Evaluation of Some Wheat Hybrids under Normal and Heat Stress Conditions  

Jehanzeb Farooq1 , Ihsan Khaliq2 , Abid Mahmood3
1. Cotton Research Institute, Ayub Agricultural Research Institute, Faisalabad, Pakistan 2. Department of Plant Breeding and Genetics, University of Agriculture, Faisalabad, Pakistan 3. Director General Agri. Research Ayub Agricultural Research Institute, Faisalabad, Pakistan
Author    Correspondence author
Triticeae Genomics and Genetics, 2014, Vol. 5, No. 2   doi: 10.5376/tgg.2014.05.0002
Received: 30 Nov., 2014    Accepted: 30 Dec., 2014    Published: 26 Jan., 2015
© 2014 BioPublisher Publishing Platform
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Preferred citation for this article:

Farooq and Khaliq, 2014, Evaluation of Some Wheat Hybrids under Normal and Heat Stress Conditions, Triticeae Genomics and Genetics, Vol.5, No.2 1-11 (doi: 10.5376/tgg.2014.05.0002)

Abstract

Heat shocks at anthesis are one of the major limitations in wheat productivity in many countries worldwide including Pakistan. Present investigations were designed to battle out this calamity which may further be exploited in breeding heat tolerant cultivars. Seven parents including tolerant, moderately tolerant and susceptible but high yielders were utilized for this purpose and later hybridized in a 7 ×7 diallel fashion. Analysis of variance revealed significant variability for all the parameters studied. Highest value of negative and desirable significant heterosis and heterobeltiosis for plant height was shown by the cross combinations Weebli-1 × Uqab-2000, Inqilab-91 × Shalimar-88 under normal conditions and under stress by the hybrids Uqab-2000 × Weebli-1, Shalimar-88 × Maya/Pavon. Maximum positive heterosis for tillers per plant was recorded by the cross combination Shalimar-88 × Chenab-2000 (12.04%) and maximum positive and highly significant heterobeltiosis was recorded by the cross combination Shalimar-88 × Chenab-2000 (11.46%). Under stress for tillers per plant none of the crosses showed positive estimates. Maximum positive heterosis for 100- grain weight was shown by the cross combination Chenab-2000 × Inqilab-91 (28.22%). Maximum positive heterosis under stress for 100-grain weight was shown by the cross Inqilab-91 × Weebli-1 (23.35%) followed by Chenab-2000 × Inqilab-91 (22.22%). Maximum positive heterosis for grain yield under normal conditions was manifested by the cross combinations Uqab-2000 × Punjab-85 (28.70%) and under stress maximum positive heterosis was recorded by cross combination Shalimar-88 × Uqab-2000 (27.02%).  The results of heterosis and heterobeltiosis estimation revealed that the hybrid vigor is available in the current experiment for all the traits and selection of desirable hybrids is the best way to improve the grain yield of bread wheat. Many cross combinations in the current studies may be utilized following pedigree or bulk method to develop heat tolerant wheat varieties because of their ability to perform well under normal and heat stress conditions.

Keywords
Terminal heat stress; Heterosis; Bread wheat; Cereals; Pakistan

Bread Wheat is among the leading cereals grown in Pakistan and all over the world. Its cultivation is most suitable in areas with cool environmental conditions (Farooq et al., 2013; Modhej et al., 2008). Its trend of cultivation is rising in areas that are too warm for favorable production (Farooq et al., 2011). Terminal heat stress in many areas of the world in the recent past resulted in poor grain filling, which is the most decisive stage of grain development. A little heat stress (≥35°C) during this point reduces starch contents ultimately decreasing grain quality and weight (Sial et al., 2005).

