Fundamentals of Crop Physiology PDF (HINDI,ENGLISH,BOOK,PPT,WORD,QUIZ)
TEMPERATURE STRESS
Temperature
stress includes both high temperature stress and low temperature stress. Low
temperature stress causes chilling injury and freezing injury.
1. Chilling injury
The
tropical origin plants are injured when the temperature drops to some point
close to 0°C. The injury which occurs
due to low temperature but above zero degree centigrade is called chilling
injury.
2. Freezing injury
Freezing
injury occurs when the temperature is 0°C or below.
Effect of freezing
and chilling injury plants
- The lipid molecules
in cell membrane get solidified i.e. changed from liquid state to solid
state. Hence, the semi-permeable
nature of the membrane is changed and the membrane becomes leaky.
- Inactivation of
mitochondria
- Streaming of
protoplasm is stopped
- Accumulation of
respiratory metabolites which become highly toxic
- Ice formation inside
the cell occurs.
Prevention of cold
injury
- Some plants change
the pattern of growth.
- The growth is
completely arrested during this period.
- In cell membrane,
unsaturated fatty acid content is increased.
- Intracellular ice
formation is reduced.
- The quantity of free enzymes, sugars and proteins increases.
High temperature
stress
The
effect of high temperature is heat Injury.
Heat Injury occurs when plant temperature is higher than that of
environment (exceeds 35°C).
General effects of
high temperature
- Seedling growth and
vigour
- Water and nutrient
uptake
- Solute transport
- Photosynthetic
activity is more sensitive than respiration to higher temps
- Fertilization and
maturation
- at warmer temps,
lipids are too fluid and can lead to ion leakage
- Warmer temps also
tend to denature/inactivate proteins
Resistance
mechanisms
- Plants that are
adapted to warmer temps tend to have higher concentration of saturated
lipids in the membranes
- Reflecting infrared
radiation (cuticle, trichomes reflect more)
- Convection cooling
by cooler air around the leaf
- Evaporative cooling
by transpiration (evaporation of water absorbs heat)
- Acute heat stress
induces the synthesis of heat shock proteins (HSPs)
- HSPs preserve
protein structure and assembly at higher temperatures
ALLEVIATING HIGH
TEMPERATURE STRESS
1. Shade:
It may used for high cash crops (Ornamentals), typically a cloth or lathe
house.
Shading decreases leaf temperature, not air temperatures.
2. Green house:
It should be whitewash, provide fans, evaporative coolers (Where
humidity
allows)
3. Overhead Irrigation:
As water evaporates heat is absorbed. Cools plant body, but
encourages
disease.
4. GA3 and
proline application exhibit positive effects on
stress alleviation through
the
stimulation of ฮฑ – amylase expression
5. Zeatin Riboside
is the most effective in slowing leaf senescence and alleviating
heat induced lipid peroxidation of cell membranes.
6. The inhibitory effect of high temperature on seed
germination can be overcome by
exogenous
application of ethylene.
7. Application of Glycine
Betaine under heat stress appreciably reduced the leakage
of all these
ions, particularly Ca2+, K+ and NO3.
8. Exogenous application of salicylic acid
enhanced the thermo tolerance ability of
both roots
and hypocotyls in intact seedlings
SALT STRESS
Salt stress occurs due to excess
salt accumulation in the soil. As a
result, water potential of soil solution decreases and therefore exosmosis
occurs. This leads to physiological
drought causing wilting of plants.
Classification
of saline soil: 1.
Saline soil 2. Alkaline soil
1. Saline soil
In saline soils, the electrical
conductivity is greater than 4 dS/m, exchangeable sodium percentage is less
than 15% and pH is less than 8.5. These
soils are dominated by Cl- and SO2-4 ions.
2.
Alkaline soil
Alkaline soils are also termed as
sodic soils wherein, the electrical conductivity is less than 4 dS/m,
exchangeable sodium percentage is greater than 15% and pH of the soil is
greater than 8.5. These soils are dominated
by CO-3 and HCO-3 ions.
Classification
of plants
Plants are classified into two
types based on the tolerance to salt stress. They are halophytes and
glycophytes.
1.
Halophytes
Halophytes are the plants that grow
under high salt concentrations. They are again divided into two types based on
extreme of tolerance.
Euhalophytes: can tolerate extreme
salt stress
Oligohalophytes: can tolerate
moderate salt stress
2.
Glycophytes
Glycophytes are the plants that
cannot grow under high salt concentration.
Effect of
salt stress on plant growth and yield
1. Seed
germination
Salt stress delays seed germination
due to the reduced activity of the enzyme, a-amylase
2.
Seedling growth
The early seedling growth is more
sensitive. There is a significant
reduction in root emergence, root growth and root length.
3.
Vegetative growth
When plants attain vegetative
stage, salt injury is more severe only at high temperature and low
humidity. Because under these
conditions, the transpiration rate will be very high as a result uptake of salt
is also high.
4.
Reproductive stage
Salinity affects panicle
initiation, spikelet formation, fertilization and pollen grain germination.
5.
Photosynthesis
Salinity drastically declines
photosynthetic process. Thylakoid are
damaged by high concentration of salt and chlorophyll b content is drastically reduced.
Mechanism
of salt tolerance
1.
Some
plants are able to maintain high water potential by reducing the transpiration
rate.
2.
Salts
are accumulated in stem and older leaves in which metabolic processes take
place in a slower rate.
3.
Na+
(sodium ion) toxicity is avoided by accumulating high amount of K+
ions.
4.
Accumulation
of toxic ions in the vacuole but not in the cytoplasm.
5.
Accumulation
of proline and abscissic acid which are associated with tolerance of the plants
to salt.
Relative
salt tolerant crops
Tolerant crops: Cotton, sugar cane,
barley
Semi tolerant crops: Rice, maize,
wheat, oats, sunflower, soybean
Sensitive crops: Cow pea, beans, groundnut and grams
Alleviation
of salt stress
1. Leaching of salts with
adequate water
2. Application gypsum to convert the highly injurious carbonates to
less injurious sulphate
3. Selection of salt tolerant crops and Use of FYM and other organic manures
๐Crop Physiology
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