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ABSTRACT

                        In the scenario of
increasing global warming, heat stress received more importance.
Un-fortunately, Pakistan is also in the line of most heat affected country in
the world. In this regard, wheat being most important staple crop of Pakistan
is highly affected by heat stress. For combating this situation, a research was
carried-out on fifteen bread wheat genotypes viz. NIA-Amber, Mehran, Khirman,
Imdad-05, Sehar-2006, AS-2002, SKD-1, TD-1, TJ-83, NIA-Sarang, Benazir, Anmol,
Kiran-95, NIA-Sunahri and Moomal, at the Experimental Field, Department of
Plant Breeding & Genetics, Sindh Agriculture University Tandojam. The
experiment was laid-out in a Randomized Complete Block Design with three
replications during Rabi season, 2015-16 in order to assess the response of
wheat genotypes for terminal heat stress tolerance. In this context, wheat
genotypes were evaluated in two sowing dates viz., normal planting on 25th
November and late planting on 25th December, 2015 considered as
normal and heat stress conditions, respectively. The
analysis of variance revealed significant differences among the genotypes under
normal and high temperatures indicating suitability of the experiment to
improve bread wheat genotypes for heat tolerance. Reductions in various
traits were observed due to late planting which indicated visible effects of
high temperatures on physio-yield traits. On an average physiological maturity,
grains spike-1, grain yield plant-1, harvest index,
relative water content and cell membrane stability showed the reductions of
7.01, 20.50, 37.87, 9.38 and 16.95%, respectively under the heat stress
conditions. While the wheat genotypes like Imdad-05, NIA-Sarang and TD-1 showed
minimum reductions under heat stress conditions for various traits suggesting
their heat tolerance, nonetheless cultivars Khirman and AS-2002 expressed
maximum declines under heat stress expressing their susceptibility to heat
stress conditions. The remaining genotypes were moderately heat stress
tolerant.

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INTRODUCTION

In recent past, average global temperature is
predicted to rise by about 2 0C over the next 50 years, making many
cereals growing regions less suitable, based on predicted temperature (Wrigley,
2006). Climate change is not an issue now a day’s but it has become a challenge
to deal with Agriculture and climate change. These both are interrelated
processes, both of which take place on a global scale. The adverse effects of
temperature on plants, higher than optimal temperature is considered as heat
stress (Kumar et al., 2015). High
temperature is a major problem in field cropping systems worldwide, with
unexpected spatial and temporal variations causing reduced plant growth,
development and productivity (Parent et
al., 2010). It has been estimated that a rise in temperature of just 1 0C
in wheat during the growing season reduces wheat yields by about 3–10% (You et al., 2009).

                        Wheat is the major staple
food crop of Pakistan, where the estimated consumption is about 124 kg per
capita which is among highest in the world. In order, to achieve the local
demand for food in Pakistan, an increase in wheat production of at least 4% is
required, to keep up pace with increasing population growth (Khan et al., 2015). In Pakistan, wheat
varieties are very sensitive to heat stress during the grain filling stage.
During this period, heat stress shortens the growth cycle and forces premature
ripening of crop, thus, reduces the number of grains spike-1,
declines seed index, and ultimately results in grain yield and quality deterioration
of wheat crop (Din et al., 2010).

                        Wheat is a winter cereal
crop which requires relatively low temperatures ranging from 12 to 22 0C
and these temperatures are considered optimum for its reproductive development
(Farooq et al., 2011).  Exposure to high temperatures can cause
considerable morpho-physiological damage which hastens leaf senescence (Wang et al., 2011), reduces photosynthesis
(Ristic et al., 2007) and reduces
starch biosynthesis (Zhao et al.,
2008). Both physiological and morphological traits like chlorophyll content,
canopy temperature depression, biomass, thousand grain weight, grain yield and
yield associated traits are affected by heat stress (Singh et al., 2016). Bala et al.
(2014) showed that heat stress significantly decreased grain yield, number of
grains per spike, plant height, grain-filling period, peduncle length, peduncle
weight and 1000-grain weight due to heat stress. Heat stress at terminal stage
is responsible for shortening of grain filling period, consequently improper
grain filling affects over-all yield of wheat crop (Rane et al., 2007). The grain yield per plant, biological yield per
plant and grain yield per spike suffered under late sown conditions (Singh et al., 2011). The total biomass at
maturity and yield / m² decreased significantly with delay in sowing. Higher
temperatures observed as further associated with limitation of water cause and
imposing rapid shrinkage of grain volume (Mitra and Bhatia 2008).

