Crop yield response to water - Cra

Response to StressesBecause rice evolved from a semi-aquatic ancestor, it is extremely sensitive to water shortage.The main reason is its shallow root system; in terms of sensitivity of rice organs to low waterpotential, it is actually not that different from many other crops (Hsiao et al., 1984). Leaf andcanopy expansion are reduced soon after the soil dries below saturation in most cultivars; evenin upland cultivars, expansion begins to be inhibited when only a small fraction of the totalavailable water (TAW) has been depleted (Lilley and Fukai, 1994; Wopereis et al., 1996). Rice issusceptible to large yield losses at the time of flowering because of reduced water availability.The spikelets scheduled to pollinate on a day when panicle water potential is low (e.g. -1.8MPa) do not open to shed pollens, causing spikelet sterility and reducing the harvest index (HI).Another stress is combined high temperature and strong wind at flowering time. Spikelets ofnewly emerged panicles have low epidermal resistance to water vapour apparently related tothe slow formation of epicuticular wax. On days of high temperature and wind, such spikeletsdesiccate and die and turn white (O’Toole et al., 1984), symptom referred to as ‘white heads’or ‘blasting’. This again reduces HI. A large part of rainfed lowlands are frequently affectedby drought, the largest and most frequently and severely affected areas being eastern India(about 20 million ha) and northeastern Thailand and Laos (7 million ha).Although rice is adapted to waterlogging, complete submergence can be lethal. Most ricevarieties can survive complete submergence of only 3-4 days though some rainfed lowlandrice varieties can survive up to 14 days (depending on depth, temperature, and turbidity of thewater). Recently, a gene has been discovered (sub1) that confers tolerance to submergence inthe early vegetative growth stage of up to 14 days, and which has successfully been introducedinto a number of popular lowland varieties using marker-assisted breeding techniques. Tallplants tend to lodge when the water level recedes, resulting in additional yield losses andpoor grain quality.Rice is salt-sensitive (Shannon, 1997). Some 9-12 million ha of lowland rice area in South Asiais estimated to be affected by salinity and/or alkalinity (Bouman et al., 2006) either from seawater intrusion in the coastal areas or from water and/or soil salinity inland. The thresholdfor yield reduction is 3 dS/m of soil electric conductivity (EC e ), with 90 percent yield loss at10 dS/m EC e . Rice is relatively salt tolerant during germination, tillering, and toward maturity,but is sensitive during early seedling and at flowering and grain filling.Fertilizer needs depend on targeted yield, the fertility of the soil, residue management,and the amount of nutrients coming into the rice field by irrigation water and atmosphericdeposition. The rice crop needs the following uptake of major nutrients to produce 1 tonne ofgrain per hectare: 15-20 kg N/ha, 2-3 kg P/ha, and 15-20 kg K/ha (Rice Knowledge Bank, IRRI).Irrigation practiceLowland riceIrrigated lowland rice is mostly grown with supplementary irrigation in the wet season(monsoon), and is entirely reliant on irrigation in the dry season. Fields are bunded with smalldykes about 0.20 m high and 0.20-0.30 m wide to keep ponded water in the field (basin).Farmers with access to irrigation aim to maintain 50-100 mm of ponded water (‘floodwater’)108crop yield response to water