Influence of enhanced UV-B radiation on wheat production in relation with abiotic, biotic and socioeconomics constraints

Abstract

The light is one of the most important factors that regulate growth and development of plants. However, the increase of the ultraviolet-B radiation due to the anthropogenic action can have negative impacts on these processes, producing a decreased photosynthesis and biomass production. Zonal average ultraviolet irradiance (flux ultraviolet, FUV) reaching the Earth's surface has significantly increased since 1979 at all latitudes except the equatorial zone. Depletion of the stratospheric ozone layer leads to an increase in ultraviolet-B (UV-B: 280- 320 nm) radiation reaching the Earth's surface, and the enhanced solar UV-B radiation predicted by atmospheric models will result in reduction of growth and yield of crops in the future. Breeding wheat cultivars with increased grain yield potential, enhanced water-use efficiency, heat tolerance, end-use quality, and durable resistance to important diseases and pests can contribute to meet at least half of the desired production increases. The remaining half must come through better agronomic and soil management practices and incentive policies.

Headnote

Abstract

The light is one of the most important factors that regulate growth and development of plants. However, the increase of the ultraviolet-B radiation due to the anthropogenic action can have negative impacts on these processes, producing a decreased photosynthesis and biomass production. Zonal average ultraviolet irradiance (flux ultraviolet, FUV) reaching the Earth's surface has significantly increased since 1979 at all latitudes except the equatorial zone. Depletion of the stratospheric ozone layer leads to an increase in ultraviolet-B (UV-B: 280-320 nm) radiation reaching the Earth's surface, and the enhanced solar UV-B radiation predicted by atmospheric models will result in reduction of growth and yield of crops in the future. Over the last two decades, extensive studies of the physiological, biochemical and morphological effects of UV-B in plants, as well as the mechanisms of UV-B resistance, have been carried out. In this review we didn't obtain evidences to show that the increased UV-B radiation influences the oscillations observed in the wheat production in major producing countries in the world. The most important constraints observed on wheat production are heat (affecting up to 57% of the entire wheat area in surveyed countries), competition with weeds, and diseases (both affecting up to 55% of wheat area). Of the socioeconomic constraints listed and evaluated, the access to mechanization and availability of credit were the most often highlighted. The way to improve wheat production in the new scenarios consequence of global environmental changes is the genetic breeding. Breeding wheat cultivars with increased grain yield potential, enhanced water-use efficiency, heat tolerance, end-use quality, and durable resistance to important diseases and pests can contribute to meet at least half of the desired production increases. The remaining half must come through better agronomic and soil management practices and incentive policies.

Key words: Climate changes, Genetic breeding UV-B radiation, Yield production, Wheat

Introduction

Solar radiation is one of the major environmental factors that affect life on our planet. This radiation controls the functioning of terrestrial and aquatic ecosystems by controlling photobiological processes (photosynthesis, photoperiod, phototropism, etc.), through its action on other environmental factors (temperature, humidity, etc.) and natural cycles (cycles per day, annual, water, etc.) which ultimately affect the distribution of organisms. High radiation intensities and spectral composition changes may affect important processes in organisms, especially plants that can not move are leftto adapt to such changes.

One of the main changes that happened this last time has been increased UV-B (Blumthaler and Ambach, 1990). That is due to the destruction of the ozone layer by polluting compounds such as chlorofluorocarbons (CFCs), oxides of nitrogen, chlorine, bromine, etc. These compounds tend to form stable compounds with ozone (O3) with a half-life of 50 to 150 years. UV-B radiation is comprised between the wavelengths 290 and 320 nm. Other components of the UV radiation are the UV-C, between 220 and 290 nm, and the UV-A between 320 and 400 nm. This last radiation is bit absorbed by O3, so that arrives in greater quantity to the surface of the Earth and is an important photomorphogenic signal in plants and is the least harmful. By contrast, the UV-C is the most energetic and harmful to the DNA. However, since is mostly absorbed by oxygen (O2) and O3 in the stratosphere hardly reaches the Earth's surface.

