Terminal heat tolerance in wheat

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Terminal heat tolerance in wheat

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See discussions stats and author profiles for this publication at: https://www.researchgate.net/publication/332803235 Terminal heat tolerance in wheat: An overview Article · April 2019 DOI: 10.25174/2249-4065/2019/79252 CITATIONS 0 READS 38 4 authors including: Some of the authors of this publication are also working on these related projects: NPTC-Functional Genomics in Wheat View project Geminivirus and RNAi Suppressors View project Girish Chandra Pandey Banasthali University 15 PUBLICATIONS   26 CITATIONS    SEE PROFILE Pradeep Sharma Indian Institute of Wheat and Barley Research Karnal 142 PUBLICATIONS   854 CITATIONS    SEE PROFILE All content following this page was uploaded by Pradeep Sharma on 29 June 2019. The user has requested enhancement of the downloaded file.

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1 Journal of Cereal Research 111: 1-16 Review Terminal heat tolerance in wheat: An overview Girish Chandra Pandey 12 Geetika Mehta 2 Pradeep Sharma 2 and Vinay Sharma 1 3 1 Department of Bioscience and Biotechnology Banasthali Vidyapith Rajasthan India 2 ICAR-Indian Institute of Wheat and Barley Research Karnal India 3 Amity Institute of Biotechnology Amity University Jaipur Rajasthan Article history Received: 11 May 2018 Revised : 20 Jan. 2019 Accepted: 23 Jan. 2019 Citation Corresponding author Email: © Society for Advancement of Wheat and Barley Research Abstract Since the beginning of the century ambient temperatures have increased and are predicted to rise further under climate change. Heat stress is a rigorous threat particularly during reproductive and grain-flling phases that results into yield loss. Reductions in dry matter accumulation and grain yield caused by reduced plant photosynthetic capacity through metabolic limitations and oxidative damage to chloroplasts due to heat stress. Wheat pre- breeding and breeding may be based on secondary traits like chlorophyll content chlorophyll fuorescence and grain number under heat stress. On the other hand grain yield under heat stress can be improved by selecting genotypes for the rate of grain flling and for grain size. Wheat varieties with improved grain yield can be developed by integrating physiology and biotechnological tools with conventional breeding techniques during reproductive and grain-flling phases. In this review we have discussed the effect of heat stress on wheat reproductive and grain-flling stages and the strategies to improve terminal heat stress tolerance in wheat. Keywords: Terminal heat tolerance agronomic traits physiological parameters QTLs Wheat 1. Introduction Heat stress is a serious threat to crop production globally. Mean temperature of 0.3°C will globally rise per decade Jones et al. 1998 Hossain et al. 2013 above the present ambient leading to approximately 0.3 and 1 0 C for years 2025 and 2100 respectively as stated by Inter-governmental Panel on Climatic Change IPCC 2014 report . Increasing temperature may alter the crop season and crop gets mature earlier Porter 2005 Gupta et al. 2013. Tripathi et al. 2016 also reported that heat stress has deleterious effect on crop production and it is very challenging for the world food security and it is expected by increasing temperature that prolonged wheat production will be very much effected. The grain flling duration GFD in cereals is measured by temperature pattern at the crop growth period related to grain development Sofeld et al. 1977 Slafer and Rawson 1994 Wheeler et al. 1996a Almeselmani et al. 2009 Muhammad et al. 2011. The overall performance of genotype is also affected by genotype × environment interactions Dia et al. 2016. The environments in that the plants live plays an important role in preparation of the key frame decisions that includes sustainable farm management and cultivar selection for understanding of possible interactions among crop system with the soil Dia et al. 2016. The suffcient amount of nutrients and minerals are very much necessary in plants exposed to temperature stress Waraich et al. 2012. Heat stress causes physiological changes by reducing chlorophyll content that leads to leaf senescence in cool- season cereal species Almeselmani et al. 2011 Dhyani et al. 2013. High temperature 31°C post anthesis can decrease the rate of grain-flling in wheat Al-Khatib and Paulsen 1990 Randall and Moss 1990 Stone et al. 1995 Wardlaw and Moncur 1995 while the same imposed before anthesis can also decrease yield Wardlaw et al. 1989 Tashiro and Wardlaw 1990 Hunt et al. 1991 by reducing the number of grains per spike Wheeler et al. 1996b. Heat stress in wheat has affect on the plant growth and development physiological functions reduced grain formation and yield Mondal et al. 2013 Iqbal et al. 201 7 Challinor et al. 2014. Heat stress causes reduction Homepage: http://epubs.icar.org.in/ejournal/index.php/JWR Pandey GC G Mehta P Sharma and V Sharma. 2019. Terminal heat tolerance in wheat: An overview. Journal of Cereal Research 111: 1-16 doi. org/10.25174/2249-4065/2019/79252 vinaysharma30yahoo.co.uk Pradeep.Sharmaicar.gv.in

