RETRACTED: Strategies for Producing More Rice with Less Water

C H A P T E R

S I X

Strategies for Producing More Rice with Less Water M. Farooq,*,†,1 N. Kobayashi,† A. Wahid,‡ O. Ito,§ and Shahzad M. A. Basra} Contents 352 353 354 358 362 364 364 372 373 374 375

1. Introduction 2. Genetic Improvement for Water Productivity 2.1. Selection and breeding strategies 2.2. Molecular and biotechnological approaches 2.3. Water-use efficiency and transpiration efficiency 3. Crop Management 3.1. Production systems 3.2. Other management practices 3.3. Physiological implications 4. Future Thrusts References

Abstract Rice is life for more than half of humanity. It is the grain that has shaped the cultures, diets, and economies of billions of people in the world. Food security in the world is challenged by increasing food demand and threatened by declining water availability. More recently, the increase in area under biofuel crops at the cost of food crops is also threatening. Exploring ways to produce more rice with less water is essential for food security. Water-saving rice production systems, such as aerobic rice culture, system of rice intensification (SRI), ground-cover rice production system (GCRPS), raised beds, and alternate wetting and drying (AWD), can drastically cut down the unproductive water outflows and increase water-use efficiency (WUE). However, these technologies can sometimes lead to some yield penalty, if the existing lowland varieties are used.

* { { } } 1

Department of Agronomy, University of Agriculture, Faisalabad 38040, Pakistan International Rice Research Institute (IRRI), Metro Manila, Philippines Department of Botany, University of Agriculture, Faisalabad 38040, Pakistan Japan International Research Center for Agricultural Sciences, Tsukuba, Japan Department of Crop Physiology, University of Agriculture, Faisalabad 38040, Pakistan Corresponding author: [email protected]; [email protected]

Advances in Agronomy, Volume 101 ISSN 0065-2113, DOI: 10.1016/S0065-2113(08)00806-7

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2009 Elsevier Inc. All rights reserved.

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Other new approaches are being explored to increase water economy without compromise on yield. These include the incorporation of the C4 photosynthetic pathway into rice to increase rice yield per unit water transpired, the use of molecular biotechnology to develop rice varieties with improved water-use efficiency, transpiration efficiency (TE), drought tolerance, and the development of varieties for aerobic system, to achieve high and sustainable yields in nonflooded soil. Through the adoption of water-saving irrigation technologies, rice land will shift away from being continuously anaerobic to being partly or even completely aerobic. These shifts will produce profound changes in water conservation, soil organic matter turnover, nutrient dynamics, carbon impounding, weed flora, and greenhouse gas emissions. Although some of these changes can be positive, for example, water conservation and decreased methane emission, others might be negative, for example, release of nitrous oxide from the soil and decline in soil organic matter. The challenge will be to develop effective integrated natural–resource–management interventions, which would allow profitable rice cultivation with increased soil aeration, while maintaining the productivity, environmental safety, and sustainability of rice-based ecosystems. This chapter discusses the integrated approaches like genetics, breeding, and resource management to increase rice yield and to reduce water demand for rice production.

1. Introduction Food security depends on the ability to increase production with decreasing availability of water to grow crops. Rice, as a submerged crop, is a prime target for water conservation because it is the most widely grown of all crops under irrigation. To produce 1 kg of grain, farmers have to supply 2–3 times more water in rice fields than other cereals (Barker et al., 1998). In Asia, more than 80% of the developed freshwater resources are used for irrigation purposes; about half of which is used for rice production (Dawe et al., 1998). Rapidly depleting water resources threaten the sustainability of the irrigated rice and hence the food security and livelihood of rice producers and consumers (Tuong et al., 2004). In Asia, 17 million hectare (Mha) of irrigated rice areas may experience ‘‘physical water scarcity’’ and 22 Mha may have ‘‘economic water scarcity’’ by 2025 (Tuong and Bouman, 2002). There is also much evidence that water scarcity already prevails in rice-growing areas, where rice farmers need technologies to cope with water shortage and ways must be sought to grow rice with lesser amount of available water (Tuong and Bouman, 2002). Rice is very sensitive to water stress and attempts to reduce water inputs may tax true yield potential (Tuong et al., 2004). The challenge is to develop novel technologies and production systems that would allow rice production to be maintained or increased at the face of declining water availability.

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Former requires a possible shift from the traditional system of flooded rice to growing rice aerobically and the latter needs the development of highyielding varieties that thrive under aerobic conditions (Castan˜eda et al., 2003). Several strategies are in vogue to reduce rice water requirements, such as saturated soil culture (Borrell et al., 1997), alternate wetting and drying (AWD; Li, 2001; Tabbal et al., 2002), ground-cover systems (Lin et al., 2003a,b), system of rice intensification (SRI; Stoop et al., 2002), aerobic rice (Bouman, 2003), raised beds (Singh et al., 2003), etc. Development of rice varieties through conventional breeding, marker-assisted selection (MAS), and employing biotechnological tools for water-limited conditions are the areas of current research (Atlin and Lafitte, 2002; Babu et al., 2003; Cattivelli et al., 2008; Ku et al., 2000). This chapter discusses strategies and options to make rice production more water-efficient with integrative use of crop improvement and management tools.

2. Genetic Improvement for Water Productivity Genetic improvement for adaptation to water-limited conditions is addressed through conventional approach by selecting for yield and secondary traits contributing to water saving (Farooq et al., 2009). The effectiveness of selection for secondary traits to improve yield under water-limiting conditions has been demonstrated in maize (Chapman and Edmeades, 1999), wheat (Richards et al., 2000), and sorghum (Tuinstra et al., 1998). Many studies have been undertaken to find genetic variation in traits that are expected to influence the response of rice to water deficit, including deeper and thicker roots (Yadav et al., 1997), root-pulling resistance (Pantuwan et al., 2002), greater root penetration (Ali et al., 2000; Clark et al., 2000; Fukai and Cooper, 1995), osmotic adjustment (OA; Lilley and Ludlow, 1996), and membrane stability (Tripathy et al., 2000). For the newly introduced aerobic rice culture, promising varieties should possess improved lodging resistance and higher harvest index. Medium-stature and moderately drought-tolerant cultivars are preferred for aerobic rice culture (Atlin et al., 2004, 2006). At IRRI, early heading type of a popular variety IR64 is being developed to provide suitable breeding materials for watersaving rice cultivation (Fujita et al., 2007). Development of early maturing and high-yielding rice varieties has substantially increased the average rice yield and reduced crop duration. This has contributed to a threefold increase in water productivity with respect to total water inputs (Bouman et al., 2006). Hybrid rice has 9% greater yield potential than inbred, with comparable growth duration when grown under flood-irrigated conditions in the tropics (Peng et al., 1999). This yield advantage offers another opportunity to increase the water productivity of flood-irrigated lowland

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rice (Peng and Bouman, 2007). The power of molecular biology for locating important gene sequences and introgressing QTL, or even selecting for genetically important QTL to develop cultivars more efficiently utilizing water, strongly depends upon our understanding of yield-determining physiological processes (Kirigwi et al., 2007). Improvement of genetic resistance to biotic stress is also important and effective breeding approach to water-saving cultivation of rice. Rice blast disease, one of the most destructive diseases of rice, is important problem under water-limited conditions, because blast is known to tend to occur in irrigated rice in tropical upland areas (Bonman, 1992) and rainfed lowland prone to drought (Mackill and Bonman, 1992). Actually, severe blast damage was observed in aerobic rice field at IRRI (Kobayashi et al., 2006). In Brazil, blast resistance is the most important target trait for breeding program of aerobic rice variety (Breseghelo et al., 2006). In the following lines, use of different selection and breeding strategies, functional genomic approaches, and biotechnological tools to develop the suitable protocols for water-saving cultivation has been presented.

