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Seeding, seedling production and transplanting
CHAPTER
Seeding, seedling production and transplanting
21
Osamu Nunomura, Toyoki Kozai, Kimiko Shinozaki, Takahiro Oshio Japan Plant Factory Association, c/o Center for Environment, Health and Field Sciences, Chiba University, Kashiwa, Chiba, Japan
21.1 Introduction A standard procedure for producing seedlings in a plant factory with artificial lighting (PFAL), consisting of preparation, seeding, seedling production, and transplanting, is described in this chapter. The procedure described here needs to be modified depending on the plant species, culture system, daily production capacity of the PFAL, etc. The numerical values given in the description are merely examples and should be modified to suit the particular situation. Leaf lettuce (Lactuca sativa L. var. crispa) is chosen as the plant material in this chapter. The objective of this chapter is to provide the standard procedure for achieving a percent seed germination of 99% or higher (the percentage achieved by beginners would be around 90%), and percent transplantable seedlings from germinated seeds exceeding 99%. Accordingly, the percent transplantable seedlings from seeds sown exceeds 98% (¼ 99 99/100) or higher. The percent salable harvests of seeds sown and of transplanted seedlings should exceed, respectively, 98% or higher and 99%.
21.2 Preparation (1) Choose either untreated or treated seeds (Fig. 21.1). Treated seeds include naked (shell-less) and coated seeds. Treated seeds for enhancing rapid and uniform germination are preferable to untreated seeds. (2) Caution is necessary regarding whether or not to treat the seeds with a fungicide. The choice depends on the purpose of seedling production and the biological characteristics of the seeds. (3) Eliminate low-quality seeds based on their size, shape, color, and (specific) weight through visual checking or by using an automatic grading/sorting machine. (4) Prepare a sponge-like or foamed urethane seeding mat (called “mat” hereafter) (28 58 2.8 cm) (Fig. 21.2) consisting of 300 cubes or cuboids (2.3 2.3 2.8 cm) each with a small hole (7e10 mm in diameter, 5e10 mm in depth) on the upper surface of the cube. Each cube can be separated easily by hand from the mat (Fig. 21.3). Plant Factory. https://doi.org/10.1016/B978-0-12-816691-8.00021-2 Copyright © 2020 Elsevier Inc. All rights reserved.
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FIGURE 21.1 Lettuce seeds untreated (left) and coated (right). Treated seeds are classified into naked (shell-less) seeds, coated seeds, and coated naked seeds.
FIGURE 21.2 White-colored seeding mat (sponge-like foamed urethane mat) (28 cm wide, 58 cm long, and 2.8 cm high) consisting of 300 cuboids (2.3 2.3 2.8 cm).
Crossing slit Crossing slit
FIGURE 21.3 The cuboids each with a hollow at the center of the upper surface. There is a crossing slit at the center of each hollow to encourage the radicle of the germinated seed to grow downward smoothly.
21.2 Preparation
287
(5) At the center of each hole, there is a crossing slit (each 10 mm long) reaching the bottom of the cube to encourage the radicle (the youngest primary root) of a germinated seed to grow downward easily. (6) Prepare a foamed plastic tray (outside dimensions: 30 60 4.0 cm) to hold the mat (Fig. 21.4). Measure the weight of the empty tray prior to putting the mat in the tray. (7) Prepare a predetermined volume (3 L per tray) of nutrient solution, the strength of which is 1/4e1/8th that of the nutrient solution to be used after the second transplanting. (8) Press down the mat surface uniformly using a flat plate (30 60 cm) with many small holes or a press machine (Fig. 21.5) with a stacking and automatic feeding unit to drive out all the air from the mat. This step can be conducted by hand if only a few mats are to be pressed.
FIGURE 21.4 Formed, foamed polystyrene tray (relatively hard) to store the seeding mat (outer dimensions: 30 cm wide, 60 cm long, and 4 cm high; inner dimensions: 28 cm wide, 58 cm long, and 2.8 cm high).
FIGURE 21.5 A press machine for foamed mats to drive out all the air from the mat and fill the pores with water. The machine can be connected to a stacking and automatic feeding unit. Sasinami Seisakujo Corporation, Aichi Prefecture, Japan.