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2 Journal of Cereal Research in photosynthetic capacity Almeselmani et al. 2012 Ashraf and Harris 2013 change of plant water relations Hasanuzzaman et al. 2012 2013 hormonal changes Krasensky and Jonak 2012 decreases of metabolic activities Farooq et al. 2011 production of oxidative reactive species Wang et al. 2011 2012 2014 2016 increases of pollen mortality reduction of pollen tube development and promotion of ethylene production in wheat Oshino et al. 2011. During the period from 1880 to 2012 the Earth’s system warmed by 0.85°C IPCC 2014. Genes associated with terminal heat tolerance traits are a major focus area for defning future strategy to characterize heat tolerance in wheat. We need to understand how heat tolerance can be improved and how crops respond to prominent temperatures for the new crop varieties to the future climate Halford 2009. The important traits associated with terminal heat tolerance are helpful in screening of wheat. These techniques largely determine the effciency of screening to simulate heat stress of the target environment. Through perturbations in metabolism membrane fluidity protein conformation and assembly of the cytoskeleton plants detect changes in ambient temperature Ruelland and Zachowski 2010. Rehman et al. 2009 also screened genotypes for heat tolerance at terminal growth stage of wheat germplasm. Abrupt temperature rise at grain flling duration GFD negatively affect the plant growth grain-quality and yield. During grain flling duration GFD sudden temperature rise is a major limitation to wheat production in many environments Hays et al. 2007. Under heat stress 36 million ha area is covered woredwide and out of this a major portion is located in south Asia Reynolds et al. 2001. In India 13.5 ha -1 Joshi et al. 2007a area is covered under heat stress. Eastern Gangetic plains cover eastern UP Bihar Jharkhand and W est Bengal while central and peninsular India covers MP Maharashtra and Karnataka state of the India and Bangladesh are considered as extreme heat stress location of the south Asia. The north- western parts of the Indo-Gangetic plain are considered as moderate heat stress zone Joshi et al. 2007b Singh et al. 2007. T raits associated with morpho-physiological yield and different growth stage of plants were affected by high temperature. Post anthesis temperature rise affected the grain flling rate Stone and Nikolas 1994. Short duration 3-4 days sudden temperature rise in form of heat wave 35-37°C alter the morphology and size of grain W ardlaw and Wrigley 1994. Yield is also reduced by up to 23 due to short duration 4 days of heat exposure 35°C Stone and Nikolas 1994 and 3-day heat treatment 38°C from 8 am to 5 pm reduced individual yield component up to 28.3 Mason et al. 2010. Terminal heat stress is major concern for India Rane et al. 2007 Joshi et al. 2007b. Global warming in form of climate change increased the risk for yield losses in wheat Lillemo et al. 2005. Improving heat tolerance in wheat is major goal for wheat breeder and morpho-physiological traits in plants are affected by heat stress Nachit et al. 1998. T emperate cereals are prone to rising temperature when compared to tropical cereals. Many traits contributing for heat tolerance are heritable additive nature and show variation these are indicative for scope of wheat improvement under terminal heat stress Tuberosa and Salvi 2006. Physiological traits such as higher photosynthetic rates stay-green chlorophyll content chlorophyll fluorescence etc. play signifcant role in heat tolerance. Crossing programmes are being carried out for such traits to assist in the breeding for abiotic stress tolerance in wheat. Heat tolerance is a complex phenomenon and is controlled by multiple genes regulating different biochemical and physiological traits. T o analyse the genetic basis of thermo- tolerance the promising approaches are molecular markers Maestri et al. 2002. Marker assisted selection MAS has been considered as helpful approach to develop crop stress tolerance because of the diffculty in phenotypic selection for tolerance and general complexity of abiotic stress tolerance. Identifcation of genetic markers that are associated with gene or quantitative trait loci QTLs or affecting whole plant stress tolerance or individual contributing components are required for MAS. The powerful tool for dissecting the genetic basis underlying complex traits into individual components is QTL analysis and the QTL analysis is based on high density molecular linkage maps. Limited information regarding the heat- responsive microsatellite markers in wheat is available on public domain. On the other hand Akpinar et al. 2017 reported 68 592 SNPs from the Aegilops tauschii genotype MvGB589 mapped on to the chromosome 5D through genomic and transcriptomic approaches. High temperature also increases the rate of grain flling to compensate for genetic dissection of wheat kernel size and shape. In this review we explained common morphological physiological and molecular responses of wheat to high temperatures during the reproductive and grain- flling stages and investigate how these responses can be exploited to improve heat tolerance. Wheat Triticum aestivum L. sensitivity have already been reported for the major wheat-producing regions to high temperature and trends in rising temperatures during the growing season Gaffen and Ross 1998 Alexander et al. 2006. 2. Mechanism of heat tolerance in wheat Crops have three different mechanisms to adapt for heat stress conditions namely heat avoidance heat tolerance and heat escape mechanisms. These mechanisms are

