NEET Biology Notes Plant cells Development
Development Process in Plant Cells
Development is a term that includes all changes that an organism goes through during its life cycle from germination of the seed to senescence.
The sequence of processes, which constitute the development of a cell of a higher plant is given in figure below :
Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This is called plasticity, e.g. heterophylly in cotton, coriander and larkspur.
Growth Regulators or Plant Hormones
The Plant Growth Regulators (PGRs) are small, simple molecules of diverse chemical composition. These may contain indole compounds, adenine derivatives, derivatives of carotenoids, terpenes, etc. Plant growth regulators can be described as plant growth substances, plant hormones or phytohormones. PGRs like auxins, gibberellins and cytokinins are called growth promoters as these are involved in growth promoting activities such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation.
Auxins
- Auxins or Indole-3-Acetic Acid (IAA) are generally produced by growing apices of the stems and roots, from where they migrate to the regions to their action.
- They initiate rooting in stem cuttings, promote flowering, i.e. in pineapples, prevent fruit and leaf drop at early stages, promote abscission of older mature leaves and fruits, induce parthenocarpy, i.e. in tomatoes, act as herbicides and controls xylem differentiation and helps in cell division.
- Apical dominance is a phenomenon in which growing apical bud inhibits the growth of lateral buds in most higher plants.
- Thimann and Skoog reported that IAA is responsible for apical dominance. Auxin like IAA and IB A (Indole Butyric Acid) have been isolated from plants.
- NAA (Nephathalene Acetic Acid) and 2, 4-D (2, 4-diehlorophenoxyacetic are synthetic auxin.
Gibberellins
Gibberellins are promotory PGR. There are more than 100 gibberellins reported from widely different organjsms like fungi and higher plants. They are denoted as GA,, GA2, GA3 and so on.
They produce a wide range of physiological responses in plants.
- Increase the length of stalks.
- Fruit elongation and improve shape.
- Delay senescence.
- Speed up malting process in brewing industry (mainly GA3).
- Hastens the maturity period leading to early seed production.
- Promote blotting (intemode elongation) in many plants.
- In lettuce, seed germination occurs by application of GA even in dark. On female plant, male flower are produced by application of GA.
Cytokinins
Natural cytokinins (kinetin) are synthesised in regions where rapid cell division occurs, e.g. root apices, developing shoot buds, young fruits, etc.
Several naturally occurring cytokinin, e.g. zeatin and some synthetic compounds with cell division promoting activity have been identified.
Cytokinins help to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation. These help overcome the apical dominance and promote nutrient mobilisation, which helps in delay of leaf senescence.
Ethylene
It is the only gaseous phytohormone It is synthesised in large amount by tissues undergoing senescence and ripening fruits. It induces horizontal growth of – seedlings, swelling of the axis and apical hook formation in dicot seedlings.
Ethylene promotes senescence and abscission of plant organs, fruit ripening, enhances respiration rate (during ripening of the fruit). It breaks seed and bud dormancy, sprouting of potato tubers, promotes rapid internode/petiole elongation.
Its broad spectrum effects make them widely used in agriculture. Ethepton is a most widely used compound of ethylene in agriculture. It hasten fruit ripening in tomatoes and apples and accelerates abscission in flowers and fruits.
Abscisic Acid
Abscisic Acid (ABA) acts as a general plant growth inhibitor and of plant metabolism. It has wide ranging effects on plant growth and development.
ABA inhibits seed germination, stimulates the closure of stomata, increases the tolerance of plants to various types of stresses (as stress hormone). It also play an important role in seed development, maturation and dormancy, helps seeds to withstand dessication and other factors unfavourable for growth. It also acts as an antagonist to gas.
Photoperiodism
The response of plants to periods of day/night is termed as photoperiodism.
On the basis of day/night plants axe divided into three parts :
- Long-day plants Those plants, which require light for a period exceeding a well-defined critical duration,
e.g. henbane, radish. - Short-day plants Those plants, which are exposed to light for a period less than this critical duration before the flowering is initiated in them, e.g. Xanthium.
- Day-neutral plant Those plants where no relation between exposure to light duration and induction of flowering response occurs are called day-neutral plants, e.g. Pisum sativum.
Not only the duration of light period but that the duration of dark period is also of equal importance.
The site of perception of light/dark duration are the leaves.
It is said that a hormonal substance migrates from leaves to shoot apices for inducing flowering only, when the plants are exposed to the necessary inductive period.
Vernalisation
The dependence of plants for flowering either quantitatively or qualitatively on exposure to low temperature is called vernalisation.
It prevents precocious reproductive development late in the growing season and enables the plant to have sufficient time to reach maturity.
Term vernalisation was first given by TD Lysenko.
As a result of vernalisation, a flowering hormone called vernaline is formed, but it has never been isolated.
Seed Dormancy
The internal inhibition of germination of a normal or viable seed even, when it is placed under most favourable conditions required for its germination is called seed dormancy.
Dormant seeds remain under non-germination condition only for a specific period of time that may vary from days to years. This specific period is called dormancy period. Dormancy in seeds is may be due to the presence of some block(s) within the seeds, which act as barriers for germination.
Seed dormancy naturally breaks due to rupturing of tough seed coat, action of digestive enzymes inside alimentary canals of animals, completion of over-ripening period, destruction of inhibitors by heat, light or oxidation, formation of growth hormones, e.g. cytokinin, gibberellin, auxin, etc.
Biological Significance of Dormancy
- Keeps seeds in viable state.
- Dormant seeds can present in soil for several years.
- Dormancy due to the impermeable seed coat helps small edible seeds remain viable even after they pass through enzymatic effects in the alimentary