ABSTRACT In the scenario ofincreasing global warming, heat stress received more importance.Un-fortunately, Pakistan is also in the line of most heat affected country inthe world. In this regard, wheat being most important staple crop of Pakistanis highly affected by heat stress. For combating this situation, a research wascarried-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 ofPlant Breeding & Genetics, Sindh Agriculture University Tandojam. Theexperiment was laid-out in a Randomized Complete Block Design with threereplications during Rabi season, 2015-16 in order to assess the response ofwheat genotypes for terminal heat stress tolerance. In this context, wheatgenotypes were evaluated in two sowing dates viz.
, normal planting on 25thNovember and late planting on 25th December, 2015 considered asnormal and heat stress conditions, respectively. Theanalysis of variance revealed significant differences among the genotypes undernormal and high temperatures indicating suitability of the experiment toimprove bread wheat genotypes for heat tolerance. Reductions in varioustraits were observed due to late planting which indicated visible effects ofhigh 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 of7.01, 20.50, 37.
87, 9.38 and 16.95%, respectively under the heat stressconditions. While the wheat genotypes like Imdad-05, NIA-Sarang and TD-1 showedminimum reductions under heat stress conditions for various traits suggestingtheir heat tolerance, nonetheless cultivars Khirman and AS-2002 expressedmaximum declines under heat stress expressing their susceptibility to heatstress conditions. The remaining genotypes were moderately heat stresstolerant. INTRODUCTIONIn recent past, average global temperature ispredicted to rise by about 2 0C over the next 50 years, making manycereals 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 challengeto deal with Agriculture and climate change.
These both are interrelatedprocesses, both of which take place on a global scale. The adverse effects oftemperature on plants, higher than optimal temperature is considered as heatstress (Kumar et al., 2015). Hightemperature is a major problem in field cropping systems worldwide, withunexpected spatial and temporal variations causing reduced plant growth,development and productivity (Parent etal., 2010). It has been estimated that a rise in temperature of just 1 0Cin wheat during the growing season reduces wheat yields by about 3–10% (You et al.
, 2009). Wheat is the major staplefood crop of Pakistan, where the estimated consumption is about 124 kg percapita which is among highest in the world. In order, to achieve the localdemand for food in Pakistan, an increase in wheat production of at least 4% isrequired, to keep up pace with increasing population growth (Khan et al., 2015). In Pakistan, wheatvarieties are very sensitive to heat stress during the grain filling stage.
During this period, heat stress shortens the growth cycle and forces prematureripening of crop, thus, reduces the number of grains spike-1,declines seed index, and ultimately results in grain yield and quality deteriorationof wheat crop (Din et al., 2010). Wheat is a winter cerealcrop which requires relatively low temperatures ranging from 12 to 22 0Cand these temperatures are considered optimum for its reproductive development(Farooq et al.
, 2011). Exposure to high temperatures can causeconsiderable morpho-physiological damage which hastens leaf senescence (Wang et al., 2011), reduces photosynthesis(Ristic et al., 2007) and reducesstarch biosynthesis (Zhao et al.
,2008). Both physiological and morphological traits like chlorophyll content,canopy temperature depression, biomass, thousand grain weight, grain yield andyield associated traits are affected by heat stress (Singh et al., 2016).
Bala et al.(2014) showed that heat stress significantly decreased grain yield, number ofgrains per spike, plant height, grain-filling period, peduncle length, peduncleweight and 1000-grain weight due to heat stress. Heat stress at terminal stageis responsible for shortening of grain filling period, consequently impropergrain filling affects over-all yield of wheat crop (Rane et al.
, 2007). The grain yield per plant, biological yield perplant and grain yield per spike suffered under late sown conditions (Singh et al., 2011). The total biomass atmaturity and yield / m² decreased significantly with delay in sowing. Highertemperatures observed as further associated with limitation of water cause andimposing rapid shrinkage of grain volume (Mitra and Bhatia 2008).Furthermore,the scenario of increasing world population FAO (2009) projected that anincrement of 71% in grain yield of wheat is required to match the food demanduntil 2050. Expansion of agriculture area have many limitations so, currentgoal could only be achievable through enhancing crop productivity by bettermanagement and evolving stress resistant genotypes development (Reynolds etal.
, 2011). Optimum planting date for different varieties vary withcropping systems depending on growing conditions of a specific region that may beassessed by planting them at different sowing dates. The other essential factorto combat the challenges of heat stress is selection by genotypes which producehigher yields and provide tolerance to adverse conditions and mature earlier(Kumar et al.
2013). Wheat plant has the capability to show a widerscope of compensating, escape, and tolerance mechanisms for heat throughdifferent molecular, biochemical, physiological, developmental and growthadaptation mechanisms (Barnabás et al., 2008). For combating the heatstress condition, an experiment was conducted with following objectives. i.
