11th Bio Botany

. Characteristics of Growth • Growth increases in protoplasm at cellular level. • Stem and roots are indeterminate in growth due to continuous cell division and is called open form of growth . • The primary growth of the plant is due to the activity of apical meristem where, new cells are added to root and shoot apex causing linear growth of plant body.

• The secondary vascular cambium and cork cambium add new cells to cause increase in girth. • Leaves, flowers and fruits are limited in growth or determinate or closed form growth. • Monocarpic annual plants produce flowers only once during lifetime and dies. Example: Paddy and Bean The Banyan tree continues to grow for thousands of years and some others particularly annual plants cease growth within a season or within a year.

Can you understand the reasons? How does a zygote give rise to an embryo and an embryo to a seedling? How does a new plant structure arise from the pre-existing structure? Growth is defined as an irreversible permanent increase in size, shape, number,volume and dry weight.

Plant growth occurs by cell division, cell enlargement, differentiation and maturation. Growth is measurable, it is amazing to know that one single maize root apical meristem can give rise to more than , new cells per hour and cells in a watermelon may increase in size upto , , times. Bamboos are evergreen grasses and certain species of it can grow at the rate of growth cm per day. The Saguaro Cactus is a tree like cactus and is a slow growing plant.

The rate of growth is one inch in the first ten years and it does not begin to flower until it is about years old. It’s lifespan exceeds years and takes – years to grow a side arm. (Functional Organisation) . Types of growth rate The increased growth per unit time is termed as growth rate.

An organism or part of an organism can produce more cells through arithmetic growth or geometric growth or both. i. Arithmetic Growth Rate If the length of a plant organ is plotted against time, it shows a linear curve and this growth is called arithmetic growth . • The rate of growth is constant and it increases in an arithmetic manner.

• Only one cell is allowed to divide between the two-resulting progeny cell. • One continues to divide but the other undergoes cell cycle arrest and begins to develop, differentiate and mature. • After each round of cell division, only a single cell remains capable of division and one new body cell forms. For example, starting with a single cell after round of cell division there is one dividing cell and one body cell.

After round there are two body cells, after round there are three and so on (Figure . ). Dividing cell Body Cell Figure . : Arithmetic Growth Rate The plants single dividing cell would undergo one million rounds of nuclear and • Monocarpic perennials produce flowers only once during life time but the plants survive for many years.

Example: Bamboo. • Polycarpic perennials produce flowers every year during life time. Example: Coconut. .

. Kinetics of growth It is an analysis of the motion of cells or expansion. . Stages in Growth rate The total period from initial to the final stage of growth is called the grand period of growth.

The total growth is plotted against time and ‘S’ shaped sigmoid curve (Grand period curve) is obtained. It consists of four phases. They are: i. Lag phase ii.

Log phase iii. Decelerating phase iv. Maturation phase i. Lag phase In this phase new cells are formed from ­pre-existing cells slowly.

It is found in the tip of the stem, root and branches. It is the initial stage of growth. In other words, growth starts from this period. ii.

Log phase or exponential growth Here, the newly formed cell increases in size rapidly by deposition of cell wall material. Growth rate is maximum and reaches top because of cell division and physiological processes are quite fast. The volume of protoplasm also increases. It results in rapid growth and causes elongation of internode in the stem.

iii. Decelerating phase or Decline phase or slow growth phase The rate of growth decreases and becomes limited owing to internal and external or both the factors because the metabolic process becomes slow. iv. Steady state period or maturation phase In this phase cell wall thickening due to new particle deposition on the inner surface of the cell wall takes place.

The overall growth ceases and becomes constant. The growth rate becomes zero. The large plant or animal parts are produced this way. In fact, it is common in animals but rare in plants except when they are young and small.

Exponential growth curve can be expressed as, Mother cell Progeny cells Progeny cells Progeny cells Figure . : Geometric growth W = W e rt W = Final size (weight, height and number) W = Initial size at the beginning of the period r = Growth rate t = Time of growth e = Base of the natural logarithms Here ‘ r’ is the relative growth rate and also a measure of the ability of the plant to produce new plant material, referred to as efficiency index. Hence, the final size of W depends on the initial size W . iii.

Arithmetic and Geometric Growth of Embryo Plants often grow by a combination of arithmetic and geometric growth patterns. A young embryonic plant grows geometrically and cell division becomes restricted to certain cells at the tips of roots and shoots. After this point, growth is of the slower arithmetic type, but some of the new cells that are produced can develop into their mature condition and begin carrying cellular division. If each round requires one day, this type of arithmetic increase would require one million days or .

years. This arithmetic rate is capable of producing small number of cells present in very small parts of plants. For example the hair on many leaves and stems consists of just a single row of cells produced by the division of the basal cell, the cell at the bottom of the hair next to other epidermal cells. Hair may contain to cells by the division of the basal cell.