Understanding of relationships between crop and environment has substantially improved during the last few decades of the 20th century. Anthropogenic factors are continuously changing the environment, and projections are that atmospheric CO2 concentrations will double and temperatures will increase by 5.5°C by the end of current century (Houghton et al., 2001).

Current stratospheric ozone levels are at the lowest point since measurements began in 1970s (figure 1) and global terrestrial UV-B radiation levels range between 0 and 12 kJm-2 on a given day with near Equator and mid-latitudes receiving higher doses (Total Ozone Mapping Spectrometer, 2005, http://ozoneaq.gsfc.nasa.gov/measurements. md).

The destruction of the ozone layer has been more intense at high latitudes, particularly in Antarctica, where ozone concentrations have decreased by 40-50% compared to the values obtained in 1980 and minor changes in the area of Ecuador in 3-6% (35-60° N and 35-60° S), where UV radiation is intense in nature (UNEP, 2002). Accordingly, from 1980 to present, the flux of UVB, mainly within the range of 290-315 nm, has increased in the troposphere on average 6-14% (Kakani et al., 2003). Therefore, since the discovery of so-called "hole" in ozone in the Antarctic, the main interest in studying the effects of ultraviolet-B radiation on plants has increased considerably.

Although UV-B comprises a small region of the electromagnetic spectrum, its effect on plants and animals is considerable. Thus, plants early in their evolution have had to adapt to their presence and develop mechanisms to reduce adverse effects. It emerged recently that there is an interrelationship between drought and ultraviolet-B radiation in plant responses, in that both stresses provoke an oxidative burst. All living organisms of the biosphere are exposed to UV-B at intensities that vary with the solar angle and the thickness of the stratospheric ozone layer. In plants, wide interand intraspecific differences have been reported in response to UV-B radiation with respect to growth, production of dry matter and biochemical changes (Kramer, 1991). Some plant species are unaffected by UV-B radiation and several are apparently stimulated in their growth, but most species are sensitive and undergo damage results (Teramura, 1983). Furthermore, numerous environmental factors have also been shown to weaken or enhance the responses of plants to UV radiation.

Drought is an important environmental constraint that limits the productivity of many crops and affects both yield quality and quantity (Boyer, 1982). Drought stress reduces growth rate, stem elongation, leaf expansion and stomatal movements (Hsiao, 1973). Furthermore, it causes changes in a number of physiological and biochemical processes governing plant growth and productivity (Daie, 1988). Under field conditions, plants usually experience several stresses simultaneously, which may cause a variety of plant responses that can be additive, synergistic or antagonistic.

Evidence of interaction between UV-B exposure and drought stress in plants has emerged in recent years, but the mechanisms involved have received little attention. Some investigations have been carried out on agricultural or model plants, despite the fact that crops account for only 6% of the plant productivity worldwide (Teramura et al., 1983; Tevini et al., 1983; Sullivan and Teramura 1990; Balakumar et al., 1993; Schmidt et al., 2000). Elucidation of the interaction between drought and UV-B stresses would help in understanding the potential impact of partial stratospheric ozone depletion on plant adaptation to changing environmental condition. However, the mechanisms of sensitivity or tolerance of crop plants, either in growth, yield, or combined stresses remains unknown.

Interaction of UV-B radiation with other factors of climate change

The combined effect of high UV-B radiation and water deficit has been approached in several studies, which are showing a reduction of plant growth and alteration of several physiological and biochemical processes (Alexieva et al., 2001). Both environmental factors act synergistically on plant secondary metabolism by increasing the production of flavonoids (Hofmann et al., 2003). In addition, drought and high flow UV-B radiation induce the production of cuticular waxes thus facilitating the reflection of light and water conservation (Steinmüller and Tevini, 1985). Enhanced UV-B radiation has a significant negative effect on growth and biomass production of wheat plants. Many studies agree that wheat was considered sensitive to UV-B radiation (Biggs et al., 1984, Rozema et al., 1991; van Staaij, 1994). The reduction in plant dry weight may be explained by UV-B induced changes in morphological and physiological processes. High UV-B radiation changes the structural characteristics of wheat plants, particularly by increasing Specific Leaf Area (SLA) indicated by altered leaf morphology. The trend to higher SLA indicates that UV-B radiation decreases the leaf thickness (Correia et al. 1999; Santos et al., 1993). This decrease may be important for two reasons. Primary, because it changes the light inside the leaves, which could also explain the decrease in the Chla/Chlb ratio (Deckmyn and Impens, 1995), secondly because increased SLA has been correlated with decreased photosynthetic rates, contributing to lower relative growth rates (Poorter, 1989).

