THE INFLUENCE OF ROOT-ZONE TEMPERATURE ON PHOTOSYNTHETIC ACCLIMATION TO ELEVATED CARBON-DIOXIDE CONCENTRATIONS

Soybean (Glycine max 'Clark') was grown from germination to 21 d after sowing (DAS) at ambient (similar to 360 mu mol mol(-1)) or elevated ( similar to 720 mu mol mol(-1)) carbon dioxide (CO2) at either one of t wo soil temperatures, 25 or 30 degrees C to determine the influence of root zone temperature on root growth and photosynthetic stimulation a t ambient and elevated concentrations of carbon dioxide. Although the photosynthetic rate became less stimulate over time, a significant sti mulation of whole plant photosynthesis and plant dry weight was observ ed at the elevated CO2 concentration during the experimental period ir respective of soil temperature. At neither carbon dioxide concentratio n did the warmer soil temperature (30 degrees C) stimulate whole plant growth compared to a soil temperature of 25 degrees C, but it did inc rease root growth relative to shoot (top) growth with a subsequent inc rease in root/shoot ratio. Increasing soil temperature at either carbo n dioxide concentration also significantly stimulated whole plant phot osynthetic rate. However, the degree of stimulation was reduced with t ime irrespective of carbon dioxide concentration so that at 21 DAS no difference in photosynthesis between ambient and elevated soil tempera tures was observed. Data from this experiment indicate that for soybea n, a higher soil temperature stimulates root/shoot ratio and enhances photosynthetic response to elevated carbon dioxide in the short-term ( i.e. days), but increasing root/shoot ratios does not provide a satisf actory explanation of long-term stimulation of photosynthesis at eleva ted levels of carbon dioxide. (C) 1988 Annals of Botany Company.