Abstract
To clarify the physiological reasons why the two lowland weeds cannot invade into a dry habitat, drought resistance among Echinochloa weeds was compared by exposing potted plants to a progressive drought condition after drainage. Drought resistance markedly differed between the two lowland weeds, E. oryzicola and E. crus-galli var. formosensis, and the upland weed, E. crus-galli var. praticola in every aspect determined. E. crus-galli var. crus-galli having a widespread habitat from lowland to upland was always intermediate between them. The midday leaf water potential decreased similarly in all the weeds as the drought con-dition progressed, but in the lowland weeds it dropped to -23 bars while in upland weed to -15 bars on the 6th day after drainage. All the weeds showed wilting symptoms such as leaf rolling and senescence of the lower leaves, but the upland weed showed much milder symptoms than the other weeds. When compared to the lowland weeds at the same midday leaf water potential of -15 bars, the upland weed had 10 times larger stomatal resistance, 7 times smaller transpiration rate and 5 times larger leaf growth rate in length. The ratio of photosynthesis to transpiration measured on the 5th day was exclusively higher for the upland weed. The lower drought resistance of the two lowland weeds, which relates to their dull response of stomata to decreasing leaf water potential, is suggested to be one of the physiological reasons why they cannot invade into habitats with dry condition.
