1. Introduction

Plants or leaves in their natural state are frequently subjected to large and rapid fluctuations in irradiance. Photosynthetic performance under fluctuating irradiation (pulsed light or irregular sunflecks in forest floor, Figure 1) is different from steady-state photosynthesis under continuous or nonfluctuating irradiation at constant light intensity [1]. For example, poplar leaves receive 15% of their light in flecks lasting between 0 and 200 ms and a further 35% in flecks

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

found critical not only to growth but also to biosynthesis of secondary metabolites in lettuce plants and especially the supplemental irradiation of green LEDs with the combination of red and blue LEDs can improve the growth [6]. Radiation mixture of blue, red, or far-red light with LEDs improved vegetable growth and enhanced the number of floral buds of

LEDs illumination system can blink or flash with a very short period, in which they can be turned fully on and fully off extremely rapidly (μs interval), emitting pulsed light with high intensity. Pulse light by adjusting the frequency and duty ratio (light on period per frequency) of LEDs resulted in optimal growth of potato plantlets in vitro with electricity savings and an effective illumination system adjusting light intensity, quality, frequency and duty ratio was developed [8–10]. In the pulsed light technique for growing tomato plants, low frequencies (0.1, 1, 10 Hz) had higher quantum efficiency in PSII than higher frequencies (50, 100 kHz) and continuous light, but the electron transport rate decreased when the frequency of pulse increased [11]. On the other hand, pulsed light of lower duty ratios, combined with lower frequencies, makes the CO2 uptake rate of cos lettuce lower than that attained in continuous light, inferring that pulsed illumination with such a condition is less advantageous than

The objectives of this study were to determine the effects of pulsed light with various frequencies of LEDs illumination system on the growth of leaf lettuce under controlled

thetic responses to pulsed light in comparison with continuous light (PPFD, 0–

2. Measurements of CO2 assimilation rates and chlorophyll fluorescence of

Leaf lettuce (Lactuca sativa L. crispa 'Bio Saradana', Nakahara Seed Product Co., Ltd., Fukuoka, Japan) seeds were sown on watered sponge blocks (10 10 20 mm). After germination, 20 seedlings that had grown uniformly with three leaves were each transplanted into a polyvinyl pot (90 mm diameter, 80 mm depth) filled with vermiculite and grown for 30 days by bottom irrigation with commercial liquid fertilizer (OAT Agrio Co., Ltd., Tokyo, Japan, EC 1.3

maintained at 22C and 60%, respectively. The photosynthetic photon flux density (PPFD) was

ing a peak wavelength of red (660 nm) and blue (455 nm) with a 16-h day length (Figure 2). Fourteen irradiation treatments were examined to determine the effect of wide-range frequencies of pulsed lighting (20, 10, 4, 2, 1.3, 1 kHz, and 500, 50, 5, 2.5, 1.3, 1, 0.5 Hz at 50% duty ratio) and continuous lighting. After measurements of leaf photosynthetic parameters at the end of culture period, all plants were sampled and fresh weights of leaves and roots and total

, pH 6.0). The air temperature and relative humidity throughout the cultivation were

<sup>1</sup> supplied by LEDs lamps (Legu LED, HRD Co., Ltd. Tootori, Japan) provid-

1

). The study was aimed to provide valuable information regarding the

), and to investigate the leaf photosyn-

Growth and Photosynthesis under Pulsed Lighting http://dx.doi.org/10.5772/intechopen.75519 19

ornamentals under controlled environmental conditions [7].

continuous light for photosynthesis [12].

1

500 μmol m<sup>2</sup> s

intact leaves

mS cm<sup>1</sup>

200 μmol m<sup>2</sup> s

leaf area were recorded.

environmental conditions (PPFD, 200 μmol m<sup>2</sup> s

possibility of electricity savings in running plant factory system.

Figure 1. Pulsed light or irregular sunflecks in forest floor.

between 200 and 400 ms [2]. Photosynthesis consists of light reaction and dark reaction as a continuous reaction in this order. The former, which is a chain redox reaction of photosystem II (PSII) and I (PSI), works as light energy harvesting and producing utilizable chemical energy products in the later CO2 assimilate reaction cycle, which reacts only enzymatically and light independently if adequate amount of these chemical energy products are supplied. The complex web of reactions in photosynthesis have different response times, so that fluxes through some reactions can be much faster than others resulting in fluctuating pool sizes. Furthermore, each reaction process seems to occur very rapidly in nanosecond to millisecond rates in the light reaction [3] as compared to seconds to minute rates in the dark reaction [4]. In recent years, photosynthetic responses to intermittent irradiation have been investigated again by using a developed illumination system with light-emitting diodes (LEDs). Measurement techniques also have a range in response times so that different reactions can be monitored with different temporal resolution.

