Drought risk assessment is a vital part of drought risk management, which plays an important role in drought mitigation. Due to its complexity, drought risk is difficult to define and challenging to quantitatively assess, as the drought impacts associate with many social sectors. This contribution method the issue by quantitatively evaluating the yield loss due to drought as a function of the drought severity indicator in Liaoning province, China for spring maize using logarithmic regression. As crop water deficit is essence to identify agricultural drought, it developed a drought severity indicator using the crop water stress coefficient and duration. The Agricultural Production Systems sIMulator (APSIM) crop model was employed to simulate the spring maize growth to obtain daily water deficit during the growth period (May to September) and yield. The relationship between drought severity frequency and yield loss rate due to drought was established to assess the drought risk of spring maize when drought severity frequency is equal to 20%, 10%, 5% and 2%. The results show that Chaoyang and Fuxin have the highest drought risk in four levels of drought severity frequency whilst the lowest drought risk was identified in Tieling. The central Liaoning province has a moderate drought risk. For a specific drought severity frequency, drought risk increases from east to west in Liaoning province whilst it varies in each city at different drought severities. This method can predict yield loss due to drought for drought early warning. Drought risk maps presents spatial characteristics that can help to agricultural drought mitigation and the development of drought preparedness plan in Liaoning province.

Introduction

Drought is slow-onset and one of the most widespread natural hazards. Drought impacts are nonstructural and the occurrence of drought is associated with significant impacts in water resources, environment, energy, and human lives, especially in agricultural production (Wilhite, 2005;Wilhite and Pulwarty, 2018). These characteristics make it particularly challenging to quantify drought risk and capture drought impacts. In China, the average annual yield loss due to drought has increased from 4.35 million tons in the 1950s, to 34.9 million tons in the early twenty-first century (Lv, 2013). Drought affected approximately 60% of the maize sown area during 1990-2007, which resulted in a 20%-30% reduction in production (Jia et al., 2012). The widespread and costly nature of drought has naturally led to an interest in drought risk assessment. Methods to quantify drought risk help decision makers in drought risk management and drought mitigation. It also has a great significance in the theory and practice of quantitative drought risk assessment (Bachmair et al., 2017;Botterill and Hayes, 2012). To date, a number of previous studies have evaluated the drought risk of different regions and climates across the world at different spatial scales, most of which focus on agricultural drought risk (Xie et al., 2016). Agriculture is directly affected by the occurrence of drought as it reliable on precipitation, temperature and evapotranspiration, which can decrease the soil moisture (Sruthi and Aslam, 2015).

Limited non-farm opportunities in the rural areas of the developing world, coupled with population growth, means agriculture will continue to play a dominant role as a source of livelihood in these areas. Thus, while rural transformation has dominated recent literature as a way of improving welfare through diversifying into non-farm sectors, improving productivity and resilience to shocks in smallholder agricultural production cannot be downplayed. This is especially so given the changing climatic conditions affecting agricultural production, and thus threatening many livelihoods in rural areas. Farm diversification is an important strategy for creating resilience against climatic shocks in farm production. Using cross-sectional data from northern Namibia, the study assesses the barriers and success factors related to effective crop and livestock enterprises diversification and the effect of these on food security outcomes. A Seemingly Unrelated Regression model is used to assess the joint factors explaining total farm diversification, while a step-wise error correction model is used to evaluate the conditional effect of diversification in each of the two farm enterprises on two measures of food security: food expenditure and dietary diversity. We find that past exposure to climate shocks informs current diversification levels and that access to climate information is a key success factor for both livestock and crop diversification. In terms of food security, greater diversification in either crop or livestock production leads to higher food security outcomes, with neither crop nor livestock diversification showing dominance in affecting food security outcomes. However, an overall higher level of diversification in both livestock and crop enterprises is dominant in explaining food security outcomes.

