Transpiration (General) and Stomata Questions in English

(C) Transpiration is the process of water loss from the aerial parts of plants,primarily through stomata.
To reduce excessive water loss,plants have evolved various adaptations.
One such adaptation is the presence of a waxy,hydrophobic layer on the surface of leaves and stems known as the cuticle.
The cuticle is primarily composed of a fatty substance called $Cutin$.
This layer acts as a barrier,significantly reducing the rate of transpiration by preventing non-stomatal water loss.

(A) Initially,an increase in temperature and wind velocity enhances the rate of transpiration by increasing the evaporation of water from the leaf surface.
However,if the temperature becomes excessively high,it leads to the closure of stomata (mid-day closure) to prevent excessive water loss.
Similarly,extremely high wind velocity can cause the stomata to close due to mechanical stress or by reducing the water potential gradient,which ultimately slows down the transpiration process.
Therefore,the correct reason for the initial increase followed by a decrease is the closure of stomata.

(D) Light affects the rate of transpiration directly by inducing the opening of stomata.
In the absence of light or when there is a decrease in light intensity,the stomata tend to close.
Since stomatal transpiration accounts for the majority of water loss in plants,the closure of stomata significantly reduces or checks the rate of transpiration.
Therefore,a decrease in light intensity leads to a reduction in the rate of transpiration.

(A) Transpiration is the process of water loss from plants in the form of water vapor. The rate of transpiration is inversely proportional to the relative humidity of the atmosphere.
$(a)$ When the atmosphere is highly humid,the water potential gradient between the leaf interior and the outside air is reduced,which significantly slows down the rate of transpiration.
$(b)$ High wind velocity generally increases the rate of transpiration by removing the humid air layer around the leaf.
$(c)$ Excess water in the cell typically promotes higher transpiration rates.
$(d)$ Dry environmental conditions increase the water potential gradient,leading to higher transpiration rates.
Therefore,transpiration is lowest when humidity is high.

(A) Phenyl mercuric acetate $(PMA)$ is a well-known antitranspirant.
It works by inducing the partial closure of stomata in plants.
By reducing the aperture of the stomata,it significantly lowers the rate of transpiration without severely affecting other physiological processes like photosynthesis or respiration.
Therefore,the correct option is $A$.

(D) The rate of transpiration is primarily determined by the difference in vapour pressure between the internal atmosphere of the leaf (sub-stomatal cavity) and the external environment. This difference is known as the Vapour Pressure Gradient $(VPG)$.
Even if the stomata are fully open,if the relative humidity of the external air is $100\%$,the vapour pressure gradient becomes zero,and no transpiration occurs. Thus,the vapour pressure gradient is the direct driving force for transpiration.

(A) Anti-transpirants are chemical substances that reduce the rate of transpiration in plants without significantly affecting the process of photosynthesis.
$Phenyl$ $mercuric$ $acetate$ $(PMA)$ is a well-known chemical that acts as an anti-transpirant by inducing the closure of stomata.
By reducing the number of open stomata,it helps the plant conserve water,especially during periods of drought or water stress.

(D) The chemical substances that reduce the rate of transpiration by increasing the resistance of leaves to water vapour diffusion,without significantly affecting the gaseous exchange required for photosynthesis and respiration,are known as anti-transpirants. These substances can be applied to any type of plant,including fruit plants,vegetable plants,and various crop plants,to conserve water during periods of drought or water stress. Therefore,the use of anti-transpirants can check transpiration in all the mentioned plant types.

(B) When the available water in the soil is insufficient,the rate of transpiration decreases.
Under conditions of internal water deficiency,the plant experiences turgor loss in guard cells,causing the stomata to partially or completely close.
This closure significantly reduces the loss of water vapor from the leaves,thereby decreasing the overall rate of transpiration.

(C) Stomata open in response to light.
Blue light is significantly more effective in inducing stomatal opening compared to other wavelengths of light.
This phenomenon was demonstrated by Mouravieff in $1958$.

