Plant Growth and Movement

Growth is defined as the irreversible permanent increase in size of an organ or its parts of organisms including plants. Growth involves metabolic processes occurring at the expense of energy. Growth is followed by development. The development is about all the changes that occur in the life cycle of a plant from germination of the seed to senescence.

The development in plants is controlled by different factors categorized as intrinsic and extrinsic factors. The intrinsic factors are both the intracellular (genetic) or intercellular factors (chemicals such as plant growth regulators). The extrinsic factors include light, temperature, water, oxygen, and nutrition, etc.

Concept on seed germination

Seed germination is the fundamental process by which different plants grow from a single seed into a complete plant. It brings about the sprouting of a seedling from a seed of angiosperms and gymnosperms.

Process of seed germination

Step 1: Imbibition

Firstly, the dry seed takes up water rapidly and swelling and softening of seed coat at an optimum temperature happens; the process is known as imbibition.

Step 2: Activation of enzymes

The absorbed water activates different enzymes present inside the seed. This is called a lag phase of seed germination.

Step 3: Growth

After different enzymes have been activated, the rate of respiration increases and seed coat ruptures. Then, the radicle emerges to form a primary root, whereas the plumule develops into a shoot. During this period, the enzymatic activity remains high.

Stage 4: Morphogenesis

It is the final stage of seed germination. The cell of the seeds becomes metabolically active, elongates and divides to give rise to the seedling.  Gradually, tiny leaves sprout from the shoot ends, called foliage leaves.

A new seedling starts synthesizing its own food by photosynthesis once the morphogenesis completes.

Necessary conditions for seed germination

Seeds require water, oxygen, temperature, and light or darkness for its germination.

Water: It provides dissolved oxygen for the growing embryos, softens the seed coats, and increases the seed permeability. It also helps in the rupturing of seed and also converts the insoluble food into the soluble form.

Oxygen: Germinating seeds require oxygen for its metabolism. Oxygen in  the pores of soil particles is available for plants to be used in the aerobic respiration by seeds.

Temperature: The optimum temperature of about 25-30 degree C is required for the seed to germinate.

Light or darkness: Seeds require darkness to germinate. Light or darkness acts as an environmental trigger.

Factors affecting the seed germination

Some major factors affecting the seed germination are water, temperature, and oxygen.

Water: The poor or additional supply of water affects the germination of seeds.

Temperature: Temperature affects both the metabolism and growth rate of seed. In some cases, a temperature below the moderate level slows down the seed germination and promotes the growth of fungi.

Oxygen: As germinating seeds respire vigorously and release energy for their growth, the deficiency of oxygen affects the seed germination.

Seed Dormancy in plants

Seed dormancy is a condition or state in which seeds are prevented from germinating even under the favourable environmental conditions for germination. It is caused by the hard seed coat in many seeds. The hard seed coat is impermeable to water, gases, and other essential nutrients. During dormancy, the metabolic activity of seeds either ceases or is drastically reduced.

Causes of seed dormancy

The major causes of seed dormancy are light, temperature, hard seed coat, germination inhibitors, immaturity of seed embryo, and mechanically resistant seed coat, etc.

Types of seed dormancy

Innate dormancy, enforced dormancy, and induced dormancy are the types of dormancy in plants.

Innate dormancy: The seeds are incapable of germinating even with the supplied suitable conditions for seedling growth. This inability to germinate may be due to the immature embryo at the time of dispersal.

Enforced dormancy: The seeds are unable to germinate due to the environmental restraints such as an adequate amount of moisture, oxygen, light, and a suitable temperature.

Induced dormancy: The seeds are dormant after they have imbibed water, but have been kept under the extremely unfavourable conditions for the germination.

Breaking the seed dormancy

Seed dormancy can be broken by natural and artificial methods.

Natural method: The seed coat becomes permeable due to the rupturing and smoothing action of natural agents like microorganisms, temperature, and abrasion by the digestive tract of birds and animals that feed on these seeds. Then, the seed dormancy breaks when the seed receives appropriate moisture and temperature.

Artificial method: Seed dormancy can be artificially broken by following ways:

• Rupturing seed coats by chipping or threshing through machines;

• Treating hot water to terminate waxes and surface inhibitors;

• Exposing heat, cold or light, depending upon the type of seed dormancy;

• Treating seed coat with concentrated sulfuric acid;

• Weakening the tough seed coats by applying hydraulic pressure.

Importance of seed dormancy

Seed dormancy has following significance in plant life:

• It helps in the storage of seeds for later use by animals and humans.

• It helps in the dispersal of the seeds through the unfavourable environment.

• It helps seeds to remain alive in the soil for several years and provides a continuous source of new plants.

Photoperiodism in plants

Photoperiodism is the physiological and developmental responses of plants to the relative lengths of light and dark periods. Most of the flowering plants use photoperiodism to determine when to flower.

Plants require exposure to light for a critical duration to induce or inhibit flowering. The critical duration is different for the different plants. Based on the critical duration, the plants are grouped into three categories - long day plants (LDP), short day plants (SDP), and day neutral plants (DNP).

Long day plants (LDP): Plants which require the exposure to light for a period exceeding a well defined critical duration to induce flowering response are known as long day plants. For example, spinach, radish, and sugar beets, etc.

Short day plants (SDP): Plants which require the exposure of light for a period less than the critical duration before the flowering is initiated are called short day plants. For such plants, exposure of light exceeding the critical duration suppresses the flowering response.  For example, rice, sunflower, tobacco, and soybeans, etc.