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Terminal heat tolerance in wheat: An overview 3 in turn governed by different associated traits viz. leaf rolling waxiness in heat avoidance production of stress responsive genes in tolerance and adjusting the phenology of plants once it senses the heat stress to escape. By leaf rolling reducing leaf size leaf shedding thickening leaves transpirational cooling reducing growth duration in morphology and ontogeny plants tend to reduce heat-induced damage W ahid et al. 2007. 3. Grain flling rate and duration Grain-flling rate and duration are influenced directly by high temperature and it directly affects the process of grain development Kumar et al. 2012 Lobell and Gourdji 2012 Gourdji et al. 2013. The degree of heat- driven damage is dependent on the intensity of heat stress. Grain flling duration is shortened because heat stress accelerates the rate of grain flling Dias and Lidon 2009. For instance 5°C increases in temperature above 20°C by 12 days in wheat increased the rate of grain flling and reduced the grain flling duration as the supply of photo assimilates may be limited under these conditions Yin et al. 2009 Calderini et al. 2006. The RILs mapping populations derived from parental crosses of genotype RAJ 4014 and WH 730 showed clear-cut segregation pattern for differences in GFR between timely and late sown conditions. About 75 of the progenies showed no difference while 25 showed signifcant difference in GFR under high temperature stress created by late sown condition Pandey et al. 2013 Hossain ad Teixeira de Silva 2012 Hakim et al. 2012. Other studies have also reported that the grain growth rate and duration were reduced by heat stress in genotypes differing in grain weight stability Viswanathan and Khanna-Chopra 2001. Wu et al. 2018 conducted feld trials during 2011–2013 in three locations on 10 wheat genotypes. Observations were recorded on grain weight grain-flling duration grain-flling rates and the lag phase from flowering to the initiation of grain flling. The grain-flling duration and rate were ftted by Richard’s equation in `thermal time growing degree-days GDD base temperature 9°C. Grain flling takes place between 362 to 400 GDD and range of the lag phase from 67 to 86 GDD. There is found a positive correlation of lag phase with rates of grain flling and grain weight while a signifcant negative correlation of days to anthesis with the lag phase and rates of grain flling. There’s a negative correlation between temperature during grain flling and the lag phase. A positive correlation associated between variation in grain weight and the rate of grain flling which consecutively was related to the grain number per unit area. Wang et al. 2018 in their study studied different verities of wheat for the effects of heat stress on grain flling at different levels of thermotolerance.They demonstrated a widely distributed mechanism for heat adaptation of metabolism where the assimilation and energy needed for metabolism and protein synthesis are redirected to heat protection and accumulation of reserves as a result storage protein accumulation is increased and a stable flling rate. Rampinoa et al. 2009 studied the durum wheat Triticum durum Desf. cultivars for the attainment of thermotolerance and HSPs gene expression 4. Agronomic traits 4.1. Grain Number and weight: Grain weight and number both are severely affected by increasing temperature Ferris et al. 1998. Each of these components of grain yield is affected by high temperature depends on the developmental phase at which the high temperature occurs Balla et al. 2012 Hampton et al. 2013. Ovule abortion due to high temperature was also reported Saini and Aspinall 1982 Thorne and Wood 1987 Tashiro and Wardlaw 1990 Mitchell et al. 1993 Wheeler et al. 1996b.Wheeler et al. 1996a showed that high temperatures at anthesis reduced grain numbers per spike in winter wheat ‘Hereward’. Saini and Aspinall 1982 showed that low grain fertility was induced by exposure to high temperature for as little as 1°C during critical periods between booting and anthesis. When abiotic stress coincides with meiosis the frst phase of gametogenesis may be further impaired Ji et al. 2 0 1 0. According to Ferris et al. 1998 maximum temperatures increase over four consecutive days near to anthesis affects grain number and that directly reduced grain yield at maturity. Around floral initiation heat stress has severe effects on grain number. For example grain number per spike decreased by 4 for every 10°C from 15–22°C increase in the 30 days after anthesis Fischer 1985. The development of spikes is increases by heat stress but reducing spikelet number and due to this number of grains per spike also reduces Porter and Gawith 1999 Saini and Aspinall 1982. There is an inverse relationship between grain number per spike and duration of heat stress Rawson and Bagga 1979. The reason for this sensitivity is that each spikelet meristem starts to produce florets and spikelets begin to form in the spike from the double ridge stage. However the reduction in the spikelet number per spike and grain number per spikelet because of the reduction in the duration of appearance to double ridge and double ridge to anthesis McMaster 1997. During floret development high temperature 30°C may cause complete sterility Saini and Aspinall 1982 variation among wheat genotypes has also been observed Anjum et al. 2008 Zhao et al. 2008 Gibson and Paulsen 1999. Heat stress severely effects the grain number around floral initiation. The one factor determining grain number for