Introduction

Risk is inherent in small-scale rain-fed agricultural production. Farmers have to contend with seasonal weather uncertainties, the threat of pests and diseases, and post-harvest losses, among other risks. These risks are being exacerbated by the effects of a changing climate; for example, the severity and distribution of important livestock and crop diseases is changing, while incidents of droughts and floods are on the rise (Elad & Pertot, 2014; Thornton, van de Steeg, Notenbaert, & Herrero, 2009; Wetherald & Manabe, 2002). These effects of climate change are expected to increase poverty incidences in most developing countries and create new poverty pockets in countries with increasing inequality (IPCC, 2014). Agricultural production has been stagnant in Sub-Saharan Africa (SSA), and there is consensus that the current trend in productivity cannot guarantee food security in the region (Jayne, Chamberlin, & Headey, 2014; Kyalo Willy, Muyanga, & Jayne, 2019; Onyutha, 2018). Climatic shocks that further adversely affect food production are a serious threat to food security and livelihoods in the region. While there are adaptation options that can create resilience in agricultural productivity, studies continue to show low adoption rates across the region (Bradshaw, Dolan, & Smit, 2004; Di Falco, Veronesi, & Yesuf, 2011; Mulwa, Marenya, Rahut, & Kassie, 2017; Singh et al., 2017; Smit & Wandel, 2006). In crop farming, such adaptation measures include using seeds adapted to climate-stressors (for example drought resistant seeds) and spreading risks across different crop types (Howden et al., 2007).

Increased biodiversity can make valuable contributions to food production and security around the world. The role of plant species diversity for “ecological intensification” of agriculture has been widely recognised, but the potential contribution of multi-trophic-level production systems, such as rice-fish coculture, has received less attention. A continuous 4-year experiment (2015e2018) was conducted comparing rice-fish (yellow finless eel and loach) co-culture, and mono-rice planting practices on the Chongming Eco-island of China. During the experiment, pests (insect herbivores and weeds), arthropods, pesticides, grain and marketable fish yield were sampled, soil quality (available nitrogen, phosphorus and potassium, total nitrogen, organic matter content and pH) and rice grain quality (protein content, chalkiness, gel consistency, amylose content) were evaluated, and an economic analysis were performed. Fish decreased herbivore insect abundance by 24.07%, reduced weeds abundance, richness and biomass by 67.62, 62.01 and 58.88% respectively, increased invertebrate predator abundance by 19.48%, and reduced the need for pesticide by 23.4%. Co-culture practice produced an average economic values 10.33% higher than in the mono-rice farming. In addition, rice-fish co-culture enhanced both soil and rice quality. Our results confirm that rice-fish co-culture can be an effective form of ecological intensification, incorporating and contributing ecosystem services in agricultural production and increasing sustainability.

Introduction

Agricultural intensification has been recognised one of the main reasons for biodiversity loss and related decline in ecosystem functioning due to the conversion of natural habitats into monoculture farming areas (Gagic et al., 2012; Batary et al., 2017 ; Hass et al., 2018; Plue et al., 2018). Intensive use of agrochemicals is reducing environmental quality (Stehle and Schulz, 2015; He et al., 2017; Ruiz and MarAa Dolores, 2018 ~ ), damaging local plant communities (Cassman, 1999; Kremen et al., 2012), and reducing the abundance and richness of beneficial arthropods (Gagic et al., 2012; Kovacs-Hosty anszki et al., 2017 ). The concept of “ecological intensification” has been promoted to redesign agroecosystems, based on the increased use of ecological processes and biodiversity, using resource more efficiently, and decreasing anthropogenic inputs (Bommarco et al., 2013; Pywell et al., 2015; Bowles et al., 2016). In this framework, it has been showed that plant-diversified farming practices can contribute to ecological intensification of agriculture by providing multiple ecosystem services, promoting biological pest control (Redlich et al., 2018; Wan et al., 2018), decreasing the use of pesticides (Tscharntke et al., 2005; Gurr et al., 2016; Zhao et al., 2016), improving soil quality (Cassman, 1999), and enhancing crop yields (Tittonell and Giller., 2013; Gurr et al., 2016). The potential contribution of multi-trophic-level production systems, such as rice-fish co-culture, to ecological intensification has not received as much attention.

