
(d) Human Being Pollinated Fruits- Data palm.
7. Pollen Viability:
Initial fruit set depends upon pollination and fertilization of egg cell. For fertilization of egg cell, besides the compatibility factors, pollen should be viable and should have capacity to germinate on stigmatic surface so that pollen tube elongates sufficiently and fertilize the egg cell. Therefore, to obtain higher fruit yield, the knowledge of pollen viability and germinability is very important.
Pollen Viability:
Pollen viability is found out through acetocarmine test. In this test, pollen grains are taken on cavity slide and stained with 1-2 per cent acetocarmine. The pollen grains which get stained are considered viable whereas the unstained pollen grains are considered non-viable. The living protoplasm gets the stain. The percentage of viable pollen is calculated by counting viable and non-viable pollen under microscope.
Pollen Germination:
The germination of pollen grains is studied in cavity slides in different concentration of sugar solution viz. 15, 20 and 25 per cent. Other media like agar and Boron can also be used to enhance pollen germination. For this purpose, sugar solutions of different concentrations are prepared and pollen grains are incubated in sugar solution for varying times.
Pollen grains can also be germinated in distilled water but to enhance the pollen tube growth sugar is added. The cavity slides are then seen under microscope repeatedly and number of germinated pollen grains is counted and calculation of percentage of germination is made. The size of pollen tube is also measured with micrometer.
Pollen Viability and Germinability of Fruits:
In ber, pollen grains are highly viable, the viability ranging between 87 to 91 per cent in different cultivars. The pollen grains in pomegranate that germinated were over 90 per cent. In sapota, the highest pollen viability of 95.8 to 97.6 per cent was noted in cultivar Kalipatti. The pollen germination was observed maximum i.e., 45.4 -57.5 per cent in 15 per cent sucrose solution.
Poor pollen grain germination has been reported in all the species of Annona. In Japanese plum, pollen viability and germinability ranged from 30 to 90 per cent and 30 to 70 per cent respectively. In aonla pollen viability and pollen grain germinability varies from 82-96 and 16.7 to 37 per cent, respectively.