Effects and heat stress
tolerance studies in Cotton (Gossypium hirsutum L.)
Cotton is a major and cash crop
of Pakistan because of source of economy boost up ,Totally textile industry
based on cotton that improve Pakistan economy and provided maximum job to
human.Hirsutum is tetraploid specie which belong to family Malvaceae and contain
2n=52 chromosomes. This specie also known by American cotton.
Global warming is the major A biotic stress that occur due to increase in
temperature that severely effecting the plant germination, vegetative and
reproductive growth. Heat stress often is defined as where temperatures are hot
enough for sufficient time that they cause irreversible damage to plant function
or development. In addition, high temperatures can increase the rate of
reproductive development, which shortens the time for photosynthesis to
contribute to fruit or seed production. I also will consider this as a
heat-stress effect even though it may not cause permanent (irreversible) damage
to development because the acceleration does substantially reduce total fruit or
grain yield.
The extent to which heat stress occurs in specific climatic zones is a complex
issue. Plants can be damaged in different ways by either high day or high night
temperatures and by either high air or high soil temperatures. Also, crop
species and cultivars differ in their sensitivity to high temperatures.
At higher temperature, when seed sown in soil, the soil heat up which cause seed
injury, rapture the cell membrane, stored food in seed is loss and embryo will
be die. Usually the delinted cotton seed show negative response to higher
temperature tolerance. Of the different low molecular weight organic osmotica
determined in this study, leaf soluble proteins, soluble sugars and proline were
significantly tolerance higher temperature stress in cotton crop.
The reduction in maximal emergence of annual crops due to hot soils can be so
pronounced that yield of the economic product is reduced substantially. This can
be a major problem for cotton crop. Heat stress at emergence is a major problem
in cotton crop if that are sown in the hot temperature and Plant growth
efficiency, photosynthesis activity reduced due to increase in temperature.
The acceleration of reproductive development by high temperatures may effect and
create problem in flower droping,boll dropping and boll bursting but not
opening. These problems severely reduced the yield Higher temperature during the
vegetative and reproductive stage significantly reduces crop vigor and yield.
The cellular membrane Thermostability (CMT) show indirectly measures integrity
of cellular membranes through quantifying electrolyte leakage.
This also implied that in upland cotton these two traits were independent of
each other, the presence or absence of heat stress determined their
relationship. The differential ability of cotton cultivars and Hybrids to adjust
to CMT under heat-stressed conditions points towards physiological adaptation to
heat stress or heat Dark respiration is a continuous process in which
mitochondria within a plant's cells oxidize carbohydrates to create energy.
Cotton plants make more starch during the day than they require for growth. The
excess starch is stored in plant cells' chloroplasts, where photosynthesis
occurs. At night, that starch is broken down via respiration and other metabolic
processes and used to support new growth, such as cotton bolls.
The cultivars with the greatest heat tolerance generally have lower rates of
dark respiration and more efficient use of carbohydrates. If ongoing studies
support these observations, the scientists may be able to use these traits to
improve the cotton breeding program.
The extent of heat stress that can occur in a specific climatic zone depends on
the probability of high temperatures occurring and their duration during the day
or night. Where global climate change is occurring these probabilities may not
be predicted well based only on historical records for specific locations. Heat
stress is a complex function of intensity (temperature degrees), duration and
rate of increase in temperature. The magnitude of heat stress rapidly increases
as temperature increases above a threshold level and complex acclimation effects
can occur that depend on temperature and other environmental factors.
During the vegetative stage, high day temperatures can cause damage to
components of leaf photosynthesis, reducing carbon dioxide assimilation rates
compared with environments having more optimal temperatures. Sensitivity of
photosynthesis to heat mainly may be due to damage to components of photosystem
II located in the thylakoid membranes of the chloroplast and membrane properties
(Al-Khatib and Paulsen 1999). More stable membranes exhibit slower electrolyte
leakage. Floral bud development also can be damaged by heat such that plants do
not produce flowers. For cotton, two weeks or more of consecutive or interrupted
hot nights during the first month after germination caused complete suppression
of floral bud development. In extreme cases the floral buds become necrotic and
die.
Surface and internal tissues of cotton fruit can be damaged by the combination
of high temperatures and intense solar radiation. High tissue temperatures also
can damage cambium layers in exposed trunks and branches.
It is concluded that degree of heat tolerance in cotton at germination and later
growth stage does not vary. Membrane thermostability is a successful measure of
heat tolerance in cotton. Leaf soluble proteins, Heat shock gene influence when
temperature exceed from 30 oC and produces heat shock protein that influences
embryogenesis, germination, plant growth, flowering, pollen development,
pollination, fruiting setting, fruit maturation in heat stress condition.
General response to heat stress involves signaling via redox system; Ca+2 and
plant hormone activate the genomic re-programming via signal cascade. Proline
and soluble sugars are important adaptive components of heat tolerance of this
crop. Physiological genetic investigation indicate that heat tolerance trait are
polygenic and they are highly influenced by environmental variation. Heat stress
problem can also be minimizing by cultural practices.
There some morphological traits which tolerant heat stress. Linted cotton seed
because fibrous material present on seed surface that resist high temperature.
Good Plant growth - plant complete vegetative growth in less time because of
minimum heat stress. Short life span-short duration variety complete life cycle
in less time because less heat stress effect. Long root length which has good
ability to uptake water and nutrients from the soil service. Hairiness on stem
and leaf—that provide partially shade to cell wall, cell membrane and repel high
energy sun rays. Crown red color. Small leaf area--that resist evaporation due
to shortening of stomata. Leaf orientation-- enhances the photosynthetic
efficiency and develops tolerance against heat stress. Good pollen viability and
fertilization. Balance macro and micro nutrients in soil.
In 2003 to onward in Pakistan, mostly cotton research institute actively busy
for the development of heat stress tolerant by using conventional and modern
breeding techniques , there is positively result shown by scientist
i.e.CIM-506,FH-114,BT-886 And SK-280. as a result of their collaboration Cotton
Breeders released five upland cotton lines with superior fiber quality and heat
stress tolerance. Those lines have been picked up by commercial seed companies
and breeders for further development and majorlly play role to boost up
Pakistani economy.