Abstract

The root growth of most crop plants is inhibited by soil salinity. Roots respond by modulating metabolism, gene expression and protein activity, which results in changes in cell wall composition, transport processes, cell size and shape, and root architecture. Here, we focus on the effects of salt stress on cell wall modifying enzymes, cellulose microfibril orientation and non-cellulosic polysaccharide deposition in root elongation zones, as important determinants of inhibition of root elongation, and highlight cell wall changes linked to tolerance to salt stressed and water limited roots. Salt stress induces changes in the wall composition of specific root cell types, including the increased deposition of lignin and suberin in endodermal and exodermal cells. These changes can benefit the plant by preventing water loss and altering ion transport pathways. We suggest that binding of Na+ ions to cell wall components might influence the passage of Na+ and that Na+ can influence the binding of other ions and hinder the function of pectin during cell growth. Naturally occurring differences in cell wall structure may provide new resources for breeding crops that are more salt tolerant.

Introduction

Plant evolution has resulted in a large array of mechanisms to tolerate the stresses associated with increased soil salinity. However, for most cereal crops the growth of roots is disrupted when soil salinity exceeds 4 dS/m, equivalent to about 40 mM NaCl. Increased soil salinity exposes plants to ionic sodium (Na+) and chloride (Cl−), which leads to a cascade of responses in the plant due to the ionic and osmotic components of salt stress [1,2].

Salt stress can indirectly affect cell wall properties by causing changes in gene expression, but Na+ can also physically interact with the cell wall components directly, and change their chemical properties [3]. An increase in soil salinity results in accumulation of Na+ in the apoplast, which can lead to an increase in interactions between Na+ and negatively charged sites within cell wall polymers, and also influence apoplastic pH. Salinity causes transient alkalinisation of the apoplast, and this could limit growth in the context of the acid growth theory [4,5]: Auxin activates plasma membrane H+-ATPases and protons are extruded into the apoplast, apoplastic acidification induces cell wall loosening by activating expansins and other remodelling enzymes resulting in loosening of the cell wall. Hence, growth could be limited by a decrease in free apoplastic protons causing a shift in the apoplastic pH away from the range that favors cell-wall loosening [4], although in maize the inhibition of growth as a result of salinity was not associated with the capacity of the epidermal cells to acidify their walls [6].

Glyphosate (N-(phosphonomethyl) glycine) is a post-emergence, non-selective, foliar herbicide. It is the most used herbicide in Argentina, accounting for 62% of the commercialized pesticides in the market (CASAFE, 2012). It is used as a weed controller in no-till (NT) systems, and it is also applied on various genetically modified crops (e.g. soybean, corn, cotton). Glyphosate has an amine, carboxylate, and phosphonate group. It is a highly water-soluble compound (11.6 g L1 at 25º C), but it also has a high affinity to soil particles, such as clays and aluminum and iron oxides (Vereecken, 2005). The biological degradation of a pesticide in the soil is dependent on various factors such as: microbiological activity, organic matter (OM) content, nutrient availability, pH, salinity, temperature, oxygen content, humidity and bio-availability (Aislabie and Lloydjones, 1995; Walker et al., 2001; Borggaard and Gimsing, 2008). Glyphosate is mainly degraded by biological activity, although evidence of an abiotic pathway via metal interaction has been recently reported (Ascolani Yael et al., 2014). In general, microorganisms use glyphosate aerobically or anaerobically, as a source of organic phosphorous (Zboinska et al., 1992; Dick and Quinn 1995; Obojska et al., 2002; Ermakova et al., 2008). However, some species have been described to be capable of using glyphosate as a carbon or nitrogen source (Kryosko and Lupicka, 1997; Obojska et al., 1999). There are two pathways known for the degradation of glyphosate (Fig. 1). In one pathway, the herbicide is hydrolyzed to inorganic phosphorous and sarcosine, by the activity of a C-P lyase enzyme. Sarcosine is then metabolized into glycine and formaldehyde by a sarcosine oxidase, until its complete mineralization to CO2 and NH3. Some examples of bacteria that use this pathway are Arthrobacter spp., Rhizobium spp. and Pseudomonas spp. (Dick and Quinn, 1995; Kishore and Jacob, 1987; Liu et al., 1991). Other organisms degrade glyphosate by an oxidoreductase, producing glyoxylate and aminomethylphosphonic acid (AMPA). Glyoxylate enters the glyoxylate bypass of the Krebs cycle, while AMPA is exported into the extracellular space (Jacob et al., 1988). Some bacteria are capable of metabolizing AMPA by a C-P lyase, yielding inorganic phosphorous and methylamine (Pipke et al., 1987; Kertesz et al., 1994), which further fully metabolizes to CO2 and NH3. Examples of microorganisms that use this pathway are Ochromobactrum anthropi (Sviridov et al., 2012) and Geobacillus caldoxylosilyticus (Obojska et al., 2002).