Wheat yield progress worldwide

Wheat is one of the most important cultivated crops in the world with a worldwide production of 676 million tons in 2011, representing a growth of 3.4 percent from 2010 (FAO - Crop Prospects and Food Situation report, 2011). In figure 2 is represented the yield progress since 1985 to 2010. It was consider the prospects for continued yield growth, in particular that resulting from plant breeding, which it is believed is becoming a proportionally larger component of yield growth. Possible yield changes result from shifts in cropping regions or proportions irrigated changes in cropping intensity, or climate change, factors that need to be considered for a complete understanding of wheat yield changes.

Figure 3 shows the highest production of wheat countries in the world, with more than 4 t/ha.

Wheat cultivated area in many countries have increased or are expected to increase in 2011 in response to strong prices, while yield recoveries are forecast in areas that were affected by drought in 2010 (FAO - Crop Prospects and Food Situation report, 2011).

The most important wheat producers in the world

When studying a period of 10 years (2000- 2010), it appears that the production of wheat in major producing countries is uneven, with the exception of India and China that additionally is increasing its production in the last years.

Much of the observed fluctuations in yields in most producing countries, are primarily related to abiotic constraints, especially in Australia where the climate impact is more evident in the yield average obtained. FAO, through the report "Crop Prospects and Food Situation" (2011), estimate an increase in wheat production worldwide mainly supported by the yield of most countries shown in Figure 3 and 4, but it is interesting to highlight that there is a significant number of other constraints that can also influence the increased production in countries with lower rates of development.

Major constraints to wheat production

The actual crops productivity not only depends on the sensitivity of different species showing the effect of UV-B radiation (figure 5) but also depends on the interaction with other biotic and environmental factors (Caldwell and Flint, 1994). Accordingly, in the context of climate change experienced in recent times, has been observed, in addition to increased UV-B, an increase of CO2 and temperature as well as significant changes in the frequency and quality of precipitation (Caldwell et al., 2007).

The scientific consensus predicts a global temperature increase between 1.5 and 4.5°C over the next 100 years in agreement with the temperature anomalies already verified from 1992 to 2011 presented on figure 6, plus the existing increase of 0.6°C that has experienced the atmosphere since the industrial revolution (Caldwell et al., 2007).

A survey, conduct by Kosina et al. (2007), covered nineteen developing countries, including major wheat producers, prior to the 2006 International Symposium on Increasing Wheat Yield Potential in Ciudad Obregon, Mexico. Collectively these countries represent 102 million hectares of wheat (47% of the global wheat area or 89% of the wheat area in developing countries) and 285 million tons of wheat production (45% of the global wheat production or 92% of wheat production in developing countries (FAO, 2006). The results emphasize the substantial yield losses associated with a number of critical abiotic, biotic and socioeconomic constraints, and indicate their global prevalence.

Abiotic stress

A major constraint that is estimated to affect up to 58.7 million hectares of wheat area in sample countries (57.3% of entire wheat area in surveyed countries) is heat. Average estimated yield loss caused by extreme temperatures varies between 14.7 and 31.3%, depending on the region. The total estimated loss amounts to 21 million tons. The largest areas affected by heat stress were identified in Central, South and Southeast Asia. The major threat is terminal heat stress at anthesis and during grain filling period, which accelerates maturity and reduces significantly grain size, weight, and yield (Kosina et al., 2007; Dias and Lidon, 2009).