(B) The rate of transpiration is directly proportional to the temperature.
As the temperature increases,the kinetic energy of water molecules increases,which leads to faster evaporation of water from the leaf surface into the atmosphere.
Therefore,an increase in temperature results in a higher rate of transpiration.

(D) The action spectrum of transpiration refers to the range of wavelengths of light in which the rate of transpiration is maximum.
Experimental studies have shown that the rate of transpiration is highest in the blue and red regions of the visible light spectrum.
This is primarily because stomatal opening,which facilitates transpiration,is most effectively triggered by blue light,and these wavelengths also contribute to the heating of the leaf surface,thereby increasing the rate of evaporation.

(C) Transpiration is the process of water loss from the aerial parts of plants in the form of water vapor.
Factors that increase the rate of transpiration include high temperature,low humidity,high wind velocity,and high light intensity.
High temperature increases the rate of evaporation of water from the mesophyll cells into the intercellular spaces and also increases the diffusion gradient between the leaf and the atmosphere.
Therefore,among the given options,high temperature is the primary factor that directly increases the rate of transpiration.

(B) The rate of transpiration is influenced by several environmental factors such as light,temperature,humidity,and wind speed.
$1$. Placing a plant in the dark closes the stomata,which decreases transpiration.
$2$. Placing a plant in a cold room reduces the temperature,which decreases the rate of evaporation and thus transpiration.
$3$. Placing a plant outside during rain increases humidity,which reduces the water potential gradient between the leaf and the atmosphere,thereby decreasing transpiration.
$4$. Placing a plant in the draught of an electric fan increases the wind speed around the leaves. This removes the humid air layer (boundary layer) surrounding the leaf surface,thereby increasing the water potential gradient and significantly increasing the rate of transpiration.
Therefore,option $B$ is the correct answer.

(C) Transpiration is the process of water loss from the aerial parts of plants,primarily through the stomata.
Stomata are microscopic pores found on the epidermis of leaves,and their opening and closing are regulated by specialized cells known as guard cells.
Since the majority of transpiration occurs through stomata,any factor that interferes with the functioning of guard cells will directly affect the rate of transpiration.
Therefore,interfering with guard cells is a direct way to influence the transpiration process.

(B) The most important factor regulating transpiration is the opening and closing of stomata.
Stomatal movement is primarily controlled by light intensity.
During the day,light triggers the opening of stomata to facilitate gas exchange for photosynthesis,which simultaneously allows water vapor to escape,thereby regulating the rate of transpiration.
Therefore,light is considered the most significant factor among the given options.

(A) When the relative humidity of the atmosphere is $100\%$,the water potential gradient between the leaf interior and the atmosphere becomes zero.
Since transpiration is the loss of water in the form of water vapor from the aerial parts of the plant,it depends on the vapor pressure deficit between the leaf and the surrounding air.
At $100\%$ relative humidity,the air is saturated with water vapor,meaning there is no driving force for water loss.
However,stomata generally remain open during the day to facilitate gas exchange ($CO_2$ uptake for photosynthesis),even if transpiration is negligible or absent due to the saturated atmosphere.

(B) Stomatal transpiration is primarily regulated by the opening and closing of stomata.
An increase in the concentration of $CO_2$ in the sub-stomatal cavity or the surrounding atmosphere promotes the closing of stomata.
When $CO_2$ levels are high,the guard cells lose turgidity,leading to the closure of the stomatal pore,which results in a steep decline in the rate of transpiration.
Therefore,high $CO_2$ concentration is a key factor that inhibits stomatal transpiration.

(C) Transpiration is the process of water loss from the aerial parts of plants in the form of water vapor.
Temperature and wind speed are directly proportional to the rate of transpiration.
As temperature increases,the kinetic energy of water molecules increases,leading to faster evaporation.
Similarly,wind removes the humid air surrounding the leaf surface,maintaining a steep water vapor concentration gradient,which increases the rate of transpiration.
Conversely,relative humidity is inversely proportional to transpiration because high humidity reduces the concentration gradient between the leaf interior and the atmosphere.

