5. Conclusions

The photosynthetic responses to sudden changes in light conditions (lightflecks) were studied and the CO2 uptake rate found to be maintained at a certain level for several seconds after a sudden decrease in light intensity [4, 1]. When photosynthetic rates have been measured with flashing light, considerable rate increases have been observed at high irradiances in the absence [19] or presence [20] of continuous background light. This has been attributed to post-illumination CO2 fixation consuming the pools of RuBP [21] as well as triose phosphate that requires some extra ATP synthesis from the ΔpH gradient to convert it to RuBP [22]. If the time between flashes is sufficiently long, the ΔpH gradient is unable to build up as it is dissipated by the post-illumination CO2 fixation's demand for ATP synthesis. This means that electron transport can occur more rapidly during the pulses of light than in continuous light because plastoquinone reoxidation is not restricted by the ΔpH gradient. If a sufficiently large ΔpH gradient exists at high irradiances even with high CO2 partial pressures in leaf, then one would expect that flashing light could further enhance the photosynthetic rate. Stitt [23]

26 Photosynthesis - From Its Evolution to Future Improvements in Photosynthetic Efficiency Using Nanomaterials

cycles (10s) were used in an oxygen electrode with 5% CO2, the leaf was kept for 10 s at each light condition. Roden and Pearcy [2] found that the efficiency of post-illumination CO2 fixation only declined once the intervening dark period exceeded about 1 s. Kriedemann et al. [19] also showed that fluctuating light with 200 ms dark intervening periods enhanced photosynthesis. Electron transport during the flash reduces NADPH and builds up the pool of RuBP and triose phosphate. These pools, termed assimilatory power by Laisk et al. [21] enable postillumination CO2 fixation to occur and were equivalent to 5 s of photosynthesis in sunflower

consumption of these pools during and after lightflecks. For Phaseolus leaves grown in sunlight, the RuBP pool was 5 μmol m<sup>2</sup> and the total post-illumination CO2 fixation was 12 μmol m<sup>2</sup> when triose phosphate was included. The latter requires additional ATP synthesis that comes from the proton pool stored in the thylakoid lumen [22]. Steady-state pool sizes of 100 μmol m<sup>2</sup> for RuBP have been measured in Raphanus leaves [24], which are certainly adequate to cope with the maximum of 5 μmol m<sup>2</sup> observed here per flash. The balance between Rubisco activity and electron transport rate is effectively increased by the ratio of intervening time to flash length up to the limit set by the pool sizes of RuBP and triose phosphate. Therefore, in flashing light, the dependence of electron transport rate on CO2 should be small. Thus, photosynthetic intermediates (PIs) were quickly produced by photochemical reactions during lightflecks and consumed thereafter in the CO2 fixation occurred during next dark or dim light period. An actual estimation of PIs content is difficult under pulsed light, especially at high frequencies. A kinetic model to estimate Pn was developed by considering that photosynthetic intermediates were pooled during light periods and then consumed by partial photosynthetic reactions during dark periods [25]. According to this model, they quantitatively estimated the effects of pulsed light frequency and duty ratio on photosynthetic rates of cos lettuce leaves. The estimated Pn was lower, especially under pulsed light at lower frequencies and did not exceed Pn under continuous light. Accordingly, they concluded that, compared with a constant PPFD, fluctuation in PPFD can theoretically be disadvantageous to photosynthesis, even though the time-averaged PPFD are identical. In this study, lettuce leaves grown under pulsed light at low frequencies (2–0.5 Hz) maintained higher Pn compared to

<sup>1</sup> with spinach when long flashing

<sup>1</sup> PPFD. Sharkey et al. [22] showed the rapid buildup and

observed a 30% increase in the rate at 1500 μmol m<sup>2</sup> s

leaves at 100–200 μmol m<sup>2</sup> s

Pulsed light at high frequencies (2–20 kHz, 50% duty ratio, 200 μmol m�<sup>2</sup> s �1 ) positively affected the growth of lettuce leaves under controlled environment. The photosynthetic performances showed differences between leaves developed under pulsed light and leaves developed under continuous light, when the CO2 uptake rates and chlorophyll fluorescence parameters were measured at lower frequencies (<2 Hz). In the pulsed light technique, it is important to determine both optimal frequency and duty ratio for plants to attain the most efficient use of harvested light. The reason why growth was enhanced under pulsed light at high frequencies has not been resolved by analyzing photosynthetic performances in this study. Further research is required for detecting the pool size of PIs in leaves during their exposure to intermittent radiation. We propose that the pulsed lighting technique by using LEDs could become a useful for the production of leafy vegetables controlled plant factory systems in the near future.
