Unit 9: Plant Physiology — Short Questions

Plants get their food from a process called photosynthesis. But for the synthesis of other biomolecules, they need other materials from soil. Such materials are called mineral nutrients and the process through which these special chemicals absorbed from soil that are essential for the plants to grow is called mineral nutrition.

The minerals which are required in larger quantities are called macronutrients e.g. carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulphur, calcium, and magnesium. While, the minerals which are required in lower quantities are called micronutrients e.g. iron, molybdenum, boron, copper, manganese, zinc, chlorine, and nickel.

Nitrogen i. Nitrogen is a necessary part of all proteins, enzymes,
ii. It also activates many plant enzymes needed for growth.
iii. It also helps in fruit formation and germination of seeds.
iv. Plant roots absorb magnesium in ionic form (Mg+2).

Magnesium i. Magnesium is part of the chlorophyll.

Definition
The loss of water in the form of vapours from plant surface is called transpiration.

Types 1. Stomatal transpiration
2. Cuticular transpiration
3. Lenticular transpiration

Transpiration creates a pull called transpiration pull which is principally responsible for the conduction of water and salts from roots to the aerial parts of the plant body.

Transpiration is a necessary evil. It is harmful during the condition of drought. As water loss cause wilting of the plant.
But at the same time it is important for plant as it cause cooling effect, generates transpirational pull and helps in gaseous exchange.

i. Xylem and phloem

Xylem 1. Xylem conducts water and mineral salts and provide support and strength.
2. Xylem consists of two main types of cells namely tracheids and vessel elements.

Phloem 1. Phloem conducts prepared food from leaves to stem and roots etc.
2. Two main types of cells in the phloem namely sieve tube element and companion cells.

ii. Transpiration and guttation

Transpiration i. Plants absorb water from the soil by the roots. This absorbed water moves in the aerial parts of the plant from where the most of this water (approx. 99%) has been lost in the form of vapours into the atmosphere.

Guttation i. The appearance of drops of water on the tips or edges of leaves is called guttation.
ii. Guttation is not to be confused with dew, which condenses from the atmosphere on to the plant surface.
iii. Some plants such as sea grasses and strawberry force this water through special pores present at leaf tips or edges and form drops.

iii. Hydrophytes and Halophytes

Hydrophytes i. These plants live in water-rich environments.
ii. Rate of transpiration is highest.
iii. Stomata are present on the upper surface of leaf.
iv. These plants have thin cuticle.

Halophytes i. These plants live in sea water and are adapted to salty environments.
ii. Salts enter in the bodies of such plants due to their higher concentration in sea water, water tends to move out of their cells into the hypertonic sea water.

Examples Many sea grasses are included in this group of plant.

iv. Hydrophytes and xerophytes

Hydrophytes i. They live in water-rich environments.
ii. Rate of transpiration is highest.

Xerophytes i. These plants are adapted to extreme dry conditions.
ii. They exhibit lowest rate of transpiration.

v. Lenticular transpiration and stomatal transpiration

Stomatal Transpiration Most of the transpiration occurs through stomata.

Lenticular Transpiration This is the water loss of plant in form of vapours by the lenticels present on the bark of woody stems.
And is called stomatal transpiration. In leaves, water moves from the xylem into the cell walls of mesophyll cells.

Plants deposit many metabolic wastes in their bodies as harmless insoluble materials:
i. Latex are removed by rubber plants.
ii. Gums are removed by keekar.

(i) Osmotic adjustment
Osmotic adjustment, also known as osmoregulation, is like, plant's way of maintaining the right balance of water and solutes in its body.

(ii) Transpiration
Plants absorb water from the soil by the roots. This absorbed water moves in the aerial parts of the plant from where the most of this water (approx. 99%) has been lost in the form of vapours into the atmosphere. This loss is called transpiration.

(iii) Translocation
The transport of prepared food (organic solutes) to different parts of the plant through the phloem tissue is translocation.

(iv) Micronutrients
The seven elements are needed in traces or small amounts (less than 0.05% dry weight) for normal plant growth and development that are known as micronutrients. These include iron, boron, manganese, copper, molybdenum, chlorine, and zinc.

(v) Excretophores
Plant cells have large vacuoles that can store useful stuff or waste. Sometimes, these stored substances can build up and form crystals in the vacuoles. Leaves are key players in this process. When the leaves are loaded with large amount of pigmented compounds, they turn yellow. Remember, this yellowing is not due to lack of chlorophyll as happens in chlorosis.

