the stomatal opening for a period of two or more weeks without influencing other metabolic processes. The most
promising of these inhibitors is phenyl mercuric acetate. Another is abscisic acid (ABA).
Film forming chemicals check transpiration by forming a thin film on the transpiring surface. They are sufficiently permeable
to carbon dioxide and oxygen to allow photosynthesis and respiration but prevent movement of water vapours through
them. The important chemicals of this group are silicon emulsions, colourless plastic resins and low viscosity waxes.
Transpiration and photosynthesis - A compromise
An actively photosynthesizing plant has an insatiable need for water. Photosynthesis is limited by available water which
can be swiftly depleted by transpiration. The humidity of rainforests is largely due to this vast cycling of water from
root to leaf to atmosphere and back to the soil.
The evolution of the C 4 photosynthetic system is probably one of the strategies for maximizing the availability of CO 2
while minimizing water loss. C 4 plants are twice as efficient as C 3 plants in terms of fixing carbon (making sugar).
However, a C 4 plant loses only half as much water as a C 3 plant for the same amount of CO 2 fixed.GuTTATION
Loss or excretion of water in the form of liquid droplets from the tips and margins of leaves is called guttation. It
was first studied by Bergerstein in 1887.
All plants do not show guttation. It is restricted to about 345 genera of herbaceous and some woody plants. Common
examples are garden nasturtium, oat and other cereals, balsam, tomato, cucurbits etc. It occurs in warm, moist soils with
humid environment or when warm days are followed by cool nights.
In general, guttation occurs when transpiration rate is very low as compared to rate of water absorption.
Due to this, root pressure is developed and water is pushed out through specialized pores at vein endings called
hydathodes. So guttation is not due to activity of hydathodes but due to root pressure. Each hydathode consists of a
group of loosely arranged colourless parenchymatous cells called epithem. It lies over the tip of a vascular strand and
communicates with the outside through a permanent pore in epidermis called water pore or water stoma.
The guttated liquid is never pure water. It contains 0.6-2.5 gm/litre of solutes– both organic (carbohydrates, organic
acids, amino acids, enzymes) and inorganic (Ca2+, Mg2+, K+, Na+, CO 3
2–
, SO 4 2–, Cl–). These salts sometimes are redissolved
back into leaves and cause ‘salt injury’.
Gutttion takes place either at night or early in the morning. Dry soils, poorly aerated soils, heavily salted or mineral
deficient soils and the atmospheric conditions promoting transpiration inhibit guttation.
Table : Differences between transpiration and guttationTranspiration Guttation- Loss of water is in vapour form. Loss of water is in liquid form.
- It occurs during day time. It occurs during night or early morning.
- Loss of pure water. Loss of impure water.
- Through stomata or epidermis or cuticle or lenticels. Through hydathodes.
- Controlled phenomenon. Uncontrolled phenomenon.
- Associated with regulation of temperature. No such role has been given to guttation.