Furthermore,
the scenario of increasing world population FAO (2009) projected that an
increment of 71% in grain yield of wheat is required to match the food demand
until 2050. Expansion of agriculture area have many limitations so, current
goal could only be achievable through enhancing crop productivity by better
management and evolving stress resistant genotypes development (Reynolds et
al., 2011). Optimum planting date for different varieties vary with
cropping systems depending on growing conditions of a specific region that may be
assessed by planting them at different sowing dates. The other essential factor
to combat the challenges of heat stress is selection by genotypes which produce
higher yields and provide tolerance to adverse conditions and mature earlier
(Kumar et al. 2013). Wheat plant has the capability to show a wider
scope of compensating, escape, and tolerance mechanisms for heat through
different molecular, biochemical, physiological, developmental and growth
adaptation mechanisms (Barnabás et al., 2008). For combating the heat
stress condition, an experiment was conducted with following objectives.

       
i.           
To identify the potential
source of terminal heat tolerance in wheat genotypes for future breeding
programme

     
ii.           
To study the effect of terminal
heat on different agro-physiological traits in bread wheat

   
iii.           
To assess the genetic
variability among the heat tolerant genotypes of wheat

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MATERIALS AND METHODS

The
present research was carried-out at the Experimental Field, Department of Plant
Breeding & Genetics, Sindh Agriculture University, Tandojam. The experiment
was laid-out in Randomized Complete Block Design with three replications during
Rabi season, 2015-16, to assess the response of wheat genotypes for terminal
heat stress tolerance. In this context, the experimental material was evaluated
in two sowing dates viz., normal planting (25th November, 2015) and
late planting (25th December, 2015), considered as normal and heat
stress conditions, respectively.

Treatments = Two factors (A and B)

Factor – A: Sowing dates (D) = 2

D1 =    Normal
sowing (25th November)

D2 =    Late sowing (25th December)             

Factor – B: Genotypes = 15

1.     
NIA-Amber                            6. AS-2002                              11. Benazir

2.     
Mehran                                    7. SKD-1                                 12. Anmol

3.     
Khirman                                  8. TD-1                                    13. Kiran-95

4.     
Imdad-05                                9. TJ-83                                   14. NIA-Sunahri

5.     
Sehar-2006                              10. NIA-Sarang                      15. Moomal

The data
were collected from ten randomly tagged index plants from each genotype per
replication for the following traits.

Physiological maturity (75%):

This
character was taken when the peduncle of 75% of plants turned in yellow colour,
thus reached at 75% physiological maturity.

Grains spike-1:

                        The
total numbers of seeds in main spike were counted and data were recorded as
grains spike-1.

Grain yield plant-1 (g):

After
harvesting, each plant was threshed separately by hand and grains were weighed
on electric digital balance and yield plant-1 was recorded in grams.

Harvest index (%):

The harvest index (H.I.%) was taken by the
ratio of grain yield to biological yield. Harvest index (%) was calculated
according to following formula.

                                     H.I. %    =        Grain yield plant-1
(g)       x 100                                                                                        Biological yield plant-1
(g)

 

Relative water content (RWC%):

For the
determination of RWC, the next to flag leaves were sampled in polythene bags
and transported to the laboratory as quickly as possible in order to minimize
water losses due to evaporation. The samples were also weighed immediately for
fresh weight (FW), then sliced into 2 cm sections and floated on distilled
water for 4 hours. The turgid leaf discs were then rapidly blotted to remove
surface water and weighed to obtain turgid weight (TW). The leaf discs were
dried in the oven at 60 0C for 24 hours and then dry weight (DW) was
obtained. The calculation of RWC was carried out by using the formula suggested
by Barrs (1968).

RWC (%) = (FW-DW) / (TW-DW) x 100

Meteorological data:

Minimum and maximum temperatures on daily basis
were recorded during entire cropping season at the experimental site from SAU,
Tandojam.

Statistical analysis:

The data of different parameters for each genotype were averaged
and subjected to statistical analysis. Standard analysis of variance technique
was applied to determine significant difference among the means.

The analysis of variance for all the traits was carried out
separately as described by Gomez and Gomez (1984) to establish the level of
significance among various bread wheat genotypes planted under non-stress and
heat stress conditions with the following statistical model.

