By Shehzad Ahmad Kang

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 belongs to family Malvaceae and contains 2n=52 chromosomes. This specie is also known as American cotton.

Global warming is the major a biotic stress that occurs due to increase in temperature that severely effecting the plant germination, vegetative and reproductive growth. High temperatures can increase the rate of reproductive development, which shortens the time for photosynthesis to contribute to fruit or seed production.

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 heats up which causes seed injury, rapture the cell membrane, stored food in seed is loss and embryo will die. Usually the delinted cotton seed shows 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.

The acceleration of reproductive development by high temperatures may effect and create problem in flower dropping, 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.

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.

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.

It is concluded that degree of heat tolerance in cotton at germination and later growth stage does not vary. Membrane thermo stability is a successful measure of heat tolerance in cotton. Leaf soluble proteins, Heat shock gene influence when temperature exceed from 30°C 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 indicates that heat tolerance traits 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.

In 2003 onward, mostly cotton research institutes in Pakistan 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,BT-456,BT-222 And SK-280. As a result of their collaboration cotton breeders released seven uplannd 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 majorly play role to boost up Pakistani economy.

The author is associated with the Department of Plant Breeding and Genetics, University of Agriculture, Faisalabad, Pakistan. He can be reached

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