Shinji TAZAWA
Light Source Division, Iwasaki Electric Co., Ltd.
(Gyoda, Saitama, 361-0021 Japan)
---Abstract-------------
In Part 1 of this report, we introduced fundamental aspects of the use of artificial light in horticulture, giving an outline of a number of different artificial light sources and discussing recent research trends(such as the use of microwave-powered lamps, light-emitting diode and laser diode devices) in Japan.
Discipline: Agricultural facilities/Crop production/Horticulture
Additional key words: artificial light source, supplemental lighting, plant factory
< url removed >
1...32):References
(Received for publication, December 18, 1998)
---Introduction-------------
Most terrestrial plants grow by selective absorption of natural light from the sun. In plant factories and indoor living spaces, artificial light is necessary as a source of light energy. Therefore, it is necessary to develop technologies to control the light environment and provide effective and economical irradiation for plants. Part 1 of this report covers basic issues related to plant growth and light.
---Wavelengths for effective plant growth---------------
Solar radiation is subject to extensive scattering and absorption by the atmosphere before it reaches the surface of the earth. Direct solar radiation has wavelengths ranging from 300 to 3,000 nm, and is divided into 3 bands: ultraviolet radiation, visible radiation and infrared radiation. The wavelengths of visible radiation for humans are in the range from 380 to 780 nm, and the peak of the visibility curve(photopic vision) is at 555 nm. Similarly, plants have a range of wavelengths that are physiologically effective. There are 2 types of effective radiation for plants: physiologically active radiation and photosynthetically active radiation(PAR). These 2 types of radiation, ranging from 300 to 800 nm, are physiologically effective in photosynthesis, pigment biosynthesis, photoperiodism, phototropism and photomorphogenesis8).
Physiologically active radiation is divided into 5 wavebands: near ultraviolet light(UV)300-400 nm, blue light(B)400-500 nm, green light(G)500-600 nm, red light(R)600-700 nm, and far-red light(FR)700-800 nm (Fig. 1(20KB)) . Photosynthesis, which uses PAR(waveband 400 to 700 nm), requires an energy source with high intensity. The units of PAR radiation are expressed as total photon fluxes in this waveband, since this radiation induces chemical reactions. The total energy emitted from the light source is designated as photosynthetic photon flux(PPF). On the other hand, the energy actually received by plants is designated as photosynthetic photon flux density(PPFD), and its S. I. units are expressed as ?mol?m-2?sec-1 . Although quantum sensors are preferable for measuring the photon flux, because of their high cost, radiation is often measured by PPFD with conversion factors for illuminance.
---Light intensity suitable for photosynthesis--------------
Light intensity suitable for photosynthesis depends on the light adaptation and acclimation properties of the plants, which in turn depend on the environment of their place of origin. The effect of the light intensity can be estimated to some extent by changes in morphology. Generally, plants which grow in the shade or at low light intensities (shade plants) have large, and thin leaves. Inside their leaves, parenchymatous cells do not adequately develop, resulting in an increase of the development of the grana structure and of the chlorophyll content in chloroplasts. The same morphological changes also occur with exposure to red light. On the other hand, plants which grow at high light intensities (sun plants) have thick leaves. Inside their leaves, parenchymatous cells are remarkably developed, resulting in a lower development of the grana structure. However, many enzymes important for photosynthesis can be observed. The same morphological changes occur with exposure to blue light. These differences in the morphology can also be observed in a single plant. Leaves that grow at low light intensities are referred to as shade leaves, and leaves that grow at high light intensities are referred to as sun leaves. Accordingly, leaves in the upper and lower parts of trees have different photosynthetic capabilities9). Morphological adaptation through changes of the light environment is related to the speed of photosynthesis. Plants growing at high light intensities (for example, watermelons, tomatoes, cucumbers, melons and C4 plants)have high saturation points, and they show a maximum photosynthetic rate at the light saturation point. Therefore, a large amount of light energy is required to cultivate plants that grow better at high light intensities. Fig.2(21KB) was obtained by measuring the absorption and release of carbon dioxide during photosynthesis, and indicates the light adaption capability for photosynthesis. When the light intensity is low, the amount of carbon dioxide released by plant respiration is higher than the amount of that absorbed for photosynthesis, resulting in a net release of carbon dioxide. As the light intensity increases, absorbed and released amounts of carbon dioxide change and reach an equilibrium at point A where a net release of carbon dioxide is no longer observed. This point is referred to as the compensation point. If the light intensity increases further, the amount absorbed reaches point B. This point is the saturation point. A suitable light intensity can be determined somewhere between these points A and B according to the particular requirements. On the other hand, since plants that grow under a low light energy (for example, lettuce, Cryptotaenia japonica , herbage crops, and most of the indoor ornamental plants) have low saturation and compensation points, it is relatively easy to cultivate them, to provide them with supplemental lighting and to maintain growth with artificial lighting. Table 1(77KB) shows the saturation and compensation points of major crops, and Table 2(109KB) shows the saturation and compensation points of ornamental plants. Indoor ornamental plants, most of which are derived from jungle undergrowth, can maintain growth at a relatively low light intensity.
