Introduction

Plants are often faced with a wide range of biotic (pathogens and diseases) and abiotic stresses including high temperature, salinity, drought, ozone, flooding, etc., among which drought stress is considered to be a major threat to sustaining food security under current and more so in future climates. Although plants to a certain extent can withstand limited water conditions, a wide geno-typic variability in response to drought stress is seen in most of the cultivated crops (for rice -Rang et al. 2011; Kumar et al. 2007; Venuprasad et al. 2008; maize - Atteya 2003; barley -Samarah 2005) . Drought stress or water limited condition is a result of insufficient amount of water available for the basic up keep and maintenance of normal physiological processes such as photosynthesis and overall cell, tissue, organ, and plant homeostasis. With the predicted increase in precipitation variability across major cropping regions in the world, the intensity and duration of extreme drought stress are likely to increase, putting additional pressure to meet the demographic demand for food. Drought stress moreover occurs across regions, countries, and continents, for example in South Asia; severe drought years during 1987 and 2002/2003 affected more than 50% of the total cropped area and almost 300 million people in India (Pandey et al. 2007), in south East Asia; drought in 2004 affected 20% of the rice land and more than eight million people in Thailand (Pandey et al. 2007) and among the globally recorded disasters over the past three decades, 20% are accounted by Africa with nearly half of them caused by extreme weather, particularly drought (Cornford 2003). The World Economic Forum (2009) at Davos published a "Water Initiative" report, which estimated global crop production losses up to 30% by 2025 compared to current yields due to water shortage, if unsustainable use of water for agriculture continues (Zhang 2011). In addition, the steeply increasing temperatures could lead to a rapid loss of soil water, bringing forward severe water stress to coincide with critical developmental stages like flowering mainly due to increased evapotranspiration losses.

Crop plants in order to survive have evolved to withstand water limited conditions by (a) escaping, that is, completion of fertilization and life cycle before the on set of stress (b) avoidance through minimized water loss (e.g. stomatal regulation) or through efficient exploration of soil moisture reserves (e.g. deep root system) and (c) tolerance by maintaining normal photosyn-thetic activity under water deficit conditions or efficient water uptake (e.g. hydraulic conductivity). Breeding for drought tolerant crop varieties having the ability to withstand harsh conditions and simultaneously producing yields (i.e. grains in rice and wheat, lint in cotton, etc.), closer to stress-free conditions are constantly increasing in demand. The demand for these developed products is going to get bigger as we begin to see devastating effects of droughts at a much higher frequency. A wide range of negative impacts of drought on the rice crop during different developmental stages is

Impaired germination Poor crop stand Reduced (i) growth, (ii) leaf expansion, (iii) photosynthesis (iv) leaf area (v) plant height

Seedling & Tillering

Reduced flowering rate Extended flowering duration Incomplete panicle exertion Reduced anther dehiscence Spikelet abortion 4-►

Impaired germination Poor crop stand Reduced (i) growth, (ii) leaf expansion, (iii) photosynthesis (iv) leaf area (v) plant height

Seedling & Tillering

Panicle initiation & Booting

Reduced (i) spikelet number, (ii) viable pollen Dead microspores Pollen abortion Abnormal organogenesis

Grain abortion Chaffiness Reduced yield Poor grain quality (i.e. increased chalkiness)

Panicle initiation & Booting

Reduced (i) spikelet number, (ii) viable pollen Dead microspores Pollen abortion Abnormal organogenesis

Grain abortion Chaffiness Reduced yield Poor grain quality (i.e. increased chalkiness)

Fig. 7.1 Negative effects of drought stress during different developmental stages. Studies from which the information has been derived are listed below. Seedling and Tillering -Singh et al. (1996), Boonlertnirun et al. (2007), Harris et al. (2002), Tripathy et al. (2000), Manickavelu et al. (2006);

Panicle initiation and Booting - Nguyen et al. (2009), Sheoran and Saini (1996), Liu and Bennett (2010); Flowering and Fertilization - Jongdee et al. (2006), Rang et al. (2011), Liu et al. (2006); Grain filling and yield - Liu et al. (2006), Kumar et al. (2007), Venuprasad et al. (2008)

presented in Fig. 7.1 and similar effects have been documented with other crops (pea - Okcu et al. 2005; sunflower - Kaya et al. 2006; Hussain et al. 2008; maize - Cattivelli et al. 2008; wheat -Wardlaw and Willenbrink 2000; Ahmadi and Baker 2001). One of the immediate responses of crops to drought stress is partial stomatal closure which limits the entry of CO2 for photosynthesis, further increasing the production of reactive oxygen species (ROS) (Sgherri et al. 1993; Loggini et al. 1999; Boo and Jung 1999). It has been extensively documented that crop varieties having the ability to enhance their potential to reduce the harmful effects of the oxidative damage caused by drought and other abiotic stresses result in significantly higher yields. Hence the main aim of this chapter is to discuss the different aspects of ROS production, scavenging, and signalling which results from drought induced oxidative stress in plants.

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