Morphological and physiological responses of rice roots and shoots to varying water regimes and soil microbial densities

To understand the responses of rice roots and shoots to non-continuously-flooded soil–water regimes together with varied levels of soil microbial density, studies were conducted evaluating three different water regimes: intermittent flooding (IF) through the vegetative stage (IF-V), IF extended into the reproductive stage (IF-R), and soil maintained with no standing water (NSW); and three levels of soil microbial density: normal, reduced (autoclaved), and enhanced (EMS). At flowering, EMS-treated plants were found to have increased root-length density and greater root oxidizing activity rates under both IF-V and IF-R and more available soil nitrogen (ASN) under IF-V. Earlier senescence of plants grown with EMS was also observed under all water treatments. Water regime was seen to have a major effect on grain yield with a number of causal mechanisms involved. Significant relationships were observed between root oxidizing activity rate (ROA) and ASN, and between ROA and the chlorophyll content of lower leaves. The EMS effects were apparently not caused by microorganisms directly, but rather by differences in ASN facilitated by interaction between water regime and soil microbial activity. The latter would probably vary with different levels of soil organic carbon, but this additional parameter was not investigated. Further study on the role of soil organic matter under similar conditions is warranted for better understanding of these relationships, which have significant effects on rice grain yield.