Adoption of conservation agricultural (CA) practices including no-tillage (NT) has been widely embraced in grain growing regions of Queensland (Thomas et al. 2007) and across Australian agriculture (Llewelyn et al. 2012). However, within CA farming systems, strategic or occasional tillage (ST) is being utilized for a number of reasons by landholders in the Northern Grains Region (NGR) of Australia (Argent et al. 2013). These can include but are not limited to the control of herbicide resistant weed populations and soil- and stubble-borne diseases, and to address stratification of nutrients and organic carbon near the soil surface (Dang et al. 2015a). It can be misconstrued as to what the ST process consists of, or if there is even a place for it within CA. Strategic tillage can be best described as an opportunistic use of tillage operation/s (adequate tillage implement for desired outcome at adequate soil water level) in an otherwise NT system (Crawford et al. 2015, Dang et al. 2015a). A potential benefit is that it can provide a circuit breaker in herbicide use, addressing the build-up of resistant weed populations. However, the risk is, that poor timing or too many tillage application can lead to detrimental soil health impacts. Palm et al. (2014), states that CA is a system of agronomic practices that includes reduced tillage (RT) and NT. Reduced tillage occurs during seed bed preparation and hence a more regular tillage application, therefore strategic tillage should be a natural inclusion, as it involves far less tillage frequency than RT. Reviews such as Dang et al. 2015a, Dang et al. 2015b, and Busari et al. 2015, depicted a good representation of the process involved within the CA management system and the risk/benefit of including strategic tillage in these systems. These reviews also discussed the potential for productivity decline with time if a strategic tillage process is not included within CA systems. This sentiment is supported in Kirkegaard et al. (2014) which stated ‘the avoidance or exclusion of ST within the CA process makes little sense if considerable overall benefits are put at risk’. Nationally, increasing herbicide resistant weed populations have been reported as threatening crop-production profitability and sustainability across 20 million ha (Walsh and Powles 2007). Werth et al. (2011), stated that within the sub-tropical cropping region of north-east Australia, up to five weed species have evolved glyphosate resistance; namely annual ryegrass (Lolium rigidum), awnless barnyard grass (Echinochloa colona), liverseed grass (Urochloa panicoides), flaxleaf fleabane (Conyza bonariensis), and windmill grass (Chloris truncate). While efficient and effective herbicide use is still the preferred management style within the NGR, utilizing tillage as stated previously has re-entered CA potential management options. Other methods such as increased seeding rates and decreasing row spacing (Bajwa et al. 2016; Lemerle et al. 2004) have demonstrated positive results for productivity while addressing weed competition. While there are large amounts of literature on the benefits of CA (inclusive of NT, RT, and minimal tillage (MT) compared to aggressive tillage), very little has been published on the impacts of ST in NT systems, especially within Australia. The purpose of undertaking this tillage frequency study was to address literature gaps on the impacts of strategic tillage on productivity of CA systems.

