The effects of light quality and non-steady-state, localized 14CO2 pulse labeling on net assimilation and 14C translocation profiles in Heracleum lanatum

Red, green, blue, and white light of equivalent intensities of photosynthetically active radiation (PAR) (400–700 nm) had no effect upon leaf diffusion resistance, net assimilation, or translocation of leaves of Heracleum lanatum Michx. at 20 °C. Regardless of light quality at 80 μE m−2 s−1 PAR, 5 min and 40 min after the start of a 2-min application of 70 μCi of 14CO2 to a 1.33-cm2 spot on a leaflet, the major ethanol-soluble labeled assimilates were sucrose and glutamic acid while sucrose was the only labeled translocate. Regardless of light quality at 80 μE m−2 s−1 PAR, 5 min after the start of a 2-min spot tag, well defined ‘wave-like’ translocation profiles consisting of five peaks, 3 to 6 cm apart, were observed along 20 cm of petiole. Regardless of light quality, in 40-min experiments, the ‘wave-like’ profiles were replaced by typical, linear semilogarithmic translocation profiles. It is concluded that the initial ‘wave-like’ profiles were induced by perturbing the steady-state kinetics of synthesis and vein loading labeled sucrose by pulse labeling a small spot with an unusually high concentration of 14CO2. It is possible that pulsed profiles reported by others may have originated in similar fashion.