Photoperiodism

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Photoperiodism

Trees take several years for initiation of flowering whereas an annual herb flowers within few months. Each plant requires a specific time period to complete their vegetative phase which will be followed by reproductive phase as per their internal control points through Biological Clock.

The physiological mechanisms in relation to flowering are controlled by

  1. Light period (Photoperiodism) and
  2. Temperature (Vernalization)

The physiological change on flowering due to relative length of light and darkness (photoperiod) is called Photoperiodism. The term photoperiodism was coined by Garner and Allard (1920) when they observed this in ‘Biloxi’ variety of soybean (Glycine max) and ‘Maryland mammoth’ variety of tobacco (Nicotiana tabacum).

The photoperiod required to induce flowering is called critical day length. Maryland mammoth (tobacco variety) requires 12 hours of light and cocklebur (Xanthium pensylvanicum) requires 15.05 hours of light for flowering.

1. Classification of Plants Based on Photoperiodism

(i) Long Day Plants:
The plants that require long critical day length for flowering are called long day plants or short night plants. Example: Pea, Barley and Oats.

(ii) Short Day Plants:
The plants that require a short critical day length for flowering are called short day plants or long night plants. Example: Tobacco, Cocklebur, Soybean, Rice and Chrysanthemum.

(iii) Day Neutral Plants:
There are a number of plants which can flower in all possible photoperiods. They are also called photo neutrals or indeterminate plants. Example: Potato, Rhododendron, Tomato and Cotton.

2. Photoperiodic Induction

An appropriate photoperiod in 24 hours cycle constitutes one inductive cycle. Plants may require one or more inductive cycles for flowering. The phenomenon of conversion of leaf primordia into flower primordia under the influence of suitable inductive cycles is called photoperiodic induction. Example: Xanthium (SDP) – 1 inductive cycle and Plantago (LDP) – 25 inductive cycles.

3. Site of Photoinductive Perception

Photoperiodic stimulus is perceived by the leaves. Floral hormone is synthesised in leaves and translocated to the apical tip to promote flowering. This can be explained by a simple experiment on Cocklebur (Xanthium pensylvanicum), a short day plant. Usually Xanthium will flower under short day conditions. If the plant is defoliated and kept under short day conditions it will not flower.

Flowering will occur even when all the leaves are removed except one leaf. If a cocklebur plant is defoliated and kept under long day conditions, it will not flower. If one of its leaves is exposed to short day condition and rest are in long day condition, flowering will occur (Figure 15.10).
Photoperiodism img 1

4. Importance of Photoperiodism

  • The knowledge of photoperiodism plays an important role in hybridisation experiments.
  • Photoperiodism is an excellent example of physiological pre-conditioning that is using an external factor to induce physiological changes in the plant.

5. Phytochrome

Phytochrome is a bluish biliprotein pigment responsible for the perception of light in photo physiological process. Butler et al., (1959) named this pigment and it exists in two interconvertible forms:

  • Red Light absorbing pigment which is designated as Pr and
  • Far red light absorbing pigment which is designated as Pfr. The Pr form absorbs red light in 660nm
    and changes to Pfr.

The Pfr form absorbs far red light in 730nm and changes to Pr. The Pr form is biologically inactive and it is stable whereas Pfr form is biologically active and it is very unstable. In short day plants, Pr promotes flowering and Pfr inhibits the flowering whereas in long day plants flowering is promoted by Pfr and inhibited by Pr form.
Photoperiodism img 2

Pfr is always associated with hydrophobic area of membrane systems while Pr is found in diffused state in the cytoplasm. The interconversion of the two forms of phytochrome is mainly involved in flower induction and also additionally plays a role in seed germination and changes in membrane conformation.
Photoperiodism img 3

Pentose Phosphate Pathway

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Pentose Phosphate Pathway

During respiration breakdown of glucose in cytosol occurs both by glycolysis (about 2/3) as well as by oxidative pentose phosphate pathway (about 1/3). Pentose phosphate pathway was described by Warburg, Dickens and Lipmann (1938). Hence, it is also called Warburg-Dickens-Lipmann pathway. It takes place in cytoplasm of mature plant cells. It is an alternate way for breakdown of glucose (Figure 14.15).
Pentose Phosphate Pathway img 1

It is also known as Hexosemonophosphate shunt (HMP Shunt) or Direct Oxidative Pathway. It consists of two phases, oxidative phase and non-oxidative phase. The oxidative events convert six molecules of six carbon Glucose-6-phosphate to 6 molecules of five carbon sugar Ribulose-5 phosphate with loss of 6CO2 molecules and generation of 12 NADPH + H+ (not NADH).

