Strategy of Echinochloa oryzicola Vasing. for survival in flooded rice [Oryza sativa]

Ecliinocliloa oryzicola Vasing. (= Ecliinocliloa phyllopogon Stapf ex Kessenko) is an obligate weed@with an elaborated survival strategy in the flooded rice of Japan. In this review various adaptive characters of the weed, which comprise the survival strategy, are discussed through the@life cycle. The weed is distributed only in flooded rice. Seeds (spikelets) buried in the soil@exhibit annual cycles between dormant and non-dormant state, and non-dormant seeds@recurrently appear in spring when rice growers start to prepare seedling beds and fields for@rice transplanting. The non-dormant seeds have unique characters metabolically adapted to@submerged conditions to germinate and grow by the anaerobic respiration through alcohol@fermentation. The weed has seemingly perfect mimicry of the rice plants throughout its@development from seedling to heading, by which the weed escapes from manual weeding. In@a rice paddy, the weed starts heading coincidentally with the rice plants at the period when@the growers are reluctant to walk in the rice paddy to weed. Irrespective of plant height@of the rice cultivar, the weed develops a few upper leaves above the rice canopy during@the heading period of rice. This phenotypic plasticity of E. oryzicola in plant height is one of@the characters conferring its competitive aggressiveness in flooded rice. When weeding@is begun again after heading, the dormant weed seeds escape weeding by shattering and@join the soil seedbank. The dormant seeds express the gene of an enzyme catalyzing@ATP synthesis through the mitochondrial oxidative phosphorylation more abundantly,@and have larger oxygen absorption and enzyme activity of the aerobic respiration than@the non-dormant seeds, suggesting that the dormant seeds maintain viability by the con-ventional aerobic respiration in the paddy soil drained from rice harvesting in fall to@the next early spring. The various adaptive characters comprising the survival strategy of E.@oryzicola in flooded rice consist of those inherited from the wild progenitor and those@selected by the crop cultivation pressure. It is suggested that both the mimicry of the weed@and the heading coincident with the rice plants have been acquired by the large selection@pressure of frequent weeding, which has been done over the past hundred years. However,@today, the manual weeding is substituted with herbicides, which cannot detect the mimicry@and heading photoperiodic sensitivity. As a result, the dominant species of Ediinochloa weeds@in flooded rice is changing from E. oryzicola to Ecliinocliloa crus-galli var, cnis-galli that has@neither mimicry nor photoperiodic sensitivity synchronizing to that of rice, but is more@competitive against rice.