2.1. Selection and breeding strategies Rice breeding over the last decade has increased water productivity by increasing yields together with reducing crop growth duration, and hence reducing seasonal transpiration (Tuong, 1999). Grain yield is characterized by (a) the amount of biomass produced by photosynthesis and (b) the amount of biomass partitioned to grains, usually expressed as harvest index (HI). Most of the increases in rice yield in the last decades were achieved by improvement in HI. Some scientists argue that HI may now be approaching its theoretical limit in major crops (Richards et al., 1993). The photosynthesis process generally governs biomass production per unit water transpired; referred as water-use efficiency (WUE). Although there is little difference in photosynthetic rate among different commonly grown rice varieties, Peng et al. (1998) reported that WUE was some 25–30% higher for tropical japonica than for indica rice. This implies that significant variation exists in rice germplasm for photosynthesis-to-transpiration ratio, and this could be investigated further to enhance water productivity of rice. Conventional breeding has been based on empirical selection for yield (Atlin and Lafitte, 2002). However, this approach is far from being optimal, since yield is a quantitative trait and characterized by a low heritability and a high genotype  environment interaction (Babu et al., 2003). It is strongly believed that understanding of physiological and molecular basis may help target key yield-limiting traits. Such an approach may complement conventional breeding programs and hasten yield improvement (Cattivelli et al., 2008).

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For a breeder, individual or combination of traits, that would directly or indirectly be associated with enhanced plant survival, are likely to improve economic yield (with or without stability), which may constitute potential target(s) for study and selection (Kirigwi et al., 2007). It is therefore imperative that utility of trait(s) for enhancing water productivity must be manifested as enhanced plant survival and better grain and dry matter yield under conditions of drought stress when assessed at the level of whole plant and crop community. Finally, the magnitude of expression of each trait and its ability to blend with other causal- or causally related traits will contribute toward its utility in plant-breeding programs (Kirigwi et al., 2007). The root characters such as biomass, length, density, and depth are very important in contributing to water saving (Subbarao et al., 1995; Turner et al., 2001). Deep and thick root system is helpful in extracting water from considerable depths (Kavar et al., 2007). Glaucousness or waxy bloom on leaves helps in reducing water losses and the maintenance of high tissue water potential, and is, therefore, considered as a desirable trait for drought tolerance (Ludlow and Muchow, 1990; Richards et al., 1986). Plant growing under water deficit conditions must conserve available water by reducing transpiration while, at the same time, fixing sufficient CO2 to meet the energy needs of the plant. Studies suggest that transpiration losses can also be reduced even in sunny, dry environments by reducing the leaf size. The plant size as expressed mainly in terms of single plant leaf area or leaf area index (LAI) has a major control over water use under stress. Short stature and small leaf area are generally conducive to low productivity while they limit water use. Botanists have long recognized plants bearing small leaves as typical ecotypes of xeric environments. Such plants withstand drought very well although their growth rate and biomass are relatively low (Ball et al., 1994). Henson (1985) used leaf size as a criterion for selection for water saving in rice. Leaf area is product of leaf length and width, and variation in both these has been recorded in rice lines. Leaf pubescence is a xeromorphic trait that helps protect from excessive heat load. Hairy leaves reduce leaf temperature and transpiration (Sandquist and Ehleringer, 2003), whilst interand intraspecific variation exists for the presence of this trait. Under high temperature and radiation stress, hairiness increases the light reflectance and reduces water loss by increasing the boundary layer resistance to water vapor movement away from the leaf surface. Although drought stress also induces the production of trichomes on both sides of wheat leaves, they had no significant influence on boundary layer resistance. The possession of a deep and thick root system allows access to deep water in the soil, and is considered important in determining drought resistance in upland rice (Kavar et al., 2007). Evidence suggests that it is quality (distribution and structure), not quantity, of roots that determines the most efficient strategy for extracting water during the crop-growing

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season. Rice, evolved from a semiaquatic ancestor, was domesticated into lowland or aerobic cultivars. These cultivars developed aerenchyma in roots, a superficial root system, and high levels of nonstomatal water loss from leaves (Lafitte and Bennet, 2003). There is a similar tendency to conserve adaptations to excess water at the cellular and molecular levels. For rice to succeed as an aerobic crop, breeders must overcome the legacy of anaerobic adaptation, enabling the crop to tolerate intermittent water deficits, high soil impedance, and low humidity of air. Extensive genetic variation exists within cultivated rice and additional variation exists in wild relatives. Dramatic contrasts are observed among rice cultivars in the response of root growth to soil drying; some cultivars cease root development, some increase root mass in superficial layers, and others show increased and deep root growth. Genetic variation is also observed in the sensitivity of rice leaf area expansion to both soil and atmospheric water deficit, and in the relative reduction in spikelet number and fertility that occurs in aerobic conditions. The processes like drought rhizogenesis, leaf expansion, and sink pruning are expected to reflect differences in signal reception and transduction in rice compared with other crops. Improved understanding of the molecular basis and genetic control of these signaling processes is likely to develop successful aerobic cultivars that would respond to the environment more like other upland crop species (Lafitte and Bennett, 2002). Rice plants completing life cycle in shortened period of time use less amount of water. Crop duration is interactively determined by genotype and the environment, and determines the ability of the crop to complete growth cycle in lesser time (Dingkuhn and Asch, 1999). Time of flowering is a major trait of crop adaptation to environment, particularly when the growing season is restricted by low water availability and high temperatures during later growth stages. Developing short-duration varieties has been an effective strategy for minimizing yield loss from terminal drought, as early maturity helps the crop to avoid the period of stress (Kumar and Abbo, 2001). However, yield is generally correlated with longer crop duration under favorable growing conditions, and any reduction of crop duration below optimum would tax yield (Turner et al., 2001). Aerobic systems are likely to require cultivars that have been selected from early generations under high-input aerobic management to produce genotypes that combine moderate tolerance of moisture stress with high HI and lodging resistance. Cultivars that perform well under aerobic management usually contain germplasm from both traditional upland and eliteirrigated parents, but some cultivars without elite-irrigated high-yielding variety parentage, and some developed for irrigated systems also display high yields in aerobic systems (Atlin and Lafitte, 2003).

