Esculantum is a diploid specie,
belong to family solanaccae and chromosome number is 2n=24.It is day neutral
plant. Tomato fruit commercially use for different purpose
i.e.cooking,salad,ketchup.The red color due to presence of lycopene.It contain
93% water content and 1.9% protein content. Normally best tomato plant growth at
temperature range 15-25 oC and flowering temperature 30 oC in Pakistan. In 2007
tomato crop grown under the area of 53000 ha and yield obtained 536.22MT.
Average yield can be obtained 6-13 Ton /ha at field condition and 17-21 Ton /ha
by cultivation of hybrid varieties in tunnel farming in Pakistan. Nagina, Roma
and Garden delight are partially heat tolerance varieties under 40oC–50oC. Heat
stress due to increase temperature is a major agricultural problem in many areas
in the world. Constantly high temperature cause an array of morpho-anatomical
change in plant which effect the seed germination, plant growth, flower
shedding, pollen viability, gametic fertilization, fruit setting ,fruit size and
fruit weight. These problems can also be minimizing by the improvement of
cultural practices and breeding approaches. so there are different type of
morphological traits which help for heat tolerance in convential breeding
approaches.
1) Long root length which has good ability to uptake water and nutrients from
the soil surface.
2) Short life span which help to minimize the temperature affect on plant.
3) Hairiness which provide partially shade to cell wall, cell membrane and repel
sun rays.
4) Small leave size that resist evaporation due to shortening of stomata.
5) Leaf orientation enhance the photosynthetic activity and produce tolerance
against heat stress.
Physiologically heat stress enhance the kinetic energy and movement of molecule
across a membrane thereby breakage of chemical bond within molecule of
biological membrane. This makes lipid bilayer of biological membrane more fluid
by either denaturation of protein so integrity and function of biological
membrane are sensitive to high temperature. The hormone play major role in life
stability and biosynthesis i.e. Abscisic acid and ethylene are heat tolerant
hormones which are involved to regulate the many physiological role by acting as
signal molecules, and expression of different type of heat tolerant proteins
which soluble in water and therefore contribute to heat stress tolerance by the
hydration of cellular structure. Several genes responsible for producing heat
shock protein and the expression of heat shock gene influence when temperature
exceed from 30oC. i.e. Heat shock protein that influences such as embryogenesis,
germination, plant growth, flowering, pollen development, fruit formation and
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. It is suggested that changing membrane
fluidity play a central role in the sensing and influencing gene expression
under low and high temperature. Modeling for temperature sensing and regulation
of heat shock response integrates observed membrane alteration. So the thylakoid
membrane in plant acts as a heat sensor is physiologically crucial. Recent
research has suggested that plant experiences oxidative stresses during the
initial period of adjustment to any stress. Each gene show specific response to
specific stress. However genomic in plant including genetics (direct mutation)
and epigenetic (methylation, chromatin remodeling, histone acecylation),
transfer of gene in plant response to heat stress tolerance. Physiological
genetic investigation indicate that heat tolerance traits are complex (polygenic),it
means that traits are controlled by more than one gene and they are highly
influence by environmental variation. With the improvement through convential
breeding scientist select two cultivars which both desire characters i.e. heat
tolerance and high yielded genes and hybridize it and select desirable plants
from segregating generation. However scientists use molecular and
biotechnological strategies for the purpose of heat tolerance in tomato by using
such techniques i.e.
Genetic engineering use to transfer of heat tolerant gene in plant cell which
have all other desirable characters is called genetic engineering, High
temperature tolerance has been genetically engineered in plants mainly by
over-expressing the heat shock protein genes or indirectly by altering levels of
heat shock transcription factor proteins. Apart from heat shock proteins, thermo
tolerance has also been altered by elevating levels of osmolytes, increasing
levels of cell detoxification enzymes and through altering membrane fluidity. It
is suggested that HSPs may be directly implicated in thermo tolerance as agents
that minimize damage to cell proteins.
Tissue culture technique use to make plant from transformed cell. Molecular /
genetic marker used to identify the gene in plant cell or in F2 generation that
has ability to tolerance against high temperaure. Now a days biotechnology
contributed significantly to better understanding of heat tolerant gene and heat
tolerance mechanism in tomato plants. Biotechnology play positive role to take
genetic homozygosity in plant in very short period to make model plant for
mankind.