Low Temperature Photosynthesis in Successional Winter Annuals

Winter annuals usually germinate in summer and fall, overwinter as vegetative rosettes, and then dominate 1—yr fields, suppressing spring—germinated summer annuals. Net photosynthetic response to light and temperature of Erigeron canadensis, E. annuus, Rorippa sessiliflora, Capsella bursa—pastoris, and Lactuca scariola were measured three times during the year. Optimum temperature for photosynthesis shifted from ° 25 degrees—30 degrees C during the summer to 15 degrees C during winter. Absolute rates of photosynthesis at 15 degrees C during winter approached maximum summer rates indicating temperature compensation. In the field winter photosynthesis is maximized because (1) rosette leaf temperatures, at air temperatures of 0 degrees —10 degrees C, were up to 10 degrees C above air temperature, (2) low light compensation point of photosynthesis shifts form 75 μE m(—2) s(—1) at 25 degrees C to 18 μE m(—2) s(—2) at 5 degrees C, (3) start up time for photosynthesis after dark period was only a few minutes and maximum photosynthesis was reached within 12 min, and (4) temperature compensation for photosynthesis approached 100% in a number of individuals. Photosynthesis over winter gives the species competitive advantage and may be critical to the dominance of winter annuals in succession during spring and summer.