A simple new equation for the reversible temperature dependence of photosynthetic electron transport: a study on soybean leaf

The temperature response of J(max), the irradiance-saturated potential rate of photosynthetic electron transport in the absence of Rubisco limitation, has usually been modelled by a complicated, modified Arrhenius type of equation. Light saturation can be difficult to achieve and reduces the precision of fluorescence measurements. Consequently, we calculated the rate of electron transport at 1200 mumol photosynthetically active radiation (PAR) quanta m(-2) s(-1) from chlorophyll fluorescence measurements on intact soybean leaves [Glycine max (L.) Merr] as temperature increased from 15 to 43degreesC with 1250 mumol mol(-1) ambient [CO2]. Electron transport rate was maximal around 37degreesC and the decline in rate following further increases in leaf temperature to 43degreesC was found to be completely reversible immediately upon return to lower temperatures. We report a convenient, new equation for the temperature dependence of the rate of electron transport under high irradiance: J(T-L) = J(T-o)e(-(TL-To/Omega)2), where T-L is the leaf temperature (degreesC), J (T-o) is the rate of electron transport at the optimum temperature T-o, and Omega is the difference in temperature from T-o at which J falls to e(-1) (0.37) of its value at T-o. Increased supply of nitrate increased J (T-o). Acclimation to growth temperature was observed, with T-o increasing from 35.4degreesC to 39.2degreesC for soybean plants grown at 20/15degreesC and 32/27degreesC (day/night), respectively. The average value of Omega was 18 +/- 0.6degreesC and was unaltered by growth conditions. A comprehensive review of the literature revealed a slight tendency for Omega to increase with T-o across species.