Water as an Ecological Factor by Salman Saeed Lecturer Botany

Course Title:
Plant Physiology and
Ecology
Course Instructor:
SALMAN SAEED
Botany department
UNIVERSITY college of management & Sciences, Khanewal,
PAKSITAN

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Salman Saeed Lecturer (Botany)

The Water
Factor
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WATER CYCLE IN NATURE
 Water exhibits cyclic flow between atmosphere , land
and sea , and between living organisms and their
environment referred as water cycle or hydrological
cycle.
 Without cycling of water ecosystem could not
function , and life could not maintained.
 Watre cycle is characterized by :
1. Precipitation.
2. Interception.
3. Infilteration.
4. Evaporation.
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STAGES OF WATER CYCLE
1. Precipitation:
 It results from condensation of water vapours
derived from surface of oceans. It is the major
source of soil moisture and driving force of the
water cycle.
 Precipitation begins as water vapour in
atmosphere . When the air rises it is cooled and
when it rises above the temperature level at which
condensation take place ,clouds form.
 The condensing moisture aggregate to droplets ,
when diameter reach about 1mm ,they fall as rain.
5

CONT
2. Interception:
 As the precipitation reach earth , some water
reaches ground directly , some is intercepted by
vegetation and by building etc.
 Because of interception various amount of water
evaporates into atmosphere without reaching the soil
surface.
3.Infilterartion:
 The precipitation that reaches the soil moves in to
ground by infilterartion. More intense the rain, greater
would be the infiltration.
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CONT.
 Water seeps down and called gravitational water.
 A great amount of water is used by humans and for
domestic purposes.
 A part of water is held in soil by capillary forces between the
soil particles called capillary water.
 4.Evaporation:
 Water on the surface of ground on the surfaceof vegetation,
and in stream, lakes and oceans atmosphere by
evaporation.
 Some water is lost by roots of plants .
return to
 The total loss of water from the surface of ground is called as
evapotranspiration.
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PRECIPITATION
 Precipitation is condensation of water vapours in
the form of rain ,hail and snow etc.
 It is source of soil moisture.
 It is major driving force of water cycle as well.
 Precipitation directly or indirectly affects the plant
life through humidity and water content of the soil ,
therefore important ecological factor.
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FORMS OF PRECIPITATION
 Normally precipitation takes three forms given
below:
1. Snow:
 When condensation takes place under very low
temperature ( below 32°F) the moisture released
after condensation is frozen in the form of
hexagonal crystals called snow.
2. Hail and steel:
 Sometime drops become solidified in to small
rounded piece of ice called hail.
 The falling snow or hail mixed with rain is steel.
9

CONT.
3. Rainfall:
 Clouds consist of millions of tiny water droplets
.when clouds become older the droplets coalesce
to form large drops, reach certain size , and fall
out as rain.
 It is determined by:
a) Geography.
b) Pattern of air movement.
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MEASUREMENT OF PRECIPITATION.
 Rainfall is measured in inches or millimeters.
 Measured by rain gauge.
 Simple rain gauge consist of funnel with vertical
collar around rim of funnel which collects water
.the volume of water can be measured by pouring
in to graduated cylinder. And a calibrated stick is
inserted to measure collected water.
 Daily rainfall is measured then monthly rainfall is
taken by average of daily rainfall.
11

CONT.
 The mean annual rainfall is derived from the mean
of monthly rainfall.
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IMPORTANCE OF PRECIPITATION TO
PLANTS
1. Snow:
 It may be injurious or beneficial.
 Snow that melt is a source of water.
 By wind and by rain break off tree branches.
 At high altitude and latitude it damage vegetation.
 Snow press seedlings down-may attacked by
fungus easily.
13

CONT.
2. Rainfall:
 Source of moisture for soil.
 Helps to crack open thin skinned fruits e.g. cherries
and tomatoes and release seeds.
 The amount of rain determine vegetational zones
e.g. Subtropical forests ,rain forest etc.
3. Hail and steel:
 Cause damage to aerial parts of plants.
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DROUGHT AND DROUGHT RESISTANCE
OF PLANT DROUGHT
 It can be defined as : “an extreme fluctuations from
the mean rainfall (less than 75%) at certain place
on particular area” .
 Xerophytes and succulents are adapted to water
stress.
 If rainfall is far enough below than normal, it badly
effects vegetation. This is referred as drought.
Drought leads to:
 Reduction in size.
 Reduction in yield and vigour.
 Leads to death.
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CONT.
 When dry season is accompanied with higher
temperature, that causes the plants to use up soil
moisture very rapidly, promoting the effects of
drought.
 plant response to drought:
 plants exhibits responses to drought variously such
as :
i. In some plants leaves curl , in other these wilt , or
stomata closed.
ii. Prolonged drought inhibit production of
chlorophyll so leaves turn yellow.
16

CONT.
iii. Deciduous trees may shed leaves prematurely.
iv. Water stressed plants reduce osmotic potential
due to accumulation of ions calcium, sodium,
magnesium in leaves.
v. Conifers and evergreen species- winter drought –
browning of needles and die back of twigs from
dehydration.
17

CONT.
5. Root adaptations:
a) Deep roots can reach underground water.
b) Spreading shallow root- cover large area.
6. Ephemerals life cycle:
 Seeds germinate quickly- bloom and produce
seeds before moisture gone- drought escapers –
survive as dormant seed.
7.Halophytes:
 Have salt secreting glands and leaves coated with
waxes.
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ECOLOGICAL CLASSES OF PLANTS BASED
ON WATER RELATIONS
1.Hydrophytes:
 It include aquatics which normally grow in water.
 Presence of spongy tissues.
 Presence of air spaces.
 Phytoplanktons , Floating (wolffia) ,Submerged
(Elodea , Emergents (typha).
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Cont.
2. Mesophytes:
 Plants can not inhabit water or wet soil and soils
where water is depleted.
2. Xerophytes:
 Plants live in dry habitat.
 Complete life cycle in short duration.
 Succulents contain water storing tissues.
 E.g. cacti.
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PLANT ADAPTATIONS TO DROUGHT
 Plants in arid and semi arid regions have evolved
adaptation to survive and reproduce in drought
.These include :
1. Reduction in leaf area of canopy:
 Plants drop their leaves and reduce leaf area
canopy, and develop new leaves at the onset of
rainy season .
 It affects photosynthesis but it is compensated by
green stem.
21

CONT.
2. Change of angle of leaves:
 When water is limited , plants reorient leaves so
surface is parallel to sun rays and reducing the
transpiration and evaporation.
3. Succulence:
 Development of fleshy tissues for storage and
retention of water in plant body are called
succulent plants e.g. cacti .they absorb water in
rainy season and use this water in dry season.
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CONT.
4. Leaf adaptations:
 Various leaf adaptations:
a) Small leaves .
b) Cell walls thickened.
c) Stomata modified.
d) Palisade tissues are more developed.
e) Vascular system denser.
f) Coated with waxes and resins.
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Salman Saeed Lecturer (Botany) Email: Salmanbotanist@gmail.com

Water as an Ecological Factor by Salman Saeed Lecturer Botany

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