So, all its cells could be produced in just five to ten days. In the figure . , on plotting the hight of the plant against time a linear curve is obtained. Mathematically it is expressed as: C D Time Height of the plant Figure .

: Constant Linear Growth L t = L o + rt L t = length at time ‘t’ L o = length at time zero r = growth rate of elongation per unit ii. Geometric growth rate: This growth occurs in many higher plants and plant organs and is measured in size or weight. In plant growth, geometric cell division results if all cells of an organism or tissue are active mitotically. Example: Round three in the given figure .

, produces cells as = and after round there are = , , cells. Figure . : Diagrammatic comparision of absolute and relative growth rates Measurement of Growth Experiment: . Arc auxanometer: The increase in the length of the stem tip can easily be measured by an arc auxanometer which consists of a small pulley to the axis of which is attached a long pointer sliding over a graduated arc.

A thread one end of which is tied to the stem tip and another end to a weight passes over the pulley tightly. As soon as the stem tip increases in length, the pulley moves and the pointer slide over the graduated arc (Figure . ). The reading is taken.

The actual increase in the length of the stem is then calculated by knowing the length of the pointer and the radius of the pulley. If the distance travelled by the pointer is and the radius of the pulley is inches and the length of the pint is inches, the actual grown is measured as follows: Actual growth in length = (Distance travelled by the pointer × radius of the pulley) / Length of the pointer. For example, actual growth in length = ( × inches)/ inches = inches Arc Pointer Pulley Weight Potted plant Stand Figure . : Arc auxanometer out specialized types of metabolism (Figure .

). Plants are thus a mixture of older, mature cells and young, dividing cells. Figure . : Arithmetic and geometric growth of embryo Quantitative comparisons between the growth of living system can also be made in two ways and is explained in the table .

In figure . , two leaves A and B are drawn at a particular time. Then A and B are drawn after a given time. A and B = Area of leaves at a particular time.

A and B = Area of leaves after a given time. (A -A) and (B -B) represents an absolute increase in area in the given time. Leaf A increases from cm to cm ; cm in a given time. Leaf B increases from cm to cm ; cm in a given time.

Hence, both leaves A and B increase their area by cm in a given time. This is absolute growth. Relative growth is faster in leaf A because of initial small size. It decreases with time.

. Plant Growth Regulators Plant Growth Regulators (chemical messenger) are defined as organic substances which are synthesized in minute quantities in one part of the plant body and transported to another part where they influence specific physiological processes. Five major groups of hormones viz. , auxins, gibberellins, cytokinins, ethylene and abscisic acid are presently known to coordinate and regulate growth and development in plants.

The term phytohormones is implied to those chemical substances which are synthesized by plants and thus, naturally occurring. On the other hand, there are several manufactured chemicals which often resemble the hormones in physiological action and even in molecular structure. Recently, another two groups, the brassinosteroids and polyamines were also known to behave like hormones. .

Plant growth regulators – classification Plant Growth Regulators are classified as natural and synthetic based on their source and a detailed flow diagram is given in Figure . . Plant Growth Regulators (PGRs) Natural (Phytohormones) Synthetic Plant Growth Promoters Growth inhibitors Auxin Gibberellin Cytokinin Ethylene Abscisic acid NAA , -D , , - T Figure . : Classification of Plant Growth Regulators .

Characteristics of phytohormones i. Usually produced in tips of roots, stems and leaves. ii. Transfer of hormones from one place to another takes part through conductive systems.

iii. They are required in trace quantities. iv. All hormones are organic in nature.

v. There are no specialized cells or organs for their secretion. vi. They are capable of influencing physiological activities leading to promotion, inhibition and modification of growth.

. Synergistic and Antagonistic effects i. Synergistic effects : The effect of one or more substance in such a way that both promote each others activity. Example: Activity of auxin and gibberellins or cytokinins.

ii. Antagonistic effects : The effect of two substances in such a way that they have opposite effects on the same process. One accelerates and other inhibits. Example: ABA and gibberellins during seed or bud dormancy.

ABA induces dormancy and gibberellins break it. . . Auxins .

Discovery During , Charles Darwin noted the unilateral growth and curvature of Canary grass ( Phalaris canariensis ) coleoptile to light. The term auxin ( Greek : Auxin – to Grow) was first used by F. W. Went in using Oats ( Avena ) coleoptile and isolated the auxin.

F. W. Went in collected auxin in agar jelly. Kogl and Haugen Smith ( ) isolated Auxin from human urine, and called it as Auxin A .

Later on in , similar active substances was isolated from corn grain oil and was named as Auxin B . Kogl et al., ( ) found heteroauxin in the plant and chemically called it as Indole Acetic Acid (IAA) . Occurrence Auxin is generally produced by the growing tips of the stem and root, from where they migrate to the region of the action. .