Low rainfall (moisture stress) is the second most significant abiotic constraint to wheat production in terms of area potentially affected, concerning 42.6 million hectares (41.6% of wheat area in surveyed countries). Estimated yield loss (in average) caused by low rainfall varies between 19.3 and 50.4%, and overall is estimated to cause losses of 31 million tons. Areas potentially affected by low rainfall are present in four regions: South and Southeast Asia, Central and West Asia, Northern Africa, Sub-Saharan Africa and Latin America. The most common threat is yearly fluctuation (periodically occurring 'dry years') and irregular seasonal distribution of rainfall.

A third important constraint to wheat production, potentially affecting up to 38.4 million hectares of wheat is the declining availability of irrigation water. Average estimated yield losses caused by declining availability of irrigation water varies between 20 and 37.2% and can cause losses of up to 21.8 million tons of wheat annually. The largest proportion of potentially affected areas appears to be in South and Southeast Asia. Reasons for declining availability of irrigation water include overexploitation of ground water resources, competition with other crops (cash crops), restrictive governmental policies, and deterioration of irrigation infrastructure. Factors such as lodging, physical soil degradation and microelement deficiencies affect approximately 28-30 million hectares. Potential losses in terms of wheat production oscillate between 7.7 and 20%, which represents an aggregate loss of 6-8 million tons of wheat for each of the three constraints.

The main causes of lodging include tall varieties (weak straw), poor crop management, high yield (over 6 t/ha) in wet years (excessive irrigation), heavy rains, and windy conditions. Soil degradation is reported to occur mainly due to heavy tillage and mismanagement causing soil compaction, organic matter depletion, soil erosion, and water logging. Micronutrient deficiencies, such as an unavailability of zinc and boron, often stem from pH imbalances. Relatively smaller areas of wheat production are affected by other factors such as cold (15.8 million hectares), mainly in Central and West Asia, China and South America; salinization (11 million hectares) in Central and South Asia; and microelement toxicity (1.2 million hectares) mainly in Turkey and Brazil. These three constraints may cause annual losses of 5, 3.5, and 0.5 million tons of wheat, respectively. Cold refers to sporadic frost damage to susceptible varieties, particularly in the case of winter wheat and in mountain areas. Saline soils are a growing problem, especially in arid and semiarid areas and in fields exposed to excessive irrigation. Problems with microelement toxicity (Al, Mn, and Bo) occur mainly in areas with low pH conditions.

Biotic stress

Biotic stresses are reported to affect roughly the same area as heat stress. Estimated yield loss caused by weeds varies between 8.5 and 23.9%, depending on the region, and overall could cause up to 24 million tons in losses annually. Among the most often mentioned weeds are Avena spp., Phalaris spp., Chenopodium spp., Rumex spp., Medicago spp., Amaranthus spp., Lolium spp., Polypogon spp., Convonvulus spp., and Echinochloa spp. Likewise, diseases are rated nearly equally in importance, affecting roughly 56 million hectares. The most serious diseases cited are leaf and stripe rusts (Puccinia triticina and P. striiformis), Fusarium head blight (Fusarium spp.), Septoria blotch (Septoria tritici), powdery mildew (Erysiphe graminis), tan spot (Pyrenophora tritici repentis), spot blotch (Bipolaris sorokiniana), bunts (Tilletia spp.), and eyespot (Cercosporella herpotrichoides). Although pests (especially insect pests) are usually reported as a less binding constraint in wheat, potentially affected areas cover approximately 47 million hectares. Estimated yield loss caused by pests varies between 12.2 and 22% and can overall cause up to 20 million tons of loss annually. The most often mentioned insect pests include aphids, sunn pest (Eurygaster spp.), Hessian Fly (Mayetiola destructor), weevils, termites and some other species of minor importance (Kosina et al., 2007).

