The Primary productivity and physiology of Ceratophllum demersum. II. Micro primary productivity, pH, and the P/R ratio

Abstract

Photosynthesis in Ceratophyllum demersum has been measured in a Warburg respirometer containing a diethanolamine carbon dioxide buffer. Except in May, the photosynthetic activity of leaves is always greater than the buds. Production in the leaves rises from the minimum to the maximum during the period from May to October (winter to spring), probably as a response to increasing day length, while that in the buds remains practically constant throughout the year. With the onset of warmer temperatures in summer, vegetative growth declines. It is concluded that turions of Ceratophyllum are resistant not to cold but to heat and high light intensities of summer. Leaf photosynthesis occurs within a temperature range of 10-30�c and reaches a maximum at 20°C. The extracted chlorophyll content is greatest in plants collected from the surface of the lake and the ratio of chlorophyll a to chlorophyll b reaches a peak at 5 m depth. In Ceratophyllum it is suggested that the phenomenon of chromatic adaptation is occurring, and it is concluded that Ceratophyllum is a shade plant, being adapted to the quantity as well as the quality of light. Production in the leaves is depressed on the surface of the water and is greatest between 2.5 and 5.0 m depth. Also it is suggested that the compensation point for Ceratophyllum in Lake Ohakuri cannot be at a depth of much greater than 8 metres. The photosynthetic activity expressed on a dry weight basis is the greatest in the first mature leaves (6 cm behind the apex) and becomes progressively lower in the buds, leaves (1 ft behind the apex), and the stems. Furthermore, the rate of gross photosynthesis in leaves, buds, or stems exceeds the rate of respiration at all months of the year. Respiration in the dark is greater in the presence of carbon dioxide than in its absence. It is also greater in the dark than in the light. The rate of photosynthesis in Ceratophyllum is greatest at pH 5.0. From this, it is concluded that photosynthesis takes place at the greatest rate in a medium containing free CO2 rather than the bicarbonate ion. With only bicarbonate ion carbon dioxide present photosynthesis will still take place, although it is at a much lower rate.