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 μmol 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 43°C with 1250 μmol mol^-1 ambient [CO₂]. Electron transport rate was maximal around 37°C and the decline in rate following further increases in leaf temperature to 43°C 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: (Equation Presented) /ZLzi)2 J(T1) =J(T0)e\ ° , where T^L is the leaf temperature (°C), J(T_o) is the rate of electron transport at the optimum temperature T_o, and Ω 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 35.4°C to 39.2°C for soybean plants grown at 20/15°C and 32/27°C (day/night), respectively. The average value of Ω was 18 ± 0.6°C and was unaltered by growth conditions. A comprehensive review of the literature revealed a slight tendency for Ω to increase with T_o across species.