There that plants are subjected have either direct

There exist some abiotic stresses which include
salinity, drought and temperature that affect the growth, survival and
reproduction of plants. As a results, plants respond to such unfavorable
conditions by physiological, developmental and biochemical ways. In order to
respond appropriately to these changes, it demands expression of stress-
response genes. These stress response genes are regulated by a network of
transcription factors (TFs). These include the heat stress transcription
factor, abbreviated as HSFs). This factor plays an important role in response
of plants to various abiotic stresses. They accomplish this by regulating the
expression of stress responsive genes like the heat shock proteins (Hsps).

The stresses that plants are subjected have either
direct or indirect effect on their productivity. Some of the abiotic stresses
like high temperature, salinity and drought results in a deadly economic loss
in agricultural sector. According to Rodziewicz et al., (2014), they
established that there is an estimate of 50% losses in crop yield worldwide as
a result of these stresses. In addition, Edmeades (2009) established that
abiotic stresses that result from either cold, salinity; high temperature or drought
hinders crops from realizing their full yield potential.

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It is a complex phenomenon for plants to respond to
these abiotic stresses because of plants at different development stages can be
affected by a particular stress and therefore, simultaneous occurrence of
stresses can affect the plants, as established by Chinnusammy et al, (2004).
Heat stress affect the photosynthetic rate of plants directly and indirectly
hence changing the structural organization and psycho-chemical properties of
thylakoid membrane of the plant, as suggested by Lichtenthaler et al., (2005). According
to Sage et la. (2007), there is an increase in the rate of photorespiration
with the increase in heat (temperature) which then results in reduction of
photosynthesis.

Flowing stage in a plant is the most sensitive stage
that is affected by high temperature as high temperatures damage it. This is
because of the high level of vulnerability in the event of development of pollen;
fertilization and anthesis in a plant which will alter reduce the final yield
of the plant (Kumar, 2015). He found out that for every unit increase in
temperature result in a reduction in the plant yield by about 3 to 7% (Kumar,
2015).

With the increasing incidences of abiotic stresses, it
has necessitated the creation of a new genotype or screen that can be used for
the existing germplasm that is favorable for the changing conditions.

Stress

Stress can be defined as a change in the environment
that is sudden and the change can exceed the optimal conditions of the organism
and the change result in a change in homeostatic imbalance (Taiz et al., 1991).
Plants that grow under the field conditions experience various environmental
conditions that are exposed to them and these conditions affect their macro and
microenvironment as suggested by Larcher, (2003). Stress can be caused by
different factors including abiotic factors which include variation in temperature,
strong light, and salinity among others. It can also be caused by biotic
factors like insects, bacteria, virus among others. Plants can at times face a
condition of combination of both biotic and abiotic factors. Yadav et al (2014)
found out that abiotic stress is the main agents that cause failure in most of
plants. They found out that they lower the average production of plants by
about 60% which then threaten sustainability and food security.

Abiotic Stress

These are non-living factors that affect negatively
plants on a specific environment. These factors influence the environmental
conditions beyond their normal range required by plants hence affective
adversely the performance of the population of plants. They include extreme
temperatures which can be too much heat or freezing, drought, reduction in the
nutrients in the soil, too much light and excessive toxicity in the soil.

The response of plants to stress depends on the area
it was affected by the stress. Several signaling pathways come up due to the
molecular response of the plant to any stress. These include RNS or ROS and the
hormonal changes which include ethylene and ABA as established by Cramer et
al., (2010). He also commented on the use of time series analysis in order to
study multiple phases that relate to stress responses that were important in
analysis that aims at distinguishing between the primary and the secondary
stress responses by plants. Some of these mechanisms that can be used to
respond to stress include signal transduction, stress perception and
transcriptional activation of stress response genes. Others include synthesis
of proteins that relate to the stress and other molecules that can help plants
to cope with adverse environmental conditions.

Heat Stress

According to Rao et al (1992), they found out that
high temperature enhances development in plants and result in abortion of
flower leading to a significant loss in the yield of seeds. They suggested that
the duration of flowering in plants have a strong impact on the yield of seeds
and a rise in temperature result in causes a decline in the yield. These
findings were obtained from a study of Indian Mustard seed.  They also agreed that flowering is the most
sensitive stage of a plant and any change in environmental conditions result in
stress damage.

In addition, Cramer et al. (2010) established that the
main cause of the rise in sterility in plants when subjected to heat stress are
as a result of the impaired meiosis in both the female and male organs of
plants. It is also due to the impairment of the pollen germination and the
growth of pollen tube, anomaly in position of style and stigma and disturbance
in the process of fertilization among others.

According to the research done by Mishra et al. (2011),
they reported that in case of a progressive shortfall in precipitation in conjunction
with high level of evapotranspiration rate that is caused by high heat result
in agricultural drought. Drought adversely affect the growth of plants and
their development which in turn lower the rate of growth in plants and hence
biomass accumulation. Mishra et al. (2011) further argued that heat stress is
one of the main factors that limit the production of crops in the world.

Araus et al., (2002) concluded that heat stress is one
of the main abiotic factors that seriously affect crop productivity worldwide. He
found out that long term exposure of plants to high temperatures affect their
biochemical, metabolic and molecular functioning of plants. This have a serious
effect on various parts of the plant like the leaves, flowers, roots and the
buds hence affect productivity.

Another research conducted by Al-khatib et al. (2004)
found out that in case of an increase in temperature, it results in premature
senescence of plants which can lead to an increased rate of photorespiration. They
also observed that there is an increased in the rate of photorespiration with
an increase in the temperature. This in turn lowers the rate of photosynthesis which
in turn affects the yield. 

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