Socioeconomic constraints

Many socioeconomic constraints are related to agricultural policies and institutions that potentially affect the entire wheat crop (Kosina et al., 2007). The first reported lack was the access to mechanization (suitable machinery) as a constraint, mainly related to high purchasing and operational costs, and unavailability of small-scale and zero tillage machinery. The second most socioeconomic constraint is availability (and level) of credit. High interest rates, insufficient credit resources, lack of timely access in rural areas, and unwillingness of financial institutions to provide credit to the agricultural sector (particularly to subsistence/staple crops) were the most frequently reported constraints. Seed availability/quality and fertilizer availability is also an important constraint (Kosina et al., 2007). Such constraints will affect more significantly the developing countries not having the same impact in developed countries; however, the impact in the global production of wheat will be important.

The importance of genetic breeding in a global environmental change

Although more food is needed for the rapidly growing human population, food quality also needs to be improved, particularly for increased nutrient content. In addition, agricultural inputs must be reduced, especially those of nitrogenous fertilizers, in order to reduce environmental degradation caused by emissions of CO2 and nitrogenous compounds from agricultural processes. Furthermore, there are now concerns about the ability to increase or even sustain crop yield and quality in the face of dynamic environmental and biotic threats that will be particularly challenging in the face of rapid global environmental change (Tester and Langridge, 2010). This scale of sustained increase in global food production is unprecedented and requires substantial changes in methods for agronomic processes and crop improvement.

Certain aspects of global environmental change are beneficial to agriculture. Rising CO2 acts as a fertilizer for C3 crops and is estimated to account for approximately 0.3% of the observed 1% rise in global wheat production (Fisher and Edmeades, 2010), although this benefit is likely to diminish, because rising temperatures will increase photorespiration and nighttime respiration. A benefit of rising temperatures is the alleviation of low-temperature inhibition of growth, which is a widespread limitation at higher latitudes and altitudes. Offsetting these benefits, however, are obvious deleterious changes, such as an increased frequency of damaging high-temperature events, new pest and disease pressures, and altered patterns of drought. Negative effects of other pollutants, notably ozone, will also reduce benefits to plant growth from rising CO2 and temperature. Particularly challenging for society will be changes in weather patterns that will require alterations in farming practices and infrastructure; for example, water storage and transport networks. Because onethird of the world's food is produced on irrigated land (Munns and Tester, 2008), the likely impacts on global food production are many. Along with agronomic- and management-based approaches to improving food production, improvements in a crop's ability to maintain yields with lower water supply and quality will be critical. Put simply, there is a need to increase the tolerance of crops to drought and salinity.

In the context of global environmental change, the efficiency of nitrogen use has also emerged as a key target. Human activity has already more than doubled the amount of atmospheric N2 fixed annually, which has led to environmental impacts, such as increased water pollution, and the emission of greenhouse gases, such as nitrous oxide. Nitrogen inputs are increasingly being managed by legislation that limits fertilizer use in agriculture. Furthermore, rising energy costs means that fertilizers are now commonly the highest input cost for farmers. New crop varieties will need to be more efficient in their use of reduced nitrogen than current varieties (Peoples et al., 1995). Therefore, it is important that breeding programs develop strategies to select for yield and quality with lower nitrogen inputs.

Current approaches to crop improvement

Questionably, increased yield in conditions of abiotic stresses, such as drought and salinity, could be best achieved by selecting for increased yield under optimal production conditions: plants with higher yields in good conditions are more likely to have higher yields in stressed conditions (Richards, 1992). Such an approach will also increase yield in high-yield environments. However, it is becoming increasingly apparent that specific selection for stressed environments is efficient. Given that average global yields of wheat are more or less 4 t/ha and that there are some areas with yield as high as 7 t/ha, the majority of land cropped to wheat delivers yield below 3 t/ha (Figure 7).