For the development of new varieties having desirable traits choice of parents is the most important step. Utilization of heterotic effects for grain yield was mostly related to cross pollinated crops. In wheat hybrid vigour is directly related to the effective selection of the parents. But results of different researchers on hybrid vigour do not show such parallelism. According to the findings of Cox & Murphy (1990) and Picard et al., (1992) prospect of developing superior genotype is more if both parents used in crosses have parallel performance instead of one parent being lesser or greater in terms of one or more traits. However, genetic variation between parents is vital to develop superior hybrids (Morgan, 1998; Fabrizius et al., 1998; Baric et al., 2004). The parents with better yielding potential show less heterosis for yield because parents already have many valuable genes in homozygous state Morgan et al., (1989). But according to Fabrizius et al., (1998) more the genetic dissimilarities among parents more will be heterosis for yield in a hybrid. Similarly Singh et al., (2004), recommended that heterobeltiosis can be valuable for decisive true heterotic cross combinations. The researchers like Chowdhry et al (2001) reported significant positive heterosis for peduncle length. Both positive and significant negative heterosis and heterobeltiosis for tillers per plant, plant height, peduncle length, 100 grain weight and grain yield was found in the studies of Farooq et al 2004.
The experiment was carried out aiming to study some yield contributing traits in wheat hybrids under normal and heat stress conditions.
1 Results
1.1 Genotypic differences among the parents
Analysis of variance indicated significant differences (P<0.01) for all the traits among 49 genotypes under both regimes. The mean squares for the traits are represented in Table 1 describing high significance of the ‘F’ test for all the traits under study.


Table 1 Mean squares of various plant traits in a 7×7 diallel cross under normal and heat stress conditions of Triticum aestivum L.

1.2 Heterosis for plant height under normal and heat stress conditions
Plant height is among one of the most important characters in wheat production. For this trait reduced plant height is desirable because of the reason that short stature varieties are more responsive to fertilizes than the taller ones. Heterosis studies under normal conditions for plant height revealed that 21 cross combinations showed positive heterosis. Nine of the crosses were highly significant and 2 crosses were significant while remaining 10 crosses were non-significant. Among crosses showing negative values 5 crosses were highly significant, 3 were significant and 13 were negative and non-significant Table 2. Maximum positive heterosis was shown by the cross Maya/Pavon × Shalimar-88 with a value of (8.30%), followed by Shalimar-88×Maya/Pavon (7.76%) and hybrid Punjab-85 × Maya/Pavon (5.57%). Maximum negative heterosis was shown by the cross combination Weebli-1×Uqab-2000 (-3.60%) followed by Inqilab-91×Shalimar-88 (-3.30%).Under heat stress conditions 30 crosses showed reduction in plant height. Out of these 30 crosses 16 showed negative and highly significant results and 4 showed significant results while 10 crosses showing negative heterosis were non-significant. Twelve crosses showed increase in height as compared to parents. Out of these 12 hybrids 10 showed positive but non-significant results. One cross showed highly significant results and one cross showed significant results. The highest negative heterosis as compared to the parents was showed by the cross combinations i.e. Weebli-1 × Maya/Pavon (-10.88%) followed by the crosses Uqab-2000 × Weebli-1 (-7.64%) and Punjab-85 × Weebli-1 (-7.18%). The highest positive heterosis was shown by the cross Punjab-85 × Shalimar-88 (4.50%).


Table 2 Heterosis and heterobeltiosis for various traits in Triticum aestivum L. under normal and heat stress conditions

