Effect of Maize Production in a Changing Climate: Its Impacts, Adaptation, and Mitigation Strategies through Breeding

Over the next 50 years agriculture must provide for an additional 3.5 billion people Borlaug [1]. Production of the three major cereal crops alone (maize, wheat and rice) will need to increase by 70 % by 2050 in order to feed the world’s growing rural and urban populations. However, climate change scenarios show that agricultural production will largely be negatively affected and will impede the ability of many regions to achieve the necessary gains for future food security Lobell et al. [2]. Climate change refers to the increase of earth’s temperature due to the release of gases such as CO, CH, CFCs, NO and O into the earth’s atmosphere IPCC [3]. Climate variability has been and continues to be, the principal source of fluctuations in global food production in countries of the developing world and is of serious concern. The mean annual rainfall is considerably low in most parts of the world and temporal variability is quite high. Climate change impacts on agricultural crop production vary from place to place and from crop to crop. Climatic factors such as temperature, precipitation, moisture and pressure affect the development of plants, either alone or by interacting with other factors. This implies that rural sustenance and food security is under threat along with socioeconomic stability Burke et al. [4], and ecological integrity Walker and Schulze [5]. These risks are particularly high for the less resilient impoverished countries. Considerable research work has been carried out on the effects of weather/climate on agricultural production, but few works have been specific on the effects of climate change on maize production. Maize is one of the most important staple food crops in the world after wheat and rice and belongs to the family Poaceae. Maize occupies an important position among the crops, both as food and feed as well as raw material in industrial production of starch, oil, protein, alcoholic beverages, biofuel, food sweeteners, pharmaceuticals, cosmetics, films, textiles, gums, and also in packaging and paper industries, etc. It is the most versatile photoinsensitive crop with high adaptability which is why maize Abstract


Introduction
Over the next 50 years agriculture must provide for an additional 3.5 billion people Borlaug [1]. Production of the three major cereal crops alone (maize, wheat and rice) will need to increase by 70 % by 2050 in order to feed the world's growing rural and urban populations. However, climate change scenarios show that agricultural production will largely be negatively affected and will impede the ability of many regions to achieve the necessary gains for future food security Lobell et al. [2]. Climate change refers to the increase of earth's temperature due to the release of gases such as CO, CH, CFCs, NO and O into the earth's atmosphere IPCC [3]. Climate variability has been and continues to be, the principal source of fluctuations in global food production in countries of the developing world and is of serious concern. The mean annual rainfall is considerably low in most parts of the world and temporal variability is quite high. Climate change impacts on agricultural crop production vary from place to place and from crop to crop.
Climatic factors such as temperature, precipitation, moisture and pressure affect the development of plants, either alone or by interacting with other factors. This implies that rural sustenance and food security is under threat along with socioeconomic stability Burke et al. [4], and ecological integrity Walker and Schulze [5].
These risks are particularly high for the less resilient impoverished countries. Considerable research work has been carried out on the effects of weather/climate on agricultural production, but few works have been specific on the effects of climate change on maize production. Maize is one of the most important staple food crops in the world after wheat and rice and belongs to the family Poaceae. Maize occupies an important position among the crops, both as food and feed as well as raw material in industrial production of starch, oil, protein, alcoholic beverages, biofuel, food is referred to as "Miracle Crop". Being a C4 plant, it is physiologically more efficient, has higher grain yield potential compared to other grass family members and is also regarded as "Queen of Cereals".
Maize crop as such has multiple uses. The kernel contains about 77 per cent starch, two per cent sugar, nine per cent protein, 2 per cent ash on a water-free basis, five per cent Pentosan (sold for the relief of many medical conditions including thrombi and interstitial cystitis in humans and osteoarthritis in dogs and horses) and five per cent oil. Maize Oil is considered as the highest containing poly unsaturated fatty acid (PUFA), linoleic acid (61.9%). So, it remains a liquid at fairly low temperatures which is helpful in combating heart diseases ( Figure 1). Maize oil is also low in linolenic acid (0.7%) and contains a high level of natural flavour. Maize is used primarily   Climate variability affects maize yield and the various crop processes and activities in maize production. There has been a significant fluctuation in maize yield and production.