The remaining reactions known as non-oxidative pathway, convert Ribulose-5-phosphate molecules to various intermediates such as Ribose-5-phosphate(5C), Xylulose-5-phosphate(5C), Glyceraldehyde-3 phosphate(3C), Sedoheptulose-7-Phosphate (7C), and Erythrose-4-phosphate (4C). Finally, five molecules of glucose-6-phosphate is regene-rated (Figure 14.16). The overall reaction is:

6 × Glucose-6-Phosphate + 12NADP+ + 6H2O

5 × Glucose-6-Phosphate + 6CO2 + Pi + 12NADPH + 12H+

Pentose Phosphate Pathway img 2
The net result of complete oxidation of one glucose-6-phosphate yield 6CO2 and 12NADPH + H+. The oxidative pentose phosphate pathway is controlled by glucose-6-phosphate dehydrogenase enzyme which is inhibited by high ratio of NADPH to NADP+. Significance of pentose phosphate pathway.

  1. HMP shunt is associated with the generation of two important products, NADPH and pentose sugars, which play a vital role in anabolic reactions.
  2. Coenzyme NADPH generated is used for reductive biosynthesis and counter damaging the effects of oxygen free radicals
  3. Ribose-5-phosphate and its derivatives are used in the synthesis of DNA, RNA, ATP, NAD+, FAD and Coenzyme A.
  4. Erythrose is used for synthesis of anthocyanin, lignin and other aromatic compounds.
  5. It plays a role on fixation of CO2 in photosynthesis through RUBP.

Factors Affecting Respiration

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Factors Affecting Respiration

The eight environmental factors effecting the rate of respiration are:

  1. Oxygen Content of the Atmosphere
  2. Effect of Temperature
  3. Effect of Light
  4. Effect of Water Contents
  5. Effect of Respirable Material
  6. Effect of Carbon Dioxide Concentration
  7. Protoplasmic Conditions and
  8. Other Factors

The process of respiration is influenced by a number of external and internal factors. The main external factors are temperature, light, oxygen supply, water supply, CO2 concentration, toxic and stimulating substances and disease and injury.

For most plant species temperature, acidity, salt concentration and the amount of moisture, carbon dioxide and oxygen are some of the additional important factors which affect respiration. Let’s see the factors affecting the rate of respiration.

Temperature:
At a very high temperature, the rate of respiration decreases with time, and at very low temperature, the respiration rate is insignificant.

Carbon Dioxide Concentration:
The higher the carbon dioxide concentration, the lower the rate of respiration.

Explanation:
The rate of breathing is affected by many chemical factors like the level of carbon dioxide and oxygen in the blood. The increase in levels of the carbon dioxide will lower the blood pH this will direct the medulla of the brain to increase the breathing rate to obtain more amount of oxygen in the body.

Brainstem Rhythmicity Center. Breathing usually takes place outside of your conscious awareness. Blood Carbon Dioxide. The amount of carbon dioxide in the blood exerts a strong influence on respiratory rate. Blood pH.

The main factors affecting breathing rate are the levels of carbon dioxide and oxygen in the blood, and the blood’s pH. The main factors affecting rate of photosynthesis are light intensity, carbon dioxide concentration and temperature.

Several factors can affect the rate of photosynthesis:

  • Light Intensity
  • Carbon Dioxide Concentration
  • Temperature

Chemical – carbon dioxide, hydrogen ions and oxygen levels are the most important factors that regulate respiration. CO2 levels are the main influence, oxygen levels only affect breathing with dangerously low.

Oxygen, carbondioxide, temperature, light, availability of respirable materials etc., affect the rate of respiration. Oxygen is most important for aerobic respiration.

The temperature, light, materils of respiration such as carbohydrates, fats, proteins, etc., affect the rate of aerobic respiration.

External Factors:
Many external factors like temperature, light, carbon dioxide etc., affect the rate of respiration.

Temperature:
Temperature significantly affects the rate of respiration. Usually, the rate of respiration increases with the increase in temperature in the range of 0-45 degree centigrade.

