Ecophysiology of Ambrosia Artemisiifolia: A Successional Dominant

Ambrosia artemisiifolia L. dominates abandoned fields after spring plowing in many parts of the eastern United States. Its seeds germinate at or near the soil surface in early spring, and the cotyledons become green shortly after they emerge. They are photosynthetically active, with a net photosynthesis dark respiration ratio (P/R) of 1.9. The primary leaves are photosynthetically more active than the cotyledons with a P/R ratio of 3. Photosynthesis of young A. artemisiifolia seedlings is 21 mg CO2 dm—2 hr—2 lux, 25°C, and 300 ppm CO2. They approach light saturation at 1,076 X 102 lux (1.1 cal cm—2) and exhibit appreciable photosynthesis at temperatures from 5° to 35°C. Their rate of photosynthesis increases with an increase of the ambient CO2 concentrations comparable to that measured near the soil surface in the field. Although seedling photosynthesis is quite sensitive to decreasing water potential (°w) between 0 and —10 bars, there is no further decrease between —10 and —20 bars. Furthermore, at this range of °w photosynthesis is about 20% of that at 0 bars. In a plowed field these seedlings experience high light intensities, appreciable variation in daily temperature, high CO2 concentrations due to its flux from the soil, and low water potentials since the roots are in the upper part of the soil. Field—grown, mature A. artemisiifolia plants do not light—saturate even at 1,076 x 102 lux. Their photosynthetic rate at optimum light intensity, temperature, °w, and 300 ppm CO2 is about 35 mg CO2 dm—2 hr—1. This high rate of photosynthesis is accompanied by a high rate of transpiration (3 g H2O dm—2 hr—1). Photosynthesis is these plants is sensitive to temperature and reaches a peak at 20°C. The effect of CO2 enrichment on the rate of photosynthesis is not as pronounced in mature plants as in seedlings. Photosynthesis rates decline sharply between —10 and —15 bars °2; however, appreciable rates are maintained even at °w of —20 bars for several hours. Photosynthesis recovery from water stress is rapid after watering and is attributed to the very low plant resistance to water transport (1 x 106 sec cm—1). Mature plants have moderate photorespiration since their CO2 compensation point is reached at 50 ppm CO2. These data indicate that A. artemisiifolia can not only efficiently exploit the environmental resources of the field during seedlings and mature plant stages of its life cycle but also effectively function under the sometimes extreme conditions encountered in the field. These ecophysiological attributed coupled with complex germination behavior and genetic plasticity enable the species to be a successful pioneer in early successional ecosystems where the environment can be severe and unpredictable.