In cultivation facilities for plants utilized for salad, and lettuce in closed-system type plant factories in Japan, a light intensity of about 300 to 400 ?mol?m-2?sec-1 is used. Factories where a higher light intensity is needed are hybrid type plant factories where supplemental lighting of 100 to 150 ?mol?m-2?sec-1 is provided. For indoor ornamental plants, supplemental lighting of 10 to 50 ?mol?m-2?sec-1, depending on the variety, has been gradually employed.
---Photosynthesis action spectrum--------------
The efficiency of plant photosynthesis is not the same throughout the 400 to 700 nm waveband. Just as human eyes have visual curves, plants have sensitivity curves over a wide range. Plants select effective wavelengths from white light and utilize them. Fig. 3(21KB) shows the photosynthesis action spectra described by Inada(1976)7) . Curve 1 shows the average values for 26 species of herbaceous plants, and curve 2 shows the average values for arboreous plants. Fig. 4(21KB) shows the photosynthesis action spectra described by McCree(1972)14) . Curve 1 shows the average values for 20 species of plants in chambers, and curve 2 shows the average values for 8 species of plants in fields. The sample plants used are listed in Table 3 . Each of these 4 photosynthesis action spectra has a large peak composed of 2 peaks at about 675 and 625 nm in the red light region, and a small peak between 440 and 450 nm. All 4 photosynthesis action spectra show that red light has a strong action and blue light a weak action. Fig. 5(18KB) shows the average values for the 4 photosynthesis action spectra, and is used to evaluate light sources for plant growth.
Table 3. Plant materials used for the determination of photosynthesis action spectra
Plants Species
Inada 1. (26 species of
herbaceous plants,
1976)
rice, maize, wheat, barley, oat, soybean, peanut, kidney, bean, pea, cabbage, turnip, radish, tomato, eggplant, cucumber, squash, lettuce, garland, chrysanthemum, spinach, onion, sugar beet, sweet potato, perilla, buckwheat, strawberry
India 2. (7 species of arboreous plants, 1976)
peach, Japanese pear, grape, satsuma mandarin, tea, Japanese black pine, ginkgo
McCree 1. (20 species tested in chamber, 1972)
Maize, sorghum, wheat, oat, barley, secalotricum, sunflower, soybean, tampala, peanut, lettuce, tomato, radish, cabbage, cucumber, oriental melon, squash, clover, sugar beet, castor-oil plant
McCree 2. (8 species tested in field, 1972)
Maize, wheat, oat, secalotricum, rice, sunflower, squash, cotton
----Photomorphogenesis----------------
Light acts on plant morphogenesis, including germination, flowering, stem growth, and leaf opening. Light is also a source of stimuli or information in different ways depending on the plant species and the stage of growth. In general, light with blue, red, and far-red components acts on plants. Table 4(57KB) shows the action of each range of wavelengths31).