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4 Journal of Cereal Research Yields are reduced 3–4 per 1°C rise above the optimum temperature 15–20°C during grain flling Wardlaw et al. 1989. In India most commercially sown wheat cultivars lost their yield about 50 when exposed to 32–38°C temperature at the fundamental grain formation stage can be calculated by using this factor 3–4 loss per 1°C above 15–20°C. Among wheat germplasm a signifcant variation exists for heat tolerance Reynolds et al. 1994 Joshi et al. 2007bc due to lack of understanding of dependable direct criteria until now very slight progress has been made for selecting heat tolerant genotypes. Willey and Dent 1969 suggested that grain yield is neither limited by assimilate supply nor by the storage capacity of the ear alone but by both. Grain yield in wheat could reduce if the temperature increased in variably during anthesis. According to Ferris et al. 1998 and Huang et al. 2012 further threat to crop production also occur if higher temperatures makes deleterious effects on root growth. The overall growth process of wheat is negatively affected by higher temperature thus resulting in rigorously hindered productivity Janjua et al. 2010. Temperature also effects spring wheat at period of grain formation under late sown conditions. They also observed the comparison of reduction in grain yield 53.75 and reduction in tiller number 15.38 Din et al. 2010.There are number of ways by which high temperature affects the crop some of them are poor germination less photosynthesis leaf senescence and all fnally leads to less yield Asseng et al. 2011. 5. Physiological traits 5.1. Leaf senescence Avacuolarcollapse disruption of cellular homeostasis and plasma membrane integrity lost are some structural changes in chloroplast during leaf senescence Lim et al. 2007 Khanna-Chopra 2012. Delay in the expression of senescence-related genes permits stay green genotypes to maintain photosynthesis Lim et al. 2007. Leaf senescence is enhanced if plants are exposed to heat stress during maturity Haque et al. 2014. While stay green is recognized as an adaptive mechanism for stress conditions chlorosis is an essential part of programmed senescence where there are essential tradeoffs between preserved photosynthetic area and remobilization of nitrogen to the maturing grain Vijayalakshmi et al. 2010. QTLs for the association of yield and stay green have been shown in mapping populations Kumar et al. 2010 Vijayalakshmi et al. 2010. Since chlorosis is expressed heterogeneously on all above ground organs and it is not simple to consider even within a single organ such as leaf. However effortless and integrated approaches to assess stay green via spectral reflectance. Normalized Difference V egetation Index NDVI explain substantial floret development is availability of carbohydrates Abbate et al. 1995 Demotes-Mainard and Jeuffroy 2004 because insuffcient availability of assimilates may cause floret most sensitive period to heat stress. Maintaining optimum kernel weight under heat stress is used as a measure of heat tolerance Tyagi et al. 2003 Singha et al. 2006. High temperature during grain- flling period decreases yield by decreasing kernel weight Warrington et al. 1977 T ashiro and W ardlaw 1990 Stone and Nicolas 1994. Kernel weight was decreased by 85 when the temperature rose from 20/16°C day/night to 36/31°C from 7 DAA until maturity Tashiro and Wardlaw 1989. The distribution of differences in thousand grain weight TGW in RILs showed normal curve. About 15 progenies 18 RILs had signifcant difference in TGW under late sown condition whereas 20 RILs had higher TGW under stress conditions Pandey et al. 2014. In the hard red winter wheat Karl 92 which is adapted to great plains conditions kernel number was reduced by 63 kernel weight by 29 and grain yield was reduced by 78 when a temperature regime of 35/20°C was imposed from 10 DAA until maturity Gibson and Paulsen 1999. The duration between anthesis and physiological maturity are reduced by the increasing temperature Warrington et al. 1977 which is related with reduction in grain weight Warrington et al. 1977 Shpiler and Blum 1986. For every 1°C above 15–20°C grain weight Reduces 1.5 mg per day Streck 2005. High temperature variability effects on wheat grain size and number appears to be related to genotypic differences in heat tolerance Viswanathan and Khanna-Chopra 2001 Tahir and Nakata 2005. The main stem grain weight reduces by 20 to 44 during reproductive phase by increasing temperature from 30 to 38°C Tahir and Nakata 2005. If the standard temperature increases up to one degree throughout reproductive phase in wheat can cause greater loss of yield Bennett et al. 2012 Yu et al. 2014. Shrinkage of grain through ultra structural changes in the aleuronic layer and endosperm cells can be caused by high temperature as observed by Dias et al. 2008 when day/night temperatures increased from 25/14°C to 31/20°C.Viswanathan et al. 2008 studied that when the varieties of wheat is exposed to heat the stability of grain weight differed by that rate and duration of grain growth is also affected. 4.2. Yield Integration of high temperatures i.e. accumulated thermal time above 31°C over 8 d of the treatment during anthesis has been reported to cause 50 reduction in grain yield Ferris et al. 1998. Even a brief period of exposure to high ambient temperature 35°C can drastically reduce the grain yield in wheat Randall and Moss 1990 Hawker and Jenner 1993 Stone and Nicolas 1994 Sharma et al. 201 7.