1.3 Heterobeltiosis for plant height under normal and heat stress conditions
Heterobeltiosis studies under normal environment for plant height indicated that 12 crosses showed increase over better parental value. Eight out of 12 crosses showed positive and highly significant heterobeltiosis and 4 crosses showed non-significant results. However, 30 crosses showed negative heterobeltiosis Table 2. Among them 12 were highly significant, 6 were significant and 12 were non-significant. Maximum positive heterobeltiosis was shown by the cross Maya/Pavon×Shalimar-88 with a value of (7.26%) followed by Shalimar-88×Maya/Pavon (6.73%). Maximum negative value was shown by the cross combination Inqilab-91 × Shalimar-88 (-5.47%).
As far heterobeltiosis is concerned under stress environment 24 crosses showed highly significant but negative heterobeltiosis over better parental values. Among these 24 crosses Weebli-1×Maya/Pavon showed the highest value of negative heterobeltiosis (-14.41%) followed by (-12.02%) by cross combination Punjab-85×Uqab-2000 and Maya/Pavon×Chenab-2000 (-11.76%). Three crosses showed significant but negative values. While 9 crosses showed negative and non-significant results. Five crosses showed positive but non-significant results and one cross i.e. Punjab-85×Shalimar-88 showed significant and positive heterobeltiosis.
1. 4 Heterosis for peduncle length under normal and heat stress conditions
Heterosis studies for peduncle length showed that under normal conditions out of 42 crosses only 9 combinations showed positive results in which only 1 cross showed highly significant heterosis and 1 cross showed significant results. While 7 crosses showed positive and non-significant results. Thirty three crosses revealed negative heterosis. Highly significant negative heterosis was shown by 8 crosses and 9 crosses showed significant results, while 16 crosses showed negative and non-significant results. Maximum positive heterosis was shown by the cross combinations Maya/Pavon × Shalimar-88 (13.67%) followed by the cross combination Punjab-85× Shalimar-88 (9.46%). However, maximum decrease was shown by the cross combination Weebli-1 × Chenab-2000 (-18.22%) followed by the cross Uqab-2000×Inqilab-91 (-15.33%). For heterotic effects of peduncle length under heat stress conditions 30 crosses showed negative values in 42 crosses studied. Among these 30 crosses 18 showed highly significant results, 3 were significant and 9 were negative and non-significant. Among remaining 12 crosses which showed positive results 1 cross showed positive and highly significant results while 2 crosses were positive and significant and 9 were non-significant. Following crosses i.e., Maya/Pavon× Weebli-1 (-28.02%) followed by Uqab-2000 × Inqilab-91 (-20.60%) and Inqilab-91×Weebli-1 (-20.14%) showed maximum negative values. However, maximum positive heterosis was shown by the cross Weebli-1×Chenab-2000 (14.36%) followed by Chenab-2000×Punjab-85 with a value of (12.11%).
1.5 Heterobeltiosis for peduncle length under normal and heat stress conditions
Percent increase or decrease of first filial generation crosses as compared to their better parental values under normal conditions indicated that 39 out of 42 crosses showed decrease in heterobeltiosis values. However, 12 crosses were negative and highly significant, 11 were significant and 16 were negative and non-significant. Among remaining 3 crosses only one cross was positive and highly significant Maya/Pavon×Shalimar-88 (12.64%) and 2 were non-significant. Among crosses showing maximum decrease were Weebli-1×Chenab-2000 (-22.80%) followed by the cross combination Uqab-2000 × Punjab-85 (-21.98%).
Heterobeltiosis studies for stress environment revealed that 40 crosses showed negative values. Among these 40 crosses 26 showed negative but highly significant results and 10 crosses showed negative and non-significant results. Among the highly significant crosses Maya/Pavon × Weebli-1, Maya/Pavon× Chenab-2000 and its reciprocal cross showed the highest negative values of (-39.52%), (-35.69%) and (-34.83%) respectively, followed by the cross Weebli-1×Maya/Pavon (-32.74%). Positive and non-significant heterobeltiosis was shown by the crosses Chenab-2000 × Punjab-85 (3.44%) and Weebli-1 × Uqab-2000 (0.17%).
