This article throws light upon the top six processes of plant physiology. The processes are: 1. Photosynthesis 2. Respiration 3. Photorespiration 4. Transpiration 5. Growth and Development 6. Photoperiodism and Vernalisation.
Plant Physiology: Process # 1.
Photosynthesis:
“Photosynthesis is a process by which chlorophyll containing organism (green plants, algae etc.) captures energy in the form of sunlight and converts it to chemical energy”.
Photosynthesis active radiation (PAR) occurs at 400-700 nm.
Major photosynthesis active pigments of higher plants are Chlorophyll a and Chlorophyll b.
Photosynthesis completes in 2 phases:
1. C4/Light reaction/Hill reaction – take place in Graina of Chlorophyll
2. C3/Dark reaction/Calvin cycle – take place in Stroma of Chlorophyll
Mechanisms of photosynthesis:
1. C3 pathway:
A. Also known as reductive pentose pathway or Blackman reaction or Calvin cycle.
B. It takes place in Rice, Wheat, Pea and Soybean etc.
C. Rubisco is the most important enzyme involved in photosynthetic CO2 fixation in C3 plants.
D. Its final product is 3 PGA.
2. C4 pathway:
A. Also known as Hatch and slack pathway or Di-carboxylic acid pathway or B- carboxylation cycle or Cooperative photosynthesis.
B. The 1st product in C4plants is Oxalo acetic acid.
C. PEP carboxylase is the most important enzyme involved in photosynthetic CO2 fixation in C3 plants
D. Its products are ATP and NADPH2.
E. It takes place in Maize, Sorghum and Sugarcane etc.
F. C4 plants have 2 types of photosynthetic cells with Kranz leaf anatomy viz. mesophyll cell and bundle sheath cells causing higher photosynthetic rate.
CAM (Crassulacean Acid Metabolism) pathway is found in Pineapple and Opuntia.
Comparison between C3 and C4 Plants:
A. C3Plants: Rice, wheat, barley, pea, gram, mustard and rye, cotton, Arhar, soybean, sunflower, lentil, sugarbeet, tomato etc.
B. C4 Plants: Maize, sorghum, Bajra, sugarcane, millets.
C. Cam Plants: Pineapple, khajur, cactus, sisal.
D. One NADH2 produces 3 ATP molecules.
One FADH2 produces 2 ATP molecules.
Plant Physiology: Process # 2.
Respiration:
(a). Respiration involves 2 processes:
1. Physical process – by which living organism take O2, and emit waste CO2
2. Chemical process – by which fuel molecules i.e. sugars and fats are broken down within a cell to liberate energy for cellular life process. Respiration in plants consists of-
1. Glycolysis
2. Krebs cycle (Citric acid or TCA-Tricarboxylic acid)
3. ETC (Electron Transport Chain)
b. Glycolysis: 1) It occurs in the cytoplasm and in anaerobic condition.
(2) Final product is Pyrubic acid/pyruyate.
c. Phases of glycolysis: 1st phase→ Consumption of ATP → Endothermic
2nd phase →Production of ATP → Exothermic
d. ATP synthesis in glycolysis,
1. Net gain ATP →2
2. Gross production→4
a. Krebs cycle and ETC occurs in Mitochondria.
b. TCA cycle starts with Acetyl co-enzymes A and Oxalo acetate.
c. ETC is present in the Cristae of Mitochondria, where ATP synthesized during respiration.
d. From one molecule of Glucose in respiration,
a. Net gain ATP synthesis →36 ATP
b. Gross production →38 ATP
e. Products of anaerobic respiration are Ethanol and Lactic acid.
f. Cytochromes are electron carriers in the respiratory ETC.
g. One molecules of glucose yields – 686 Kcal energy
One molecule of ATP yields – 7.6 Kcal energy
One molecule of nadph2 yields – 52 Kcal energy
h. The energy currency of the cell is ATP.
Plant Physiology: Process # 3.
Photorespiration:
“Light dependent oxygen uptake and carbon dioxide production is known as photorespiration”.
(1) This occurs only in light.
(2) Photorespiration is high at 25°-35°C.
(3) This occurs only in chlorophyllous cells.
(4) This is distinct from mitochondria respiration.
(5) This is primarily more in C3 plants and is very rare in C4 plants.
(6) Respiratory substrate: Glycolate.
(7) Process of respiration occurs in chloroplast + Peroxisomes + mitochondria.
(8) During this process, no ATP is produced.
Plant Physiology: Process # 4.
