Photosynthetic response of Myriophyllum salsugineum A.E. orchard to photon irradiance, temperature and external free CO2

The photosynthetic capacity of Myriophyllum salsugineum A.E. Orchard was measured, using plants collected from Lake Wendouree, Ballarat, Victoria and grown subsequently in a glasshouse pond at Griffith, New South Wales. At pH 7.00, under conditions of constant total alkalinity of 1.0 meq dm^?3 and saturating photon irradiance, the temperature optimum was found to be 30–35°C with rates of 140 μmol mg^?1 chlorophyll a h^?1 for oxygen production and 149 μmol mg^?1 chlorophyll a h^?1 for consumption of CO₂. These rates are generally higher than those measured by other workers for the noxious Eurasian water milfoil, Myriophyllum spicatum L., of which Myriophyllum salsugineum is a close relative. The light-compensation point and the photon irradiance required to saturate photosynthetic oxygen production were exponentially dependent on water temperature. Over the temperature range 15–35°C the light-compensation point increased from 2.4 to 16.9 μmol (PAR) m^?2 s^?1 for oxygen production while saturation photon irradiance increased from 41.5 to 138 μmol (PAR) m^?2 s^?1 for oxygen production and from 42.0 to 174 μmol (PAR) m^?2 s^?1 for CO₂ consumption. Respiration rates increased from 27.1 to 112.3 μmol (oxygen consumed) g^?1 dry weight h^?1 as temperature was increased from 15 to 35°C. The optimum temperature for productivity is 30°C.