(B) Transpiration is the process of water loss from the aerial parts of plants,primarily through stomata.
During the summer months,the rate of transpiration is influenced by light intensity,temperature,and humidity.
At $1 AM$ (nighttime),the light intensity is zero,and the temperature is at its lowest point.
Most plants close their stomata during the night to prevent excessive water loss,leading to the lowest rate of transpiration at this time.

(D) In intense light,the rate of transpiration typically increases. However,if a plant exhibits a decrease in the rate of transpiration under very intense light,it is a protective mechanism to prevent excessive water loss and wilting.
This phenomenon occurs due to the partial closure of stomata,which reduces the surface area available for water vapor to escape from the leaf interior to the atmosphere.
Therefore,the correct option is $D$.

(D) Transpiration is the process of water loss from the aerial parts of plants,primarily through stomata.
During the night,most plants close their stomata to prevent water loss,as there is no photosynthesis occurring.
Since the stomata are closed,the rate of transpiration is already minimal or negligible.
Therefore,changes in external environmental factors like humidity or temperature have very little effect on the transpiration rate during the night compared to the day when stomata are open.

(D) The rate of transpiration is inversely proportional to the relative humidity of the surrounding air.
When the humidity of the atmosphere increases,the air becomes saturated with water vapor,which reduces the diffusion gradient between the leaf interior and the atmosphere,thereby decreasing the rate of transpiration.
Conversely,when the humidity decreases (i.e.,the air becomes drier),the water vapor concentration gradient between the leaf and the atmosphere increases,which leads to an increase in the rate of transpiration.

(B) Transpiration is the process of water loss from the aerial parts of plants in the form of water vapor.
$(b)$ Temperature directly affects the rate of transpiration.
As the temperature increases,the kinetic energy of water molecules increases,leading to faster evaporation from the leaf surface.
Additionally,higher temperatures decrease the relative humidity of the air surrounding the leaf,which increases the water potential gradient between the leaf and the atmosphere,thereby accelerating transpiration.

(C) The opening and closing of stomata in angiosperms are primarily regulated by changes in the turgor pressure of the guard cells.
When the guard cells become turgid due to the influx of water,the stomatal aperture opens.
Conversely,when the guard cells lose water and become flaccid,the stomatal aperture closes.

(B) The opening and closing of stomata are primarily regulated by the turgor pressure of the guard cells.
When guard cells are exposed to light,they actively accumulate potassium ions $(K^+)$,which lowers their water potential.
This causes water to enter the guard cells via osmosis,leading to an increase in their turgidity.
Due to the specific arrangement of cellulose microfibrils in the guard cell walls,this increased turgor pressure causes the guard cells to bow outwards,thereby opening the stomatal pore.

(B) Transpiration is the process of water loss from the aerial parts of plants in the form of water vapor.
This process is primarily regulated by the opening and closing of stomata.
The stomatal aperture is controlled by the turgor pressure changes within the guard cells.
When guard cells become turgid,the stomata open,facilitating transpiration.
When guard cells lose water and become flaccid,the stomata close,thereby reducing transpiration.

(B) Lewitt $(1974)$ proposed the '$K^+$ pump theory' or 'Active $K^+$ transport theory' to explain the mechanism of stomatal opening and closing.
According to this theory,the accumulation of $K^+$ ions in guard cells is responsible for the decrease in water potential,which leads to the entry of water into the guard cells,causing them to become turgid and the stomata to open.
Conversely,the efflux of $K^+$ ions leads to the loss of water,causing the guard cells to become flaccid and the stomata to close.

(D) The opening and closing of stomata are regulated by the movement of $K^+$ ions in and out of the guard cells.
When $K^+$ ions accumulate in the guard cells,the osmotic pressure increases,causing water to enter the cells via osmosis.
This increases the turgor pressure of the guard cells,leading to the opening of the stomata.
Conversely,when $K^+$ ions move out of the guard cells,water follows,leading to a decrease in turgor pressure and the closure of the stomata.
Therefore,the correct answer is $Potassium$ $(K^+)$.

