Impact of alternate wetting and drying on rice physiology, grain production, and grain quality

As the world’s population increases, demands on staple crops like rice (Oryza sativa L.) will also increase, requiring additional fresh water supplies for irrigation of rice fields. Safe alternate wetting and drying (AWD) is a water management technique that is being adopted across a number of countries to reduce the water input for rice cultivation. The impact of AWD on plant growth, yield and grain quality is not well understood. A field trial of AWD was conducted at Mymensingh, Bangladesh over two boro (dry) seasons using eight field plots, four under AWD and four continuously flooded (CF). This manuscript describes the results of check cultivar BRRI Dhan28 which was replicated in 35-40 rows per plot giving a total of 140-160 replicates per treatment. A study on the soil solution concentration of many elements indicated that manganese, iron, zinc, and arsenic were different under AWD conditions compared to CF on a number of sampling time points, but did not show a pattern related to the AWD treatment. A survey of soil strength using a penetrometer detected a small, but significant, hardening of the surface soil of the AWD plots. At harvest the shoot and grain mass was significantly greater for the plants grown under AWD (9.0-9.4% and 12.0-15.4%, respectively) with the plants grown under AWD having a greater number of productive tillers. Physiological examination in the first year showed that although AWD decreased (~21%) leaf elongation rate (LER) of recently transplanted seedlings during the first drying cycle, subsequent drying cycles did not affect LER, while tillering was slightly increased by AWD and there was evidence of higher leaf abscisic acid (ABA) in AWD plants. In the second year analysis of six phytohormones revealed that AWD increased plant foliar iso-pentenyladenine (iP) concentrations by 37% while leaf trans-zeatin concentrations decreased (36%) compared to CF plants. The elemental composition of the shoots and grains was also examined. In both years AWD decreased grain concentration of sulphur (by 4% and 15%), calcium (by 6% and 9%), iron (by 11% and 16%), and arsenic (by 14% and 26%), while it increased the grain concentration of manganese (by 19% and 28%), copper (by 81% and 37%), and cadmium (by 28% and 67%). These results indicate that plants grown under safe AWD conditions at this site have an increased grain mass compared to plants grown under CF, and this may be partly due to a high number of productive tillers. AWD decreases the concentration of arsenic in the grains in this site, but it elevates the concentration of cadmium.