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For the Australian situation, Reinke et al. (1994) argued that reducing duration could save up to 10% of irrigation water, whereas Williams et al. (1999) concluded that reduced duration will always reduce yield potential and hence water productivity. Despite some evidence for the latter argument, varieties with higher yield potential and shorter duration have been developed (Reinke et al., 2004). Short-duration varieties also facilitate increased WUE of the farming system. For example, earlier maturity allows earlier harvest, increasing the chance of timely establishment of a winter crop after rice and making efficient use of stored soil water and winter rainfall instead of losing it as deep and surface drainage or transpiration by weeds. The most efficient strategy for identifying cultivars for near saturation systems is to screen short-duration elite-irrigated varieties under nonstress management to eliminate cultivars vulnerable to soil drying (Atlin and Lafitte, 2003). Several putative traits contributing to water saving and drought resistance in rice have been suggested (Fukai and Cooper, 1995). Root characteristics such as thickness, rooting depth, root density, rootpulling force, and root penetration ability have been associated with drought avoidance in rice (Nguyen et al., 1997). OA capacity is an important, shoot-related component of drought tolerance in crop plants. OA is the active accumulation of solutes during the development of water stress in plants (Blum, 1988), allows maintenance of higher turgor potential at a given leaf water potential. OA delays leaf rolling, tissue death, and leaf senescence under water stress in rice (Hsiao et al., 1984), and has been shown to enhance grain yield under water-limited conditions in several other crops (Zhang et al., 1999). However, a yield benefit due to OA is yet to be demonstrated in rice. Despite our understanding of the role of putative traits in drought resistance, these traits are rarely selected for crop improvement programs because phenotypic selection for most root traits and OA is difficult and labor intensive. Much effort is currently being directed to developing molecular markers for various traits such as maximum rooting depth (Champoux et al., 1995), the capacity of roots to penetrate hard pans (Ray et al., 1996), and ability of the plant to osmotically adjust to water deficit (Lilley and Ludlow, 1996). Considering these limitations to efficient selection, molecular marker technology is a powerful tool for selecting such traits. QTLs have been detected for several root-related traits and OA in rice (Ali et al., 2000; Lilley and Ludlow, 1996; Ray et al., 1996; Yadav et al., 1997; Zhang et al., 2001; Zheng et al., 2000). A significant proportion of the phenotypic variability of several of these putative drought resistance traits is explained by the segregation of relatively few genetic loci, thus leading to the possibility of indirect selection of these complex traits by MAS strategy. Identification of QTLs associated with water saving is an important tool for MAS of desirable plants.

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2.2. Molecular and biotechnological approaches Recent advances in genomics, the development of advanced analytical tools at the molecular level, and genetic engineering provide new avenues for improving yield potential and enhancing drought stress tolerance. Currently, slow progress in breeding to improve water productivity may be accelerated by the discovery and subsequent manipulation of regulatory genes underlying the complex physiological and biochemical responses of rice plants to water deficit. Common research tools, tolerance mechanisms, and breeding solutions are emerging across the evolutionary diversity of crops and plants (Tuong and Bouman, 2003). Enormous public- and private-sector investments in genomic analysis of Arabidopsis thaliana, cereals, and other crops are already contributing greatly to these efforts (Bennett, 2001). Many laboratory and field studies have shown that transgenic expression of some of stress-regulated genes results in increased WUE (Table 1). These transgenic approaches are currently the mainstream method to bioengineer crop plants that would require less water (Bahieldina et al., 2005). However, enhanced expression of these genes is frequently associated with retarded growth and thus may limit its practical applications. To identify the less obvious genetic networks that respond to stress, more straightforward and sensitive methods are imperative. The advent of whole genomics and related technologies are providing necessary tools to identify key genes that respond to drought and their adaptive regulation to stress (Bruce et al., 2002). Introducing a single enzyme or even an incomplete portion of the C4 cycle is, of course, unlikely to have a large impact on photosynthesis. However, some evidence suggests that the manipulations have led to the desired redirection of fluxes (Hudspeth et al., 1992). Introduction of the gene of maize PEP carboxylase in rice indicated remarkably higher level of expression (Ku et al., 1999). The activities of PEP carboxylase in leaves of some of these transgenic rice plants were two- to threefolds higher even than those in maize, and the enzyme accounted for up to 12% of the total leaf-soluble protein. It means that increasing the amounts of PEP carboxylase in isolation does not have dramatic effects on photosynthesis, although it may alter stomatal conductance (Ku et al., 2000). Transgenic rice exhibited reduced O2 inhibition of photosynthesis, but this was probably due to effects of phosphate recycling that affect photorespiration (Table 2; Matsuoka et al., 2000). In a study, Suzuki et al. (2000) overexpressed an unregulated phosphoenolpyruvate carboxykinase (PCK) from C4 plant, Urochloa panicoides, in the chloroplasts of rice leaf. In 14CO2-labeling experiment, up to 20% of the radioactivity was incorporated into C4 acids (malate, oxaloacetate, and aspartate) in leaves of transgenic plants, as compared with about 1% in

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Table 1

Transgenic rice plants reporting effects under water-limited conditions

Gene

Effect

References

p5cr

Faster shoot and root growth was observed in transgenic seedlings than controls under water-limited conditions. Stress-inducible expression of p5cr transgene gave the greatest effect Majority of transgenics survived an episode of acute drought stress. Under cycles of drought/ recovery, the transgenics had higher biomass and were taller than the controls Transgenic lines showed more sustained plant growth, less photo-oxidative damage, and more favorable mineral balance under water-limited conditions, than controls Overexpression of OsCDPK7 had less wilting than controls Transgenic plants maintained higher growth rates than controls under drought Higher leaf RWC and tolerance to water stress by protecting cell membrane After salt and drought treatments, transgenic lines showed increased stress tolerance (cell integrity and growth), compared to the control plants Accumulation of either PMA80 or PMA1959 correlates with increased drought tolerance Transgenic plants with higher expression levels of sHSP17.7 protein recovered the growth after upon rewatering after the stress period Transgenic plants exhibited less membrane injuries than control plants

Su and Wu (2004)

coda

otsA and otsB

OsCDPK7 HVA1

HVA1

HVA1

PMA80 and PMA1959 sHSP17.7

SWPA2

Sawahel (2003)

Garg et al. (2002)

Saijo et al. (2000) Xu et al. (1996)

Babu et al. (2004)

Rohila et al. (2002)

Cheng et al. (2002) Sato and Yokoya (2007)

Wang et al. (2005)

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Transgenic C4 rice, the genes, and their effect

Source of genes

Gene

Effect

References

Maize

PPDK

Ji et al. (2004)

