1. EFFECT OF MOISTURE STRESS
PRESENTED BY:
S. ARUNA
I M.Sc., (Agri.) AGRONOMY
2. Water Deficits in Plants
Traditionally, 'plant water deficit' or 'plant water stress' has
been defined as being when plant water status is reduced
sufficiently to affect normal plant functioning (e.g., plant
growth, stomatal conductance, rate of photosynthesis).
3. EFFECT OF MOISTURE STRESS ON CROP
GROWTH AND YIELD
1) Water Relations:
Water deficit alters the water status by its influence on absorption, transpiration and
translocation, reduction in leaf water potential and relative water content
Loss of Turgour of plants which appears to be due to rapid rise of transpiration as a
result of increase in atmosphere dryness.
Increase in leaf and canopy temperature , it may be high upto 8 degrees over
ambient temperature.
Stomata gradually close down.
4. 2) Photosynthesis :
Reduced due to reduction in photosynthesis rate, chlorophyll content, Leaf area, and
increase in assimilate saturation in leaves.
3) Respiration:
Increases with mild stress, However when the water deficit becomes severe, it
decreases.
More severe water drought lowers water content and respiration.
4) Anatomical changes:
Decrease in size of cells and intercellular spaces, thicker cell wall and greater
development of cell wall.
No. of Stomata are fixed for fixed for known number of epidermal cells, but size of
cells are reduced without reduction in number of stomata per unit number of cells,
therefore, stomata per unit leaf area tends to increase under moisture stress.
5. 5) Free radicles:
Moisture stress induces oxidative damage to release of free radicals,
the activity of super oxide dismutase and catalase increases in response
to stress.
6) Harmonal relationships:
Activity & Translocation of growth promoting harmones like cytokinin
and gibbrellic acid and IAA decreases and growth regulating
harmones like ABA, ethylene, betaine increases
ABA acts as water deficit sensor to minimize the loss of tissue water
potential.
Ethylene is considered to be the cause for leaf and fruit drop in
stressed plants.
6. Betain is another harmone produced by the moisture stressed plants and it is used as
an indicator of moisture stressed plants.
7. Metabolic reactions:
Severe water deficit cause decrease in enzymatic activity. However these effects
are selective.
The amount of enzymes involved in the hydrolysis remain or increase rsulting in
the breakdown of starch and protein while peroxidase activity decreases.
Accumulation of sugars and amino acids takes place under moisture stress.
Proline accumulation takes place. Its accumulation is more in later stages of
plants and is considered as a good indicator of moisture stress
7. 8. Protoplasmic dehydration:
As the tissues become dessicated, protoplasm becomes increasingly dense and its viscosity
gradually increases. When dehydration is severe, it becomes rigid and brittle.
9. Membrane breakdown:
Membranes are the main targets of drought, and there is growing evidence for the
involvement of membrane lipid in plant adaptation to drought stress.
Biosynthesis of galactosylglycerolipids, monogalactosyl-digalactosyl-diacylglycerol,
which are the main components of chloroplast envelope and thylakoid membranes, are
important for plant tolerance to water deficit and for recovery.
8. 10) Nutrition:
Moisture stress effects fixation, uptake and assimilation of nitrogen.
Nitrogen fixation in leguminous plants is reduced by moisture stress due to reduction
in leghaemoglobin in nodules, specific nodule activity, and number of nodules. There
is negative correlation between specific nodule activity and stomatal resistance.
Moisture stress also delays nodule formation in leguminous crops, but number of
bacteroids containing cells are not affected.
Nutrient uptake is a product of nutrient content and dry matter produced. Moisture
stress may or may not reduce the nutrient content, but reduces the dry matter
production considerably. As a result nutrient uptake is reduced.
9. Nitrogen assimilation is also affected by moisture stress mainly due to reduction in
nitrate reductase activity.
Nitrogen deficiency increases stomatal resistance.
Phosphorous deficiency increases the sensitivity of the most recently expnded leaves
to moisture stress and stomata close while there is still sufficient turgour in
mesophyll cells.