Therefore, by virtue that globally low-yielding land represents much larger areas, low-yielding environments offer the greatest opportunity for substantial increases in global food production. Increasing yield by 1 t/ha in a low-yielding area delivers a much higher relative increase than does the same increase in high-yielding environments. This increase can be achieved by tackling major limitations on yield in poor environments (termed yield stability); for example, by protecting plants and yield from factors such as salinity and heat or drought periods. The local social benefits of supporting farmers on low-yielding lands would also be great. It is often thought that concentration on yield stability may come at the expense of high yield in good years; however, yield penalties in more favorable conditions do not necessarily accompany drought tolerance. Select for yield stability is harder than improved yield, because selection in breeding programs requires many years and many sites for evaluation. However, there is evidence for a genetic basis for yield stability and, hence, an opportunity for improvement (Kraakman et al., 2006). There are several clear examples where single genes have been able to substantially increase yield, notably to drive domestication (to control tiller number, branching, and seed number) and the green revolution (for dwarfing). Initial results suggest that a gene conferring increased drought tolerance may also have a widespread impact on yield (Nelson et al., 2007), which doesn't mean that efforts to maintain yield should be reduced. In particular, maintaining resistance to rapidly evolving pests and pathogens is an essential mainstay of breeding programs. Interactions between breeders, pathologists, and agronomists must be maintained to ensure that crops and cropping systems change coordinately. No-till farming, in which plowing of the soil is avoided, for example, has changed the spectrum of diseases and pests attacking crops, to the extent that a change in breeding targets was needed. The development of multiple cropping systems will also demand interactions between agronomists and breeders. However, it is clear that further is required that can be provided by traditional breeding approaches.

Expanding the germplasm base for plant breeding

The success of plant breeding over the past century has been associated with a narrowing of the available genetic diversity within elite germplasm. New sources of variation include landraces and wild relatives of crop species, and although exploiting wild relatives as a source of novel alleles has changed, it has provided notable successes in crop improvement. A particularly important example of the introgression of genetic information from a relative was the use of the short arm of rye chromosome 1R in wheat. In the early 1990s, this wheat-rye translocation was used in 45% of 505 bread wheat cultivars in 17 countries (Rabinovich, 1998). Progressively easy gene discovery, improved enabling technologies for genetics and breeding, and a better understanding of the factors limiting practical exploitation of exotic germplasm promise to transform existing, and to accelerate the development of new strategies for efficient and directed germplasm use (Tester and Langridge, 2010).

Most crop geneticists agree that enrichment of the cultivated gene pool will be necessary to meet the challenges that lie ahead. However, to fully capitalize on the extensive reservoir of favorable alleles within wild germplasm, many advances are still needed. These include increasing our understanding of the molecular basis for key traits, expanding the phenotyping and genotyping of germplasm collections, improving molecular understanding of recombination in order to enhance rates of introgression of alien chromosome regions, and developing new breeding strategies that will allow introgression of multiple traits (Feuillet et al., 2008).

Conclusions

Different mechanisms of adaptation to UV-B radiation have been documented in plants; to date research show that, primarily, plants develop strategies to prevent the penetration of this type of light. While plants have developed early in the evolution protective mechanisms efficient enough to prevent the harmful effects of natural UV radiation, the predictions of increased UV-B radiation could have a major impact on the crops productivity. Therefore, it is necessary to deal with this problem globally, with further studies that could predict changes that cause increased UV-B radiation on the distribution of vegetation and in the biocenosis associated. Moreover, in the context of climate change experienced in recent years, which not only has resulted in increased UV-B radiation but also in increasing the atmospheric concentration of CO2 and temperature, it is difficult to predict how it will affect the complex interactions that occur between ecological and climatic processes. For this reason, it is important to have more information that will allow predicting the impact it could have on the interactions of these factors.

Global wheat production must continue to increase 2% annually until 2020 to meet future demands imposed by population and prosperity growth. Moreover, this must be achieved under reduced water availability, a scenario of global warming, stricter end-use quality characteristics, and evolving pathogen and pest populations. Most of the production growth must occur in developing countries where wheat will be consumed.

In what wheat is concerned, the geographic locations of the main producing countries exclude the hypothesis that there is some negative influence of the UV-B radiation harmful effect in their physiological development with implications in yield. There will be different abiotic, biotic and socioeconomic constraints that will have a greater impact on global wheat production. In developing countries the complications with biotic stresses and socioeconomic factors will have more impact. Importance of genetic breeding is emphasized, as a multidisciplinary activity which has an important role in obtaining varieties more suitable to the constraints associated with environment, as well as any changes due to climate change.

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