Maize

NADP-ME

Maize

PEPC + PPDK

Maize

PEPC

Urochloa panicoides

PCK

Maize

PEPC

Increased net photosynthesis and decreased photorespiration in transgenic plants Photorespiration rate decreased and net photosynthetic rate increased in transgenic plants Increased net photosynthesis and decreased photorespiration in transgenic plants Photorespiration rate decreased and net photosynthetic rate increased in transgenic plants Threefold greater sucrose synthesis was observed in transgenic plants than in control plants Transgenic plants exhibited a higher photosynthetic capacity (up to 35%) than untransformed plants. The increased photosynthetic capacity in these plants was mainly associated with an enhanced stomatal

Ji et al. (2004)

Ji et al. (2004)

Ji et al. (2004)

Suzuki et al. (2000)

Ku et al. (2007)

x

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Table 2 (continued) Source of genes

Gene

Maize

PPDK

Maize

PEPC

Maize

PEPC

Maize

NADP-ME + PPDK

Effect

conductance and a higher internal CO2 concentration A higher photosynthetic capacity (up to 35%) than untransformed plants Transgenic C4 plants were 30–35% more efficient in photosynthesis Photosynthetic capacity was increased greatly (50%) under high CO2 supply. In CO2-free air, CO2 release in the leaf was less. In addition, transgenic rice was more tolerant to photoinhibition Photosynthetic rate was increased by 50%

References

Ku et al. (2007)

Ku et al. (1999)

Jiao et al. (2005)

Jiao et al. (2002)

excised leaves of control plants. When 14C-malate was fed to excised leaves the extent of incorporation of radioactivity into sucrose was threefold greater in transgenic than control plants and the level of radiolabeled aspartate was significantly lower in transgenic plants. Thus, expression of PCK in rice chloroplasts led to a partial change in carbon flow in mesophyll cells into a C4-like photosynthetic pathway. Overexpression of maize pyruvate, Pi dikinase (Fukayama et al., 2001), has been claimed to display a higher photosynthetic rate, associated with higher stomatal conductance (Ku et al., 2000).

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About 20–70-fold increase in maize NADP-malic enzyme in rice leaves (located mainly in the chloroplasts) led to an aberrant chloroplast structure with a granal thylakoid membranes, and an inverse correlation between NADP-malic enzyme activity and chlorophyll and photosystem II activity (Takeuchi et al., 2000). This is particularly interesting in relation to the presence of granal chloroplasts in the bundle sheath of NADP-malic species. However, other studies on rice overexpressing maize NADP-malic enzyme also indicated a reduction in chlorophyll content, enhanced photoinhibition, and reduced growth. This probably resulted from a greater reduction of the NADP pool as a result of a high activity of the overexpressed enzyme in vivo (Takeuchi et al., 2000; Tsuchida et al., 2001).

2.3. Water-use efficiency and transpiration efficiency While transpiration efficiency (TE) is the ratio between photosynthesis and transpiration (Tuong and Bouman, 2003), whole-plant WUE can be expressed as the ratio of total biomass or grain production to the amount of water transpired. The WUE is determined by both photosynthesis and transpiration. Increasing photosynthesis and/or decreasing transpiration would elevate plant WUE (Tuong and Bouman, 2003). Stomata play a crucial role in both these processes, and hence economizing water. Different plant species have evolved different stomata with great variations in size, density, and morphology. This rapid opening and closing strategy can save energy and increase photosynthesis and WUE (Grantz and Assmann, 1991). The physiological and molecular bases of water saving are complex and highly linked to drought tolerance mechanisms (Chaves and Oliveira, 2004). There are two major ways to increase the plant WUE. One is the engineering-based cropping system where modern irrigation techniques play an important role. Much progress has been made to improve WUE by managing irrigation (Giordano et al., 2007; Jones, 2004; Kacira and Ling, 2001; Kang and Zhang, 2004; Sun et al., 2005). The other way is ‘‘biological water saving (BWS),’’ the physiological and ecological bases of water saving by crops in agriculture (Shan, 1991). The concept of BWS was further developed by Shi (1999) and defined as ‘‘more agricultural products output with the same or less water input by exploiting the physiological and genetic potential of organisms themselves.’’ The core of BWS is to increase WUE of plants, which is used as an indicator for plant’s ability to economize water. In contrast to the irrigation method, BWS is a more efficient and economic way to increase WUE for the obvious reason of less input than engineeringbased methods. Blum (2005) pointed out that high yield under water-limited conditions is generally associated with increased WUE mainly because of high

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water use. Therefore, selection for high WUE in a breeding program will result in smaller or earlier flowering plants that use less water but have low yield potential at the same time (Blum, 2005). Thus, the challenge is to develop water-efficient genotypes that produce higher yields with limited water supply, and equal or greater yields than current varieties under favorable growth conditions without stress. Because photosynthesis and transpiration rates are generally proportional, there is only a small difference in TE among rice varieties at the single-leaf level when grown under flooded and aerobic cultivation (Singh and Sasahara, 1981). However, developing rice varieties with superior performance under water-saving technologies such as AWD and aerobic cultivation could result in a significant improvement in water productivity of irrigated lowland rice (Peng and Bouman, 2007). Peng et al. (1998) reported that improved tropical japonica rice lines had 25–30% higher TE at the single-leaf level than indica varieties when grown under flooded conditions. This was because the indica varieties had a higher transpiration rate than the tropical japonica lines, whereas the differences in photosynthesis between the two types were relatively small and inconsistent across growth stages and years compared with the differences in transpiration rate. In another study, Yeo et al. (1994) observed large differences among Oryza species in TE at the single-leaf level. Oryza australiensis had significantly greater TE than O. sativa at the same photosynthetic rates. The potential for exploiting this trait, however, has not been investigated. Varietal differences in TE at the single-leaf level and whole-plant WUE measured by gravimetric determinations of growth and water loss from individual plants were reported in rice (Flowers et al., 1988). However, a high WUE was associated with the nondwarf habit and, therefore, it may not be useful to incorporate this trait into commercial varieties to increase water productivity. Increase in waxiness of rice leaves was proposed to reduce nonstomatal transpiration but the impact on WUE was not demonstrated (Lafitte and Bennett, 2002). Transforming the C3 rice plant into C4 by genetic engineering of photosynthetic enzymes and required anatomic structures was suggested as another approach to improve TE. High-level expression of maize phosphoenolpyruvate carboxylase (PEPC) and pyruvate, orthophosphate dikinase (PPDK) and NADP-malic enzyme (NADP-ME) in transgenic rice plants has been achieved (Table 2; Agarie et al., 1998). Ku et al. (2000) reported that PEPC and PPDK transgenic rice plants had up to 30–35% higher photosynthesis than untransformed ones. However, increase in photosynthesis was associated with enhanced stomatal conductance, which reduces the potential for increasing TE by the development of C4 rice plants. Nevertheless, development of C4 rice plants seems more ambitious and very cumbersome.

