Photosynthetic and Respiratory Exchanges of Carbon Dioxide by Leaves of the Grain Amaranth

Abstract

The primary productivity of a plant community depends largely on the activity of the individual photosynthetic elements of the foliage canopy. Although an extensive literature exists on the photosynthetic capabilities of individual leaves, it is only recently that such data have been used successfully in the interpretation of community productivity (de Wit 1965; Monteith 1965; Duncan et al. 1967). In these studies, usually aided by electronic computers, the illumination of each leaf has been assessed and then the net photosynthesis resulting from that illumination has been summed for an entire community. Such simulations are highly dependent upon the physiological data which are supplied to the computer, and one needs to consider environmental variations as well as the manner in which the plants adapt to particular conditions within the canopy. An important aspect of such adaptations is that the photosynthetic capability of a leaf, as well as its activity, may vary according to its illumination within a canopy; thus photosynthetic capabilities may be assessed best from measurements with leaves adapted to various light environments. Bjorkman (1965, 1966) has shown with Solidago and other species that adaptation to a specific light environment may involve significant changes in the relative contributions of photosystems I and II and other basic reactions in photosynthesis. In addition, Ludwig, Saeki & Evans (1965) and McCree & Troughton (1966) have recently shown that the adaptation of leaves to different illumination levels also results in marked changes in their rates of dark respiration. From recent work it has become evident that interspecific and environmentally induced variations in the compensation light intensities, and the maximum photosynthetic rates of individual leaves, all depend substantially on variations in the rates of respiration, both in light and in dark (El-Sharkawy & Hesketh 1965; El-Sharkawy, Loomis & Williams 1967; Duncan et al. 1967). The very large photosynthetic rates of several tropical grasses are associated with the failure of their leaves to release respiratory CO2 when sun-adapted plants are exposed to light while maintained in an atmosphere free of CO2 (El-Sharkawy & Hesketh 1965). This contrasts with the behaviour of a number of dicotyledonous species which have much smaller maximum photosynthetic rates associated with a marked release of CO2 into C02-free air during illumination. In further studies of dicotyledonous species, El-Sharkawy et al. (1967) discovered that in respect to photosynthesis and respiration, certain Amaranthus species behave more like the tropical grasses such as corn than like other dicotyledonous species. This report is concerned with the behaviour of one of these amaranths, A. edulis, the grain amaranth. A characterization of its photosynthetic responses to light, carbon dioxide and temperature is of interest because of the potential of the species as research material for studies on maximization of primary productivity in terrestrial plant communities. The grain amaranth is a tall, erect, annual herb, and in open stands it may branch at all nodes. At Davis, California, it grows well as a summer annual and reaches a height of * Present address: Ministry of Agriculture, Cotton Production Section, Giza Station, Cairo, Egypt.