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Chapter 21 Seeding, seedling production and transplanting
(9) Concurrently, soak the mat in nutrient solution so that all the capillary tubes and/or pores in the mat are filled with nutrient solution, resulting in no air bubbles in the mat. (10) Weigh the tray containing the mat and nutrient solution, and add or remove a small amount (ca. 0.2 L) of nutrient solution to obtain the predetermined target weight of 3280 g (e.g., 215 g for tray, 3000 g for nutrient solution, and 65 g for the mat) common to all the trays. (11) Check that the surface of the horizontal mat is wet evenly and that the level of free nutrient solution is just at the bottom of each hole of the mat. An evenly wet mat surface and free nutrient solution at the bottom of holes are important to achieve uniform seed germination over the mat. If the wetness at the mat surface is uneven, often due to insufficient capillary suction within the mat, the mat needs to be replaced with a new one.
21.3 Seeding (12) Place one seed in the wet hole of each cube (Fig. 21.6) using tweezers, a seeding plate, a semiautomatic seeder (Fig. 21.7), or an automatic seeding machine (Fig. 21.8). Confirm that the seed touches the wet center of the hole. (13) Cover the mat surface with thin plastic film (0.02 mm thick) to keep its surface constantly moist during the germination stage (Fig. 21.9). (14) Move the trays to a germination space. Ensure that the trays are always horizontal to prevent the seeds and nutrient solution from moving in the tray during transportation. (15) The trays are stacked in case the seeds are negatively photoblastic (dim light is unnecessary for germination of photoblastic seeds). Apply a slight downward pressure on the mat surface of the uppermost tray using an additional thin plastic plate (0.5e1.0 mm thick). (16) If the seeds are positively photoblastic, the trays are placed in a germination rack with the vertical distance between the tiers being 7e10 cm. Dim light is applied to the surface of each mat from the side using vertically placed fluorescent tubes or string-type LED lamps.
FIGURE 21.6 Lettuce seeds sown on the mat. The seeds touch the wet center of the hole to germinate smoothly.
21.3 Seeding
289
FIGURE 21.7 Tweezers (upper left), seeding plate (lower left), and semiautomatic seeding tool (right: Minoru Industrial Co., Ltd., Okayama Prefecture, Japan). The seeding plate has two transparent plastic plates with small holes in a grid-like fashion. A handle is connected to the right-hand side of the upper plate. When one seed has been placed in each hole of the upper plate, the seeding plate is placed above the mat and the handle is pulled to the right-hand side. Then, each seed falls down at each center of the hole. The seeds are stacked in the upper container of the semiautomatic seeding tool. The machine is moved to the left-hand side manually. Then, one seed drops down from a hole at the bottom of the rotary container. The distance between the holes can be adjusted manually to suit different mats.
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FIGURE 21.8 Automatic seeding machine for sponge-like seeding mat. Nansei-Kobashi Co., Ltd., Mie Prefecture, Japan.
FIGURE 21.9 Formed polystyrene tray with seeds covered with thin plastic film (0.02 mm thick) (left) and covered with an empty foamed polystyrene tray (right) to keep the mat surface wet.
(17) Microorganisms grow easily in the germination space, so periodic cleaning and/or sterilization are required. (18) The set point of air temperature varies between 15 C and 30 C with the plant species and cultivar (15e22 C for lettuce plants). (19) Two to 3 days after sowing, remove the plastic film from the tray. (20) At seed germination, a radicle comes out first (Fig. 21.10). Then, the hypocotyl with unfolded cotyledon leaves covered with seed coat appears (Fig. 21.11). (21) Confirm that all the radicles are growing downward into the mat. By applying slight downward pressure to the germinating seeds, the radicles grow downward smoothly through the crossing slit into the bottom of the cube. If the downward pressure is too low, the radicle will not penetrate the mat (Fig. 21.12).
21.3 Seeding
291
FIGURE 21.10 Germinated lettuce seed with radicle. Hypocotyl and cotyledon are still in the seed coat.
Seed coat
Hypocotyl
Hypocotyl
Radicle
Unfolded cotyledon Radicle
FIGURE 21.11 Lettuce (Lactuca sativa L. var. crispa) seed 22 h after seeding (left) and 42 h after seeding (right). Cotyledon is still folded and partly in the seed coat.