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3. Crop Management The management strategies start from the selection of a good genotype, crop planting site; include seedbed preparation, production system, date and method of planting, sowing time, plant protection, nutrient management; and last till the crop harvesting. In rice systems aiming to have high water productivity, soil type (Tripathi, 1996), weed management (Singh et al., 2003), irrigation method (Beecher et al., 2006), and land leveling (Alam et al., 2003; Kahlown et al., 2002) are of premier importance. One obvious measure to improve the water productivity is to reduce the evaporation by shortening the land preparation period (Tuong, 1999). Moreover, early canopy closure can help to reduce evaporation after crop establishment (Tuong et al., 2000). This can be achieved by proper plant density and growing rice varieties with good seedling vigor (Tuong et al., 2000). These measures can also help the rice plants compete better with weeds, thus reducing nonbeneficial transpiration from weeds and increasing yield (Tuong et al., 2000). Amongst different rice production systems, aerobic rice (Bouman et al., 2007), AWD (Cabangon et al., 2001), raised beds ( Jehangir et al., 2002), SRI (Uphoff and Randriamiharisoa, 2003), and ground-cover rice production system (GCRPS; Dittert et al., 2003) have been recognized to possess high water productivity in different agroecological regions. Some physiological strategies including seed priming (Farooq et al., 2006a,b,c, 2007a,b; Harris et al., 2002), use of osmoprotectants (Farooq et al., 2008; Yang et al., 2007), and silicon (Si) nutrition (Ma, 2004) can be employed to further enhance the rice water productivity.

3.1. Production systems Water surfaces have a higher evaporation rate than soil. Evaporative water loss can also be reduced by adopting the production systems and technologies, which shorten the duration that the field is flooded and/or requirement for water application (Bouman et al., 2007). Rice systems such as AWD irrigation, bed planting, aerobic culture, SRI, and GCRPS are very effective in this regard (Fig. 1). In the following lines, literature available on each of these systems is discussed with emphasis on water-saving rice production. 3.1.1. Aerobic rice system Aerobic rice is a new way of production system in which specially developed, input-response rice varieties with aerobic adaptation are grown in well-drained, nonpuddled, and nonsaturated soils without ponded water

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Yield (t ha−1)

10

SRI

05

Aerobic rice system

Traditional lowland system Alternate wetting and drying system

Traditional upland system

0 Low

High Water availability Aerobic

S FC Soil condition

Flooded

Figure 1 Schematic presentation of yield responds to water availability and soil conditions in different rice production systems. Abbreviations: SRI, system of rice intensification; FC, field capacity; S, saturation point. (Adapted from Tuong et al., 2004 after modification.)

(Bouman et al., 2007). It entails growing rice in aerobic soil, with the use of external inputs such as supplementary irrigation and fertilizers, and aiming at high yields (Bouman and Tuong, 2001). Main driving force behind aerobic rice is the economic water use. A fundamental approach to reduce water inputs in rice is growing like an irrigated upland crop, such as wheat or maize. Instead of trying to reduce water input in lowland paddy fields, the concept of having the field flooded or saturated is abandoned altogether (Bouman and Tuong, 2001). The adoption of aerobic rice is facilitated by the availability of weed management tools and seed-coating technologies. Case studies showed yields to vary from 4.5 to 6.5 t ha 1, which is about double than that of traditional upland varieties and about 20–30% lower than that of lowland varieties grown under flooded conditions. However, the water use was about 60% less than that of lowland rice, total water productivity 1.6–1.9 times higher, and net returns to water use was twofold higher. Aerobic rice requires lesser labor than lowland rice and can be highly mechanized (Huaqi et al., 2003). Input water savings of 35–57% have been reported for dryseeded rice (DSR) sown into nonpuddled soil with the soil kept near

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saturation or field capacity compared with continuously flooded (5 cm) transplanted rice (Sharma et al., 2003; Singh et al., 2003). However, yields were reduced by similar amounts due to iron or zinc deficiency and increased incidence of nematodes. Contrary to the results of small plot replicated experiments, participatory trials in farmers’ fields in India and Pakistan suggest a small increase or 10% decline in yield of DSR on the flat compared with puddled transplanted rice, and around 20% reduction in irrigation time or water use (Gupta et al., 2003). In their experiments on a high-yielding lowland rice variety (IR20) like an upland crop under furrow irrigation, De Datta et al. (1973) reported that total water savings were 56% and irrigation water savings 78% compared with growing the crop under flooded conditions. However, the yield was reduced from 7.9 to 3.4 t ha 1. The WUE of the aerobic varieties under aerobic conditions was 164–188% higher than that of a lowland cultivated rice variety. Aerobic rice maximizes water use in terms of yield and is a suitable crop for water-limiting conditions (Xiaoguang et al., 2003). In a study, rice yields under aerobic conditions were 2.4–4.4 t ha 1, which were 14–40% lower than under flooded conditions (Castan˜eda et al., 2003). However, water use decreased relatively more than yield, and water productivity under aerobic cultivation increased by 20–40% (in one case even 80%) over that under flooded conditions. The aerobic rice technology eliminates puddling and flooding, and presents an alternative system in reducing water use and increase water productivity. Aerobic rice saved 73% of irrigation water for land preparation and 56% during the crop growth period (Castan˜eda et al., 2003). In a two year field experiment at Indo-Gangetic plains to evaluate various tillage and crop establishment systems for their efficiency in labor, water and energy use, and economic profitability, the yields of rice in the conventional puddled transplanting and direct-seeding on puddled or nonpuddled (no-tillage) flat bed systems were equal (Bhushana et al., 2007). Nevertheless, decline in yield was observed when aerobic rice was continuously grown and the decline was greater in the dry than in the wet season (Peng et al., 2006). In crux, aerobic rice is an attractive option to the traditional rice production system. Yield penalty and yield stability of aerobic rice have to be considered before promoting this water-saving technology. 3.1.2. Alternative wetting and drying irrigation Most rice in Asia is transplanted into puddled soils. Puddling is done for a range of reasons including weed control, ease of field leveling and transplanting, and to reduce percolation losses. The relative importance ascribed to each of the above reasons varies. For example, Tabbal et al. (2002) consider that puddling in central Luzon, Philippines, is done primarily for weed control, whereas Kukal and Aggarwal (2003) and Gajri et al. (1992) placed more emphasis on its role in reducing percolation losses in

