Adapting to Biotic and Abiotic Stresses Through Crop Breeding

One of the most challenging aspects of adapting crops to climate change will be to maintain their genetic resistance to pests and diseases, including weeds, herbivorous insects, arthropods, nematodes, fungi, bacteria and viruses. Rising temperatures and variations in humidity affect the diversity and responsiveness of agricultural pests and diseases and are likely to lead to new and perhaps unpredictable epidemiologies (Gregory et al., 2009). Legreve and Duveiller in Chapter 4 explain that, for a disease to occur, three essential components are required simultaneously: a virulent pathogen, a susceptible host and a favourable environment - often referred to as the 'disease triangle'. Climate change, as well as sometimes fulfilling the last link of that triangle, can also drive evolutionary change in pathogen populations by forcing changes in reproductive behaviour. Changes in cropping systems can lead to the development of new pathogens, for example through interspecific hybridization between introduced and endemic pathogens, and history has shown how devastating such events can be to food security. Legreve and Duveiller point out that strategies to limit the effect of climate change on pests and diseases do not fundamentally differ from existing integrated pest management practices, although there will need to be a much greater emphasis on modelling and forecasting systems, while breeding for host resistance will continue to have a pivotal role. They cite the rapid response of the scientific community to the dispersal of the Ug99 wheat stem rust race as an example of how internationally coordinated monitoring and breeding efforts can mitigate the threat of potential epidemics (Singh et al, 2008).

The major abiotic stresses that are expected to increase in response to climate change are heat, drought, salinity, waterlogging and inundation. The former are addressed by Reynolds et al. in Chapter 5. The responses of crops to these two abiotic stresses have a number of similarities, although the genetic basis is not necessarily the same. Growth rate is accelerated due to increased plant temperature, which reduces the window of opportunity for photosynthesis since the life cycle is truncated, while both heat and drought stress may also inhibit growth directly at the metabolic level. Furthermore, harvest index may be reduced if reproductive processes are impaired by stress that occurs at critical developmental stages. Genetic improvement under these environments has been achieved by incorporating stress-adaptive traits into good agronomic backgrounds (Richards, 2006). As understanding of the physiological and genetic basis of adaptation is improved, this approach can be expanded in conjunction with molecular approaches to tackle even some of the most challenging aspects of climate change, such as adaptation to higher temperatures without loss of water-use efficiency, and tolerance to sudden extreme climatic events or combinations of stress factors. Given the complexity of the target environments themselves, as well as the constant fluxes in weather and other factors such as biotic stresses, plant selection will for the foreseeable future require empirical approaches such as multi-location testing. A number of crop-specific examples of successful breeding approaches are discussed as well as the potential of biotechnology to improve the efficiency of breeding through marker assisted selection (MAS), and the use of genetic resources to broaden the genetic base of crop species.

In Chapter 6, Mullan and Barrett-Lennard explain that climate change is expected to reduce water availability in general making the use of low-quality water resources more common. Water-stressed hydrological basins already affect approximately 1.5-2.0 billion people (Bates et al., 2008), a figure expected to increase substantially leading to problems of soil salinity and sodicity. Climate change will also bring inundation in low-lying landscapes associated with increased runoff from tropical storms while sea level rise will increase levels of salinity, waterlogging and inundation in coastal regions. The authors go on to explain that soil salinity affects plant growth and survival because ions (mainly Na+ and Cl-) increase in the soil solution, causing osmotic stress, while their accumulation in plant tissue impairs metabolism. Waterlogging leads to the displacement of air from the soil pores, leading to hypoxia (O2 deficiency, which is especially detrimental to root growth and eventually impairs all aspects of plant growth). A range of adaptive traits is discussed; however, large areas of land subject to salinity and waterlogging are still to benefit from plant breeding. Climate change is likely to increase these areas, making it imperative to address the genetic challenges of productivity in such environments.

It is important to remember that waterlogging and salinity, which already constrain productivity on hundreds of millions of hectares worldwide, also have potential engineering solutions (Bhutta and Smedema, 2007). Although beyond the scope of this book, given the scale of the problem and the challenges ahead associated with population growth and climate change, engineering interventions will require major investment; failure to do so will lead to desertification and an overall net reduction in potential global productivity.

Development and dissemination of new germplasm can be a slow process without public sector investment that provides new genotypes to seed companies. The most comprehensive germplasm development and deployment exercise ever undertaken was that associated with the Green Revolution rice and wheat cultivars, and its legacy includes some of the largest and most effective breeding programmes in the world for the major cereal crops. Chapter 7 by Braun et al. describes how these global breeding programmes function - using examples drawn from maize, rice and wheat - and their unique remit to provide useful new cultivars for a range of environments that already encompasses many of the stress factors that climate change will make more widespread in years to come. The authors explain the benefit of genetic resources as a global public good, implemented through an extensive system of international nursery trials with a breeding hub, free sharing of germplasm, collaboration in information collection, the development of human resources, and an international collaborative network. Broad-based, widely adapted, stress-tolerant cultivars, coupled with sustainable crop and natural resource management, will provide means for farmers to cope with climate change and benefit consumers worldwide. Chapter 7 also provides an overview on climate change impacts on the three main cereals that feed the world as well as ongoing breeding research to adapt the crop to the expected warm and drought-prone environments where they will grow. The authors end their chapter by discussing the future of crop mega-environments (MEs) as a breeder's tool. MEs are broad, often non-contiguous or transcontinental areas with similar biotic or abiotic stresses, cropping systems, consumer preferences and volumes of production. Braun et al. conclude that under new climate change scenarios the ME can be refined geographically to address evolving needs of various production systems.