Effects of Climate Variability and Change on Maize Growth
The occurrence of extreme climate variability such as may be characterized by a prolonged dry period or heavy rainfall spell coinciding with the critical stages of crop growth and development may lead to significantly reduced crop yields and extensive crop losses ( Figure 2). Maize production has been on steady decline due to erratic rainfall variability and the area planted to maize has also been reduced to adapt to the anticipated drought period.

Effects of Climate Variability in Relation to Biotic and Abiotic Stress
Due to global warming, and potential climate abnormalities associated with it, crops typically encounter an increased number of abiotic and biotic stress combinations, which severely affect their growth and yield. Concurrent occurrence of abiotic stresses such as drought and heat has been shown to be more destructive to crop production than these stresses occurring separately at different crop growth stages. Abiotic stress conditions such as drought, high and low temperature and salinity are known to influence the occurrence and spread of pathogens, insects, and weeds. They can also result in minor pests to become potential threats in future Duveiller et al. [6]. These stress conditions also directly affect plant-pest interactions by altering plant physiology and defence responses ( Figure 3). Additionally, abiotic stress conditions such as drought enhance competitive interactions of weeds on crops as several weeds exhibit enhanced water use efficiency than crops.

Abiotic Stresses of Maize Under the Changing Climate
Drought: Drought is the most pervasive limitation to the realization of yield potential in maize (Edmeades et al. [7]). Average annual global losses due to drought in maize range from 15% in temperate zone to 17% in tropical zone as estimated by empirical methods. A precise measurement of yield losses worldwide is not possible due to a range of occurrences of drought from individual fields to regional in extent, with severity from slight to catastrophic.
Losses are greatest in parts of the world where soils and weather patterns are less favourable than US Corn Belt, which is named for its long-term suitability for growing maize at relatively low level of risk of crop failure.
Heat: By the end of this century, growing season temperatures will exceed the most extreme seasonal temperatures recorded in the past century Battisti and Naylor [8]. Using crop production and meteorological records, Thomson [9] showed that a 6°C increase in temperature during the grain filling period resulted in a 10% yield loss in the US Corn Belt. A later study in the same region showed maize yields to be negatively correlated with accumulated degrees of daily maximum temperatures above 32°C during the grain filling period.
Lobell and Burke [10] suggested that an increase in temperature of 2°C would result in a greater reduction in maize yields within sub-Saharan Africa than a decrease in precipitation by 20%. A recent analysis of more than 20,000 historical maize trial yields in Africa over an eight year period combined with weather data showed for every degree day above 30°C grain yield was reduced by 1 % and 1.7% under optimal rainfed and drought conditions, respectively Lobell et al. [11]. The temperature threshold for damage by heat stress is significantly lower in reproductive organs than in other organs Stone [12]. Successful grain set in maize requires the production of viable pollen, interception of the pollen by receptive silks, transmission of the male gamete to the egg cell, and initiation and maintenance of the embryo and endosperm development Schoper et al. [13]. High temperature during the reproductive phase is associated with a decrease in yield due to a decrease in the number of grains and kernel weight. Under high temperatures, the number of ovules that are fertilized and develop into grain decreases.
Water Logging: Over 18% of the total maize production area in South and Southeast Asia is frequently affected by floods and water logging problems, causing production losses of 25-30% annually Zaidi and Singh [14]. Although the area of land in sub-Saharan Africa affected by water logging is lower than in Asia, it is a risk in a few areas. Water logging stress can be defined as the stress inhibiting plant growth and development when the water table of the soil is above field capacity. The diffusion rate of gases in the flooded soil could be 100 times lower than that in the air, leading to reduced gas exchange between root tissues and the atmosphere Armstrong and Drew [15]. As a result of the gradual decline in oxygen concentration within the rhizosphere, the plant roots suffer hypoxia (low oxygen), and during extended water logging (more than 3 days), anoxia (no oxygen) Zaidi and Singh [14]. Carbon dioxide, ethylene and toxic gases (hydrogen sulphide, ammonium and methane) also accumulate within the rhizosphere during periods of water logging. A secondary effect of water logging is a deficit of essential macronutrients (nitrogen, phosphorous and potassium) and an accumulation of toxic nutrients (iron and magnesium) resulting from decreased plant root uptake and changes in redox potential. Nutrient uptake is reduced as a result of several factors. Anaerobic conditions reduce ATP production per glucose molecules, thereby reducing energy available for nutrient uptake. Reduced transport of water further reduces internal nutrient transport. Reduced soil conditions decrease the availability of key macro nutrients within the soil. Under water logging conditions nitrate is reduced to ammonium and sulphate is converted to hydrogen sulphide, and both become unavailable to most of the non-wetland crops, including maize. Availability of phosphorous may increase or decrease depending upon soil pH during water logging.