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Terminal heat tolerance in wheat: An overview 5 relationship with yield in two large mapping populations under heat stress making it a consistent tool for large-scale screening and gene discovery work Lopes and Reynolds 2012. While NDVI is correlated with the heat tolerance which is a consistent indicator of greenness combine all chlorophyll. Leaf senescence in wheat may speed up by inhibition of chlorophyll biosynthesis under high temperature 34°C Asseng et al. 2013. Photosynthesis mainly affected by heat stress effects photosynthetic machinery premature leaf senescence decreased leaf area expansion and ultimately less yield Mathur et al. 2014 Feng et al. 2014. Grain yield can be increased by foliar spray of potassium orthophosphate KH 2 PO 4 after anthesis because of the delay in the heat stress-induced leaf senescence Dias and Lidon 2010. 5.2. Spike Fertility Grain set is actually sensitive and conservative towards carbohydrate supply even underrelatively optimal conditions Fischer 2011. The observation for reduction in grain number is more than expected by reduction in biomass leading to quite low harvest index during heat stress from hot wheat-growing environments Reynolds et al. 2007. Programmed cell death at high temperature induced by ethylene level that leads to grain abortion Hays et al. 2007. Structurally and functionally abnormal anthers are produced in 80 of the florets if wheat plants are exposed to 30°C during a 3-d period around anthesis. Heat stress affects the morphology pollen tube growth rate metabolism chemical composition and viability of pollens Hedhly et al. 2009. In other cereals like sorghum the carbohydrate metabolism has been linked to reduced pollen viability under heat stress Jain et al. 2007 Prasad and Djanaguiraman 2011. Abnormal ovary development resulted in reduced pollen tube growth and seed set when heat stress coincided with meiosis Barnabás et al. 2008. 5.3. Starch biosynthesis Signifcant decrease in quantity of starch in wheat grains can be resulted by heat-shock treatment above 30°C Liu et al. 2011. Due to the small amount of soluble starch synthase at 40°C about 97 of activity was lost and in result of this grain size and accumulation of starch is reduced in wheat Chauhan et al. 2011. Soluble sugar content and biomass is reduced by increased temperature stress 35/27°C given at seedling stage W ang et al. 2014. Starch biosynthesis is signifcantly restricts by heat stress in wheat grains although caused a signifcant increase in total soluble sugar and protein Asthir and Bhatia 2014. 5.4. Canopy temperature Cooler canopy temperature CT is strongly associated with yield in drought and heat stress and also appear to have some common genetic basis under both the conditions Pinto et al. 2010. Recent data show canopy temperature to be associated with deeper roots under drought and heat stress Lopes and Reynolds 2010. While the genetic variation in stomatal conductance under heat is also linked with the cooler canopy selection for canopy temperature can be helpful for the improvement of heat tolerance Reynolds et al. 1994 2007. 5.5. Chlorophyll fuorescence and Chlorophyll content Chlorophyll fluorescence CFL is one of the traits used extensively in feld phenotype after CT and chlorophyll content Chl. The roles of chlorophyll fluorescence CFL in relation to grain yield under water-stress conditions were suggested for selecting heat and drought tolerant genotypes of wheat. CFL is directly proportional to the yield i.e genotypes with higher yield have higher CFL representing that in screening for tolerant genotypes CFL can be used. Photosynthetic efficiency can be calculated indirectly by CFL in wheat genotypes mainly in terms of photosystem II PSII function. A study on RILs mapping populations derived from cross of heat sensitive genotype RAJ 4014 and heat tolerant genotype WH 730 showed that out of 112 RILs 17 RILs had their heat susceptibility index HSI less than 1 for all three traits viz. CFL Chl and TGW. By virtue of low HSI 0.43 for thousand grain weight group of genotypes showing tolerance to terminal heat stress were also found having values less than zero -0.52 of Chl HSI indicating substantial increase in the Chl value under stress Pandey et al. 2015. However contrasting HSI values of 2.07 and 2.19 were observed for TGW and Chl in another set of RILs depicting heat sensitivity Pandey et al. 2015. Sharma et al. 2012 screened 1274 contrasting wheat cultivars for heat tolerance of diverse origin based on the maximum quantum effciency of PSII Fv/Fm and physiological trait. The role of CFL and Chl in relation to grain yield under water-stress conditions was suggested for the selection of heat and drought tolerant wheat plants Blum 1988 1989 Krause and Weis1991. Wang et al. 2008 reported that the maintenance of the higher Chl in wheat plants is an effective strategy to increase its yield and biomass. Chl around flowering time were positively associated with yield in near-isogenic lines NILs of wheat Quarrie et al. 2005. 6. Molecular mechanism Signifcant contribution of the molecular science for the understanding of the mecanism of stress response under heat stress conditions spread light on the information of protective mechanisms and molecular pathways to raise the plants that are tolerant to heat stress Asthir 2015ab. At different growth stages the genotypic

