Increase in global warming poses a severe threat on agricultural production thereby affecting food security. A drastic reduction in yield at elevated temperature is a resultant of several agro-morphological, physiological and biochemical modifications in plants. Heat tolerance is a complex mechanism under polygenic inheritance. Development of tolerant genotypes suited to heat extremes will be more advantageous to tropical and sub tropical regimes. A clear understanding on heat tolerance mechanism is needed for bringing trait based improvement in a crop species. Heat tolerance is often correlated with undesirable traits which limits the economic yield. In addition, high environmental interactions coupled with poor phenotyping techniques limit the progress of breeding programme. Recent advances in molecular technique led to precise introgression of thermo-tolerant genes into elite genetic background which has been reviewed briefly in this chapter.
- global warming
- high temperature
- polygenic inheritance
- breeding approaches
- thermo-tolerant genes
Increase in global temperature had major impact on crop productivity especially in tropical and sub tropical regimes. Based on climate model predictions, around 1.8–4.0°C rise in air temperature was expected in 21st century . The increase in temperature beyond a certain threshold level tends to induce detrimental effects in plant growth and development. In general, the elevation in temperature of 10–15°C above ambient triggers heat shock in crop plants. The extent of induced heat stress depends on the duration, intensity and rate of increase in global air temperature . Indian lowlands share 15 per cent of global wheat production. The change in global climate would shift these fertile lowlands into heat stressed unproductive environment . Similarly, the cultivation of cereals in Southern Africa and South East Asia was predicted to be heat stressed zone in near future . Around 4–14% yield decline in rice was encountered due to elevated temperature of 1°C in South-East Asia . The declined productivity due to elevated temperature imposes the urgent need for development of climate resilience genotypes. Evolving heat tolerant cultivars would highly benefit the livelihood of developing countries as around 70–80% of population relies on agriculture. Understanding the effect of heat stress on crop plants and its adaptation mechanisms would help in framing out the breeding strategies for high temperature tolerance.
Heat tolerance in crop plants is a complex mechanism involving adaptations through altered physiological process, morpho-anatomical features and induction of several biochemical pathways. On exposure to high temperature, several signal transduction pathways were triggered leading to changes in gene expression. As a result, varied stress related proteins were synthesized contributing heat tolerance in plants . The tolerance mechanism to high temperature stress varies within genotypes of a plant species. The existing variation between and within species provide scope for evolving heat tolerant lines through conventional breeding approaches . Dissecting out genetic information through molecular tools would hasten the development of climate resilient cultivars contributing to food security in near future. A brief review on plant response, adaptation mechanisms and genetic approaches to combat heat stress were presented in this chapter.
2. Effect of heat stress on crop plants
Heat stress had varying impact on different phenological stages
3. Adaptation mechanisms
Plants tend to adapt several complex mechanisms through phenological and morphological changes to combat high temperature stress (Figure 1). On heat stress regimes, plants exhibit varied short term escape/avoidance mechanisms
4. Thermo-tolerance through breeding strategies
4.1 Screening criteria
Breeding for high temperature tolerance requires an essential knowledge on plant adaptation response to heat shocks. In general, the genotypes exhibiting less detrimental effect on photosynthesis and reproductive development tend to survive well under heat prone areas . Involvement of these two components in selection criteria would be beneficial in evolving thermo tolerant cultivars. Tolerant genotypes evolve several morphological, physiological and biochemical alterations in response to heat shocks. Knowledge on sensitivity of several phenological stages to high temperature will pave way for trait specific improvement. High temperature is often correlated with other environmental factors which poses a major limitation for selection under field conditions. At present, varied selection criteria has been developed by scientists, which favors delineation of superior variety at prevailing environment . Heat tolerant index has been evolved for sorghum which depicts the proportion of growth recovery after exposure to high temperature stress. It is the ratio of increase in coleoptile growth in a heat stress environment [50°C] to the enhancement in coleoptile length under normal environment (non-stress) . It proves cost effective and rapid method to screen a large population size within shorter period. A proper validation of such technique would facilitate the development of tolerant lines in other crop species. Pollen viability and fruit set was considered as major selection criteria to predict yield under high temperature stress in tomato . Physiological based trait selection such as harvest index, photosynthetic efficiency, respiration rate, delayed senescence and canopy architecture will also contribute towards increased tolerance to heat stress [31, 32].