Biotic Stresses of Maize Under the Changing Climate:
Abiotic stresses account for a significant proportion of maize yield losses worldwide. The predominant insect-pests and diseases vary across environments and a major challenge in adapting crops to climate change will be the maintenance of genetic resistance to pests and diseases Reynolds and Ortiz [16]. Changing climates will affect the diversity and responsiveness of agricultural pests and diseases. Studying and understanding the drivers of change will be essential to minimize the impact of plant diseases and pests on maize production.  Miller [20]. Mycotoxin contamination is a serious problem with long-term consequences for human and animal health. Sub-lethal exposure to mycotoxins suppress the immune system, increase the incidence and severity of infectious diseases, reduce child growth and development, and reduce the efficacy of vaccination programs Williams et al. [21]. Consumption of high doses of mycotoxins causes acute illness and can prove fatal. In 2004, more than 125 people died in Kenya from eating maize with aflatoxin B1 concentrations as high as 4,400 parts per billion -220 times the Kenyan limit for foods Lewis et al. [22]. The maize implicated in this outbreak was harvested during unseasonable early rains and stored under wet conditions conducive to mold growth and therefore aflatoxin contamination CDC [23]. Previous outbreaks in Kenya and India have also been attributable to unseasonable, heavy rain during harvest Krishnamachari et al. [24]; Ngindu et al. [25].

Insect-Pests:
The dynamics of insect-pests are also strongly coupled with environmental conditions. Insects do not use their metabolism to maintain their body temperature, and are dependent on ambient temperature to control their body temperature. It has been estimated that a 2°C increase in temperature has the potential to increase the number of insect life cycles during the crop season by one to five times Petzoldt and Seaman [26]; Bale et al.
[27]; Porter et al. [28]. The feeding rate of many arthropod vectors increases at higher temperatures, thus increasing exposure of crops to mycotoxigenic fungi thereby increasing the spread of mycotoxins Bale et al. [27]; Dowd [29]. The increased global warming and drought incidences will favour insect proliferation and herbivory, which will likely increase the incidence and severity of insect  [30]. However, the population structure of most maize pathogens remains inadequately characterized. Also, concerted efforts are required to widely test the available sources of resistance in multiple and relevant environments to expose them to a wide spectrum of pathogen strains and to facilitate identification of the most suitable resistance genes/alleles for use in the breeding programs [31]. Research at CIMMYT is focused on multi-location phenotyping of a common set of 500 maize inbred lines for some prioritized diseases, namely GLS (gray leaf spot), TLB (Turcicum leaf blight), MSV (maize streak virus), and ear rots, across more than 15 locations in Sub-Saharan Africa, Latin America and Asia. This will help identify stable sources of resistance to key diseases and identify key phenotyping sites for future research. Using a common set of genotypes across environments will also provide the ability to monitor and detect emergence of new pathogen strains that will be registered as shifts in disease pressure and emerging new diseases, and how the environmental characteristics impacts pest biology and prevalence. CIMMYT has also developed several insectpest resistant populations, inbred lines, and varieties, especially for the stem borers and post-harvest insect pests (weevils and grain borers) through projects such as Insect Resistant Maize for Africa (IRMA). In addition, several inbred lines have been developed combining resistance to stem borers and storage pests.

Conclusion
Adaptation to climate change requires cross-disciplinary solutions that include the development of appropriate germplasm and mechanism to facilitate to farmers access to germplasm. Seed production and deployment, effective policies and management strategies at the country, regional and international levels will all be required to ensure that the technologies reach the intended beneficiaries and make the desired impacts. Varieties with increased resilience to abiotic and biotic stresses will play an important role in autonomous adaptation to climate change. Over fifty years ago scientists were able to offset yield losses by up to 40% through the development of improved germplasm and management options.
Today, scientists are faced with an even harder challenge-to meet the needs of future generations in the face of both population growth and climate change. While this challenge is immense, the advancement in molecular and phenotyping tools combined with the vast accumulated knowledge on mechanisms responsible for yield loss will provide a solid foundation to achieve increases in productivity within maize systems.

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