Photosynthesis in relation to growth and dormancy in Ceratophyllum demersum

Abstract

Growth and photosynthetic activity in the different development stages of Ceratophyllum demersum L. were measured with particular reference to the physiological maxima of these processes and the factors influencing the carbon fixation rates. The elongation of the main axis and growth of the adventitious buds were used as parameters for growth. The rate of photosynthesis was determined by measuring the uptake of radioactivity from 14C-bicarbonate labelled medium. The growth of directly harvested and preconditioned plant material was measured under standard laboratory conditions. In the directly harvested plants, maximum elongation was observed in February, whereas sprout growth occurred particularly in April. Subsequently, the growth slowed down until November and the plants went into innate dormancy. In the preconditioned plants, the sequence of development events and the increase in photosynthetic activity happened earlier. The photosynthetic carbon fixation rate of Ceratophyllum plants was measured in several stages of post-dormancy under standard conditions. From the dormant period up to the spring, the rate of photosynthesis increased steadily. A sharp increase occurred in spring after the maximum development in elongation and the sprout growth. In the preconditioned plants, variations in length of the incubation period did not affect the carbon fixation rates, though such variations caused significant differences in full-grown plants. When the plants were adapted for 24 h to the standard conditions, the carbon fixation rates were, however, affected. In the preconditioned plants adapted for a long period to standard conditions, respiration rate was much higher in the dark than in the light, indicating efficient refixation of the 14CO 2 evolved in the light. In directly harvested, full-grown plant material, however, the fixation rate was much higher than that in preconditioned plants, which might explain the decrease in the carbon fixation rate commonly reported in midsummer. Only a small percentage of the released radioactivity from the plants consisted of the excreted non-gaseous organic products.