The agricultural sector produces food, fuel, and fiber but is also an important source of greenhouse gas (GHG) emissions. Agriculture contributes around 9% of total United States GHG emissions, with carbon dioxide (CO2) making up the majority (81%), followed by methane (CH4) (11%) and nitrous oxide (N2O) (6%) (EPA, 2016). The global warming potential (GWP) of N2O and CH4 is 298 and 25 times greater than that of CO2, respectively. Global warming potential is a measure of the amount of energy one kilogram of a certain GHG will absorb over a given time period, usually 100 years, relative to CO2 (EPA, 2016). Agricultural soil management which includes synthetic fertilizer application and use, tillage practices, and crop rotation systems accounts for around 80% of total N2O emissions in the U.S. annually (EPA, 2016) (Venterea et al., 2011). Nitrous oxide emissions are directly affected by N application rate as well as fertilizer source and crop type (Eichner, 1990; FAO, 2001). Likewise, fertilizer application technique and timing, use of other chemicals, irrigation, and residual N and C from previous crops and fertilizer all affect N2O emissions (Eichner, 1990). Application of N fertilizer stimulates N2O production by providing a substrate for microbial N conversion through nitrification and denitrification (Venterea et al., 2005; Norton, 2008). Nitrification occurs when ammonium is either added to the soil in the form of fertilizers, as N fixation by legumes, or as mineralized soil organic matter (SOM) (Paustian et al., 2016). During this microbial process, ammonium is converted to nitrite and eventually to nitrate, yet small quantities can be lost as N2O (Snyder et al., 2009). Likewise, in conditions olow soil oxygen, denitrifiers use nitrate as a terminal electron acceptor and N2O is an intermediate step in complete denitrification to N2 gas (Aulakh et al., 1992; Robertson et al., 2007; Paustian et al., 2016). Since spring fertilizer application in the United States Corn Belt (Illinois, Iowa, Indiana, Ohio southern and western Minnesota, and eastern Nebraska) occurs when saturating rains are common, the soil may easily become water-logged, promoting large denitrification events wherein a large proportion of annual N2O flux can occur over short time scales (Venterea et al., 2012). Tillage studies often have mixed results with no-till (NT) or reduced till having less, more, or no effect on N2O emissions compared to conventional tillage systems (T) (Venterea et al., 2005; Rochette et al., 2008; Snyder et al., 2009). Snyder et al. (2009) compared various cropping rotation studies and found that continuous corn (Zea mays L.)- (CCC) had higher yields compared to a corn-soybean [Glycine max (L.) Merr.]-wheat (Triticum aestivum L.) (CSW) rotation. While CCC resulted in a two to three times higher N2O emissions, it produced four to five times the food yield in caloric value compared to the CSW rotation. Parkin and Kaspar (2006) observed that a corn-soybean (CS) rotation did not differ in N2O emissions between T and NT, but corn in the rotation emitted more N2O than did soybeans. In a meta-analysis by Pittelkow et al. (2015) studying the long-term effects of no-till on yield in several agroecosystems, the authors found that after 5+ years of notill, soybean and wheat yields matched that of conventional tillage; however, corn yields did not improve over time compared to conventional tillage. Relatively few studies have compared side-by-side crop rotation effects as influenced by tillage, and since both of these practices tend to influence soil properties more over time, long-term assessments are needed which allow for soils to stabilize.

