EFFICIENCY OF PHOTOSYNTHESIS IN CONTINUOUS AND PULSED-LIGHT EMITTING DIODE IRRADIATION

The light utilization efficiency and relative photon requirement of ph otosynthesis in pulsed and continuous light from light emitting diodes (LEDs) has been measured. First, we characterized the photon requirem ent of photosynthesis from light of LEDs that differ in spectral quali ty. A photon requirement of 10.3 +/- 0.4 was measured using light from a 658 nm peak wavelength (22 nm half band width) LED over the range o f 0-50 mu mol photons m(-2) s(-1) in 2 kPa O-2 in leaves of tomato (Ly copersicon esculentum Mill., cv. VF36). Because the conversion of elec trical power to photons increased with wavelength, LED lamps with peak photon output of 668 nm were most efficient for converting electricit y to photosynthetically fixed carbon. The effect of pulsed irradiation on photosynthesis was then measured. When all of the light to make th e equivalent of 50 mu mol photons m(-2) s(-1) was provided during 1.5 mu s pulses of 5000 mu mol photons m(-2) s(-1) followed by 148.5 mu s dark periods, photosynthesis was the same as in continuous 50 mu mol p hotons m(-2) s(-1). When the pulse light and dark periods were lengthe ned to 200 mu s and 19.8 ms, respectively, photosynthesis was reduced, although the averaged photon flux density was unchanged. Under these conditions, the light pulses delivered 10(17) photons m(-2), which we calculate to be equivalent to the capacitance of PS I or PS II. Data s upport the theory that photons in pulses of 100 mu s or shorter are ab sorbed and stored in the reaction centers to be used in electron trans port during the dark period. When light/dark pulses were lengthened to 2 ms light and 198 ms dark, net photosynthesis was reduced to half of that measured in continuous light. Pigments of the xanthophyll cycle were not affected by any of these pulsed light treatments even though zeaxanthin formation occurred when leaves were forced to dissipate an equal amount of continuous light.