Australia contributes approximately 12% of the world’s total cotton production, and is the third largest exporter of cotton fibre. Most Australian cotton is cultivated in New South Wales, (70% of the total production), with the remainder cultivated in Queensland, an area that extends from Emerald in Queensland to Hay in New South Wales (Hearn and Fitt, 1992). Australian cotton is generally furrow irrigated with only a small proportion rainfed. There has been a dramatic increase in cotton production in Australia from 45,000 tonnes in 1970s to 600,000 tonnes in 2000s, with an average increase in lint yield of 1.8% per year (Constable, 2004). Despite this enormous improvement in cotton production systems, the cotton yield in Australia remains substantially subject to various abiotic stress factors including drought, heat, waterlogging and cloudy conditions.
Waterlogging is an important factor that adversely affects cotton yield. Australian cotton is cultivated on heavy clay soils with inherently low drainage and a summer dominant rainfall pattern poses significant risk of intermittent waterlogging. In addition, the reproductive phase of cotton, which starts by late December through January, often coincides with heavy summer rains in cotton producing regions. As the reproductive phase of cotton growth is most sensitive to stress-induced damage, exposure to waterlogging at this phase can significantly reduce yield. A degree of damage to cotton is expected if heavy rainfall occurs just after an irrigation event. Heavy lint yield losses have been recorded in Australian cotton under persistent rainfall and cloudy weather during the 2009-2010 and 2010-11 cotton seasons (CRC, 2010-11).
Waterlogging-induced growth and yield reduction are the result of a complex syndrome caused by O2 deficiency in the soil. Soil hypoxia impairs root growth and subsequent water and nutrient uptake. An inhibited supply of nutrients and water influences leaf development, light interception and photosynthetic efficiency leading to growth reduction. In addition, soil waterlogging alters the level of phytohormones in root tissues; specifically it accelerates biosynthesis of 1- aminocyclopropane 1-carboxylic acid (ACC). This ACC is converted into ethylene in the presence of O2 and ACC oxidase in aboveground plant parts (Bradford and Yang, 1980). Elevated ethylene accumulation in cotton tissues can stimulate leaf senescence and fruit abortion (Lipe and Morgan, 1973).
Tolerance to waterlogging in plants is a complex phenomenon that depends on tolerance to by-products of anaerobiosis and elemental/molecular toxicities. Plants exhibit a variety of modifications to survive in O2-deficient environments. Development of aerenchyma is one of the most common responses in many plant species at the anatomical level. Aerenchyma facilitates oxygen diffusion into root tissues (Jackson et al., 2008). Other morphological changes include increased root porosity via development of adventitious root and hypertrophied lenticels, and rapid shoot elongation in some waterlogging-tolerant species. Modifications of water relations, stomatal changes, decreased transpiration and photosynthesis are the physiological adaptive responses in plants. Metabolic adaptations, including energy production via fermentation, metabolic adjustments and anaerobic protein synthesis are also crucial for survival of plants exposed to low O2 concentration.
Absence of any apparent changes in cotton roots in terms of aerenchyma formation (Conaty et al., 2008), as well as the slow rate of energy production through anaerobic respiration, make cotton relatively sensitive to waterlogging. Cotton roots rapidly respond to soil O2 deficiency, showing symptoms of growth inhibition under mildly hypoxic conditions (O2 < 10%) within a short time (Huck, 1970). Inhibited root growth restricts nutrient uptake and interferes with various physiological process, causing overall yield reduction. Yield loss in cotton is directly associated with the duration for which root roots remain under O2 deficient environments. For example, an inundation period of 4 to 16 h (when soil O2 < 10 %) caused a 8% reduction in cotton lint yield, while prolonging inundation time to 32 h increased yield losses to 18% (Hodgson, 1982). Similarly, 27 – 30% yield reduction was recorded in response to 4 to 9 d of waterlogging, respectively (Wu et al., 2012). Despite significant improvements in cotton production systems, limited effort has been made in improving tolerance to waterlogging. Waterlogging tolerance in cotton is a complex trait, which depends on several environmental and physiological factors. Screening and breeding for waterlogging tolerance alone may not be adequate, as the waterlogging-tolerant cultivars identified in one experiment may appear intolerant in other trials. Therefore, understanding the impact of environmental factors and plant adaptation to waterlogging is critical for developing efficient waterlogging tolerance strategies. Physiological and biochemical modifications can provide clues to understanding plant tolerance mechanisms to waterlogging and assist in devising techniques for reducing yield losses under stressful conditions.