11. Growth:
Generally, the organ growing most rapidly at the time of stress is one most affected.
The expansion of cells and cell division are reduced due to moisture stress resulting
in decrease in growth of leaves, stems and fruits.
Moisture stress affects germination, leaf area, leaf expansion and root development.
10. 12) Development:
In general, moisture stress delays maturity.
However, the response will vary with the stage of occurrence of stress. If stress
occurs before flowering the duration of the crop increases and when it comes after
flowering, the duration decreases.
13) Reproduction and Grain growth:
Photosynthetic Intensity (PI) in cereals, flowering and anthesis stages are most
effected with water stress.
Flowering: No.of fruits produced are reduced.
Fruit development: Results in small shriveled grains
Ripening: Length of ripening period reduced ( affected)
11. 14) Yield:
Effect of water stress on yields depends on largely on what proportion of dry
matter produced is considered as useful material to be harvested.
When the yield is biological yield, he effect of moisture stress on yield are much
the same as that on total growth .
When the Yield consists of storage organs as in potato, sugarbeet it will be as
sensitive as such as that on total growth.
When the Yield consists of seeds as in cereals, moisture stress at flowering is
detrimental.
When the fibre or chemicals where economic products is small fraction of TDM,
moderate tress on growth does not have adverse effect on yields.
12. Crop Adaptations to Moisture stress
Drought Adaptation:
The ability of the crop to grow satisfactorily under
water stress is called drought adaptation.
Adaptation is a structural or functional modification
in plants to survive and reproduce in a particular
environment.
Escaping Drought
Drought resistance
13. A) Drought Avoidance
1) Conserving water (Water stress)
2) Improving water uptake (Water spenders)
B) Drought Tolerance
1) Mitigating stress
2) High Tolerance
14.
15.
16. 1) Stomatal mechanism:
Drought resisting varieities are capable of regulating the stomatal mechanism and closing the stomata more
rapidly in the early morning when moisture stress at its minimum and photosynthesis can proceed with
least loss of water.
2) Lipid deposists on leaves:
Soyabean, sorghum reduce water loss by depositing lipids on the plant surfaces under moisture stress.
3) Reduction in leaf surface:
Plants reduces transpiration by reducing leaf area.
The individuals leaf size is reduced as leaf expansion is very sensitive to moisture stress.
In grasses, the leaves roll due to moisture stress and thus reduce the area exposed to solar radiation.
Leguminous plants show parahelionastic movements i.e. the leaves are oriented parallel to sun rays thus
avoiding the load of solar radiation.
17. 4) Leaf surface characteristics:
Various leaf morphological characteristics of leaves to reduce the transpiration rate and
may influence survival of plants under drought conditions.
Leaves with thick cuticle, waxy surface and spines are common and effective.
5) Increased Photosynthetic Efficiency:
C4 and CAM plants are highly drought tolerant.
6) Effect of Awns:
Awned varieties are more resistant than awnless varieties.
7) Water storage in plants :
This is an adaptive mechanism, the leaves of pineapple contain substantial amounts of
water in special non-chlorophyllous tissues which is utilized during drought.
18.
19. 1) Efficient root system:
Deep, well branched and rapidly growing root system helps in absorbing more
moisture by exploiting more volume of soil.
It helps in drought resistance without losing productivity.
2) Root shoot ratio:
Higher the root shoot ratio more the plant resistant.
Because of more absorption area than that of transpiring area
3) Increase in lipid phase conductance:
Lowering of resistance to water can be achieved by increasing their diameter of
xylem vessels or their diameter.
20. 4) Osmotic adjustment:
Plants which have the ability to lower their osmotic
potential in response to slowly developing deficits are
useful.
Leaves, hypocotyls, roots & spiklets have shown to
adjust osmotically in some species.
Osmotic adjustment helps to maintain turgor as water
deficits develop.
Osmotic adjustment increases the tanslocation & helps
in increasing grain yield.
21. (B) High degree of Tolerance : The death of plants can be avoided either by
reducing metabolic strain or plastic strain.
Adverse influence of drought on metabolism of plants Is known as metabolic
strain.