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6 Journal of Cereal Research differences in cell membrane thermo stability tolerance were also reported in wheat Kumar et al. 2013b Asthir et al. 2013. Identifcation of three known and one novel genome rearrangement of wheat is confrmed by Clavijo et al. 2016. Heat tolerance is quantitative and irregular in nature so effcient selection of the traits is very much diffcult for breeders. The normal distribution was typical of traits controlled by multiple genes Prasad et al. 1999 and supported the hypothesis of Blum 1988 that heat tolerance is quantitatively inherited. In several studies of wheat QTL for yield and its components such as thousand grain weight TGW and grain numbers per ear GNE have been previously reported Huang et al. 2006 Kumar et al. 2006. T o record agronomically essential characters and yield enhancing traits in wheat QTL analyses have been conducted Campbell et al. 2003 Kato et al. 2000. TGW is used to estimate yield of wheat genotypes because it is also one of the yield contributing traits and can be measured easily. Related to many studies of the yield components of wheat recognition of QTL associated with TGW on different wheat chromosomes Borneret al. 2002 Huang et al. 2006 Wang et al. 2009. Signifcant association of differences in TGW between optimum and late sown of RILs with two markers viz. Xpsp3094 and Xgwm282 with coeffcients of determination R 2 values of 0.14 and 0.11 respectively analysed by regression analysis Pandey et al. 2014. In the earlier studies this had been reported that the markers e.g. Xpsp3094 were found to be associated with dTGW in single marker analysis Quarrie et al. 2005. On the other hand by using Xgwm282 marker reported QTL for single kernel weight Mason et al. 2010. In wheat there is a major role of grain flling rate GFR in the fnal yield Beiquan and Kronstad 1994 and is positively associated with fnal grain weight. Three signifcant genomic regions on 2B 7B and 7D were found to be associated with heat tolerance. Using single-marker analysis Yang et al. 2002 found QTLs associated with markers Xgwm11 1B and Xgwm293 5A for heat tolerance in an F 2 population 166 plants that contributed 23 of phenotypic variation. According to Campbell et al. 2003 a marker linked to a QTL for grain yield on wheat chromosome 3A was found to interact signifcantly with the temperature from emergence to anthesis. Mason et al. 2010 reported fve stable QTL for HSI of single grain weight 1A and 2A grain weight 3B and grain number 2B and 3B contributing 11.1–22.6 of phenotypic variation. However their phenotypic assessment was performed on 65 lines of F 5 generation material in controlled conditions for a short period of heat stress 3 days. Another study was carried out by Mason et al. 2010 for detailed characterization of the effect of heat shock on plant yield and yield components during early grain- flling and to identify molecular markers linked to QTL for heat susceptibility index HSI and other potentially benefcial phenotypic traits. HSI was calculated based on difference in yield and yield components between the heat stressed and control treated RIL replications and used as a phenotypic measure of heat tolerance for QTL mapping. Paliwal et al. 2012 reported 3 QTLs from using RILs populations derived from Indian parental lines under terminal heat stress. Groos et al. 2003 found that QTL affecting yield was located on chromosome 7D and QTL affecting kernel weight were located on chromosomes 2B 5B and 7A. Borner et al. 2002 reported 210 QTL controlling 20 morphological and physiological traits. In an analysis of spring wheat populations for heat tolerance loci on chromosomes 2B and 5B were most important Byrne et al. 2002. Bhusal et al. 201 7 mapped maximum number of QTLs on chromosome 2A followed by 6D and 2B for grain yield components in wheat under heat stress. In another study HSP 18 got accumulated in developing grains of heat tolerant varieties more than susceptible types where several wheat varieties were exposed to 3.2 to 3.6 0 C higher than normalSharma-Natu et al. 2010. Dehydrins help to stabilize macromolecules against heat-induced damage and belong to Group 2 late embryogenesis abundant proteins LEA Brini et al. 2010. In wheat the key enzymes to start metabolism is protect and stabilized by DHN-5 Brini et al. 2010. The RuBisCo enzyme is deactivated by heat stress and the deactivity of RuBisCo has been observed within 7 days of imposed heat shock treatment Kumar et al. 2016. Staygreen has also been studied extensively in different crops. Major components for identifying terminal heat stress resistant genotypes of wheat are canopy temperature depression and stay green Jangid and Srivastava 2018. Xu et al. 2000 found that regions that contained the QTL for stay-green coincided with the genes for key photosynthetic enzymes heat shock proteins and ascorbic acid response. For heat tolerance in wheat during the reproductive phase several QTLs have been identifed. Three QTLs are identifed for stay-green character Kumar et al. 2010. A QTL on chromosome 4A was also identifed for canopy temperature under heat stress Pinto et al. 2010. On the other hand 15 and 12 QTLs were also identifed associated with yield and yield associated traits Mason et al. 2010. The results suggest that the main spike should be used for the identifcation of QTLs genomic regions for heat tolerance Mason et al. 2011. The QTL influencing stay-green under high temperature stress in wheat have been mapped to date. Christopher et al.2018 identifed QTL for stay-green traits in well-watered and water-limited environments.by using doubled haploid population. For stay-green traits