Source of variation                D.F                              Mean
square              Mean squares expectations

Replicates (R)                         (r-1)                             MSR                           

Genotypes (G)                         (g-1)                            MSG                           ?²e + r
?²g

Treatment (T)                          (t-1)                             MST                            ?²t

  G x T                                     (g-1)
(t-1)                    MSGT                         ?²g + ?²t

Error                                        (r-1)
(g x t-1)               MSE                            ?²e

­­­­­­­­­­­­­­­Where:

 =
Environmental variance,  =
Genetic variance and  =
Treatment variance.

 

Estimation of least significance difference
(LSD):

L.S.D at
P ?0.05 for pair wise comparisons was used to determine the critical
differences between the means of fifteen genotypes by using the following
formula:

                        L.S.D (5%) = S.E x t
value by using error degrees of freedom.

Estimation of relative decrease (%):

Relative
decrease (%) was measured by the subtraction of mean value of stress from the
mean value of non-stress, divided by mean value of stress, and multiplied by
100 as under.

Relative
decrease (RD%) = (non-stress – heat stress) / non-stress x 100

 

 

 

 

 

 

 

 

 

 

 

RESULT
AND DISCUSSIONS

Temperature:

The
meteorological data on daily basis for minimum and maximum temperatures
measured during entire cropping season (2015-2016) at experimental site are
given in figure 1. The high temperature was noticed during the sowing of
experiment in the November, however, the temperatures decreased in the months
of December and January. From February to May, the temperature rises about by 5
0C averagely for each month. 
At the time of grain filling period during the month of February and March,
temperature reached at 35 0C (figure-2) which exceeded up to 40 0C
at 1st week of April, 2016. This situation of temperature is above
from the threshold value (25 0C) of wheat crop. Therefore, late sown
wheat crop faced terminal heat stress.

Analysis of Variance

Mean
squares from analysis of variance (Table 2) indicated that heat stress imposed
significant impact on physiological maturity (75%), grains spike-1,
grain yield plant-1 (g), harvest index (%) and relative water
content (%). There also existed significant differences among the genotypes for
all the yield and physiological traits studied that could allow wheat breeders
to select the heat tolerant genotypes for one or more morpho-physiological
attributes. The mean squares due to genotypes were also significant for all the
traits under non-stress as well as in heat stressed conditions. The mean
squares from analysis of variance (Table 2) for genotype x treatment
interaction for all the studied traits were also significant. The significance
of genotype x treatment interaction showed that genotypes performed differently
over the stress condition. These interactions could help wheat breeders to
select the best performing varieties based on one or more reliable heat
tolerant indicators.

 

Mean
Performance and Relative Decrease

Physiological maturity (75%)

In our
experiment, the decline was recorded averagely by 7.01% due to heat stress
(Table 2). The maximum reduction however was observed in Khirman (10.33%)
followed by AS-2002 (8.62%) under heat stress condition. The best performance
was shown by Imdad-05 with minimum relative decrease of 4.30%, and the 2nd
and 3rd better performing were NIA-Sarang and TD-1 (4.63 and 5.48%),
respectively, with less decline in heat stress condition. In non-stress,
physiological maturity ranged from 113.67 to 119.33, while in heat stress
condition, it was ranged from 104 to 111.33 days. The present findings are in
agreement with Nahar et al. (2010)
who reported up to 15% reduction in maturity period of wheat genotypes due to
the effect of heat stress. The reduction in maturity days were also found in
the research of Hossain et al. (2015)
with the decrement of 13.04% under the late sowing dates. Ishaq et al. (2015) also observed that
terminal heat stress significantly affected the physiological maturity and
shortened from 10.46 to 12.67% maturity grown under heat stress conditions.

Grains spike?1

                        A grain
per spike is major yield contributing trait. Terminal heat stress reduces the
number of grains per spike to a significant extent in wheat. Heat stress caused
the decline of 20.50% averagely, in all the varieties grown under late sowing
condition (Table 2). The highest reduction was observed in Khirman (38.71%)
followed by AS-2002 (35.99%) under the heat stress condition. The lowest
reduction was noticed in Imdad-05 (6.15%) closely followed by NIA-Sarang and
TD-1 (6.72 and 9.75%, respectively) under heat stress condition. In normal
condition, grains spike-1 ranged from 48.20 to 67.13, while in heat
stress condition, it ranged from 36.40 to 63.00 grains per spike. These finding
are supported by Hamam et al. (2015)
who found a decline of 18.13% in grain numbers per spike due to heat stress. El-Ameen
(2012) Abd El-Majeed et al. (2005)
and Sial et al. (2005) also reported
that heat stress caused a significant reduction in the number of grains per
spike under heat stress conditions.