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northwestern India, where soils are highly permeable. Puddling is not essential for rice growth and yield. Many studies (but not all, e.g., Singh et al., 2001) reported similar yields for transplanted or direct-seeded rice with and without puddling (e.g., Aggarwal et al., 1995; Humphreys et al., 1996; Kukal and Aggarwal, 2003). The high-yielding rice cultural systems of Australia and California (USA) are not puddled. Although it is widely recognized that puddling reduces percolation, there are surprisingly few reports of quantitative field comparisons of percolation losses in puddled and nonpuddled soils. These indicated that the effect of puddling on percolation rate ranges from little to reductions from 30 to 13 mm day 1 on flooded sandy loam soils and from 17 to 3 mm day 1 on flooded clay soils (Humphreys et al., 1996; Kukal and Aggarwal, 2003). Despite reducing percolation losses during the rice crop, puddling does not necessarily reduce the total water input for rice (Tabbal et al., 2002; Tuong et al., 1996). However, there are only a few reports showing the comparison of total water use or percolation losses in puddled and nonpuddled systems that include the whole period from pre-irrigation to harvest, and that use the same water management after planting. An exception was the study of Singh et al. (2001) which compared water use and yield of water-seeded rice with and without puddling on a sandy loam soil in India, with water depth maintained at 5 cm in both treatments. Averaged over 3 years, there was irrigation water saving of only 75 mm with puddling out of a total irrigation water application of 1537 mm. Thus, even on this highly permeable soil, the irrigation water saving with puddling was relatively small in comparison with the total water use. Puddling for rice induces high bulk density, high soil strength, and low permeability in subsurface layers (Aggarwal et al., 1995; Kukal and Aggarwal, 2003), which can restrict root development and water and nutrient use from the soil profile for wheat after rice (Gajri et al., 1992). Continuous flooding had the highest irrigation water inputs, followed by AWD irrigation, saturated soil culture in raised beds, flush irrigation in aerobic soil, and rainfed treatments. Rice yields did not differ significantly among watering treatments (Lu et al., 2003). AWD has been commonly used as a water-saving practice in many parts of the world for more than a decade (Cabangon et al., 2001). In this system, the soil is allowed to dry for a few days within irrigation events depending on plant developmental stages (Cabangon et al., 2001; Shi et al., 2003). Some success has been reported as far as yield and water demand is concerned (Gani et al., 2003; Lu et al., 2003); however, unproductive water losses could not be totally avoided by AWD. Hence, the water consumption is still high in AWD since the soils need to be submerged at least during the irrigation period. Savings in irrigation water in the AWD treatments were 53–87 mm (13–16%) compared with the continuously submerged regime. Rice grain yields ranged from 7.2 to 8.7 t ha 1 and were not markedly affected by the water regimes.

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Water productivity was significantly higher in the AWD regime than in the continuously submerged regime (Belder et al., 2003). 3.1.3. System of rice intensification SRI that evolved in the 1980s and 1990s in Madagascar permits resourcelimited farmers to realize paddy yields of up to 15 t ha 1 even on infertile soils, with greatly reduced rates of irrigation and without external additional inputs (Stoop et al., 2002). The main features of this system are transplanting young seedlings singly in a square pattern with wide spacing, using organic fertilizers and hand weeding, and keeping the paddy soil moist during the vegetative growth phase. Significant phenotypic changes occur in plant structure and function and in yield and yield components under SRI cultivation. SRI increased yields substantially (50–100% or more), while requiring only about half as much water as conventional rice (Uphoff and Randriamiharisoa, 2003), whilst not needing the purchase of additional external inputs. SRI is difficult for most farmers to practice because it requires significant additional labor inputs at a time of the year when liquidity to hire labor is low and family labor effort is already high. This poses the challenge to researchers and policymakers concerned with the promotion of watersaving rice technologies. Even though the yields can be increased while saving water, adoption by farmers is still far from assured (Moser and Barrett, 2003). SRI methods are able to enhance yields of any rice variety, but the highest yields have come from improved high-yielding varieties. Factorial trials in Madagascar explain synergistic dynamics among the SRI practices that account for 100–200% increases in yield (Uphoff and Randriamiharisoa, 2003). A large increase in the productivity of irrigation water use with SRI can make water savings more attractive, compensating farmers well for the extra labor or expenditures involved. The returns to land, labor, capital, and water are all increased by the use of SRI practices (Uphoff and Randriamiharisoa, 2003). Lu et al. (2004) evaluated some modifications in traditional SRI, viz. transplanting three separated seedlings in one hill in a triangular pattern with the leaf age extended to 3–4 weeks; application of herbicide before transplanting; mulching the spaces between plants with straw; adding chemical fertilizers to promote plant growth vigorously when needed; making shallow furrows before transplanting in the zero-till fields; and applying the AWD method for water management with midseason drainage to inhibit tillering. With these modifications, grain yield exceeded 12 t ha 1, being 46% greater than in control using field comparison along with water saving. McHugh et al. (2003) conducted a survey of farmers in Madagascar to investigate farmer implementation of AWD as part of SRI and showed that farmers have adapted AWD practices to fit the soil type, availability of water and labor. The primary drawbacks reported by farmers with implementing

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AWD were the lack of a reliable water source, little water control, and water-use conflicts. They suggested that by combining AWD with SRI, farmers can increase grain yields while reducing irrigation water demand (McHugh et al., 2003). Uphoff and Randriamiharisoa (2003) proposed that continuously flooded soils constrain root growth and contribute to root degeneration. Moreover, soil microbial life is limited to anaerobic populations. This excludes contributions to plant performance from mycorrhizal fungal associations that are of benefit to most plant species. Keeping paddy fields flooded also restricts biological nitrogen fixation to anaerobic processes, forgoing possibilities for aerobic contributions. In another study, Thiyagarajan et al. (2003) reported savings in irrigation water of 56% and 50% using conventional and young seedlings, respectively, without a significant effect on grain yield under SRI system. Twoweek-old seedlings planted one seedling per hill produced significantly higher yield (6.43 t ha 1) than the farmer’s practice of using 21-day-old seedlings (5.96 t ha 1). However, yields were similar for both age groups when the number of seedlings increased to 2 and 4 per hill. The performance of 15-day-old seedlings improved more than that of 21-day-old seedlings with the addition of well-decomposed organic matter and intermittent irrigation (Makarim et al., 2003). In a cement-box experiment in China, production characteristics, water-use efficiency, nitrogen-use efficiency, and major physiological characteristics of three alternative water management practices SRI, GCRPS, and AWD were compared with a conventional flooded rice system. Water supply in SRI and AWD was 46% and 36% lower than in conventional flooded rice system, respectively; whereas their yields were similar or significantly higher (5% for SRI and 8% for AWD), resulting in greater WUE. The higher yields of SRI and AWD compared with conventional flooded rice system were associated with higher harvest indices but not with differences in total biomass production. Water supply and yield in GCRPS were 65% and 62% lower than in conventional flooded rice system (Cao et al., 2003). 3.1.4. The ground-cover rice production system The plastic film or straw mulching rice production systems have been developed since 1990 in China to improve the tolerance to low temperatures (Shen et al., 1997). This is similar to the success in Japan in the 1960s, but now its benefits for water-saving rice production led to the adoption of this system. In plastic film mulching (PFM), also called GCRPS, lowland rice varieties are used and the soil is kept humid by covering materials (Kreye et al., 2007). In GCRPS, soil is irrigated to approximately 80% of water-holding capacity. Nevertheless, the amount of water saved with this system can be as high as 60–85% of the need in the traditional paddy systems with no adverse effects on grain yield (Huang et al., 1999). However, some