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Terminal heat tolerance in wheat: An overview 7 the associated genetic regions with QTL were identifed in both the environments on chromosomes 4A 4B 4D constitutive stay-green 2A in wetter environments and 5B dryer environments. On the other hand 3B and 7B are the regions on which the QTL for stay-green were identifed in a mixture of environment. Heat tolerance contributed traits molecular markers can be used in marker assisted selection MAS. T raits that could be selected more effciently include photosystem PS II Fv/Fm effciency Blum and Ebercon 1981 Chlorophyll content CHL Li et al. 2006 and canopy temperature are the ideal physiological selection trait in many ways since measurement is quick simple and inexpensive Cossani and Reynolds 2012. In crops like tomato and maize for the better understanding of the genetic basis of plant stress tolerance the very much contributed approach is Marker-assisted selection MAS Liu et al. 2006 Momcilovic and Ristic 2007 and in some cases has led to the development of improved abiotic stress tolerance plants Lopes and Reynolds 2010 Thomson et al. 2010. Because of the diffculty in phenotypic selection and general complexity of abiotic stress tolerance MAS are considered as an effective approach. Therefore an urgent need exists to understand the genetic factors underlying heat tolerance and to recognize consistent markers to be deployed in molecular breeding leading to enhancement in grain yield of wheat. 7. Screening and breeding for heat tolerance Various physiological approaches have been found to be effective in breeding programs. This includes screening genetic resources for identifcation of genetic bases for heat tolerance in crops. Thereby a preferred new plant types can be established following physiological crossing of new trait combinations to combat future climate that comprises high temperature events Reynolds and Langridge 2016. It is very well understood that screening wheat genotypes under natural heat stress condition in several environments is problematic. Therefore no dependable selection criterion has been established to evaluate diverse genetic materials for heat tolerance. In this concern researchers recommended some indirect selection criteria for developing heat tolerance in wheat Table 1. 8. Strategies to improve thermo-tolerance in wheat High temperature severely affects the plant growth by affecting the quality of grain and wheat productivity. One of the key step to increase the yield in wheat is to develop the thermotolerance verities therefore at present a number of techniques are being used to increase the thermotolerance in wheat. In addition to this researchers also reported superior propagation of different plants on large scale using tissue culture technique Ali et al. 2013 Hussain et al. 2016a Hussain et al. 2016b Iqbal et al. 2016a Iqbal et al. 2016b. For enhancement in germination potential of wheat there are number of growth enhancers that are exogenously applied Iqbal et al. 2016c. On the other hand transcriptomic and proteomic data play a key role for the identifcation of genes and proteins that affects quality and yield. So we need to know more about this process for the development of new wheat variety that can adapt to increasing temperature Altenbach 2012. For the improvement of heat tolerance the inoculation of seed with rhizobacteria also signifcantly used Anderson and Habiger 2012. On the other hand effects of heat stress can be managed chiefly by developing genotype collectively with modification of relevant agronomic properties Asseng et al. 2011 Chapman et al. 2012. Similarly there are several growth-promoting protectactants that are applied exogenously few of them are osmoprotectants phytohormones signaling molecules and trace elements having potential to protect the plants by unfavorable effects of heatand up regulating the antioxidant capacity Upreti and Sharma 2016 Hemantaranjan et al. 2014. Some other adaptation procedures under heat stress are surface cooling by irrigation antioxidants defense osmoprotectants choice of cultivars and change in date of sowing Lobell et al. 2008 Suzuki et al. 2011 Caverzan et al. 2016 Kaushal et al. 2016 Deryng et al. 2014. 9. Concluding remarks Table 1 Selection criteria of wheat genetic resources for tolerance to heat stress Sr. No. Selection criteria for heat stress tolerance in wheat References 1. Growth and phenology a. Rapid ground coverage b. Leaf rolling shedding and thickening c. Biomass yield d. Early heading and phe- nology Cossani and Reynolds 2012 Khan and Kabir 2014 Nawaz et al. 2013 Khan and Kabir 2014 Hussain et al. 2016 2. Physiological traits a. Photosynthesis and sto- matal conductance b. Stay green duration c. Membrane stability d. Leaf chlorophyll content and chlorophyll flouresence e. Stem reserves Radhika and Thind 2014 Lopes and Reynolds 2012 Nawaz et al. 2013 Sikder andPaul 2010 Dhanda and Munjal 2012 Talukder et al. 2014 Trethowan and Mujeeb-Kazi 2008 Bhusal et al. 2018 Pan- dey et al. 2015 Mohammadi et al. 2009 3. Yield and yield attributes a. Grain weight b. Grain flling rate and duration c. Number of fertile spikes Sareen et al. 2012 Bennani et al. 2016 Pandey et al. 2014 Nawaz et al. 2013 Khan and Kabir 2014 Song et al. 2015 Pandey et al. 2013 Khan andKabir 2014 Bennani et al. 2016