Types of Auxin Auxins are divided into two categories Natural auxins and Synthetic auxins. Anti-auxins Anti-auxin compounds when applied to the plant inhibit the effect of auxin. Example: , , -Tri Iodine Benzoic Acid (TIBA) and Napthylpthalamine. (i) Free auxin They move out of tissues as they are easily diffusible.

Example: IAA. (ii) Bound Auxin They are not diffusible. Example: IAA. .

Precursor The amino acid Tryptophan is the precursor of IAA and zinc is required for its synthesis. . Chemical structure Auxin has similar chemical structure of IAA. .

Transport in Plants Auxin is polar in transport. It includes basipetal and acropetal transport. Basipetal means transport through phloem from shoot to root and acropetal means transport through xylem from root to shoot. .

Bioassay (Avena Curvature Test / Went Experiment) Bioassay means testing of substances for their activity in causing a growth response in a living plant or its part. The procedure involves the following steps: When the Avena seedlings have attained a height of to mm, about 1mm of the coleoptile tip is removed. This apical part is the source of natural auxin. The tip is now placed on agar blocks for few hours.

During this period, the auxin diffuses out of these tips into the agar. The auxin containing agar block is now placed on one side of the decapitated stump of Avena coleoptile. The auxin from the agar blocks diffuses down through coleoptile along the side to which the auxin agar block is placed. An agar block without auxin is placed on another decapitated coleoptile.

Within an hour, the coleoptiles with auxin agar block bends on the opposite side where the agar block is placed. This curvature can be measured (Figure . ). .

Physiological Effects • They promote cell elongation in stem and coleoptile. • At higher concentrations auxins inhibit the elongation of roots but extermely lower concentrations promotes growth of root. • Suppression of growth in lateral bud by apical bud due to auxin produced by apical bud is termed as apical dominance . • Auxin prevents abscission.

• It is used to eradicate weeds. Example: , -D and , , -T. • Synthetic auxins are used in the formation of seedless fruits (Parthenocarpic fruit). • It is used to break the dormancy in seeds.

Auxin in the Avena coleoptile Coleoptile placed on Agar Block Auxin diffuses in to agar block Decapited stump Auxin containing agar block in one side of stump Diffusion of Auxin from agar block Figure . : Avena Curvature Test . . Gibberellins .

Discovery The effect of gibberellins had been known in Japan since early where certain rice plants were found to suffer from ‘ Bakanae ’ or foolish seedling disease. This disease was found by Kurosawa ( ) to be caused by a fungus Gibberella fujikuroi . The active substance was separated from fungus and named as Types of Auxin Natural Synthetic Auxin occuring in plants are called “Natural auxin” . Indole Acetic Acid (IAA) .

Indole Propionic Acid (IPA) . Indole Butyric Acid (IBA) . Phenyl Acetic Acid (PAA) These are synthesized artificially and have properties like Auxin. .

, -Dichloro Phenoxy Acetic Acid ( , -D) . , , -Trichloro Phenoxy Acetic Acid ( , , -T) . Napthalene Acetic Acid (NAA) phloem and also occur in xylem due to lateral movement between vascular bundles. .

Bioassay (Dwarf Pea assay) Seeds of dwarf pea are allowed to germinate till the formation of the coleoptile. GA solution is applied to some seedlings. Others are kept under control. Epicotyl length is measured and as such, GA stimulating epicotyl growth can be seen.

. Physiological Effects • It produces extraordinary elongation of stem caused by cell division and cell elongation. • Rosette plants (genetic dwarfism) exhibit excessive internodal growth when they are treated with gibberellins. This sudden elongation of stem followed by flowering by the application of gibberellin is called bolting (Figure .

). • Gibberellin breaks dormancy in potato tubers. • Many biennials usually flower during second year of their growth. For flowering in the first year it self these plants should be treated with gibberellins.

• Formation of seedless fruits without fertili- zation is induced by gibberellins Example: Seedless tomato, apple and cucumber. • Promotes elongation of inter-node in sugarcane without decreasing sugar content. • Promotion of flowering in long day plants even under short day conditions. • It stimulates the seed germination.

Figure . : Bolting (a) Untreated plant (b) Treated plant showing bolting. Rosette leaves . .

Cytokinins (Cytos – cell, Kinesis – division) . Discovery The presence of cell division inducing substances in plants was first demonstrated by Haberlandt in in Coconut milk (liquid gibberellin by Yabuta ( ). These are more than gibberellins reported from both fungi and higher plants. They are noted as GA , GA , GA and so on.

GA is the first discovered gibberellin. In , Yabuta and Sumiki isolated gibberellin in crystalline form. In1955, Brain et al., gave the name gibberellic acid . In , Cross et al., established its structure.