1.6 Heterosis for Tillers per plant under normal and heat stress conditions
Percentage increase or decrease of F1 over mid parental value under normal conditions showed that for tillers per plant under normal conditions only 5 crosses showed positive heterosis over mid parental values. Out of these 5 crosses 3 crosses showed highly significant heterosis and 2 crosses showed non-significant heterosis. Only one cross showed no heterosis. Remaining 36 crosses showed negative heterosis. Out of them 24 crosses were negative and highly significant, 2 crosses were significant and 10 crosses were non-significant in which one cross showed no heterosis. Maximum positive heterosis was recorded by the cross combination Shalimar-88 × Chenab-2000 (12.04%) followed by Shalimar-88 × Punjab-85 (10.19%). Maximum negative heterosis was recorded by the cross combinations Punjab-85 × Maya/Pavon (-30.06%) followed by Maya/Pavon × Uqab-2000 (-29.24%). For tillers per plant under heat stress environments 31 crosses showed negative heterosis. Twenty one showed negative and highly significant, 5 negative and significant and 5 showed negative and non-significant results. Maximum negative and highly significant values were shown by the crosses Inqilab-91 × Shalimar-88 and its reciprocal with values of (-40.74%) and (-39.26%) respectively followed by cross combination Uqab-2000 × Inqilab-91 showed a negative value of (-31.93%). Remaining 11 crosses which showed positive heterosis 6 were highly significant and 4 were non-significant and one cross showed no heterosis with a value of (0.00%). Maximum positive value was shown by the cross Chenab-2000 × Uqab-2000 with its reciprocal i.e., (25.49%) and (22.55%) respectively.
1.7 Heterobeltiosis for tillers per plant under normal and heat stress conditions
For tillers per plant heterobeltiosis studies under normal conditions indicated that only 2 crosses showed positive heterosis with one cross highly significant and one cross non-significant. Reduction in tillers per plant was shown by 40 crosses with 32 crosses highly significant, 1 cross was significant and 7 crosses non-significant. Maximum positive and highly significant heterobeltiosis was recorded by the cross combination Shalimar-88×Chenab-2000 (11.46%). Maximum negative values was shown by the cross combinations Weebli-1×Chenab-2000 (-32.89%) followed by Maya/Pavon × Uqab-2000 (-31.73%).
For heterobeltiosis under stress conditions reduction in number of tillers was shown by 36 crosses. Among these 36 crosses 28 were negative and highly significant, 3 were significant and remaining 5 negative crosses showed non-significant results. Among 6 positive crosses 3 were significant and 3 were non-significant. Maximum negative heterobeltiosis was shown by the cross combinations Inqilab-91 × Shalimar-88 and Uqab-2000 × Inqilab-91 with values of 46.67%) and (-46.00%) respectively. Maximum positive heterobeltiosis was shown by the cross Chenab-2000 × Uqab-2000 (10.34%) followed by the hybrid Uqab-2000 × Punjab-85 (8.33%).
1.8 Heterosis for 100-grain weight under normal and heat stress conditions
Heterosis studies for 100-grain weight under normal sowing indicated that all the crosses for this yield related trait showed positive heterosis. However, 31 crosses manifested highly significant results 8 crosses showed significant and 3 crosses showed non-significant results. Maximum positive heterosis was shown by the cross combinations Chenab-2000 × Inqilab-91 (28.22%), Chenab-2000×Uqab-2000 (24.82%) followed by the cross Inqilab-91 × Chenab-2000 (23.99%). For 100-grain weight under heat stress conditions 22 crosses showed positive heterosis. Among these 15 crosses showed highly significant results, 2 crosses showed positive and significant results and 5 showed non-significant results. While 20 crosses showed negative heterosis. Among them 13 were highly significant, 2 were significant and 5 were non-significant. Among crosses showing maximum positive heterosis Inqilab-91× Weebli-1 (23.35%) was at the top followed by Chenab-2000×Inqilab-91 (22.22%) and Maya/Pavon× Shalimar-88 (19.52%). Maximum negative heterosis was shown by the hybrids Punjab-85×Shalimar-88 (-25.49%), Chenab-2000×Shalimar-88 (-24.30%) and Shalimar-88 × Punjab-85 (-23.63%).