(vi) Vascular Bundle
There are two types of compound tissues in plants: (a) xylem (b) phloem. Together they form the vascular bundles. Both xylem and phloem are composed of more than one type of cells. Xylem tissue is responsible for the transport of water and dissolved substances from roots to aerial parts. Phloem are responsible for the conduction of dissolved organic matter (food) between different parts of plant body.

(vii) Xylem
(1) Xylem tissue is responsible for the transport of water and dissolved substances from roots to aerial parts.
(2) They provide support to plant body because of presence of lignin in its secondary cell walls. Lignin makes these walls thick and rigid.

(viii) Adhesion
Adhesion is the attraction between water molecules and other substances. Water is strongly attracted to the walls of the xylem cells because both water and cellulose (in cell walls) are polar molecules. This adhesion helps water move upward in the plant against gravity. It also keeps water in the xylem when transpiration is not happening.

(ix) Cohesion
Cohesion is the attraction between nearby water molecules, which is possible because water is a polar molecule.

(i) On a sunny day with strong sunlight, the air temperature rises, and this increase in temperature lowers the humidity in the air. As a result, more water evaporates from the surfaces of plant mesophyll cells, which leads to a higher rate of transpiration.

(ii) For every 10°C increase in temperature, the rate of transpiration roughly doubles.

(iii) However, when the environmental temperature becomes very high, around 40-45°C, it causes the stomata on plant leaves to close. This closure helps the plant conserve its much- needed water because excessive loss can be detrimental.

(iv) If these higher temperatures persist for an extended period and the soil doesn't have enough water, the plants may start to wilt and could eventually die

Transpiration provides evaporative cooling. As water leaves the plant tissues, it takes energy with it in the form of heat. Much like when we sweat, this allows the plants to cool and maintain the homeostasis.

Pith Pith is a small area in the centre of vascular bundles of plants.

Cortex Cortex consists of many layers of parenchyma cells, inner to epidermis.

Guard Cells A pair of guard cells form a stoma, which is involved in the gas exchange of plants.

Epidermal Cells Epidermal cells provide a protection to the plant from the external environment.

A location in a plant where sugar is being produced either by photosynthesis or by the breakdown of stored starch is called a sugar source; green leaves and stem.
A location in a plant where sugar is consumed or stored is called a sugar sink e.g., young leaves, fruits etc.

Endodermis refers to the inner layer of cortex that surrounds the vascular bundle. Pericycle is a single ring like layer internal to the endodermis.

During dawn and dusk, when light intensity is low, the rates of photosynthesis and respiration become equal. This means the carbon dioxide produced by respiration is enough for photosynthesis, and the oxygen released by photosynthesis is used in respiration. At this point, there's no net exchange of gases with the environment, and we call it the "compensation point of photosynthesis."

During the day, plants are busy with both photosynthesis (making food) and respiration. The rate of photosynthesis varies throughout the day as it mainly depends upon light intensity. Generally, the rate of photosynthesis is greater than rate of respiration, therefore, the photosynthesis needs more carbon dioxide than what respiration produces, so plants bring in extra carbon dioxide from the environment. On the other hand, photosynthesis produces more oxygen than respiration needs, so plants release excess oxygen.

As the process of respiration takes place continuously, so carbon dioxide is also produced continuously. Photosynthesis takes place at day time in the plants, during which oxygen is released.
CO2 produced during respiration at day time is used in photosynthesis. Only a small quantity of oxygen produced during photosynthesis is used by plants for respiration. The rest of the oxygen is released through stomata and lenticels. At night no photosynthesis takes place. So, CO2 is released as by product and atmospheric oxygen is used for respiration is plants.

(i) CO2
(ii) Extra Oxygen
(iii) Excess Water
(iv) Calcium oxalate
(v) Latex
(vi) Resins
(vii) Gums

The process by which metabolic wastes are eliminated from body to maintain the internal conditions of equilibrium is called excretion e.g. urea, salts of uric acid and water are eliminated out of body through excretion.

They have-developed mechanisms for the removal of extra water from their cells.
i. Hydrophytes have broad leaves with large number of stomata on their upper surfaces this characteristic helps them to remove the extra amount of water.
ii. Hydrophytes keep stomata open day and night.
iii. Hydrophytes almost have no cuticle.

It is defined as the maintenance of the amounts of water and salts in body fluids i.e. blood and tissue fluids, e.g; blood glucose level remains about 1g/L despite eating a meal rich in carbohydrate.

Plants transpire more on a windy day because wind removes the water vapours around the leaves, making it easier for more water to evaporate from the stomata. While on a humid day, the air already has a lot of water vapours, which slow down the rate of evaporation and so reduces rate of transpiration. So, wind increases the evaporation rate, while humidity slows it down.