To identify the potentialsource of terminal heat tolerance in wheat genotypes for future breedingprogramme ii. To study the effect of terminalheat on different agro-physiological traits in bread wheat iii. To assess the geneticvariability among the heat tolerant genotypes of wheat MATERIALS AND METHODSThepresent research was carried-out at the Experimental Field, Department of PlantBreeding & Genetics, Sindh Agriculture University, Tandojam. The experimentwas laid-out in Randomized Complete Block Design with three replications duringRabi season, 2015-16, to assess the response of wheat genotypes for terminalheat stress tolerance. In this context, the experimental material was evaluatedin two sowing dates viz.
, normal planting (25th November, 2015) andlate planting (25th December, 2015), considered as normal and heatstress conditions, respectively.Treatments = Two factors (A and B)Factor – A: Sowing dates (D) = 2D1 = Normalsowing (25th November)D2 = Late sowing (25th December) Factor – B: Genotypes = 151. NIA-Amber 6.
AS-2002 11. Benazir2. Mehran 7. SKD-1 12. Anmol3. Khirman 8. TD-1 13.
Kiran-954. Imdad-05 9. TJ-83 14. NIA-Sunahri5. Sehar-2006 10.
NIA-Sarang 15. MoomalThe datawere collected from ten randomly tagged index plants from each genotype perreplication for the following traits.Physiological maturity (75%):Thischaracter was taken when the peduncle of 75% of plants turned in yellow colour,thus reached at 75% physiological maturity.
Grains spike-1: Thetotal numbers of seeds in main spike were counted and data were recorded asgrains spike-1. Grain yield plant-1 (g):Afterharvesting, each plant was threshed separately by hand and grains were weighedon electric digital balance and yield plant-1 was recorded in grams.Harvest index (%):The harvest index (H.I.%) was taken by theratio of grain yield to biological yield. Harvest index (%) was calculatedaccording to following formula. H.
I. % = Grain yield plant-1(g) x 100 Biological yield plant-1(g) Relative water content (RWC%):For thedetermination of RWC, the next to flag leaves were sampled in polythene bagsand transported to the laboratory as quickly as possible in order to minimizewater losses due to evaporation. The samples were also weighed immediately forfresh weight (FW), then sliced into 2 cm sections and floated on distilledwater for 4 hours. The turgid leaf discs were then rapidly blotted to removesurface water and weighed to obtain turgid weight (TW). The leaf discs weredried in the oven at 60 0C for 24 hours and then dry weight (DW) wasobtained. The calculation of RWC was carried out by using the formula suggestedby Barrs (1968).
RWC (%) = (FW-DW) / (TW-DW) x 100Meteorological data:Minimum and maximum temperatures on daily basiswere recorded during entire cropping season at the experimental site from SAU,Tandojam. Statistical analysis:The data of different parameters for each genotype were averagedand subjected to statistical analysis. Standard analysis of variance techniquewas applied to determine significant difference among the means.
The analysis of variance for all the traits was carried outseparately as described by Gomez and Gomez (1984) to establish the level ofsignificance among various bread wheat genotypes planted under non-stress andheat stress conditions with the following statistical model.Source of variation D.F Meansquare Mean squares expectationsReplicates (R) (r-1) MSR Genotypes (G) (g-1) MSG ?²e + r?²gTreatment (T) (t-1) MST ?²t G x T (g-1)(t-1) MSGT ?²g + ?²tError (r-1)(g x t-1) MSE ?²eWhere: =Environmental variance, =Genetic variance and =Treatment variance. Estimation of least significance difference(LSD):L.S.D atP ?0.
05 for pair wise comparisons was used to determine the criticaldifferences between the means of fifteen genotypes by using the followingformula: L.S.D (5%) = S.
E x tvalue by using error degrees of freedom.Estimation of relative decrease (%):Relativedecrease (%) was measured by the subtraction of mean value of stress from themean value of non-stress, divided by mean value of stress, and multiplied by100 as under.Relativedecrease (RD%) = (non-stress – heat stress) / non-stress x 100 RESULTAND DISCUSSIONSTemperature:Themeteorological data on daily basis for minimum and maximum temperaturesmeasured during entire cropping season (2015-2016) at experimental site aregiven in figure 1.
The high temperature was noticed during the sowing ofexperiment in the November, however, the temperatures decreased in the monthsof December and January. From February to May, the temperature rises about by 50C 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 0Cat 1st week of April, 2016. This situation of temperature is abovefrom the threshold value (25 0C) of wheat crop. Therefore, late sownwheat crop faced terminal heat stress.Analysis of VarianceMeansquares from analysis of variance (Table 2) indicated that heat stress imposedsignificant impact on physiological maturity (75%), grains spike-1,grain yield plant-1 (g), harvest index (%) and relative watercontent (%).
There also existed significant differences among the genotypes forall the yield and physiological traits studied that could allow wheat breedersto select the heat tolerant genotypes for one or more morpho-physiologicalattributes. The mean squares due to genotypes were also significant for all thetraits under non-stress as well as in heat stressed conditions. The meansquares from analysis of variance (Table 2) for genotype x treatmentinteraction for all the studied traits were also significant. The significanceof genotype x treatment interaction showed that genotypes performed differentlyover the stress condition.