1.9 Heterobeltiosis for 100-grain weight under normal and heat stress conditions
Heterobeltiosis studies under normal conditions for 100-grain weight revealed that 39 crosses showed increase in mid parental value with 18 crosses showing highly significant results, 9 crosses significant results and 12 crosses non-significant results. Only three crosses showed negative and non-significant heterobeltiosis. Maximum heterobeltiosis was shown by the cross combination Chenab-2000× Inqilab-91 (27.87%) followed by Chenab-2000× Uqab-2000 (24.15%). Maximum negative heterobeltiosis was exhibited by the cross combination Punjab-85 × Weebli-1 (-3.23%).
As for the increase over better parent heterobeltiosis is concerned under the late sown conditions 23 cross combinations were negative and highly significant, 2 negative and significant and 3 negative and non-significant. Among 14 hybrids showing positive heterosis 8 crosses were highly significant 2 were significant and 3 were positive and non-significant. One cross showed no heterobeltiosis. Crosses showing highest negative values were Weebli-1 × Shalimar-88 (-34.57%), Chenab-2000×Shalimar-88 (-32.07%) and Punjab-85×Shalimar-88 (-29.48%). Maximum positive values were exhibited by the crosses Maya/Pavon × Uqab-2000 (13.28%) and Inqilab-91 × Maya/Pavon (13.11%).
1.10 Heterosis for grain yield under normal and heat stress conditions
Percentage increase of F1 over their mid parental values for grain yield per plant under normal conditions revealed that 21 crosses showed positive heterosis and 21 crosses showed negative heterosis. Maximum positive heterosis was manifested by the cross combinations Uqab-2000 × Punjab-85 (28.70%), Chenab-2000×Uqab-2000 (27.94%) followed by Uqab-2000×Chenab-2000 (26.31%). However, maximum negative heterosis was shown by the cross combinations Chenab-2000 × Inqilab-91 (-21.14%) followed by its reciprocal cross Inqilab-91× Chenab-2000 (-18.99%) and Weebli-1 × Maya/Pavon (-18.71%).
Percentage increase of F1’s over their mid parents for grain yield per plant under stress environments showed that only 10 crosses exhibited positive and statistically significant heterosis over mid parental values. While, 2 crosses showed positively significant values and 7 showed positive and non-significant results. Twenty three crosses showed negative values as compared to their mid parental values. Out of these 23 hybrids 18 were highly significant, 1 was significant and 4 were negative and non-significant. Maximum positive heterosis was recorded by cross combination Shalimar-88 × Uqab-2000 (27.02%) followed by Maya/Pavon × Weebli-1 (21.63%) and Weebli-1 × Punjab-85 (20.88%). Maximum negative heterosis was exhibited by the cross combinations Maya/Pavon×Chenab-2000 (-32.58%), Chenab-2000× Maya/Pavon (-32.01%) followed by the cross Chenab-2000 × Shalimar-88 (-28.17%).
1.11 Heterobeltiosis for grain yield under normal and heat stress conditions
Heterobeltiosis for grain yield per plant under normal conditions indicated that percentage increase of F1’s over better parents showed that 13 crosses out of 42 showed positive increase over better parental value. However, only 7 crosses were found highly significant, one cross found significant and 5 crosses showed non-significant results. 29 crosses showed negative heterobeltiosis with 21 crosses showed negative and highly significant heterobeltiosis, 3 crosses showed significant and 5 cross combinations showed non-significant results. Maximum positive heterobeltiosis values were shown by the cross combinations Uqab-2000 × Punjab-85 (15.58%) and Chenab-2000 × Uqab-2000 (13.06%). Maximum negative heterobeltiosis was shown by the cross combinations Punjab-8 × Weebli-1 (29.41%), Chenab-2000× Inqilab-91 (-29.29%) and Inqilab-91 × Chenab-2000 (-27.36%).
In case of heterobeltiosis 35 cross combinations showed decline over better parental values under heat stress conditions. Among them 32 crosses showed negative and highly significant heterobeltiosis and 3 crosses showed significant results. Only 7 crosses reported positive heterosis with 4 crosses showing positive and highly significant, 2 positive and significant and 1 positive but non-significant. Maximum negative heterobeltiosis was shown by the cross combinations Weebli-1×Maya/Pavon (-46.54%), Weebli-1 × Shalimar-88 (-38.28%) and Maya/Pavon × Chenab-2000 (-37.36%). Maximum positive he- terobeltiosis was indicated by the cross combinations Shalimar-88 × Uqab-2000 (13.62%) and Inqilab-91 × Punjab-85 (11.24%).