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8 Journal of Cereal Research Frequency of heat stress in wheat is increasing across the globe due to high temperature. The substantial effects of heat stress on grain setting duration and rate ultimately leads to reduction in grain yield. However the timing duration and intensity of heat stress determine its impact on grain yield. By developing tolerant genotypes and agronomic strategies the effect of heat stress can be minimized. Although in wheat physiological basis of heat tolerance are quite well-understood research into phenotypic flexibility and assimilate partitioning are required. Molecular basis of response and tolerance mechanisms to harvest higher grain yields on a sustainable basis must be explored by the use of functional genomics approaches. The important area for future research are conventional breeding blended with modern biotechnological tools for identifcation and introgression of heat tolerance gene into elite lines. For the understanding of genotypes and the environment interactions the most recent genomics resource combined with ecophysiological research may be useful. T o study the complex quantitative traits like yield stability under heat stress an integrated system approach should be designed. Acknowledgement We acklowledge Grants-in-Aid project No. 1008018 of Indian Council of Agricultural Research New Delhi. Authors are thankful to Prof. Aditya Shastri Vice - Chancellor Banasthali Vidyapith and Director ICAR- Indian Institue of Wheat and Barley Research Karnal for providing opportunity to complete our manuscript writing. References 1. Abbate PE FH Andrade and JP Culot. 1995. The effect of radiation and nitrogen on number of grains in wheat. Journal of Agricultural Science 124:351–360. 2. Akpinar BA S Lucas and H Budak. 2017.A large- scale chromosome specifc SNP discovery guideline. Functional Integrated Genomics 171:97–105. 3. Alexander LV X Zhang TC Peterson J CaesarB Gleason A Tank M Haylock D Collins BTrewin F Rahimzadeh A Tagipour KR Kumar JRevadekar G Griffths L Vincent DB Stephenson J Burn E Aguilar M Brunet M Taylor M New P Zhai M Rusticucci and JL Vazquez-Aguirre. 2006.Global observed changes in daily climate extremes of temperature and precipitation. Journal of Geophysics Research Atmosphere 111:1–22. 4. Al-Khatib K and GM Paulsen. 1990. Photosynthesis and productivity during high temperature stress of wheat genotypes from major world regions. Crop Science 30:1127–132. 5. Almeselmani M F Abdullah F Hareri M Naaesan MA Ammar and OZ Kanbar. 2011. Effect of drought on different physiological characters and yield component in different Syrian durum wheat varieties. Journal of Agricultural Science 3:127-33. 6. Almeselmani M PS Deshmukh and V Chinnusamy. 2012. Effect of prolonged high temperature stress on respiration photosynthesis and gene expression in wheat Triticum aestivum L. varieties differing in their thermotolerance. Plant Stress 6:25-32. 7. Almeselmani M PS Deshmukh and RK Sairam. 2009. High temperature stress tolerance in wheat genotypes: Role of antioxidant defence enzymes. Acta Agronomica Hungarica 57:1-14. 8. Altenbach SB. 2012. New insights into the effects of high temperature drought and post-anthesis fertilizer on wheat grain development. Journal of Cereal Science 56:39–50. 9. Anderson M and J Habiger. 2012. Characterization and identification of productivity-associated Rhizobacteria in wheat. Applied and Environmental Microbiology 78:4434-4446. 10. Anjum F A Wahid F Javed and M Arshad. 2008. Influence of foliar applied thiourea on flag leaf gas exchange and yield parameters of bread wheat T riticum aestivum L. cultivars under salinity and heat stresses. International Journal of Agriculture and Biology 10: 619-626. 11. Ashraf M and PJC Harris. 2013. Photosynthesis under stressful environments: an overview. Photosynthetica 51:163-190. 12. Asseng S I Foster and NC Turner. 2011. The impact of temperature variability on wheat yields. Global Change Biology 17:997-1012. 13. Asseng S R Royce and D Cammarano. 2013. T emperature routines in wheat workshop modeling wheat response to high temperature. Proceedings Vol. VIII p. 128. CIMMYT Mexico DF Mexico. Jun 19–21 14. Asthir B. 2015a. Mechanisms of heat tolerance in crop plants. Biologia Plantarum 59:620-628. 15. Asthir B. 2015b. Protective mechanisms of heat tolerance in crop plants. Journal of Plant Interactions 10:202-210. 16. Asthir B S Bala and NS Bains. 2013. Metabolic profiling of grain carbon and nitrogen in wheat as influenced by high temperature. Cereal Research Communications 41:230-242.

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