Alternate wetting and drying is an irrigation management practice in which rice crop is subjected to wetting and drying cycles at regular intervals during crop growth.
It avoids the flooded condition throughout the growing season beside this only at critical stages of crop flooded condition are maintained. It reduces water requirement up to 30% and hence, it reduce the input cost on water pumping and irrigation.
Heavy Metals in Rice:
Heavy metals are defined as those elements having a specific density of more than 5 g/cm3. Heavy metals uptake in rice under continuous flooding condition is one of the major issues in many Asian countries like Bangladesh, Pakistan, Japan and Korea.
Heavy metals contents in rice grains above its limits recommended by WHO (World Health Organization) is extremely threatening to its exporters and consumers. Decrease in accumulation of heavy metals, particularly Arsenic (As), Cadmium (Cd), Nickle (Ni) and lead (Pb) is of a great concern in present situation
AWD as Mitigation Strategy in Rice Crop:
Alternate wetting and drying in flooded rice have an ability to utilize the benefits of both aerobic and anaerobic conditions during crop growth. Growing rice in aerobic and intermittently flooded conditions results in lesser uptake of heavy metals as compared to flooded field because heavy metals uptake decreases in neutral soil pH.
While in flooded field there is reduced condition and pH is relatively lower than aerobic and intermittently flooded field, this condition results in significant increase in the uptake of heavy metals through roots. There is greater accumulation of both arsenic and cadmium. There is 64 percent less Arsenic concentration found in rice grains when crop plants are irrigated according to alternate wetting and drying strategy.
Uptake Mechanism:
Redox potential and pH are two of the major factors influencing the mobilization and immobilization of heavy metals in flooded soil and their availability to plants for being uptake. Soluble Cd in the soil suspension is significantly correlated with total plant Cd and Cd uptake by shoot. Total Pb concentration uptake, including Pb associated with the roots, decrease with an increase in suspension redox potential and pH.
Uptake of lead (Pb) by shoot decreases with an increase in pH. There is no definite redox potential effect found on Pb uptake by the shoot. Lead is less mobile than Cd in the rice plants and primarily associated with the roots.
Water-soluble Pb uptake decreases with an increase in redox potential and pH, and significantly correlated with total plant Pb uptake and Pb accumulation by shoot. The estimated daily intakes of Pb and Cd from rice grown with continuous flooded fields are higher than alternate wetting and drying strategy. 
Figure: Comparison of Arsenic uptake under continuously flooded and intermittently flooded field
Effects of Heavy Metals on Human Health:
The main danger to human health’s from heavy metals are associated with exposure to lead, mercury, arsenic and cadmium. Heavy metals have been used by humans for several years. Although several harmful health effects of heavy metals have been known. In-spite of the fact that consumption of these in smaller amounts maintains good health but if taken in higher concentrations they become toxic and dangerous.
Heavy metals can cause decline in body’s energy level and damage the proper functioning of lungs, kidney, brain, liver, blood composition and possibly also bone effects and fractures. Continuous uptake can gradually result in muscular, physical and neurological weakening process that emulate diseases like Alzheimer’s and Parkinson disease as well as muscular dystrophy.
Exposure to arsenic (As) is mainly by intake of food and water, cereals especially rice being the most important source in most populations. Long-term uptake of arsenic through drinking-water is mainly associated with increased risks of skin cancer, but also some other types, as well as other skin lesions such as pigmentation changes and hyperkeratosis.
Authors: Muhammad Hamza Latif*, Fareeha Athar and Muhammad Ishfaq
*Agro-Biology Lab, Department of Agronomy, University of Agriculture, Faisalabad