2 Discussion
Terminal heat stress in wheat is an area of major concern in Pakistan and it needs improvement. In temperate climatic conditions terminal heat shocks are restrictive factor during anthesis and grain filling (Reynolds et al, 1994). The researchers like Chen et al. (2000) utilized late sown conditions and plastic sheet tunnel to induce heat stress at the time of anthesis and they suggested weight of grains in a spike and yield as selection criterion against heat stress. Significant decrease in wheat production resulted due to average temperature above 150C during grain filling stage (Weigand and Cuellar, 1981).
In wheat breeding like other crops proper utilization and choice of parental material is a prerequisite to obtain desirable results. Different researchers have different opinions in explaining the phenomena of heterosis. According to Cox & Murphy (1990) and Picard et al., (1992) opportunity of obtaining superior genotype is dependant if both parents have at par performance instead of one parent being lesser or better in terms of one or more traits. However, in the studies of Fonseca & Patterson, 1968; Baric et al., 2004 genetic variability between parents is a criterion to develop superior hybrids. In this experiment, the spotlight was on the development of hybrids by using parental sources having tolerance ability against heat shocks so that they can be effectively utilized in future breeding programmes.
Plant height is an important trait and largely contributes in biological yield of wheat. Negative percentages of heterosis and heterobeltiosis for plant height are preferred over their mid and better parents in wheat breeding because short stature is a desirable character as it confer resistance against lodging thus produce higher yield. Desirable recombinants from such genetic material through selection can be obtained. Present results showed many desirable negative estimates which are in accordance with the results of Abdullah et al. (2002) and Rasul et al. (2002).
For tillers per plant under both conditions many crosses showed increase both in mid and better parental values which may be exploited in later generations. Increase in vigor for tillers per plants was reported by Chowdhry et al. (2001) and Shah et al. (2004), but negative heterosis for tiller number per plant was observed in the studies of Knobel et al. (1997) and Farooq and Khaliq (2004). For peduncle length under both environments positive and negative estimates were observed, however peduncle length in hybrid wheat was found to increase grain yield in many crosses (Chowdhry et al., 2001).
In the current studies many crosses offer and opportunity to increase 1000 grain weight which is another important yield component. By exploiting heterosis for this trait, researchers found this trait as direct contributor for increasing grain yield in hybrid wheat (Shah et al., 2004; Akbar et al., 2007).
Yield and yield related characters having significant positive heterosis and heterobeltiosis are important for selection of these characters in crosses for future breeding programme. Grain yield is related with various traits such as morphological, physiological and yield components. As a result of the research conducted by many researchers on hybrid wheat found the ranges of heterosis for grain yield from 6% (Borghi et al., 1986) to 41% (Zehr et al., 1997). In the current studies the values of maximum heterosis for yield was 28.07% under normal and 27.02% under stressed conditions while better parental values were from 15.58% under normal and 13.62% under stress conditions. The value of mid parent heterosis for grain yield was observed as high as 72 % under normal irrigation conditions and it increased to 127 % under water stress conditions (Solomon et al. 2006). Similarly some researchers reported negative heterosis for grain yield (Farooq and Khaliq, 2004).
