Ability of some rice (Oryza sativa L.)

Direct-seeded rice (DSR) is becoming more popular among paddy farmers but it results in irregular stand establishment and high weed infestation. Early season flooding is another constraint to rice production in tropics as well as some of the major rice-growing areas in global scale and Sri Lanka is not an exception. Ability of some rice (Oryza sativa L.) genotypes to tolerate flooding during germination and seedling growth or anaerobic germination (AG) tolerance could help in overcoming the major obstacles in DSR. The rice cultivars, their mechanisms and the major QTLs governing AG-tolerance have been identified and two genes namely AG1 and AG2 have been incorporated to popular rice varieties for direct use. However, limited attempts have been made to screen Sri Lankan rice entries for AG tolerance. This review emphasizes the progress of AG-tolerance research, current challenges, future prospects as well as the importance of screening and identifying AG-tolerant rice varieties and validating management options to use this technology in DSR system in Sri Lanka.

Weeds are the most disastrous constraint in direct-seeded rice (DSR). Anaerobic germination-tolerance (AG-tolerance), which is the ability of rice to germinate, grow and survive under oxygen-limited conditions, together with suitable management options is a better alternative to suppress weeds in DSR. This study was designed to identify the effect of seeding rate and AG-tolerant varieties on stand establishment, weed growth and yield of rice (Oryza sativa L.) germinated under submerged condition. The experiments were conducted in a split-split plot design with three replicates during the Maha season (October 2014 to February 2015) and the Yala season (March to September 2015). The two main plots consisted of different water levels; submerged (sowing into 5 cm of standing water) and saturated as in conventional DSR. Three seeding rates (100, 80 and 60 kg/ha) were allocated to sub-plots and three rice varieties viz. Ciherang AG1+Sub1 and IR64+AG1 (two standard AG-tolerant rice varieties from the International Rice Research Institute (IRRI) at The Philippines), and Bg366 (from Sri Lanka) were allocated to sub-sub plots. Submergence reduced the stand establishment, panicle density and yield in all varieties compared to the saturated condition (P<0.05). It also reduced the total weed density by more than 5-fold, and biomass of grasses and sedges by 96 % (P<0.05), however, there was no effect (P>0.05) on aquatic and semi-aquatic weeds. The highest seedling and panicle densities, and yield under submerged conditions were recorded with the seeding rate of 100 kg/ha. Under the submergence, the rice variety Bg366 showed the lowest stand establishment and panicle density, however, recorded a comparable yield to that of Ciherang AG1+SUB1 and a higher yield than IR64+AG1. The results highlighted the need of novel AG-varieties, screening Sri Lankan rice varieties for AG-tolerance and finding suitable management options capable of producing high stand establishment and yield from DSR under submerged condition.