Abstract
Ceratophyllum demersum L. remained physiologically active beneath ice of a southeastern Michigan lake. The effect of seasonally low photosynthetic photon flux density (PPFD) and cold but nonfreezing temperature on whole-plant physiology was studied. Net photosynthesis was measured at six temperatures and 12 PPFDs. Net photosynthesis, soluble protein concentration, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein concentration, and Rubisco activity of winter plants were 32, 31, 33, and 70% lower, respectively, than those of plants collected in the summer. Optimum temperatures for net photosynthesis of winter and summer plants were 5 and 30[deg]C, respectively. Dark respiration of winter plants was up to 313% greater than that of summer plants. Reduced Rubisco activity and increased dark respiration interacted to reduce net photosynthesis. Interaction of reduced net photosynthesis and increased dark respiration increased CO2 and light compensation points and the light saturation point of winter plants. Growth of C. demersum was limited by the ambient phosphorus concentration of lake water during summer. Apical stem segments of winter-collected plants had 54 and 35% more phosphorus and nitrogen, respectively, than summer-collected plants. Physiologically active perennation beneath ice enabled C. demersum to accumulate phosphorus during the winter when it was most abundant. Partial uncoupling of phosphorus acquisition from utilization may reduce phosphorus limitation upon growth during the summer when phosphorus concentration is seasonally the lowest.