(B) According to the starch-sugar interconversion theory proposed by $Sayre$ $(1926)$,the opening and closing of stomata are regulated by the interconversion of starch and soluble sugars.
During the day,the $pH$ of the guard cells increases due to the consumption of $CO_2$ in photosynthesis.
This high $pH$ triggers the conversion of starch into organic acids (like malic acid) and glucose$-1-$phosphate.
This increases the osmotic concentration of the guard cells,leading to the endosmosis of water,which causes the guard cells to become turgid and the stomata to open.
Therefore,the conversion of starch to organic acid is essential for stomatal opening.

(A) The modern and most widely accepted theory for the opening and closing of stomata is the $K^+$ (Potassium ion) pump theory,proposed by Levitt.
According to this theory,the accumulation of $K^+$ ions in the guard cells leads to a decrease in water potential,causing water to enter the cells via osmosis.
This results in an increase in turgor pressure,which causes the guard cells to swell and the stomatal pore to open.
Conversely,the exit of $K^+$ ions leads to water loss,a decrease in turgor pressure,and the closing of the stomata.

(A) Stomata open during the day because the guard cells perform photosynthesis,which produces osmotically active substances like sugars or organic acids.
This accumulation of solutes increases the osmotic pressure within the guard cells.
Consequently,water enters the guard cells from surrounding cells via endosmosis,leading to an increase in turgor pressure.
This increased turgor pressure causes the guard cells to swell and curve,thereby opening the stomatal pore.

(C) The opening and closing of stomata are primarily regulated by the turgor pressure of the guard cells. When water molecules enter the guard cells through osmosis,the guard cells become turgid. Due to the presence of thick inner walls and thin outer walls,the turgid guard cells bulge outwards,causing the stomatal pore to open wider. Therefore,the entry of water into the guard cells is the direct cause of stomatal opening.

(A) When the osmotic pressure of guard cells becomes higher than that of the adjoining subsidiary cells,water enters the guard cells through endosmosis.
This influx of water increases the turgor pressure within the guard cells,causing them to become turgid.
Due to the specific orientation of cellulose microfibrils in the guard cell walls,this turgidity leads to the opening of the stoma.

(C) The $Korper-Kappe$ theory is not related to the opening of stomata.
It is a theory related to the organization of the root and shoot apical meristems in plants.
In contrast,the $Sachs$ theory (starch-sugar interconversion),$K^+$ transport theory (active transport of potassium ions),and $Lewitt$ theory (pH-dependent starch-sugar conversion) are all established theories explaining the mechanism of stomatal opening and closing.

(C) The movement of stomata caused by changes in the water status of the guard cells is known as hydroactive movement.
When guard cells lose water,they become flaccid,leading to the closure of the stomata.
Conversely,when guard cells take up water,they become turgid,causing the stomata to open.
This process is directly dependent on the water potential and turgor pressure within the guard cells.

(D) According to the proton transport theory (also known as the active $K^+$ transport theory),the opening and closing of stomata are regulated by the movement of potassium ions $(K^+)$.
During the day,protons $(H^+)$ are pumped out of the guard cells,creating an electrochemical gradient.
This gradient facilitates the active uptake of $K^+$ ions into the guard cells from the surrounding subsidiary cells.
The accumulation of $K^+$ ions lowers the water potential inside the guard cells,causing water to enter via osmosis,which leads to the swelling of guard cells and the opening of the stomatal pore.

(C) During the day,starch is converted into sugars due to an increase in $pH$.
This conversion results in an increase in the concentration of the cell sap.
Consequently,endosmosis of water into the guard cells occurs,which increases their turgidity and leads to the opening of the stomata.

(C) Stomatal movement is regulated by the osmotic pressure of guard cells.
During the night or under water stress,the $pH$ of the guard cells decreases.
This acidic environment promotes the conversion of soluble sugar into insoluble starch.
As starch is insoluble,it does not contribute to the osmotic potential,causing water to move out of the guard cells via osmosis.
This loss of turgor pressure leads to the flaccidity of guard cells,resulting in the closure of the stomata.

