Juvenility and Flower Bud Differentiation PDF

FUNDAMENTALS OF HORTICULTURE Compiled by Sangeet Chhetri, Assistant Professor (Horticulture) School of Agriculture, Seacom Skills University JUVENILITY AND FLOWER BUD...

FUNDAMENTALS OF HORTICULTURE Compiled by Sangeet Chhetri, Assistant Professor (Horticulture) School of Agriculture, Seacom Skills University JUVENILITY AND FLOWER BUD DIFFERENTIATION  Plant growth is the process by which a plant increases in size, creating more leaves and stems.  Plant development is the process by which plants change from one stage of growth to the next. These stages include juvenility, maturity, flowering and seeding.  A plant passes through different stages in its lifecycle.  These stages are embryonic growth, juvenility. Maturity, senescence and death.  It is well known that many plants appear unable to produce flowers until they have attained a minimum of age and size. JUVENILITY  It may define as the physiological state of a seedling plant or a nursery plant during which it cannot induce flower.  This minimum growth requirement or delay before sexual reproduction become possible has been frequently termed as juvenility.  This state is followed by a transition phase in which flowering can occur but not as readily as later when the plant grows into the adult phase.  During juvenility period, they will not produce flower and it will not be possible to have yield.  Juvenile phase is characterized by the most rapid growth in plants.  In woody plants, the juvenile phase may be very short or very long depending upon both environmental and genetic factors.  The juvenile period can often be shortened by increasing the growth rate of the young seedling, because a minimum size must be attained to reach the adult state.  An extreme example of this is alpine trees (such as oak, beech, elm ) and shrubs more than 100 years old that remain juvenile because slow growth has prevented their reaching the required minimum size. Characteristics of Juvenility 1. Leaf Form: Leaf is vigorous in juvenile phase. Leaf morphology also differ in juvenile phase of some plants like Acacia sp. 2. Growth Form: Juvenile shoot growth appears whip like. The shoot grows parallel to main stem. Such shoots are known as water sprout. E.g. Citrus, Guava etc.  3. Presence of thorns: Some fruit plant seedlings show thorns in juvenile phase and these thorns disappears when attain adult phase. E.g. Apple, Pear, Citrus etc. 4. Leaf Retention: In juvenile phase plant do not fall leaves throughout the year. Whereas, it falls in adult phase or maturation phase. 5. Root emergence: In propagation use juvenile phase plant parts the root emergence faster than mature portion of plant. Table: Juvenile period of some plants Sl. no. Name of crops Juvenile period 1. Rose 20-30 days 2. Grapes 1 year 3. Apple 4-8 years 4. Mango 5 years 5. Citrus 5-10 years 6. Okra 45 days 7. Banana 1 year 8. Tomato & Brinjal 60 days 9. Cucumber 80 days FLOWER BUD DIFFERENTIATION Bud A bud is an immature shoot system which often surrounded by protective scale leaves. The bud develops into a lateral branch, a flower or an inflorescence. Types of Bud 1. Simple bud: It grows into vegetative shoot. It is also known as leaf bud. 2. Mixed bud: It grows into a shoot with a flower. It is also known as flower bud. 3. Compound bud: It grows into both leaves and flowers. 4. Flower bud differentiation:  It may defined as the transition from the vegetative growth to the generative growth and also known as flower induction.  The complex process of floral development arises in response to the integration of signals from the external environment and internal factors.  The vegetative meristematic cells in the buds convert to generative (floral) meristematic cells.  In most plants, once the transformation from the vegetative to the reproductive state has been made, the process is irreversible  Fig: Flower bud (left) and vegetative bud (right) PC: Fernando José Hawerroth Factors influencing flower bud differentiation 1. Endogenous factors a) Carbon: Nitrogen ratio (C:N ratio) High ratio of carbohyd, ates to nitrogen favours flowering. In young plant, heavy application of nitrogen which lowers C:N ratio delays flowering. The plant which are old, weak and having higher C/N ratio flowers heavily. With respect to C/N ratio, there may be four situations, such as  High nitrogen and low carbohydrates-flower forms  High nitrogen and enough carbohydrates - flower forms  Moderate nitrogen as well as carbohydrates - Good growth and abundant flower formation.  Low nitrogen and high carbohydrades-Formation of only few buds. b) Genes When plants undergo phasic change from juvenile to adult, eafl gene synthesizes. The gene is responsible for truncating juvenile and adult phases which induces early flowering. The HST(HASTY) gene promotes juvenility in plants. 2. Environmental factor a) Temperature Temperature is main environmental factor which is associated with flowering. Due to availability of a particular temperature in spring season, flowering occurs at the same time each year. Exposure of plants to low temperature favours flowering. Biennial plants like carrot, celery, cauliflower, cabbage, turnip etc. upto exposed to 0 to 10°C temperature come into flowering. This is known as vernalization. In these plants, low temperature requirement is commonly known as chilling requirement. They have chilling requirement below 7º C for about 4-60 days for the bud to bloom in spring. b) Photoperiod Some plants flowering only when their photoperiodic requirement is fulfilled. According to photoperiod plants are categorised into three broad categories as under :-   Long day plant (LDP): flowering occurs only if day length is 12 hrs or more than 12 hrs.  