Death of cells occurred either due to reduction in carbon metabolism
(photosynthesis) or nitrogen metabolism (protein synthesis).
Resistant to metabolic stain (setback)
The greater he elastic dehydration strain greater is the danger of resulting
plastic metabolic strain.
Cell dehydration is elastic and completely reversible upto a point beyond
which it is plastic, irreversible and therefore injurious.
26. Desirable characters for drought resistant crops
Rapid germination and early establishment of deep roots.
Rapid phonological development.
Developmental plasticity.
Paraheliotropic movements.
Stomata sensitive to larger vapour pressure deficits and incentive to ,ow
leaf water potential.
Ability to adjust osmotically.
Large transfer of assimilates from stem to grain and dehydration tolerance
particularly, seeding and grain filling stages.
27. Excess Water
Water is the life of plant and must be supplied in proper quantity.
Most of the soils receive water through the rain also water is added by the
way of irrigation from well, tank or canal etc.
Too much water may suffocate the plant roots and to little may not be able to
sustain the plants.
Excess water is (a) harmful to crops and soils, (b) wastage costly input
28. (A)Harmful to Crops
Normally about 80 % roots of a crop are in the first foot of the soil and the remaining go
deeper.
With deep rooted crop like cotton, orchard trees their roots deep down and utilize moisture
at greater depths.
At the field capacity stage, the crop draws water easily from the soil and grows rapidly.
The growth progressively decreases as the soil moisture approaches wilting point. It may
cause moisture stress, which may encourage root growth.
Normally the crop should not be allowed to extract more than 50 % available water
otherwise the plants beginning to wilt and their growth is checked.
High moisture level develops tenderness in plants.
29. Following harmful effects cause due to excess water-
The field will get water logged and the crop cannot get sufficient water and air, as
good aeration and warmth in the root zone are essential for proper plant growth.
Bacteria that change organic matter into plant foods cannot get necessary air and
warm temperature in the soil.
Desirable chemical reactions cannot take place and nutrient availability is not
easily to the plants.
Proper root development and absorption of nutrients is not accelerated.
Seed germination is affected due to poor aeration and warm temperature.
Plants affected by diseases and pest attack.
30. Excess water cause water logging conditions, soil may
be ill drained and causes following harmful effects
In low lying areas excess water if cannot be carried away as surface runoff
causes water logged condition which affects aeration and soil temperature.
The fine texture soils due to poor permeability the water cannot move
downward fast enough and accumulates on the surface, obstructing aeration. In
sandy soil if heavy irrigation is given the water will rapidly percolate down and
will be wasted.
The water table may be high and the additional water accumulates and chokes
the air spaces and saturates the surface and sub-soil.
31. Contd..
Many toxic salts affecting water absorption by roots and causes
damage to crops.
Agricultural operation viz. inter cultivation cannot be carried out
properly.
Weeds, which compete for nutrients, space and light, grow
rapidly and interfere, crop growth and cultural operations.
Due to excess water structure of soil get affected.
32. (B) Wastage Costly Input
i) Fertilizers loss- Due to excess water, leaching losses of fertilizer namely
urea, occur and unavailable to the plants. Also availability of other nutrients
affected.
ii) Poor germination- Due to poor aeration and warmth in the soil, seed
germination is badly affected and ultimately there is a poor stand of crop.
iii) Effect on irrigation layout- Excess water cause damage to irrigation
layouts namely basin, and furrow, which cause flowing of water and required
labour to correct the layout.
33. Cont..
iv) Effect on cultural operation- Excess water interfere with inter cultivation,
field requires frequent weeding as excess water accelerates weed growth. Due to
wetting of land harrowing, earthing-up etc.get affected and farm implements turn
to wear –tear.
v) Additional cost of labour and plant protection- Due to frequent weeding
and damage cause to layout it requires more labour for weeding and correction of
irrigation layout. As plants/crop affected due to diseases and attack of pests it add
cost of control measure of these diseases and pests.
Thus excess water causes harmful effects on soil and plant and other input it is
necessary to apply proper drainage methods and reclamation.
