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Chapter 3 – Plant Physiology

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Introduction

Plants also have an internal structure that carries out different processes. Plant physiology is the study of the physical, chemical, and biological functions of plants. Various processes like respiration, photosynthesis, nutrition and seed germination fall under this category.

A seed is a fertilised, matured ovule of a flowering plant. Simultaneously, the two integuments of an ovule develop into two seed coats, and the egg cell grows into an embryo. This embryo enclosed within the seed develops into a new plant by the process of germination. 

The central part of a seed embryo attached along with the plumule and the radicle is called a cotyledon. Depending on the number of cotyledons, seeds are classified into two types. 

Dicotyledonous SeedsMonocotyledonous Seeds
Seeds contain two cotyledons.Seeds contain one cotyledon.
It is a dicot plant.It is a monocot plant.
Examples: Gram, bean, castor, mustard, sunflower, poppy, pea.Examples: Rice, maize, wheat, orchids, and onion.

Reference: PMG Biology

A dicot seed is normally kidney-shaped, flat with a notch on the side. The seed looks fattened because it contains the endosperm to feed the embryo plant. A mature bean seed consists of two essential parts: seed coat and embryo.

  1. Seed coat: The seed coat is the outer, tough protective layer. It is divided further into the testa and the tegmen.
    • Testa: It is the outer, thicker tough layer.
    • Tegmen: It is the inner, thin, membranous layer, generally united with the testa.

Hilum: The hilum is a small scar present on the testa, and it is the point where a seed is attached to its stalk.

Micropyle: Micropyle is the opening or hole above the hilum through which the pollen tube enters the ovule. During seed germination, the micropyle absorbs water, softens the seed coats, and allows the embryo to develop. 

  1. Embryo
    The embryo is a small plant that emerges from the seed and germinates into a mature plant. It consists of:
    • Cotyledons: The two fleshy white kidney-shaped cotyledons provide nourishment to the growing embryo.
    • Radicle: It is the lower, pointed conical part of the central axis. It emerges from the micropyle during germination.
    • Plumule: It is the upper leafy end of the axis between the cotyledons that gives rise to the plant’s shoot.

The main grain is oval and usually flattened. The two main parts are the endosperm and the embryo.

Endosperm

The endosperm occupies a large portion of the grain, and it stores food mainly in the form of starch. A thin epithelial layer separates the endosperm and the embryo. The extraneous layer of the endosperm, rich in protein, is called the aleurone layer. The inner part of the endosperm is made of starch and a few lipid cells.

Embryo

It is seen below the endosperm. It consists of a single cotyledon called scutellum, plumule, and radicle. It has special cells for the secretion of digestive enzymes and provides nourishment to the developing embryo. The coleorhiza and coleoptile is the protective sheath for the radicle and plumule.

Reference: Bank of Biology

Through germination, the plant grows from a seed or spore. Seed germination is also defined as a process in which an embryo of a seed renews its metabolic activities and produces a seedling. 

Stages in Seed Germination:

Imbibition: At this stage, the seed rapidly absorbs water, and you can observe the swelling of the seed coat.

Activation: In this stage, the absorbed water activates the enzymes inside the seed, thereby starting the growth phase in the embryo. The seed begins respiration to produce necessary proteins for its growth.

Growth (Formation of Root and Shoot): As the rate of respiration increases, the seed coat begins to rupture, and the radicle emerges to form a primary root, and the hypocotyl is visible. Hypocotyl elongates and lifts the seed out of the ground while the seed coat remains below. At the same time, the plumule develops into a shoot. Here, you will notice the shoot starts growing upwards, producing primary and secondary roots. 

Morphogenesis (Formation of Seedling): In the final step of seed germination, the embryonic leaf appears. The baby plant uses the food stored within the seed, and after this stage, it synthesises its food through photosynthesis. Later, the cotyledons shrivel and fall off.

Epigeal GerminationHypogeal Germination
Cotyledons are pushed above the ground.Cotyledons remain below the soil.
Cotyledons turn green to carry photosynthesis.Cotyledons do not turn green.
Elongation of hypocotyl.Elongation of epicotyl.
Bean, sunflower, castor.Maise, rice, groundnut, mango.

Water: The presence of adequate water is crucial to start the seed’s enzymatic activity and metabolism. As it provides the necessary hydration for protoplasm, it softens the seed coat and facilitates necessary chemical changes in food material.

Temperature: The seeds can germinate at a wide range of temperatures, like 5-40°c. But the optimum temperature required for the seeds to germinate is 25-30°c. This is because the weather controls the metabolic processes of the seeds. 

Oxygen: Oxygen is necessary for the respiration process and speeds germination. You know that respiration releases energy by breaking down stored food. You can find oxygen within the pores of soil particles.

Light/Darkness: The presence/absence of light is the determining factor for seed germination. Many seeds do not germinate until sunlight falls on them; such seeds are called photoblastic.

Respiration in Plants: Respiration is the oxidation of complex organic molecules and releases energy in ATP for various cellular metabolic activities. The plants synthesise food through photosynthesis. The role of respiration in plants is to make the stored food available to the cells. Plants require oxygen for respiration, and they give out carbon dioxide. The respiration process of plants continues throughout their lives because they need the energy to survive. However, plants breathe differently through a process called cellular respiration.

In cellular respiration, plants generate glucose molecules through photosynthesis by capturing energy from sunlight. During respiration, the exchange of gas occurs. Leaves have some unique tiny pore-like structures called stomata for gaseous exchange. The movement of the stomata is controlled by guard cells. The oxygen consumed via stomata is used up by cells in the leaves to disintegrate glucose into water and carbon dioxide.

Dark respiration in plants is independent of the presence of light. After synthesising sugar molecules through photosynthesis, plant cells undergo cellular respiration to break down food molecules to release ATP.

Plant physiology is an essential topic concerning the development of seeds into plants, production of food crops, harvesting, and other functions of plants. A seed goes through different stages to germinate under optimal environmental conditions. Photosynthesis and respiration are the necessary processes for the growth of plants.

  1. What is the difference between photosynthesis and cellular respiration?Photosynthesis takes in carbon dioxide from the atmosphere, and cellular respiration puts it back. Photosynthesis releases oxygen into the atmosphere, and cellular respiration uses that oxygen to release energy from food.
  2. What is photorespiration?
    Photorespiration is a plant metabolism process where the plant takes in oxygen and gives out carbon dioxide.
  3. What is the role of transpiration in plants?
    Transpiration is the movement of water into and out of the plant. It is essential for evaporative cooling, CO₂ acquisition, and intake of mineral nutrients.
  4. What is vivipary?
    The germination of seed enclosed in the fruit while still attached to the parent plant is vivipary.

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High School Physics

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High School Biology

  • Absorption and Movement of Water in Plants
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