Photosynthesis in flashing light in soybean leaves grown in different conditions. II. Lightfleck utilization efficiency

Leaves of soybean plants grown in contrasting light and nutrient availability conditions were exposed to constant and to flashing light regimes with lightflecks of different frequencies, durations and photon flux density (PFD). The lightfleck characteristics were selected to be representative of the range of variation found for sunflecks in a soybean canopy. CO₂ fixation rates were measured using a fast-response gas-exchange apparatus. The net CO₂ fixation due to 1-s-duration lightflecks was 1·3 times higher than predicted from steady-state measurements in constant light at the lightfleck and background PFD. This lightfleck utilization efficiency (LUE) was somewhat higher at a high than at a low frequency of one second lightflecks. LUE in flashing light with very short lightflecks (0·2s) and single 1 s lightflecks was as high as 2, but decreased sharply with increasing duration of lightflecks. This decrease occurred because CO₂ fixation rates during lightflecks were constrained by carbon metabolism and induction limitations, and because the contribution of post-illumination CO₂ fixation to total CO₂ fixation decreased with increased duration of lightflecks. LUE increased with increased PFD during the lightflecks, particularly in leaves from plants grown in high-light, high-nutrient conditions. Saturation PFDs were much higher in flashing light than in constant light. Only small differences in LUE were apparent between leaves from the three growth conditions.