Yield- and photosynthesis-derived critical concentrations of tissue phosphorus and their significance for growth of Eurasian water milfoil, Myriophyllum spicatum L.

Abstract

A theoretical analysis of critical phosphorus levels in the tissue of Eurasian water milfoil, Myriophyllum spicatum L., predicts that growth-rate- and photosynthetic-derived critical levels should be equal but that these levels should exceed the critical value from typical laboratory yield experiments by a factor of 1.6. This factor is not very sensitive to changes in the assumed relationship between growth rate and tissue nutrient concentration and thus may apply to many plant species. However, the theoretical result contrasts with the greater than fourfold difference found previously between yield and photosynthetic critical levels for milfoil. Using plants harvested early, in order to minimize the effects of culture age on growth rate, we measured a critical level of 0.25 – 0.27% P at 95% of the phosphorus-saturated yield. This value differs from the critical concentration for photosynthesis by approximately the theoretical ratio of 1.6:1, but is more than three times larger than the previously reported critical concentration for yield. Culture-aging effects would result in lower assessments of yield critical levels, and probably caused those data reported previously to be unrealistically low. Adoption of the higher yield value that we report would eliminate the dilemma posed by tissue levels that occur between the previously reported yield and photosynthetic concentrations, and it would alter the conclusions of many previous studies regarding factors limiting to milfoil growth.