Test: Transpiration & Root Pressure (Old NCERT) - A Level MCQ

Hydathodes are specialized pores (openings) particularly present on the leaf margins, which exudes or secretes drops of water. The exudation of water as drops from the tip or margin of the leaves is called guttation. The process of guttation is facilitated by the hydathodes. They are also called as Water Stomata because they structurally resemble stomata and they facilitate guttation. Hydathodes are commonly found in Angiosperms, especially in grasses. The water stomata always stay opened since, they do not have opening and closing mechanism.

Stomata is a tiny pore present in the epidermal layer of leaves which is meant for gaseous exchange. Usually, gaseous exchanges takes during day time i.e, stoma gets opened with the present of light and in during dark stoma will kept closed. But reverse-phase of stomatal openings can be seen in CAM plants. For example, CAM plants like cacti and Opuntia ficus–indica attain their immense water use capability by opening their stomata during the cool, desert nights (less water will evaporate from the plant, because night is cooler) and closing them during the hot, dry days. This kind of nightly opening of stoma pore in CAM plants is due to photoperiodic circadian rhythm. 
So, the correct answer is option A.

Guttation is the excretion of drops of xylem sap on the tips or edges of leaves of some vascular plants, such as grasses. In the absence of light, plant loose excess water in the form of liquid through hydathodes because most plants have their stomata closed at night. When there is a high soil moisture level, water will enter plant roots, because the water potential of the roots is lower than in the soil solution. The water will accumulate in the plant, creating a slight root pressure. The root pressure forces some water to exude through special leaf tip or edge structures, hydathodes or water glands, forming drops. Therefore, the correct answer is option D.

The hydathodes are made of a group of living cell with numerous intercellular spaces filled with water, but few or no chloroplast and represent modified bundle ends. Therefore , hydathodes are involved in the process of guttation, in which positive xylem pressure (due to root pressure ) causes liquid to excude from the pores.

Potometer is the device used to measure the rate of transpiration. It works on the principle that the amount of water absorbed equals the amount transpired so it measures the amount of water absorbed.

A potometer sometimes known as a transpirometer is a device used for measuring the rate of water uptake of a leafy shoot. The causes of water uptake are photosynthesis and transpiration.

Absorption of water and minerals by plants directly depends on the transpiration pull generated by loss of water through stomata but transportation of sugars from source to sink is a physiological process and is not related to transpiration loss of water.

External factors affecting the rate of transpiration are: atmospheric humidity, temperature, light, wind velocity, atmospheric pressure and available soil water. Some internal factors also affect rate of transpiration e.g., leaf area, leaf structure and age of plants. Chlorophyll content of leaves does not directly affect rate of transpiration.

Passive absorption of water by the root system is the result of tension on the cell sap due to transpiration. 
Transpiration pull generates a tension in the continuous water column in the xylem elements, which lifts up the water and is also responsible for absorption of water. It does not require energy as it takes place towards the gradient.

The opening and closing of stomata is governed by the turgor movement in guard cell. The changes in guard cell turgidity is brought about by the inflow or outflow of potassium ions. An inflow of potassium ions decreases water potential of guard cells and water moves into the guard cells, guard cells become turgid and stomata open. These changes will be reversed when potassium ions flow out from guard cells. The rate of transpiration depends on vapour pressure gradient. More the difference in vapour pressure of water between leaf mesophyll and atmosphere higher will be the rate of transpiration. In this question, there will be a increase in rate of transpiration due to decreased relative humidity.