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researcher reported significant yield reductions under such conditions (Borrell et al., 1997; Castillo et al., 1992). Thereafter, to check evaporation the soil surface is covered by material, such as plastic film, paper, or plant mulch (Lin et al., 2003b). Although benefits of water-saving rice cultivation in water-limited areas have been illustrated (Gani et al., 2003; Huang et al., 1999; Shen et al., 1997), other experimental evidences suggest moderate to severe yield reduction (Borrell et al., 1997; Castillo et al., 1992) of water-saving cultivation compared to paddy. With lower soil water potentials the elongation of internodes, the number of panicles and the crop growth rate reduced in comparison to flooded conditions (Lu et al., 2000). Lin et al. (2003b) recorded up to 60% reduction in water requirements of rice crop in a GCRPS; however, grain yields were up to 10% lower than the traditional lowland rice. This was associated to micronutrient deficiency and difficulties in nitrogen fertilizer management contributed to higher yield penalty in GCRPS. Two GCRPSs using thin plastic film or straw mulch soil cover were compared to traditional paddy rice production. In the submerged rice fields, methane (CH4) emission was dominant, and only during the drainage period before panicle initiation nitrous oxide (N2O) emission was found. In contrast, CH4 emission from GCRPS was negligible but N2O emission generally increased with water-saving GCRPS, and emission events were clearly linked to fertilization (Dittert et al., 2003). In a recent study, GCRPS including soil surface was covered with 14-mm thick plastic film (GCRPS-plastic); mulched with straw (GCRPS-straw) and uncovered (GCRPS-bare) were compared with lowland rice cultivated under traditional paddy conditions (control). Compared to paddy control, only 32–54% of irrigation water was applied in GCRPS treatments. Plants in GCRPS were smaller, developed fewer panicles and had a smaller LAI than paddy control. Yield was significantly less in GCRPS-bare and GCRPS-straw compared to paddy, while yield in GCRPS-plastic was only 8% lesser than the paddy control yield. WUE in GCRPS-plastic was higher than in paddy control (Tao et al., 2006). 3.1.5. Raised beds The use of raised beds for the production of irrigated non-rice crops was pioneered in the heavy clay soils of the rice-growing region in Australia in the late 1970s (Maynard, 1991), and for irrigated wheat in the rice–wheat system of the Indo-Gangetic plains during the 1990s, inspired by the success of beds for wheat–maize systems in Mexico (Meisner et al., 1992; Sayre and Hobbs, 2004). Potential agronomic advantages of beds include improved soil structure due to reduced compaction through controlled trafficking, and reduced waterlogging and timely machinery operations due to better surface drainage. Beds also provide the opportunity for mechanical weed

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control and improved fertilizer placement. While the potential benefits of beds for wheat production in the Indo-Gangetic plains have been known for some time (Dhillon et al., 2000), evaluation of beds for rice and permanent beds in rice–wheat system systems commenced more recently (Connor et al., 2002). Farmer and researcher trials in the Indo-Gangetic plains suggest irrigation water savings of 12–60% for direct-seeded and transplanted rice on beds, with similar or lower yields for transplanted compared with puddled flooded transplanted rice, and usually slightly lower yields with direct seeded rice (Balasubramanian et al., 2003; Gupta et al., 2003; Hossain et al., 2003; Jehangir et al., 2002). However, many studies in the northwest IndoGangetic plains indicate little effect of rice on beds on water productivity (typically around 0.30–0.35 g kg 1) as the decline in water input was accompanied by a similar decline in yield ( Jehangir et al., 2002; Sharma et al., 2003; Singh et al., 2003). The causes of reduced rice yield included increased weeds and nematodes, suboptimal sowing depth due to lack of precision, and micronutrient (e.g., iron, zinc) deficiencies. Singh et al. (2003) evaluated the yield and water use of rice established by transplanting, wet and dry seeding with subsequent aerobic soil conditions on flatland and on raised beds. Transplanted rice yielded 5.5 t ha 1 and used 360 mm of water for wetland preparation and 1608 mm during crop growth. Compared with transplanted rice, dry-seeded rice on flatland and on raised beds reduced total water input during crop growth by 35–42% when the soil was kept near saturation and by 47% and 51% when the soil dried out to 20 and 40 kPa moisture tension in the root zone, respectively. Most of the water savings were caused by reduced percolation losses. Moreover, no irrigation water was used during land preparation. However, the dry seeding of rice reduced yield by 23–41% on flatland and by 41–54% on raised beds compared with transplanted rice. There was no great difference in water productivity among treatments. There appears to be little scope for saving irrigation water with furrow-irrigated rice on beds on the heavy clay soils of southern Australia. Investigations over four growing seasons showed irrigation water savings of around 10% with saturated soil culture (water continuously in the furrows), with a similar reduction in the grain yield (Thompson et al., 2003). Irrigation water use of rice grown on beds with intermittent irrigation until 2 weeks before panicle initiation, followed by continuous flooding, was similar to water use of dry-seeded rice on the flat surface with continuous flooding commencing about 1 month after sowing (Beecher et al., 2006). This is in contrast with findings on a more permeable soil in semitropical southern Queensland where irrigation water use of rice on beds with saturated soil culture was 32% less than flooded rice on the flat due to considerably reduced percolation losses (Borrell et al., 1997). Studies in the USA have also shown considerable water savings with furrow-irrigated

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rice on beds (Tracy et al., 1993; Vories et al., 2002). Beecher et al. (2006) reported no water saving from the raised bed rice cultivation compared with conventional ponded rice grown on a flat layout. When grown on raised beds, a variety needs to be able to compensate for the loss in cropped area (caused by the relatively large row spacing between the beds) by producing more productive tillers (Singh et al., 2003).

