Rice is cultivated in a wide range of environments, from tropical to temperate climates, including in aerobic soils in uplands to wet lowlands with uncontrolled flooding, e.g. flood-prone and deepwater rice areas. Water plays a pivotal role in the management of rice systems and the different rice agroecosystems are mostly classified based on their hydrology and the extent of water availability (Khush 1984
). Irrigated lowland systems comprise ~55 % of the total rice area and provide 70 % of global rice production, while rainfed lowland and flood-prone areas constitute ~35 % of the total rice area, covering 47 million ha in Asia but providing only ~25 % of global rice production because of various abiotic challenges associated with rainfed ecosystems (International Rice Research Institute 1997
Farmers establish rice crops by either transplanting or direct seeding. Transplanting of rice seedlings from nurseries into ‘well-puddled’ waterlogged or flooded soils gives an advantage to rice over weeds due to seedling size and the flooded soil in which weed species must initially germinate and establish (Rao et al. 2007
). In many areas, transplanting of rice and subsequent manual weeding have enabled the sustainability of this system as it provides good weed management, but it is labour intensive and requires considerable water for land preparation. In some rainfed areas, however, such as in Bangladesh and eastern India, where water accumulates in the field to depths exceeding 30 cm within a few days of the start of rainfall, transplanting taller and older seedlings still remains the only viable option.
Numerous variations in direct seeding are being practised based on water availability and field hydrology (Chin and Mortimer 2002
; Rao et al. 2007
). Rice can be dry seeded by broadcasting, dibble sowing or drilling dry seeds on dry or unsaturated soils. This technique is mostly used in areas where water availability is limited or uncertain during crop establishment, as in rainfed areas. Wet seeding uses pre-germinated rice seeds for sowing in saturated puddled soils, as commonly practised in irrigated areas. It is sometimes used in flood-prone areas after flood recession, when it is possible to drain additional standing water from the field, followed by soil puddling to suppress weeds. Water is then re-introduced into the fields 7–10 days after sowing, when seedlings are established, and the water depth gradually increased as the rice seedlings grow. Water seeding involves pre-germinated rice seeds broadcast in standing water, and is practised in some cooler areas, as in California, Central Asia and Australia. The main advantage of this method is that the majority of weed species are suppressed by the standing water. This is common in temperate irrigated areas but could potentially be used in flood-prone rainfed lowlands in the tropics where farmers can practise early sowing without waiting for complete floodwater recession, to minimize the risk of delayed maturity and late-season drought (Pandey et al. 2002
). Once the rice crop has established, in most direct-seeded systems and based on water availability and control, the field is flooded to suppress weed growth and water depth is then maintained at 5–10 cm through most of the season before water is gradually drained prior to harvest.
Rice farmers in rainfed and irrigated areas are shifting to direct seeding from transplanted rice as it provides opportunities to reduce costs and can result in earlier harvest (Balasubramanian and Hill 2002
). There are constraints, however, that limit its large-scale adoption, the most important of them being (i) poor germination and uneven stand establishment in areas where the land is not well levelled or water is not well controlled as in rainfed areas, and (ii) high weed infestation (Du and Tuong 2002
). Commonly, lowland fields are not well levelled, which means that they can neither be completely drained nor flooded to an even depth to control weeds. With rainfall being unpredictable, flooding of low-lying areas can result in a severe reduction in rice establishment. This is likely to be a particular problem with monsoon-season rice crops.
Weeds constitute a major problem for the large-scale adoption of direct-seeded rice, with yield losses of ~20 % of attainable yield or even total loss if not controlled (Rao et al. 2007
). Although direct seeding provides opportunities for labour and water savings, current management systems for dry- and wet-seeded rice in the tropics do not usually allow standing water to be used effectively to completely suppress weed growth. Direct-seeded rice therefore faces severe challenges from competition if weeds are not adequately managed and many direct-seeded systems are reliant on herbicide use to control weeds. The use of herbicides has attendant problems such as cost, concerns related to health and the environment, and the evolution of herbicide resistance in weeds. With good water management, though, weeds can be controlled effectively by flooding the soil after direct seeding (Tuong et al. 2000
The development of rice varieties that can germinate and emerge in flooded soils will therefore help reduce the hazards of early floods, to which rice is very sensitive (Ismail et al. 2009
), and this also provides an efficient means for weed control through early flooding. This can be achieved through effectively exploiting the genetic variation in flooding tolerance during seed germination and early establishment. Moreover, the effectiveness of flooding for weed management will depend on the responses of various weeds associated with rice to early flooding, an area that has not been studied sufficiently.
Aquatic weeds and those well adapted to flooded soils are major problems in lowland rice fields. In the Philippines, the cultivation of lowland rice in rotation with upland crops and vegetables in the same fields has resulted in the selection of ecotypes of ‘upland’ weeds such as Cyperus rotundus
that can tolerate flooded soils (Peña-Fronteras et al. 2009
; Fuentes et al. 2010
). Adaptation in these ecotypes suggests changes in metabolic and/or morphological growth processes that make these weeds more adapted to the flooded conditions of paddy fields.
Water management has long been recognized as an effective cultural weed control practice in lowland rice (Rao et al. 2007
), as flooding the soil could affect the density, vigour and uniformity of rice stands, as well as the severity of weed competition and the effectiveness of herbicides (Kim et al. 2001
). Rice farmers in California converted entirely from dry seeding to water seeding in the early part of the 20th century mainly to manage Echinochloa crus-galli
(Hill et al. 2001
). Water depth alone exerts a dominant effect on the structure of weed communities and the fate of weeds recruited into the growing crop (Janiya et al. 1999
). Flooding depth and duration also have a differential effect on the survival and growth of weed species. In Africa, Kent and Johnson (2001)
reported that flooding to 2–8 cm increased the density of Sphenoclea zeylanica
and decreased that of Echinochloa colona
and E. crus-pavonis
compared with saturated soil conditions. Increased flood duration from either 2 or 4 days within every 7 days to continuous flooding had no effect on S. zeylanica
, but decreased the numbers of E. colona
and E. crus-pavonis
. Gealy (1998)
reported that deep flooding may reduce Ipomoea
spp. infestation. Different weed species seem to respond differently to water management and either selection of tolerant ecotypes for some weeds associated with rice or shifts in weed species composition means that traditional soil puddling and flood management may no longer be effective for their control. Understanding the mechanisms associated with adaptation to such aquatic systems in rice and associated weeds could facilitate the development of management strategies that favour rice establishment but simultaneously suppress weeds in flooded soils.
Substantial progress has been made in understanding the mechanisms of tolerance of anaerobiosis in different plant species during the last few decades (Kennedy et al. 1992
; Vartapetian and Jackson 1997
). This review attempts to summarize the major adaptive features of rice for low-oxygen stress during seed germination and early growth. It further highlights some of the mechanisms probably associated with tolerance of a few wetland weeds and weeds that recently became associated with lowland paddy conditions. The review further discusses the prospects of using knowledge gained in plant adaptation to anaerobic conditions for application to optimize crop establishment in rice.