The anaerobic degradation of organic matter accounts for a significant part of the global carbon cycle and involves the production of the greenhouse gas CH4 (Reeburgh, 2003). While the terminal processes liberating CH4 have been thoroughly investigated using isotopic techniques (Conrad, 2005), the contribution of other pathways during the anaerobic degradation is not well investigated. In this study we aimed to specify the contribution of acetogenesis to the acetate pool in rice paddy soils from two geographical origins (Philippines and Italy). The unifying trait of acetogenic bacteria is that acetate is their major product, which is mainly formed by the acetyl-CoA pathway reducing CO2 to acetate. However, the usage of the acetyl-CoA pathway in acetogens is versatile (Drake et al., 2002; Schuchmann and Muller, 2016): acetogens can compete with primary fermenters for monomeric compounds, e.g., glucose, fructose or xylose; they can act as secondary fermenters using lactate, ethanol or even methoxylated aromatics; many acetogens can grow chemolithotrophically converting H2 and CO2 to acetate; and some acetogens can even grow in syntrophic associations at the expense of acetate running the acetyl-CoA pathways in the reverse direction (Hattori et al., 2000; Hattori, 2008). Even though acetate is the major end product of acetogens, their environmental role is hard to address since many anaerobic bacteria also produce acetate by other biochemical pathways than the acetylCoA pathway. To investigate the function of chemolithotrophic acetogens in rice field soils we studied their contribution to the acetate pool using isotope techniques. This is a reasonable approach since studies of microbial pure cultures have shown that the stable carbon isotope fractionation of most fermentative processes is almost negligible (< 5‰) (Blair et al., 1985; Penning and Conrad, 2006; Botsch and Conrad, 2011), while in contrast the fractionation by chemolithotrophic acetogenesis (4 H2 + 2 CO2→ CH3COOH + 2 H2O) is strong (−38‰ to −68‰) (Gelwicks et al., 1989; Preuss et al., 1989; Blaser et al., 2013). One has to keep in mind that the acetate formed by heterotrophic acetogenesis originates partially from fermentation of organic matter and partially from the reduction of CO2 (Freude and Blaser, 2016). However, while the differentiation of aceticlastically and hydrogenotrophically formed CH4 is rather straight forward (Conrad, 2005; Blaser and Conrad, 2016), the application of such techniques for the investigation of chemolithotrophic acetogenic bacteria is much more complex. In contrast to CH4, which is an end product in most anaerobic sediments, acetate is produced and consumed by different metabolic pathways, which all imprint a signature on the stable carbon isotope composition (13C/12C) of acetate (Fig. 1) (Conrad et al., 2014). Hence, the stable carbon isotopic composition of acetate is basically governed by three factors: (1) the isotopic composition of its precursors, (2) the isotopic fractionations (enrichment factors) associated with its formation and consumption, and (3) the relative rates of all acetate producing and consuming processes which influence its pool size. The knowledge of all three factors is of great importance for the evaluation of 13C values of acetate determined in environmental samples.

Plants have always been a key to survival for humans, as they are vital to meet the world’s food needs by contributing about 82% to total global food production harvested from land resources. Historically, over 3000 species of plants have been used to feed humans. Among the plants, wheat is an excellent health building food and an excellent source of minerals, dietary fber, proteins, and B-group vitamins. It is often grown in rain-fed areas (Geravandi et al. 2011) for its starch and protein that makes it the most important cereal crop globally, and, along with rice and maize, forms an integral part of diets of over 4.5 billion people. The success of wheat in temperate world regions is due to its adaptability and high yields together with its unique four dough properties which give it a distinct advantage over the other temperate crops (Shewry 2007). Drought is a polygenic stress (Kilic and Tacettin 2010) that reduces productivity and quality of crops (Waraich et al. 2010, 2011) and also limits the successful realization of land potential throughout the world (Pierre et al. 2012; Liu et al. 2016). The scarcity of water, caused by erratic and poorly distributed rainfall, causes tremendous global losses in agriculture. Russia, for example, has estimated total losses of over US$800 million due to drought and other natural challenges in 2000 alone. Drought afects all plant development stages from germination, vegetative and reproductive growth to grain flling and maturation of the crop (Hossain et al. 2012). Drought reduces nitrogen (N) uptake efciency and utilization by plants. The deprived nutrient uptake is due to impaired membrane permeability and active transport and reduced transpiration rate resulting in repressed root absorbing power. In semi-arid and arid regions of the world, drought is one of the leading causes of limited agriculture production. Many growth variables and functions of plants are afected by water stress (Nezhadahmadi et al. 2013). Under drought conditions, the intake of CO2 is reduced due to stomatal closure, and inside the stomata, a high level of oxygen produces reactive oxygen species, caused by the partial reduction of oxygen, and causes rupturing of membranes which become leaky, thereby afecting respiration, photosynthesis, and the overall development of the plant. Reactive oxygen species also seriously damages the production of cellular components such as carbohydrates, nucleic acids, lipids, and proteins (Waraich et al. 2011; Reddy et al. 2014). Drought is one of the major abiotic stresses that afect at least 60% of wheat production in high-income countries and about 32% of 99 million hectares in low-income least developed countries (Chen et al. 2012). Water defcit might decrease wheat grain yield from 17 to 70% (Nouri-Ganbalani et al. 2009). Daryanto et al. (2016) reported 20.6% yield losses in 40% reduced water. Double ridge to anthesis stage is the most sensitive growth period regarding wheat yield to water defcit because of the negative infuence on number of spikelets and ultimately kernels per spike. Water defcit decreases grain yield by afecting anthesis and grain flling period. It is well documented that plant height, biomass, and yield are more sensitive traits to water defcit in comparison with number of spikes and 1000 grain weight.