Grain yield plant-1 (g)

                        The
higher grain yield plant-1 is the ultimate goal of all the plant
breeders. The increment of all other characters provides a better background to
enhance the grain yield plant-1. Heat stress caused a decline of
37.87% averagely under the late sowing date (Table 3). Our results are nearly agreement
with those of Hossain et al. (2015)
Abd-elrahman et al. (2014) and Alam et al. (2014) who also reported that
heat stress reduced the grain yield up to 49.5, 40 and 45%, respectively. The
best performance was shown by the genotype Imdad-05 with minimum decrease of
26.46% followed by NIA-Sarang and TD-1 (28.80 and 32.32%), respectively. While
the lowest performance was recorded by genotype Khirman with increased
reduction of 56.52% in grain yield followed by AS-2002 (50.82%) under heat
stress condition. In normal condition, the grain yield per plant ranged from
5.75 to 13.34g, whereas under the heat stress condition, it ranged from 2.50 to
9.81g. El-Ameen (2012) reported that delaying the sowing date resulted in a substantial
reduction in grain yield by 63.34%, while the genotypes under favourable
conditions performed well for grain yield.

Harvest index (%)

In our
experiment, there was the difference of one month in both sowing dates. Heat
stress occurred at the anthesis stage of wheat genotypes under the 2nd
planting date because of this, the major affect was recorded on grain yield.
Thus, the harvest index was also decreased by the heat stress. The overall
average of all the genotypes in non-stress condition was 41.37%, and in
terminal heat stress condition was 37.59% (Table 3). Terminal heat stress
caused a decline of 9.38% averagely under the 2nd sowing date. The
maximum reduction was observed in Khirman (15.77%), closely followed by AS-2002
(14.51%), and minimum in Imdad-05 (5.58%) closely followed by NIA-Sarang and
TD-1 (6.01 and 6.40%, respectively) under the heat stress condition. In
non-stress condition, harvest index ranged from 34.63 to 50.68%, while in heat
stress condition, it ranged from 29.17 to 47.85%. Singh and Dwivedi (2015), Zarie
et al. (2014), Nawaz et al. (2013) and Moshatati et al. (2012), also reported a decline
in harvest index under the heat stress conditions, and their results are in
conformity with our findings.

Relative water content (%)

The relative
water content is a useful indicator of the state of water balance in crop plants
which is essential because it expresses the absolute amount of water the plant
requires to reach full saturation. Terminal heat stress caused a reduction of
16.95% averagely under the late sowing date (Table 4). Nonetheless, maximum
reduction was noticed in Khirman (26.17%) closely followed by AS-2002 (25.69%)
in heat stress condition. Whereas, minimum reduction was recorded in Imdad-05
(6.73%) followed by NIA-Sarang and TD-1 (7.76 and 9.87%, respectively) under
the heat stress conditions. The highest relative water content was measured in
genotype Imdad-05 under the both non-stress (81.92%), as well as in heat stress
(76.41%) conditions. The lowest relative water content was observed in genotype
Khirman in non-stress (70.87%) as well as in heat stress conditions (52.32%).
Our results are in conformity with the findings of Savicka and Skute (2012) who
reported the reduced relative water content under the heat stress conditions.

 

 

 

 

CONCLUSIONS:

The results revealed significant differences among all
the genotypes under normal and heat stress conditions, for all the characters,
expressing the suitability of experiment to improve bread wheat genotypes for
heat tolerance. The varieties Imdad-05, NIA-Sarang and TD-1
showed minimum reductions for various traits under heat stress conditions, thus
showing their tolerance to heat stress. The varieties Khirman and AS-2002
expressed maximum declines under terminal heat stress conditions for all the
studied traits, showing their susceptibility to heat stress. In non-stress as
well as under heat stress conditions, the highest values were recorded by the
genotype Imdad-05, NIA-Sarang and TD-1, indicating the resistance against heat
stress.  

 

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