These interactions could help wheat breeders toselect the best performing varieties based on one or more reliable heattolerant indicators. MeanPerformance and Relative DecreasePhysiological maturity (75%)In ourexperiment, 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 performancewas shown by Imdad-05 with minimum relative decrease of 4.30%, and the 2ndand 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 stresscondition, it was ranged from 104 to 111.33 days.
The present findings are inagreement with Nahar et al. (2010)who reported up to 15% reduction in maturity period of wheat genotypes due tothe effect of heat stress. The reduction in maturity days were also found inthe research of Hossain et al.
(2015)with the decrement of 13.04% under the late sowing dates. Ishaq et al. (2015) also observed thatterminal heat stress significantly affected the physiological maturity andshortened from 10.46 to 12.67% maturity grown under heat stress conditions. Grains spike?1 A grainper spike is major yield contributing trait.
Terminal heat stress reduces thenumber of grains per spike to a significant extent in wheat. Heat stress causedthe decline of 20.50% averagely, in all the varieties grown under late sowingcondition (Table 2).
The highest reduction was observed in Khirman (38.71%)followed by AS-2002 (35.99%) under the heat stress condition. The lowestreduction was noticed in Imdad-05 (6.15%) closely followed by NIA-Sarang andTD-1 (6.72 and 9.75%, respectively) under heat stress condition. In normalcondition, grains spike-1 ranged from 48.
20 to 67.13, while in heatstress condition, it ranged from 36.40 to 63.00 grains per spike. These findingare 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 reportedthat heat stress caused a significant reduction in the number of grains perspike under heat stress conditions.Grain yield plant-1 (g) Thehigher grain yield plant-1 is the ultimate goal of all the plantbreeders. The increment of all other characters provides a better background toenhance the grain yield plant-1. Heat stress caused a decline of37.87% averagely under the late sowing date (Table 3). Our results are nearly agreementwith those of Hossain et al.
(2015)Abd-elrahman et al. (2014) and Alam et al. (2014) who also reported thatheat stress reduced the grain yield up to 49.5, 40 and 45%, respectively. Thebest performance was shown by the genotype Imdad-05 with minimum decrease of26.46% followed by NIA-Sarang and TD-1 (28.
80 and 32.32%), respectively. Whilethe lowest performance was recorded by genotype Khirman with increasedreduction of 56.52% in grain yield followed by AS-2002 (50.
82%) under heatstress condition. In normal condition, the grain yield per plant ranged from5.75 to 13.34g, whereas under the heat stress condition, it ranged from 2.50 to9.81g. El-Ameen (2012) reported that delaying the sowing date resulted in a substantialreduction in grain yield by 63.
34%, while the genotypes under favourableconditions performed well for grain yield.Harvest index (%)In ourexperiment, there was the difference of one month in both sowing dates. Heatstress occurred at the anthesis stage of wheat genotypes under the 2ndplanting date because of this, the major affect was recorded on grain yield.Thus, the harvest index was also decreased by the heat stress. The overallaverage of all the genotypes in non-stress condition was 41.
37%, and interminal heat stress condition was 37.59% (Table 3). Terminal heat stresscaused a decline of 9.
38% averagely under the 2nd sowing date. Themaximum 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 andTD-1 (6.01 and 6.40%, respectively) under the heat stress condition.
Innon-stress condition, harvest index ranged from 34.63 to 50.68%, while in heatstress condition, it ranged from 29.17 to 47.85%. Singh and Dwivedi (2015), Zarieet al. (2014), Nawaz et al.
(2013) and Moshatati et al. (2012), also reported a declinein harvest index under the heat stress conditions, and their results are inconformity with our findings.Relative water content (%)The relativewater content is a useful indicator of the state of water balance in crop plantswhich is essential because it expresses the absolute amount of water the plantrequires to reach full saturation. Terminal heat stress caused a reduction of16.95% averagely under the late sowing date (Table 4).
Nonetheless, maximumreduction 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) underthe heat stress conditions. The highest relative water content was measured ingenotype 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 genotypeKhirman 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) whoreported the reduced relative water content under the heat stress conditions. CONCLUSIONS:The results revealed significant differences among allthe genotypes under normal and heat stress conditions, for all the characters,expressing the suitability of experiment to improve bread wheat genotypes forheat tolerance. The varieties Imdad-05, NIA-Sarang and TD-1showed minimum reductions for various traits under heat stress conditions, thusshowing their tolerance to heat stress. The varieties Khirman and AS-2002expressed maximum declines under terminal heat stress conditions for all thestudied traits, showing their susceptibility to heat stress. In non-stress aswell as under heat stress conditions, the highest values were recorded by thegenotype Imdad-05, NIA-Sarang and TD-1, indicating the resistance against heatstress.