Because agriculture is a potential contributor to climate change, it is pertinent to consider mitigation strategies as well as those of adaptation. This is addressed in the context of crop management in the next section of the book, while Parry and Hawkesford discuss breeding strategies in Chapter 8. Genetic manipulation to enhance the specificity of Rubisco for CO2 relative to O2 and to increase the catalytic rate of Rubisco in crop plants would increase yield potential, thereby increasing input-use efficiency of cropping systems as a whole, because efficiencies of scale can be expected in terms of use of nitrogen, diesel fuel, etc. Similarly, introducing C4 photosynthesis into C3 crops can be expected to increase yield potential at warmer temperatures and moderate levels of water deficit, though this is recognized to be a long-term research undertaking due to the need for introducing multiple structural and metabolic traits into C3 plants. Selecting for genetic mechanisms that improve N-use efficiency can also mitigate climate change by reducing greenhouse gas (GHG) emissions. Transgenic approaches that allow plant roots to release inhibitory compounds to suppress nitrification in the rhizosphere could substantially decrease the emission of nitrous oxide (N2O), one of the most potent GHGs.

Sustainable and Resource-conserving Technologies for Adaptation to and Mitigation of Climate Change

Sustainable and resource-conserving crop management technologies offer several major benefits under climate change. These include:

1. Practices such as reduced tillage in combination with crop residue retention can buffer crops against severe climatic events, for example, by increasing water harvest and thereby offsetting water shortages that will intensify as global temperatures rise.

2. In addition, by improving the overall environment for root growth, such practices permit the genetic potential of improved cultivars to be more optimally expressed helping to close yield gaps that may already exist.

3. Diversification of cropping systems helps to control soilborne diseases.

Longer-term benefits include:

4. Reduced emission of GHGs through greater precision in the application of N and water as well as reduced use of diesel fuel.

5. More robust soils, which are less prone to becoming degraded even as climate change increases the need for more intensive cultivation in still productive regions.

Ortiz-Monasterio et al. focus Chapter 9 on the management options that could mitigate methane (CH4) or N2O emissions from the intensive cropping systems where they are grown. The chapter describes the main elements of each of the cropping systems that affect the environment and what alternatives are available for reducing their impact on climate change, for example mid-season drainage in rice paddy fields, or best practices to manage N use in maize and wheat fields. The authors also explain how conservation agriculture (CA) and other sustainable farming practices can reduce GHG emissions and their potential for sequestering C. For example, one of the best options for mitigating GHG emissions from rice fields includes management that leads to greater oxidative soils, allows organic decomposition under more aerobic conditions, and uses zero tillage, which seems to be very practical due to cost and labour savings. N rates, timing, source and placement in maize and wheat cropping systems could also assist in mitigating N2O emissions. In this regard, spectral sensor-based N management can be used to establish the optimum N fertilization rates, thereby minimizing the risk of over fertilizing.

Hobbs and Govaerts in Chapter 10 point out that while resource conserving technologies help mitigate climate change by reducing GHG emissions, agronomic practices must also protect against extreme weather events such as drought, flooding, etc., and prevent further soil degradation. They provide evidence that adoption of practices such as CA can achieve both objectives through reducing the surface tillage to a minimum while introducing residue retention and crop rotations into the system. Their combined effect is to protect the soil from water and wind erosion, reduce water runoff and evaporation, increase infiltration of water thereby reducing inundation and salinity build up, and, in combination with appropriate crop rotation, enhance the physical, chemical and biological properties of the soil (Hobbs et al., 2008). Additional benefits include increased N-use efficiency and less use of fossil fuel - associated with tillage operations - and therefore reduced GHG emissions. Under CA, species diversity in the soil is increased creating more possibilities for integrated pest control. The presence of increased biological activity also improves nutrient cycling, water infiltration and soil physical properties (Verhulst et al., 2010).

As already mentioned, climate change will influence the spectrum of diseases that normally affect a crop species while increasing selection pressure on pre-existing threats. In Chapter 11, Mark Mazzola points out that, compared with diseases affecting aerial plant parts, soilborne diseases are more difficult to detect and to control. That given, it is extremely challenging to select for genetic resistance, making crop management strategies an essential component of the control of soilborne diseases. The most effective control method has been soil fumigation (mostly with methyl bromide), which has highly detrimental environmental consequences. Alternatives such as host resistance or application of microbiological control agents are generally effective towards a more limited and targeted pathogen population but operate on sound ecological principals (Weller et al., 2002). Naturally disease-suppressive soils also exist associated with the presence of resident microorganisms (Cook and Baker, 1983), and such soils can even be used to 'seed' other soils to increase their capacity for suppression. In addition, approaches such as introducing organic residues including green manures, as well as growing alternate crops in rotations can increase a soil's ability to suppress pathogens. In this context, practices associated with CA, including crop rotation and residue retention, offer some strategies that can positively influence disease-suppressive soil characteristics. Likely pressures on disease evolution associated with climate change as well as intensification of cropping systems, in conjunction with restrictions on the use of chemical control methods, make it opportune to further develop this field as a viable strategy to control soilborne diseases that are likely to escalate as agricultural systems are intensified to match growing demand.

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