GROWTH OF DUCKWEED 



UNDER CONSTANT AND VARIABLE 



TEMPERATURES 



REBECCA R. SHARITZ and JEFFREY C. LUVALL 

 Savannah River Ecology Laboratory, Aiken, South Carolina 



ABSTRACT 



Effects of variable temperatures on the growth and vegetative reproduction of an 

 aquatic plant were examined by exposing populations of Spirodela oligorrhiza to 

 cyclic, acyclic, and constant temperature regimes. Growth rates under the cyclic 

 and acyclic regimes (each of which ranged from 10 to 25 C) were not 

 significantly different from those at a constant temperature equal to the mean of 

 the range (17.5 C). Population growth was directly related to temperature in 

 both constant and variable thermal regimes. 



Laboratory studies of effects on organisms of increases in tempera- 

 ture have commonly been based on exposure to constant tempera- 

 ture regimes. Such studies do not reflect conditions generally existing 

 in nature, and data on life-history phenomena of organisms at 

 constant temperatures may be misleading if extrapolated to field 

 conditions (Hagstrum and Hagstrum, 1970). Many freshwater organ- 

 isms are confined to shallow-water habitats and exposed to both 

 diurnal and seasonal fluctuations in temperature. 



The significance of cyclic and recurring temperatures on growth 

 and development in a variety of organisms has been demonstrated. 

 For example, a diurnal thermoperiodicity with night temperatures 4 

 to 8°C lower than day temperatures enhances the growth of 

 sporelings and fronds of Porphyra (Shimo, 1977). Insects frequently 

 have been shown to develop more rapidly under fluctuating 

 temperatures than constant temperatures if the range of fluctuation 

 falls within the optimal range for the organisms' development 

 (Hagstrum and Hagstrum, 1970). Heath (1963) reported that 

 maximum tolerance to temperature extremes in the sea-run cutthroat 



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