Ecology of Phragmites communis. I. Photosynthesis of a single shoot in situ

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Abstract
Photosynthesis of a single shoot of Phragmites communis Trin. var. berlandieri (Fourn.) Fern., growing isolated from a stand, was measured in situ by enclosing it in a plastic bag of volume 107 liters and administering 1 mcurie of 14CO2 (specific activity, 5 mcurie per mmole) for 1 hour under a cloudless sky (light intensity > 100 000 lux; photoperiod, 16.5 h). The plant was 164 cm in height with 15 internodes, 11 expanded leaves, and a terminal unrolled leaf. The leaves, leaf sheaths, and internodes were removed, frozen between blocks of solid CO2, and subsequently extracted by the method of Bieleski and Turner (Anal. Biochem. 17, 278–293 (1966)). 14CO2 incorporation was determined in extracts of the plant parts by a scintillation analyzer.Photosynthetic rates were calculated per leaf, per square decimeter of leaf, per gram wet weight, and per μmole of leaf chlorophyll. Greatest incorporation of 14CO2 was found in the centrally inserted leaves. Leaf sheaths and internodes fixed only 8% of the total. Leaf area was the only useful index of photosynthesis averaging 6.1 (S.D. ± 0.62) mg CO2/dm2 per hour. On a gram wet weight basis the rate of photosynthesis increased in a hyperbolic relationship from the lowest to the uppermost leaves. Chlorophyll was determined on leaves from an adjacent plant of similar size and structure. On this basis the centrally inserted leaves were the least and the upper leaves were the most efficient, the uppermost expanded leaf having a rate of 163 µmoles CO2 per µmole chlorophyll per hour (assimilation time = 22 seconds). Productivity, in terms of carbon fixed, was 3.5 g/m2 per day based on a mean density of 12.75 plants per 0.25 m2 and a 10-hour day.