Short day plant (SDP): In these plants flowering occurs when day length is less than 12 hrs.  Day Neutral Plant (DNP): In these plants flowering is not regulated by day length.  Light  Light intensity, duration as well as quality influences bud differentiation and flowering. Plant receiving high light intensity flowers properly than low light ones. That's why outer fruit tree branches have more fruiting than the inner ones.  The red light (Sunlight) promotes flowering while far red light inhibits. So, quality of light also affects the flowering. 3. Management Factor a. Nutrients: Nutrients decide vegetative or reproductive growth of the plant. Nitrogen increases the vegetative growth of the plant. And it favours the carbohydrates utilization of the plant. Phosphorus works in plant as protein synthesis, cell division; potash in translocation of sugar. Other nutrients also involve in flowering and bud differentiation. b. Moisture: Non-availability of moisture extends the time of flower bud differentiation. Flower primordia is less in water deficit conditions. POLLINATION, POLLINATOR AND POLLINIZER Pollination The process by which pollen grains are transferred from anthers to stigma is referred as pollination. Pollination is of two types: viz. 1) Autogamy or self pollination and 2) Allogamy or cross pollination. I. Self pollination (Autogamy) Transfer of pollen grains from the anther to the stigma of same flower is known as autogamy or self-pollination. Autogamy is the closest form of inbreeding. Autogamy leads to homozygosity. Transfer of pollen grains from the anther to the stigma of another flower on the same plant is known as geitonogamy. Mechanism promoting self-pollination 1. Bisexuality. Presence of male and female organs in the same flower is known as bisexuality. The presence of bisexual flowers is a must for self pollination. All the self pollinated plants have hermaphrodite flowers. 2. Homogamy. Maturation of anthers and stigma of a flower at the same time is called homogamy. As a rule, homogamy is essential for self-pollination. 3. Cleistogamy. When pollination and fertilization occur in unopened flower bud, it is known as cleistogamy. It ensures self pollination and prevents cross pollination. Cleistogamy has been reported in some varieties of wheat, barley, oats and several other grass species. 5. Position of Anthers. In some species, stigmas are surrounded by anthers in such a way that self pollination is ensured. Such situation is found in tomato and brinjal. In some legumes, the stamens and stigma are enclosed by the petals in such a way that self pollination is ensured. Examples are greengram, blackgram, soybean, chickpea and pea. I. Cross pollination (Allogamy) Transfer of pollen grains from the anther of one plant to the stigma of another plant is called allogamy or cross pollination. This is the common form of out-breeding. Allogamy leads to heterozygosity. Mechanism promoting cross-pollination 1. Dicliny. It refers to unisexual flowers. This is of two types: viz. i) monoecy and ii) dioecy. When male and female flowers are separate but present in the same plants, it is known as monoecy. In some crops, the male and female flowers are present in the same inflorescence such as in mango, castor and banana. In some cases, they are on separate inflorescence as in maize. Other examples are cucurbits, grapes, strawberry, cassava and rubber. When staminate and pistillate flowers are present on different plants, it is called dioecy. It includes papaya, date palm, spinach, hemp and asparagus. 2. Dichogamy. It refers to maturation of anthers and stigma of the same flowers at different times. Dichogamy promotes cross pollination even in the hermaphrodite species. Dichogamy is of two types: viz. i) protogyny and ii) protandry. When pistil matures before anthers, it is called protogyny such as in pearl millet. When anthers mature before pistil, it is known as protandry. It is found in maize, sugarbeet and several other species. 3. Heterostyly. When styles and filaments in a flower are of different lengths, it is called heterostyly. It promotes cross pollination, such as linseed. 4. Herkogamy. Hinderance to self-pollination due to some physical barriers such as presence of hyline membrane around the anther is known as herkogamy. Such membrane does not allow the dehiscence of pollen and prevents self-pollination such as in alfalfa. 5. Self incompatibility: The inability of fertile pollens to fertilize the same flower is referred to as self incompatibility. It prevents self-pollination and promotes cross pollination. Self incompatibility is found in several crop species like Brassica, Radish, Nicotiana, and many grass species. 