The results of heterosis indicated that hybrid vigour is accessible for the commercial production of wheat and selection of desirable crosses having heterotic and heterobeltiotic effects. The cross combinations like Inqilab-91 × Shalimar-88, Shalimar-88 × Maya/Pavon, Chenab-2000×Punjab-85, Maya/Pavon×Chenab-2000, Shalimar-88×Uqab-2000 and Uqab-2000×Maya/Pavon are the best hybrids which maybe further exploited because of their ability to perform well under normal and even heat stress conditions.
3 Material and Methods
The experimental material developed after screening against heat was comprised of seven wheat cultivars including five locals Shalimar-88 (Tolerant), Chenab-2000 (Tolerant), Inqilab-91 (Moderately tolerant), Uqab-2000 (Susceptible but yielder) and Punjab-85(Susceptible but yielder) and two exotic CIMMYT originated cultivars Weebli-1 (Susceptible but yielder) and Maya/Pavon (Tolerant) were sown in the field on 5th of November, 2006 in the Department of Plant Breeding and Genetics and later hybridized in all possible combinations including reciprocals following diallel mating system. During next crop season, seven wheat varieties/lines (parents) and their hybrids (F1) were planted in field in two sowing dates on 10th of November, 2007 and 25th of December following a triplicated randomized complete block design. Thirty plants of each genotype were grown in a 5 m long row in each replication. The plants were spaced 15 and 30 cm apart within and between the rows, respectively. To keep uniformity in the distance and depth of the seeds, a template was used. Two seeds were dibbled per hole and after germination one healthy seedling was retained at each hole after thinning. All standard agronomic practices i.e., hoeing, weeding and irrigation etc. were adopted uniformly. For data collection ten guarded plants for each parent and cross were tagged at random for each replication in both regimes and data were recorded for plant height of each of ten randomly selected plants from base of plant to the tip of spike excluding awns of mother shoot. At maturity the peduncle length of mother shoot of the selected plant was measured from last node (bearing the flag leaf) to the base of spike.Similarly,tillers number of each selected plant was counted at maturity in each replication and average was computed. The grains from ten randomly selected plants in each replication of every genotype were bulked separately. 100-grains were counted randomly from each bulk and weighed on electric balance (Compax- Cx-600). For grain yield all spikes of individual selected plants were threshed manually and weighed using electric balance (Compax- Cx-600). The collected data were analyzed to determine significant varietal differences among the 42 genotypes under both regimes following Steel et al. (1997).
3.1 Heterosis and heterobeltiosis
The percent increase (+) or decrease (-) of F1 cross over mid parent as well as better parent was calculated to observe heterotic effects for all the parameters. The estimates of heterosis over the mid parent and better parent (heterobeltiosis) were calculated using the procedure of Matzingar et al. (1962).       
Heterosis (%) = 
 
Heterobeltiosis (%) =
Where,
MP = mid parental value of the particular F1 cross (P1 + P2) / 2,
BP = better parent value in the particular F1 cross.
Difference of F1 mean from the respective mid parent and better parent value was evaluated by using a t-test according to Wynne et al. (1970).
t =
 
Where,
F1ij = the mean of the ijth F1 cross,    
MPij = mid parent value of the ijth cross, and
σ2e = estimate of error variance
Contribution of each author
Dr. Jehanzeb Farooq is the author of the article and contributed in bulk of the work including write up and analysis of the data. This research is the part of the PhD thesis of Dr. Jehanzeb Farooq. Dr Ihsan Khaliq contributed in the review of the current manuscript as he is also the supervisor of the research conducted by Dr. Jehanzeb Farooq.
Conclusion
Wheat production in Pakistan is better than before, but terminal heat stress is still an alarming threat that significantly reduces yield. The inferences obtained from this experiment would be helpful to study the performance and relationship of F1 hybrids and parents and to select suitable parents and population for scheming effective wheat breeding regarding thermal stress tolerance in wheat.
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