3.2. Other management practices Soil type has a large influence on irrigation water requirement due to much higher percolation losses on coarser textured soils. This is particularly true for rice grown under submerged condition for most of the season. Seasonal percolation losses of 57–83% of the total input water are common in the Indo-Gangetic plains, with highest losses (up to 1500 mm) on sandy and sandy-loam soils, and lowest losses on loams and clay-loams (up to 890 mm) (Tripathi, 1996). The extent of laser leveling in South Asia and China is currently extremely small, compared with 50–80% of the rice land in Australian rice-based systems (Humphreys and Bhuiyan, 2001; Lacy and Wilkins, 2003). Land leveling can reduce evaporation and percolation losses by enabling faster irrigation times and by eliminating depressions. It also reduces the depth of water required to cover the highest parts of the field and for ponding for weed control in rice, and therefore percolation losses, more so on more permeable soils. Rickman (2002) found that rice yields in rainfed lowland laser-leveled fields were 24% higher than in nonlaser-leveled fields in Cambodia, and yield increased with the uniformity of leveling. Pressurized irrigation systems (sprinkler, surface, and subsurface drip) have the potential to increase irrigation water use efficiency by providing water to match crop requirements, reducing runoff and deep drainage losses, and generally keeping the soil drier, reducing soil evaporation and increasing the capacity to capture rainfall (Camp, 1998). There are few reports of the evaluation of these technologies in rice–wheat systems. In Australia sprinkler irrigation of rice to replace evaporative loss reduced irrigation water use by 30–70% (Humphreys et al., 1989). Even at frequencies of up to three times per week yield declined by 35–70% (Muirhead et al., 1989). Irrigation water use was reduced by about 200 mm in rice with subsurface drip commencing 2 weeks prior to panicle initiation compared with flooded rice culture. Yields with drip also decreased, although there was no increase in irrigation water productivity (Beecher et al., 2006). Reducing nonbeneficial evaporation direct from the soil or free water lying on the field is true water saving, although it may be countered to some degree by increased transpiration rates as a result of impacts on the microclimate experienced by the plant. The size of this effect has not been

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established. Evaporation from the free water surface accounted for 40% of the total evaporative loss from continuously flooded water-seeded rice (Simpson et al., 1992). Substantial irrigation water savings (25–30%) can be achieved by delaying transplantation from mid-May to mid-June (Narang and Gulati, 1995). Direct seeding could help overcome the problem of labor availability, although the optimum sowing date may need to be earlier than the optimum transplanting date, which could increase the crop water requirement. It is not clear if changing to direct seeding will increase or reduce the water requirement for rice, and the impact may vary depending on sites and systems (Dawe, 2004). Although delayed rice planting can save water, it can also delay planting of wheat beyond the optimal time, causing yield loss of 1–1.5% per day due to higher temperatures at grain filling (OrtizMonasterio et al., 1994). While delaying transplanting in the Indo-Gangetic plains to the optimum time saves water, bringing forward transplanting in Eastern India enabled more profitable use of rainfall. Here, irrigation water is scarce, and the need for irrigation can be avoided and total system productivity increased by establishing rice with rainfall supplemented by irrigation from groundwater during the premonsoon period, and by raising bund height to 20 cm to capture rainfall (Gupta et al., 2003). There are few reports of evaluation of mulching for rice, apart from those from China, where considerable input water savings of 20–90% occurred with plastic and straw mulches in combination with aerobic culture compared with continuously flooded transplanted rice (Lin et al., 2003a; Pan et al., 2003; Shen and Yangchun, 2003). Much of the water savings was probably due to higher percolation losses in the flooded systems (Lin et al., 2003a,b).

3.3. Physiological implications One of the short term and the most pragmatic approaches to overcome the drought stress effects is seed priming, which involves partial hydration to a point where germination-related metabolic processes begin but radicle emergence does not occur (Farooq et al., 2006a). Primed seeds usually exhibit increased germination rate, greater germination uniformity, and sometimes greater total germination percentage (Farooq et al., 2006b,c; Kaya et al., 2006). This approach has been applied to overcome the drought stress effects in a range of crop species. However, improvement of rice and other crops for growing in water-scant areas is of current interest. In the newly introduced aerobic rice culture, the frequency and intensity of drought may increase manifold. Du and Tuong (2002) while testing the effectiveness of different osmotica to improve the performance of directseeded rice noted that osmopriming with 4% KCl solution and saturated CaHPO4 solution was successful in improving the seedling emergence,

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crop stand establishment, and yield under stress. Harris et al. (2002) reported that in drought-prone areas, primed rice seeds germinated well and seedlings emerged faster and more uniformly leading to increased yield. Germination trial of 11 varieties of upland rice under limited water conditions revealed early and synchronized emergence owing to seed priming (Harris and Jones, 1997). Primed rice seeds emerged faster and showed greater growth, dry matter accumulation, yield and harvest index compared to un-treated ones (Farooq et al., 2006a,b). Osmoprotectants are involved in signaling and regulating plant responses to multiple stresses, including reduced growth that may be part of the plant’s adaptation against stress. In plants, the common osmoprotectants are proline, trehalose, fructan, mannitol, glycinebetaine, and others (Zhu, 2002). Osmoprotectants play adaptive roles in mediating osmotic adjustment and protecting subcellular structures in stressed plants. Yang et al. (2007) suggested that for rice, to perform well under drought stress, it should have higher levels of free spermidine/spermine and insoluble-conjugated putrescene. Si is the second most abundant element in soils and a mineral substrate for most of the world’s plant life. Ample evidence is available indicating that when Si is readily available to plants, it plays a significant role in their growth, mineral nutrition, mechanical strength, and resistance to several stresses (Epstein, 1994). Still, it has not been considered an essential element for higher plants, partly because its role in plant biology is poorly understood (Gong et al., 2003). Nevertheless, numerous studies demonstrate that Si is an important element, and plays an important role in tolerance of plants to environmental stresses (Liang et al., 2007; Richmond and Sussman, 2003; Savant et al., 1999). These beneficial effects are attributed to the high accumulation of Si on the tissue surface, although other mechanisms have also been proposed. Genotypes accumulating high amount of Si have more water productivity (Ma, 2004).

4. Future Thrusts A successful change from the traditional flooded to aerobic rice production requires the breeding of special aerobic rice varieties and the development of appropriate water and crop management practices. Although, considerable progress has been made in the improvement of transgenic rice for improved water-use efficiency and productivity; however, the achievements are not satisfactory. Nevertheless, with the study of the functional genomics of plants, considerably more information about the mechanisms by which plants perceive and transduce these stress signals to initiate adaptive responses will be obtained, and with the improvement of

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the transgenic approach, marker-free transgenic rice will be produced. Therefore, to combine novel regulatory systems for the targeted expression with useful genes, more effective and rational engineering strategies must be provided for the improvement of rice for higher water productivity. Different strategies need to be tested experimentally to genetically improve the water-use efficiency and drought stress tolerance in rice. Different strategies need to be integrated, and the genes representing distinctive approaches be combined to substantially increase rice water productivity. Wide hybridization using hardy wild rice species is another area to be emphasized. Moreover, combining the transgenic with traditional breeding methods may be an effective approach to develop abiotic stress-tolerant rice cultivar. Site-specific packages of production technologies should be developed for different rice production system in various rice production zones across the continents. Because nutrient dynamics (particularly of micronutrients) are altogether different under water-saving rice production systems, future research should also include the crop nutrition with particular reference to micronutrients. Integrated tools should be developed for weed management in different systems. Newly developed rice systems should also be monitored in ecological perspective. Varieties capable of synthesizing osmoprotectants, manifesting quicker OA and capable of synthesizing stress proteins may be introduced. Various physiological tools should be investigated in detail to harvest maximum paddy yield with minimum water input.

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