  • Factors that influence blood pressure
  • Cardiac output
  • Peripheral vascular resistance
  • Volume of circulating blood
  • Viscosity of blood
  • Elasticity of vessels walls

The factors that affects temperature are altitude, latitude and distance from sea. The height measured from sea level is called altitude. When the latitude increases, the distant from the sun also increases, so the temperature gradually decreases. When the altitude increases, the temperature also gradually decreases.

The rate of respiration is normally not affected by increase of carbon dioxide concentration in the surrounding atmosphere up to 19%, but as the concentration increases from 10% to 80%, a progressive decrease in respiration occurs.

Normally, an increased concentration of carbon dioxide is the strongest stimulus to breathe more deeply and more frequently. Conversely, when the carbon dioxide concentration in the blood is low, the brain decreases the frequency and depth of breaths.

The external or environmental factors at: A light intensity, carbon dioxide concentration and temperature. The internal factor influencing the photosynthesis is chlorophyll content of the leaves and protoplasmic factors.

The environmental factors which can affect the rate of photosynthesis are carbon dioxide, light, temperature, water, oxygen, minerals, pollutants and inhibitors.

1. Effect of Carbon Dioxide:
Being one of the raw materials, carbon dioxide concentration has great effect on the rate of photosynthesis.
Factors Affecting Respiration img 1

Anaerobic Respiration

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Anaerobic Respiration

Fermentation

Some organisms can respire in the absence of oxygen. This process is called fermentation or anaerobic respiration (Figure 14.12). There are three types of fermentation:

  1. Alcoholic Fermentation
  2. Lactic Acid Fermentation
  3. Mixed Acid Fermentation

1. Alcoholic Fermentation

The cells of roots in water logged soil respire by alcoholic fermentation because of lack of oxygen by converting pyruvic acid into ethyl alcohol and CO2. Many species of yeast (Saccharomyces) also respire anaerobically. This process takes place in two steps:
Anaerobic Respiration img 1

Industrial Uses of Alcoholic Fermentation:

  1. In bakeries, it is used for preparing bread, cakes, biscuits.
  2. In beverage industries for preparing wine and alcoholic drinks.
  3. In producing vinegar and in tanning, curing of leather.
  4. Ethanol is used to make gasohol (a fuel that is used for cars in Brazil).

2. Lactic Acid Fermentation

Some bacteria (Bacillus), fungi and muscles of vertebrates produce lactic acid from pyruvic acid (Table 14.3).

3. Mixed Acid Fermentation

This type of fermentation is a characteristic feature of Enterobacteriaceae and results in the formation of lactic acid, ethanol, formic acid and gases like CO2 and H2.

Characteristics of Anaerobic Respiration

  1. Anaerobic respiration is less efficient than the aerobic respiration (Figure 14.12).
  2. Limited number of ATP molecules is generated per glucose molecule (Table 14.4).
  3. It is characterized by the production of CO2 and it is used for Carbon fixation in photosynthesis.

Net Products from one molecule of Glucose under Glycolysis and Anaeorbic Respiration

Respiratory Quotient

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Respiratory Quotient

The ratio of volume of carbon dioxide given out and volume of oxygen taken in during respiration is called Respiratory Quotient or Respiratory ratio. RQ value depends upon respiratory substrates and their oxidation.
Respiratory Quotient img 1

1. The respiratory substrate is a carbohydrate, it will be completely oxidised in aerobic respiration and the value of the RQ will be equal to unity.
Respiratory Quotient img 2
= 1(unity)

2. If the respiratory substrate is a carbohydrate it will be incompletely oxidised when it goes through anaerobic respiration and the RQ value will be infinity.
Respiratory Quotient img 3
= ∞ (infintiy)

3. In some succulent plants like Opuntia, Bryophyllum carbohydrates are partially oxidised to organic acid, particularly malic acid without corresponding release of CO2 but O2 is consumed hence the RQ value will be zero.
Respiratory Quotient img 4
= 0(zero)

4. When respiratory substrate is protein or fat, then RQ will be less than unity.
Respiratory Quotient img 5
= 0.7 (less than unity)

5. When respiratory substrate is an organic acid the value of RQ will be more than unity.
Respiratory Quotient img 6
= 1.33 (more than unity)

Significance of RQ

  1. RQ value indicates which type of respiration occurs in living cells, either aerobic or anaerobic.
  2. It also helps to know which type of respiratory substrate is involved.