6. Male sterility: In some species, the pollen grains are non functional. Such condition is known as male sterility. It prevents self-pollination and promotes cross pollination. Study of floral biology and aforesaid mechanisms is essential for determining the mode of pollination of various crop species. Moreover, if selfing has adverse effects on seed setting and general vigour, it indicates that the species is cross pollinated. If selfing does not have any adverse effect on these characters, it suggests that the species is self-pollinated. Criteria/stages for successful pollination to occur  The vailability of an adequate source of viable and compatible pollen  The pollinating agents must effectively transfer the pollen to the stigma  The stigma must be receptive  There must be sufficient growth of pollen tube  The double fertilization must take place adequately for the development of embryo and endosperm POLLINATOR A pollinator is an agent that moves pollen from the male part of a flower to the female part, which fertilizes the flower and allows it to produce fruit, seeds, and young plants. Types of pollinator 1. Entomophilous  The pollination by insects  It involves many species of honey bees viz. A. cerana indica (Indian/Asian bee), A. florea (dwarf or red dwar bees), A. dorsata (giant bees), A. andreniforms (black dwarf bees), A. laboriosa (himalayan bees) and an introduced one A. mellifera (western bees)  In addition, several other pollinators including stingless bees, carpenter bees, bumble bees, megachilids, andrenids, syrphids etc in several agroecosystems  The fruit crops pollinated entomophilously are apple, pear, peach, plum, almond, apricot, cherries, ber, litchi, citrus, mango, annonaceous fruit crops. Even ants can pollinate dragon fruit.  Wind acts as a pollinating agent  Pecan nut, hazelnut, chestnut, papaya, date palm, coconut, pomegranate, cashewnut, sapota etc are wind pollinated fruit crops 2. Ornithophilous  Pollination carried out by birds  Most bird pollinated flowers are red and have a lot of nectar also tend to be unscented  Humming birds (Phaethornis longirostris) are the most effective bird pollinators eg. Banana and pineapple 3. Hydrophilous  Pollination where by pollen is distributed by the flow of waters, particularly in rivers and streams. E.g. water chestnut 4. Human pollination  The pollination carried out manually by hand pollination. E. g. Date palm 5. Chiropterophily  The pollination by bats  Bats are the main pollinators involved in dragon fruit.  The banana bat (Musonycteris harrisoni) is a nectarivorous species found only on the Pacific Coast of Mexico. It has a very small geographic range and is distinguishable by its extremely long nose POLLINIZER  The pollinize is a plant that provides or donates pollen to other plant, whereas, a pollinator is the biotic agent that moves the pollen, such as bees, moths, bats, birds etc.  The pollinizers are important for improving fruit set, yield and quality of commercial cultivars but essentially desired for self-incompatible varieties and where assisted pollination is needed.  It is particularly important in fruit crops. That show the mechanism of self incompatibility, self-sterility or cross- incompatibility.  Such phenomena are quite prevalent in some fruit plants like ber, aonla, loquat, mango and lemons. In mango and citrus, there is false set due to improper pollination, which drops off in the later stages.  In date palm, at least 4 to 5 male plants are required for pollination of 100 female plants Characteristics required for an ideal pollinizer  Plenty of viable pollens  Compatible pollen with the main cultivar  Synchronization of flowering with main cultivar  Regular and prolific bearer  Bearing at the same age as that of the main variety  Free from major insect-pests and diseases  Adaptable to the agro climatic region, where main cultivars grow wel  FERTILIZATION AND PARTHENOCARPY Fertilization  Fertilization can be defined as the fusion of the male gametes with the female gametes to form a zygote.  In the course of fertilization, 2 male gametes get transferred into the female reproductive organs (ovule) through pollen tube.  After entering the ovule, the pollen tube enters the synergids and rupture it. egg cell to form a diploid zygote. This process is known as ‘generative fertilization or syngamy’.  Another male gamete moves towards the centre and fuses with the secondary or central cell and produce the triploid endosperm. This process is known as ‘triple fertilization’. Types of fertilization Fertilization process can be grouped into three types and are classified mainly based on the entry of the pollen tube into the ovule. 1. Porogamy The entry of pollen tube into the ovule through the micropyle is known as ‘porogamy’. It is the common type of fertilization carried out in all angiosperms or flowering plants. 2. Chalazogamy The entry of pollen tube into the ovule through the chalaza is known as ‘chalazogamy’. This type of fertilization is carried out on all Casuarina species of plants. 3. Mesogamy The entry of pollen tube into the ovule through the integuments of the ovule is known as ‘mesogamy’. This type of fertilization is seen in all Cucurbit plants, such as pumpkin, ridge gourds, bitter gourd and other gourd plants. In this type of fertilization, the pollen tube enters the ovule through the integuments of the ovule.  Usually, in the development of fruit and seed, the following relationship can be established: Ovary Fruits Ovule Seeds Integuments Testa (seed coat) Nucellus Perisperm Two polar nuclei +sperm nucleus Endosperm (3n) Egg nucleus + sperm nucleus Zygote/embryo (2n)