Water defcit and dehydration is one of the most important environmental stress factors that greatly infuences plant growth and development and seriously reduces crop yield (Pandey et al. 2008; Ceccarelli 2010; Farooq et al. 2017) and it is the bottleneck of agricultural development in many regions. However, very few plants have subjected to biochemical and molecular studies to analyze the mechanisms of dehydration stress tolerance. They showed that the intrinsic ability of plants to tolerate various environmental stresses was a result of diferent biochemical and molecular mechanisms. What is more, elucidation of the nature of these mechanisms would be an interesting area of research. Many studies on the resistances of plants under water stress showed that plants can produce a series of changes in morphological, physiological, biochemical and molecular aspects and showed drought-resistant ability (Alam et al. 2010; Manaa et al. 2011; Chen et al. 2011; Gupta et al. 2014). Nowadays, with the rapid development of modern molecular biology, the mechanism of drought resistance in plants not only had great infuence on it in terms of morphology and physiology but also genetic engineering had increased understanding (Zhu 2002; Chaves et al. 2009; Cramer et al. 2011). Since proteins are directly involved in the plant stress response, the response proteins induced by drought have already became a hot spot in fghting drought and gene expression adaptation to adverse arid circumstances (Watson et al. 2003; Zhang et al. 2006; Farooq et al. 2009; Azooz 2010; Kosová et al. 2011; Xu and Wu 2016). Legumes are valuable agricultural, commercial and cold-tolerant crops that serve as important nitrogen sources for human diet, animal feed and farmland (Broughton et al. 2003; Boschin and Arnoldi 2011). Particularly, changing in proteomic expression during drought stress had been observed in legume crops showing diferential regulation of mechanisms, such as chick pea (Turner et al. 2005; Subba et al. 2013; Jaiswal et al. 2014), soy bean (Tran and Mochida 2010; Das et al. 2016) and mung bean (Sengupta et al. 2011). They showed that the intrinsic ability of model legume plants to tolerate various environmental stresses was a result of diferent biochemical and molecular mechanisms. Faba bean (Vicia faba L.) is one of the most important legume species for human consumption due to its highnutritional value, high protein content and beneficial healthy properties (Amede et al. 1999; Abdelmula et al. 1999). Qinghai is located in northwest China and on the northeastern part of the Qinghai–Tibet Plateau with an average altitude above 3000 m. Faba bean could adapt to the cold climate and varied land conditions. Faba bean can product rhizobia nitrogen symbionts for itself and succession crop as nitrogen sources and it could change soil structure. In Qinghai, 80% of the faba bean is distributed in the irrigated agriculture region which accounts for only 30% of the total cultivated land biome; however, 70% of the total cultivated land is a rain-fed land (dry areas or semi-arid areas) where the planting area of faba bean accounts only for 20%. Faba bean uses more water and more sensitive to drought than some other grain legumes such as common bean, pea and chickpea (Mittler and Zilinskas 1994; Martínez et al. 2007; Awasthi et al. 2014). Drought stress can lead to serious yield decrease of faba bean in arid area and afects the efcient and sustainable development of agriculture in Qinghai.