Day neutral plants (DNP): Plants which do not show the correlation between the exposure to light duration and induction of flowering response are called day neutral plants. For example, tomato and pea, etc.

Figure 1 Plants showing phototropism

Importance of photoperiodism

• Photoperiodism determines the season of a plant to flower.

• Knowledge on photoperiodism is useful in keeping some plants in vegetative growth for obtaining higher yield of tubers and rhizomes, etc.

Vernalisation in plants

The flowering response of plants on exposure to the low temperature is known as vernalisation. Plants exhibiting vernalisation remain vegetative during the summer, but grow and bear flowers and fruits during winter. This phenomenon was discovered by Lysenko, a Russian plant physiologist, in 1928.

Cereals like wheat, barley, rye, etc., have two kinds of varieties, namely, winter and spring varieties. Spring varieties are normally planted in spring and flower and produce grain before the end of the growing season. However, winter varieties, if planted in the spring, normally fail to flower or produce mature grain. Hence, they are planted in the autumn. Then, they germinate into small seedlings in the winter and resume growth in the spring. Later on, they bear flowers and fruits in the summer.

Importance of vernalisation

• Vernalisation helps in shortening the vegetative period of the plant and bringing about early flowering.

• It increases the yield and resistance to cold and diseases.

• It removes kernel wrinkles of Triticale.

Senescence in plants

Plants also have a specific life span during which they develop, grow, and finally die. Prior to their death, plants undergo natural deteriorative processes to terminate functional life. The physiological process of gradual deterioration of functional characteristics of plants with age is called senescence. Senescence in plants can best be studied in leaves or similar organs such as cotyledons, sepals, and petals, etc. The plants or plant parts that undergo senescence are called senescents.

Senescence is a normal energy dependent developmental process which is controlled by plants' own genetic programme. The senescence is not confined only to the entire plant; it may be limited to a particular plant organ like leaves and flowers. The death of the plant or plant parts as a consequence of senescence is known as programmed cell death (PCD).

According to Leopold (1961), senescence patterns are of four types - overall, top, deciduous, and progressive senescence.

Overall senescence: It occurs in the annual plants. The entire plant is affected and dies due to overall senescence.

Top senescence: It occurs in perennials. It is seen only in the parts of the plant above the ground parts and roots.

Deciduous senescence: It takes place in woody plants. It occurs in all the leaves but the stem and root systems remain alive.

Progressive senescence: It is the gradual progression of senescence of leaves from the base upwards as the plant grows.

Effect of senescence on plants

Senescence has following effects on plants:

• The increased respiration, declining photosynthesis, and an orderly disintegration of macromolecules happens in plants as the characteristics of senescence.

• Firstly, chloroplasts are disintegrated; other cell organelles gradually deteriorate.

• Concentration of growth hormones especially cytokinins decline.

• Senescence associated genes (SAGs) such as proteases, ribonucleases, and lipages, are with increased expression.

• Brilliant colours appear in the leaves of many plants due to the degradation of chlorophylls.

• The cells and tissues also lose respiratory control towards the end of senescence.

Plant movements

The capacity of plants to change their position with respect to the external or internal stimuli is  known as plant movement. The growth movement of plants are of  two types - nastic and tropic movement.

1. Nastic movement 

It is the non-directional movement of plants in response to external stimuli such as humidity, light, temperature, light irradiance, nutrients, and gravity, etc. The movement may be due to the changes in the turgor or changes in the growth.

Nastic movement of different types as:

Epinasty - the downward curvature of the leaves due to the differential growth rates, for example, the bending down of a heavy flower;

Hyponasty - An upward bending of the leaves or other parts of a plant caused by the increased growth on their lower surface.

Nyctinasty - the movement at night or in the dark;

Photonasty - a movement in response to light;

Chemonasty - a response to the chemicals or nutrients;

Hydronasty - a response to water;

Thermonasty - a response to temperature;

Seismonasty - a response to shock;

Geonasty/gravinasty - a response to gravity;

Thigmonasty/seismonasty/haptonasty - the movement in response to contact.

2. Tropic movements

The directional growth movements that are induced by the external stimuli are called tropic movements. These are also called induced or paratonic movements.These movements are shown by bryophytes, ferns, and all the seed plants. Some of their examples are the movements of the stem, the petiole, and the floral axis, etc. On the basis of external factors, they are phototropism, geotropism, chemotropism, hydrotropism, and thigmotropism.

Phototropism: It is the response of plants to unidirectional light. For example, the curvature induced in the plant organs in response to the unidirectional light.  The shoot apex moves towards the source of unidirectional light and the root away from it. The stem shows positive phototropism and the root negative phototropism.

Geotropism: The growth and orientation of the stems and the roots in response to the force of gravity is called geotropism. The stem shows negative geotropism and the root and rhizomes show positive geotropism.

Chemotropism: The movement of plant organs due to the unilateral stimulus of chemicals is called chemotropism. The growth of pollen tube from the surface of stigma into the style and ovary is due to  the chemical stimulus.

Hydrotropism: It is the response of a part of the plant to water. The roots are positively hydrotropic which means they grow towards water.

Thigmotropism (Haptotropism): The growth movement of plant organs in response to the unilateral stimulus of touch is called thigmotropism. Tendrils of many cucurbitaceous plants nutate in the air and when they are in contact with any support, they coil around it.