Can photosynthesis respond to short-term fluctuations in atmospheric carbon dioxide?

Rapid and irregular variations of atmospheric CO2 concentrations (c(a)) occur in nature but are often very much more pronounced and frequent when artificially enriching CO2 concentrations in simulating the future atmosphere. Therefore, there is the danger that plant responses at elevated CO2 in fumigation experiments might reflect the increased Frequency and amplitude of fluctuation in concentration as well as the increase in average concentration. Tests were conducted to determine whether the photosynthetic process could sense such fluctuations in c(a). Instantaneous chlorophyll fluorescence (F-t) was monitored for wheat leaves (Triticum aestivum cv. Hereward) exposed to c(a) oscillating symmetrically by 225 mu mol mol(-1) about a c(a) set point concentration of 575 or 650 mu mol mol(-1). No F-t response was detected to half-cycle step changes in c(a) lasting less than two seconds, but at half-cycles of two seconds or longer, the response of F-t was pronounced. In order to determine the in vivo linear electron transport rate (J) the O-2 concentration was maintained at 21 mmol mol(-1) to eliminate photorespiration. J which is directly proportional to the rate of CO2 uptake under these conditions, was not significantly changed at half-cycles of 30 s or less but was decreased by half-cycles of 60 s or longer. It was inferred that if duration of an oscillation is less than 1 minute and is symmetrical with respect to mean CO2 concentration, then there is no effect on current carbon uptake, but oscillations of 1 minute or more decrease photosynthetic CO2 uptake in wheat.