Plants close stomata to avoid losing water through transpiration under water limiting conditions (Chaves, 1991), a physiological response that is related to either hydraulic or chemical signals (Davies and Zhang, 1991; Christmann et al., 2007). Plant acclimation to water deficits involves morphological changes that regulate water balance, with plants showing decreases in leaf area and shoot/root ratio (Pimentel, 2004). Cell osmoregulation by solutes such as sugars, glycine-betaine, and proline is another response to water deficits, allowing the maintenance of water content and protecting cellular structures (Verlues et al., 2006). Rapid stomatal response to changes in water availability is an important feature in sugarcane (Saccharum spp.), preventing excessive loss of leaf turgor and further decreases in leaf water content (Ribeiro et al., 2013). However, it is well-known that stomatal closure causes low CO2 availability for photosynthetic enzymes (Du et al., 1996; Chaves et al., 2009; Machado et al., 2013) and then an imbalance between photochemical and biochemical reactions in the leaves. Therefore, production of reactive oxygen species (ROS) is enhanced under drought conditions, and plants should be able to control such deleterious molecules through their antioxidant system. This protective system consists of several enzymatic and non-enzymatic compounds, which prevent oxidative damage by scavenging ROS inside cells (Mittler, 2002). For instance, increases in superoxide dismutase and ascorbate peroxidase activities were associated with rapid recovery of leaf gas exchange in sugarcane plants after rehydration (Sales et al., 2013). These reported plant responses to a single drought event are quite common; however, plants are exposed to recurrent cycles of drought and rehydration in nature, and the consequences of such repetitive drought events are less understood (Walter et al., 2011). Plants can acclimate to varying water conditions through morphological and physiological changes, which favor the maintenance of plant growth or survival under stressful conditions (Chaves et al., 2002). Some changes during an acclimation period can allow faster responses and enhanced plant performances during the next stressful event. In fact, an experimental design with repeated cycles of droughts is a more realistic approach when considering plants in their natural environment, with improved plant performances under limiting conditions being found in several species when there was previous exposure to stressful conditions. While Trifolium alexandrinum was able to maintain high leaf water potential and relative water content after a second drought event (Iannucci et al., 2000), Quercus ilex exhibited reductions in leaf water potential and transpiration accompanied by osmotic adjustment after hardening (Villar-Salvador et al., 2004). Seedlings of Moringa oleifera that had previously been subjected to osmotic stress experienced increased drought tolerance, with plants showing higher water-use efficiency, higher photosynthesis and increases in activity of antioxidant enzymes under water deficit conditions (Rivas et al., 2013). However, most of these studies compared plants of differing ages under varying stress intensities and environmental conditions, which makes the study of stress memory difficult.

مقاله سال 2016

مقدمه
اگرچه گیاهان اکسیژن را از طریق فتوسنتز تولید می کنند ، اما کمبود یک سیستم کارآمد برای انتقال اکسیژن به ارگان های غیر فتوسنتزی نشان می دهد که این ارگان ها ممکن است در صورت محدود شدن اکسیژن از خارج ، از اکسیژن محدود شوند. علاوه بر این ، غوطه ور شدن کامل گیاه به واسطه ی سیل ممکن است منجر به دسترسی پایین به اکسیژن در اندام های بالاتر از زمین شود ، به خصوص زمانی که آلودگی آب فتوسنتز را محدود می کند. زمانی که اکسیژن برای تنفس محدود شود ، گیاهان هیپوکسی را تجربه می کنند ، در حالی که فقدان کامل اکسیژن (آنوکسی) برای بقای گیاه بیش تر مضر است. هم هیپوکسی و هم آنوکسی (کمبود اکسیژن) موجب برنامه نویسی مجدد ساختار ژن با القای متابولیسم خمیری می شوند و گیاه را قادر به استفاده از گلیکولیز برای تولید ATP می سازند. تغییرات آب و هوا باعث دسترسی بیش از اندازه به آب می شوند و در برخی نواحی منجر به خشکسالی می شوند ، در حالی که جاری شدن سیل به علت بارندگی شدید سایر مناطق جغرافیایی را تحت تاثیر قرار می دهد. به غیر از این در برخی از موارد انواع جدید محصولات قادر به مقاومت در برابر تنش های غیر زنده می باشند ، و حاصلخیزی به شدت تحت تاثیر قرار خواهد گرفت. تا یک دهه پیش اطلاعات کمی در رابطه با ژن هایی که تحمل در برابر غوطه ور شدن را افزایش می دادند وجود داشت ، و در طول سال های اخیر مکانیسم های ملکولی مربوط به سنجش اکسیژن و سیگنال دهی در گیاهان توسعه یافتند. در این بررسی ، یافته های اخیر در زمینه ی گیاهان غیرهوازی از اکوفیزیولوژی گیاهان که در تالاب ها رشد می کنند ، مورد توجه قرار دارند.