Most of plant factory systems for producing leafy vegetables have adopted tubular cool-white fluorescent lamps as their light source. Recently, advances in LEDs technology have contributed to grow plants as a new type of light source. For further developing and improving plant factory system, LEDs illumination systems provide a potential alternative to the tubular fluorescent lamps due to their lower energy consumption, wavelength specificity and supposed durability.

Plant growth and development are strongly affected by light intensity (PPFD), quality (wavelength), and duration (photoperiod). The photosynthetic system including chlorophyll content, stomata size and leaf area of lettuce leaves was optimized by adjusting the light spectrum (455, 660, 735 nm) and flux density with high-power LEDs [5]. Light quality was found critical not only to growth but also to biosynthesis of secondary metabolites in lettuce plants and especially the supplemental irradiation of green LEDs with the combination of red and blue LEDs can improve the growth [6]. Radiation mixture of blue, red, or far-red light with LEDs improved vegetable growth and enhanced the number of floral buds of ornamentals under controlled environmental conditions [7].

LEDs illumination system can blink or flash with a very short period, in which they can be turned fully on and fully off extremely rapidly (μs interval), emitting pulsed light with high intensity. Pulse light by adjusting the frequency and duty ratio (light on period per frequency) of LEDs resulted in optimal growth of potato plantlets in vitro with electricity savings and an effective illumination system adjusting light intensity, quality, frequency and duty ratio was developed [8–10]. In the pulsed light technique for growing tomato plants, low frequencies (0.1, 1, 10 Hz) had higher quantum efficiency in PSII than higher frequencies (50, 100 kHz) and continuous light, but the electron transport rate decreased when the frequency of pulse increased [11]. On the other hand, pulsed light of lower duty ratios, combined with lower frequencies, makes the CO2 uptake rate of cos lettuce lower than that attained in continuous light, inferring that pulsed illumination with such a condition is less advantageous than continuous light for photosynthesis [12].

The objectives of this study were to determine the effects of pulsed light with various frequencies of LEDs illumination system on the growth of leaf lettuce under controlled environmental conditions (PPFD, 200 μmol m<sup>2</sup> s 1 ), and to investigate the leaf photosynthetic responses to pulsed light in comparison with continuous light (PPFD, 0– 500 μmol m<sup>2</sup> s 1 ). The study was aimed to provide valuable information regarding the possibility of electricity savings in running plant factory system.

between 200 and 400 ms [2]. Photosynthesis consists of light reaction and dark reaction as a continuous reaction in this order. The former, which is a chain redox reaction of photosystem II (PSII) and I (PSI), works as light energy harvesting and producing utilizable chemical energy products in the later CO2 assimilate reaction cycle, which reacts only enzymatically and light independently if adequate amount of these chemical energy products are supplied. The complex web of reactions in photosynthesis have different response times, so that fluxes through some reactions can be much faster than others resulting in fluctuating pool sizes. Furthermore, each reaction process seems to occur very rapidly in nanosecond to millisecond rates in the light reaction [3] as compared to seconds to minute rates in the dark reaction [4]. In recent years, photosynthetic responses to intermittent irradiation have been investigated again by using a developed illumination system with light-emitting diodes (LEDs). Measurement techniques also have a range in response times so that different reactions can be monitored with

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

Most of plant factory systems for producing leafy vegetables have adopted tubular cool-white fluorescent lamps as their light source. Recently, advances in LEDs technology have contributed to grow plants as a new type of light source. For further developing and improving plant factory system, LEDs illumination systems provide a potential alternative to the tubular fluorescent lamps due to their lower energy consumption, wavelength specificity and supposed durability. Plant growth and development are strongly affected by light intensity (PPFD), quality (wavelength), and duration (photoperiod). The photosynthetic system including chlorophyll content, stomata size and leaf area of lettuce leaves was optimized by adjusting the light spectrum (455, 660, 735 nm) and flux density with high-power LEDs [5]. Light quality was

different temporal resolution.

Figure 1. Pulsed light or irregular sunflecks in forest floor.
