Mechanisms associated with rice tillering ability under stagnant flooding

Stagnant flooding (SF) is partial submergence with water level higher than normal but not exceeding 50 cm and maintained from few weeks to few months. SF decreases the ability of rice plants to tiller and dramatically increases lodging. In this study, four rice genotypes were used to evaluate their contrasting responses to SF under field conditions. Results showed that both tolerant (IRRII 19 and IRRII 54) and sensitive (Swarna-Sub 1 and PSBRc 18-Sub-1) genotypes were affected and their survival rate was lower under SF. Tiller number significantly decreased under SF compared to control condition. The decrease in tiller number under SF was directly related to reduction in grain yield and yield components, such as grain filling, grain weight and 1000-grain weight. The survival of all genotypes under four nitrogen levels was significantly reduced with increasing N application in the seedbed during seedling stage. Moreover, the expression of OsARG gene encoding arginase was slightly reduced when the water level was gradually increased, and it was significantly reduced under prolonged SF. Expression of OsTBI that controlled tiller bud formation was reduced during SF stress, leading to the reduction of tillering ability. Interestingly, expression of OsARFI gene involved in auxin pathway increased gradually with increasing water depth from 10 to 46 cm, and was strongly suppressed when the SF was prolonged. Similarly, expression of OsERFI gene involved in ethylene pathway was slightly increased when the water depth was increased from 10 to 46 cm, but was significantly suppressed at 50 cm depth. Understanding these complex factors that control tiller bud formation and development will help in developing varieties tolerant of stagnant flooding. Further studies are needed to fully understand how responses to environmental stresses such as SF are controlled and regulated.