Plant water relation

Grade 12

Structure of a seed

Types of seed germination

LE 38-10a

Foliage leavesCotyledon

Cotyledon

Hypocotyl

Hypocotyl

RadicleSeed coat

Hypocotyl

Cotyledon

Epicotyl

Common garden bean

LE 38-10b

Foliage leaves

Coleoptile

RadicleMaize

Coleoptile

Transport Summary

A=absorption / assimilationL=loadingU=unloadingI=interchange

Opening & closing of stomata

Day time• Carbob dioxide in guard cells utilised for

photosynthesis. • Sap becomes alkaline & in this medium starch

gets converted to sugar • Guard cells : sugar (chloroplast)• Sugar soluble in sap-osmotic potential of cell

increases• Water from sorrounding cells enter guard cells By osmosis• Water exerts pressure on thin wall of guard cells

from inside.• Hence thin walls bulge outward, pulling apart

innerthick walls , opening the stomata

Night time• No photosynthesis• Carbondioxide accumulates in guard cells• Sap becomes acidic• In this sugar gets converted to starch• Being insoluble in water osmotic potential

becomes less compared to surrounding• Surrounding cells take out water from guard cells

by osmosis making them flaccid• Hence outer thin walls come to their original

position, closing the stomata.

Significance of TranspirationAbsorption of waterTranspiration influences the rate of absorption of water from the soil. Water movementBy transpiration, water moves upwards and as it passes into the cell vacuoles, it makes the cells turgid. This gives form and shape to cells and plant as a whole. Mineral salt transportThe water stream moving upwards carries dissolved minerals with it. Transpiration also helps in distributing these minerals throughout the plant. CoolingThe evaporation of water during transpiration cools the leaves. Protection from heat injurySome plants like cacti, retain water by reducing transpiration. This saves the plants from high temperatures and strong sunlight.

Factors Affecting Rate of Transpiration - External factors

TemperatureHigher the temperature more is the transpiration. LightLight causes stomata to open and hence increase the water loss from plant

Availability of soil waterWhen the soil gets dry, soil solution becomes more concentrated and the rate of absorption by cells decreases. This leads to reduction in transpiration and stomata close quickly to keep the water loss to minimum. Atmospheric humidityHigh humidity means high water Vapour pressure outside and it results in lower rate of transpiration and as the humidity decreases rate of transpiration increases.

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WindThe wind removes water vapour and thus increases the rate of transpiration. High winds lead to stomatal closure to stop the rapid water loss and hence bring a drop in rate of transpiration. Moderate winds may also reduce transpiration by lowering the temperature of leaf. Atmospheric pressureLower the atmospheric pressure, higher is the rate of transpiration