(B) Lewitt proposed the $pH$ theory for stomatal movement. According to this theory,the opening and closing of stomata are regulated by changes in the $pH$ of the guard cells. During the day,photosynthesis reduces the $CO_2$ concentration,leading to an increase in $pH$ (alkaline condition). This high $pH$ promotes the conversion of starch into soluble sugars,which lowers the water potential of guard cells,causing water to enter via osmosis,resulting in stomatal opening. Conversely,at night,$CO_2$ accumulates,lowering the $pH$ (acidic condition),which triggers the conversion of sugar back into starch,leading to stomatal closure.

(C) The opening and closing of stomata are primarily regulated by the turgor pressure of guard cells. According to the potassium ion $(K^+)$ pump theory,during the day,$K^+$ ions are actively transported into the guard cells from the surrounding subsidiary cells. To maintain electrical neutrality,these $K^+$ ions are accompanied by malate ions,forming $K^+$-malate. This accumulation of $K^+$-malate increases the osmotic concentration (osmolite) inside the guard cells,leading to the endosmosis of water. As water enters,the guard cells become turgid,causing the stomata to open. Conversely,at night,the process reverses,leading to stomatal closure.

(C) In the evening,photosynthesis in leaves stops. Carbon dioxide concentration increases inside the leaf,which results in a decrease in $pH$. Glucose is phosphorylated to form glucose $6$-phosphate,which is then converted into glucose $1$-phosphate,from which starch is synthesized.
Starch is insoluble and does not exert any osmotic potential. Consequently,guard cells lose water to the nearby epidermal cells through osmosis.
As a result,the turgor pressure of the guard cells decreases,causing the stomatal pore to close.

(A) The opening and closing of stomata are primarily regulated by the turgor pressure changes within the guard cells.
When the guard cells take up water,they become turgid and their unique shape (due to the differential thickness of their cell walls) causes them to bulge outwards,thereby opening the stomatal pore.
Therefore,the shape of the guard cells is the critical factor in the mechanism of stomatal opening.

(B) The opening of stomata is primarily driven by the increase in the turgor pressure of the guard cells. When water enters the guard cells through the process of $Endosmosis$,they become turgid. Due to the presence of thick inner walls and thin outer walls,the turgid guard cells bulge outwards,causing the stomatal pore to open.

(D) The opening and closing of stomata are regulated by the turgor pressure of the guard cells.
When water enters the guard cells through osmosis,they become turgid.
Due to the thick inner walls and thin outer walls of the guard cells,the turgidity causes the cells to bulge outwards,thereby opening the stomatal pore.
Conversely,when guard cells lose water and become flaccid,the pore closes.

(C) Steward's theory (also known as the Starch-Sugar Interconversion Theory) proposes that the opening and closing of stomata are regulated by the interconversion of starch and soluble sugars in the guard cells.
According to this theory,the process involves the following enzymatic steps:
$1$. In light,the $pH$ of guard cells increases,which activates the enzyme $Phosphorylase$. This enzyme converts starch into glucose$-1-$phosphate.
$2$. $Phosphoglucomutase$ then converts glucose$-1-$phosphate into glucose$-6-$phosphate.
$3$. $Phosphatase$ converts glucose$-6-$phosphate into glucose,which increases the osmotic pressure,leading to water entry and stomatal opening.
$4$. In darkness,the $pH$ decreases,and $Hexokinase$ (along with other enzymes) facilitates the conversion of glucose back into starch,leading to stomatal closure.
Therefore,all these enzymes are considered essential components of the theory.

(B) The opening and closing of stomata are primarily regulated by light intensity.
During the day,light triggers the accumulation of $K^+$ ions in guard cells,which lowers the water potential,causing water to enter the cells via osmosis.
This increases the turgor pressure of the guard cells,leading to the opening of the stomatal pore.
Therefore,light is the most significant environmental factor influencing the stomatal mechanism.

(A) The opening and closing of stomata are primarily regulated by the movement of potassium ions $(K^+)$ in and out of the guard cells.
When $K^+$ ions enter the guard cells,the osmotic pressure increases,causing water to enter the cells via osmosis.
This leads to the turgidity of guard cells,which causes the stomatal pore to open.
Conversely,the exit of $K^+$ ions leads to a decrease in turgor pressure,causing the stomata to close.