Transpiration:
“The loss of water in the form of vapour from the living aerial parts of the plant is known as transpiration”.
(1) The principle organ of transpiration is Leaf.
(2) Transpiration may be Folier (through stomata) or Lenticular (through lenticels).
(3) Transpiration is usually occurs in day time.
(4) The loss of water occurs in the form of vapour.
(5) It is regulated and controlled by Stomatal activities.
(6) The after affect of transpiration is “cooling the leaf surface”.
(7) The transpiring water is pure.
Types of transpiration:
1. Stomatal transpiration – 80-90 per cent water loss
2. Cuticular transpiration – 3-9 per cent
3. Lenticular transpiration – 0.1 per cent
Stomata:
1. Stomata are specialised epidermal cells.
2. It is found mainly on lower surface of leaves.
3. Approximately 97 per cent of transpiration takes place through stomata.
4. Each opened stomata has two kidney shaped guard cells.
5. Inner wall of guard cell is thicker, while outer is thin.
6. Opening and closing of stomata are due to its turgidity and flaccidity respectively.
Types of stomata:
(a) According to distribution of stomata on leaf:
1. Apple and Mulberry type – Stomata are present on only under surface of leaf.
2. Potato type – Mostly on lower surface.
3. Oat type – Equally distributed on both surface.
4. Water lily type – Only on upper surface
5. Potamogeton type – Stomata are either absent or functionless
(b) According to daily movement of stomata:
1. Alfalfa type – Stomata open through-out day and night i.e. pea, bean, mustard etc.
2. Potato type – Stomata open through-out day and night except for a few hours in the evening i.e. onion, cabbage, pumpkin etc.
3. Barley type – Stomata open only for a few hours during day
Stomatal transpiration:
Water absorbed by root hairs and reaches in the xylem vessels and tracheids through the root cortex. From the xylem of the root, it reaches in the xylem vessels and tracheids of the leaf. This results in increase in turgor pressure of its cell as compared to mesophyll cells.
The intercellular space in mesophyll cells are filled with air. By transpiration, water vapour enters in the intercellular space then passes on into atmosphere through stomata. Lenticels are the pores in the bark of fruits and woody stems.
Guttation:
“The loss of water (contains salts and minerals) through hydathodes in liquid form during night and regulated by root pressure”.
Bleeding:
“Loss of sap (water) from the injured parts of the plant due to root pressure”.
Plant Physiology: Process # 5.
Growth and Development:
A. Growth:
Growth is the irreversible change in any plant part (s) with respect to size, form, weight, volume etc.
B. Development:
Development is the phasic change of individual cells into tissues, organs and organisms. It is the resultant of growth.
Phases of growth:
1. Lag phase:
It is the initial growth phase, where internal changes in the cell occur, but it is very slow or negligible.
2. Log phase:
It is the grand period of growth and occurs fast.
3. Decreasing growth rate:
Here growth rate gradually decrease.
4. Senescence phase:
In this phase, organism reaches to maturity and growth ceases.
5. Death of organism:
I. The growth rate is measured by Auxanometer and Cresco graph.
II. Growth Hormones are the organic substances which are produced generally in meristematic cell of the plant and trans located towards the site of action inducing a physiological process.
III. Plant Growth Regulators (PGRs) are such organic compounds occurring naturally in the plants as well as synthetic and promote, inhibit or modify any physiological process in small amount.
Types of PGR:
Growth promoter – Auxins, Gibberellins and Cytokinin
Growth inhibitors – Abscisic acid and Ethylene
Plant Physiology: Process # 6.
Photoperiodism and Vernalisation:
a. Photoperiodism:
Photoperiodism is the physiological response of plants in relation to length of light (day) and dark (night). The term ‘Photoperiodism’ was coined by Garner and Allard (1920).
b. Photoperiodic Effect:
Influence of crop growth by the relative length of day and night especially for floral initiation.
1. Long day or short night plants:
Plant requires longer day length (>14 hrs.) for floral initiation (e.g.. Wheat, barley oat sugar beet and castor generally rabi crops)
2. Short day or long night plants:
Plant requires shorter day length (<10 hrs.) for floral initiation (e.g. Rice, sorghum, soybean, generally kharif crops)
3. Day neutral plants:
Intermediate day length (12-14 hrs.) e.g. Cotton, maize, sunflower, safflower, groundnut, buck wheat, tomato etc. Vernalisation is the cold treatment to a plant bud or seedling in order to fulfill a specific low temperature requirement for accelerating the flowering. Apical buds/early stages of germination (growing point) are the sites of Vernalisation.