4.2 Genetic resources for thermo tolerance
Inter-mating among closely related individuals for improvement of economic traits resulted in decline of genetic variability in a crop species . Characterization of gene pool including land races and wild relatives would offer several tolerant genes for abiotic tolerance. Extensive efforts were made in screening of heat tolerant genotypes which can be directly introduced as a cultivar or utilized to introgress gene into new genetic background . Thermo-tolerant lines were successfully isolated from wild gene pool in wheat . High magnitude of variation was observed in wild progenitor “
4.3 Conventional breeding approaches
Evolving thermo-tolerance through conventional breeding approach proves promising in many crop species. Breeding for early maturing genotype in broccoli had improved head quality by avoiding heat stress at flowering stage . In general, breeding programmes are carried out in hotter regions which promote selection of thermo-tolerant traits. Physiological based trait breeding was practiced at International Maize and Wheat Improvement Center (CIMMYT) for development of heat tolerant cultivars in wheat. The parental genotypes were characterized through various crossing schemes and appropriate breeding programme was framed for improvement of thermo related traits . A wild ancestor “
4.4 Advanced breeding approaches for thermo tolerance
The genetic basis of thermo-tolerance is not clearly understood because of complex trait inheritance. Advances in molecular approaches such as DNA marker identification and genotyping assay had paved way in determination of several QTL’s associated with high temperature tolerance . In wheat, QTL’s were identified for canopy temperature, and chlorophyll fluorescence imparting tolerance to heat stress . A major QTL “Htg 6.1” in lettuce was involved in enhancement of seed germination capacity at high temperature . A recessive QTL for increased spikelet fertility under high temperature was dissected out in rice at chromosome 4. The identified QTL were found in several populations of heat tolerant rice cultivars . Six QTL’s were involved to enhance fruit set at high temperature in tomato . Five thermo tolerant QTL’s were identified in
The closely associated markers with targeted QTL will hasten the recovery of superior genotypes with heat tolerant traits in a population. A marker assisted breeding approach was employed in rice to derive heat tolerant line with superior grain quality. Two flanking markers
At present, transgenic approach also proves to be desirable tool for designing thermo tolerant lines
Development of thermo-tolerant lines has to be prioritized to meet out the future climatic change coupled with food demands. Knowledge on plant response and adaptation mechanisms to heat stress is required for framing out breeding strategies. It remains a challenging task in evolving resilient genotypes suited to high temperature because of less efficient screening protocols at field conditions. The existence of low genetic variation for heat response related traits limited the progress of conventional breeding approach in many crop species. Use of molecular breeding strategies had opened up several heat tolerant related QTL’s in crop species. However, still precise research work involving huge marker data is needed for attaining high breeding efficiency for thermo tolerance. Recently, the involvement of transgenic approach paved way for utilization of tolerant source from diverse gene pools. Study on induction of heat shock proteins led to increased thermo tolerance in many crop species. Similarly, other heat response related traits such as induction of antioxidant components, osmolytes, and chaperones were also included in transgenic approach for inducing heat stress tolerance. Thus, high economic yield could be realized at elevated temperature regimes with the involvement of combined breeding approaches.
The authors are highly thankful to Dr. V. Geethalakshmi, Director, Directorate of Crop Management, Tamil Nadu Agricultural University (TNAU) for her valuable suggestions towards this chapter. We also acknowledge Dr. P. Jayamani, Professor and Head, Department of Pulses, TNAU; Dr. M. Raveendran, Professor and Head, Department of Biotechnology, TNAU; and Dr. K. Ganesamurthy, Professor and Head, Department of Rice, TNAU for rendering supportive documents on high temperature tolerance.
Conflict of interest
The authors declare no conflict of interest towards this chapter.
The authors express their gratitude to the Directorate of Crop Management for providing scientific support on high temperature tolerance.