(22) At this stage, percent germination is expected to be 99% or higher. If not, it is necessary to analyze the cause to improve the percent germination. (23) Move the trays from the germination space to a seedling production space. Hereafter, the seedlings grow by photosynthesis. (24) Grow the germinated seeds to seedlings with unfolded green cotyledons at a photosynthetic photon flux density (PPFD) of about 50e100 mmol m2 s1. Green cotyledons usually expand fully in 4e7 days after seeding (4 days in the case of lettuce seeds) (Figs. 21.13 and 21.14).
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(A)
(B)
Seed coat
Radicle Radicle
FIGURE 21.12 Germinated lettuce seed with radicle which did not penetrate into the substrate. The radicle was lifted up from the substrate, (A) 2 days after seeding, and (B) 7 days after seeding.
FIGURE 21.13 Lettuce (Lactuca sativa L. var. crispa) seeds with cotyledons 72 h after seeding. The cotyledon is nearly unfolded and starts photosynthetic activity at this growth stage.
(25) Algae grow quickly on the wet mat surface under light (Fig. 21.15). To prevent this, the level of free nutrient solution in the tray needs to be lowered by 10 mm to keep the mat surface dry, when the radicle with fine roots or rootlets comes out into the nutrient solution (Fig. 21.16). (26) A black-colored sponge-like, foamed urethane seeding mat is effective to suppress algae growth, although PPFD is reduced due to its low light reflectivity (Fig. 21.17).
21.3 Seeding
293
FIGURE 21.14 Ninety-six germinated lettuce seeds on the foamed urethane mat, 5 days after seeding. For commercial plant production, the germination percentage needs to be 98% or higher. In the photograph, the two seeds in the circle germinated 2 days later than the other seeds.
(B) (A)
(C)
FIGURE 21.15 Algae grow quickly in the presence of light and nutrient solution on the wet seeding mat surface. (A) Spotted algae growth on the mat; (B) spotted algae growth on the mat; (C) algae growth on the surface and within the mat.
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FIGURE 21.16 Seedling with roots which penetrated the cuboid. At this timing, the level of nutrient solution is lowered by 10 mm to allow the seeding mat surface to dry out and thus inhibit rapid growth of algae.
FIGURE 21.17 Black-colored seeding mat (sponge-like foamed urethane mat) which suppresses the growth of algae.
21.4 Seedling production and transplanting (27) Three to 7 days after seeding, cotyledons are unfolded and green in color. In the case of lettuce seeds, cotyledons are fully unfolded 72 h after seeding. Then, small true leaves can be observed by 5 days after seeding. Seven days after seeding, two true leaves are unfolded (Fig. 21.18). (28) The seedlings start growing quickly 10e12 days after seeding. The seedlings 14e16 days after seeding (Fig. 21.19) are suitable for the first transplanting to a culture panel (30 60 1 cm) having 24e30 holes (Fig. 21.20) (29) Transplant each seedling with the cube to the culture panel for further growth after the radicle with rootlets has come out from the bottom surface of the cube and the radicle is submerged
21.4 Seedling production and transplanting
295
FIGURE 21.18 Lettuce seedlings with unfolded cotyledon 3 days (upper left), 5 days (upper right), and 7 days (bottom) after seeding.
FIGURE 21.19 Lettuce (Lactuca sativa L. var. crispa) seedlings 14 days after seeding, ready for the first transplanting. Blackcolored foamed urethane cubes are used as substrate.
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Chapter 21 Seeding, seedling production and transplanting
FIGURE 21.20 Formed culture panel (relatively hard board) with 26 holes (2.2 cm in diameter) for the first transplanting (29.8 cm wide, 59.6 cm long, 1.4 cm thick).
FIGURE 21.21 Lettuce seedlings (Lactuca sativa L. var. crispa) 24 days after seeding, ready for the second transplanting. Fresh weight: 10e12 g/plant, root fresh weight ratio: 0.40e0.45.
FIGURE 21.22 Formed culture panel (relatively hard board) with six holes (2.2 cm in diameter) for the second transplanting (29.8 cm wide, 59.6 cm long, 1.4 cm thick).
21.4 Seedling production and transplanting
(30)
(31)
(32)
(33) (34)
297
about 30 mm beneath the bottom surface. Transplanting can be done manually or using a transplanting machine. Twelve (¼ 300/25) culture panels (formed, foamed urethane board) each with 26 holes (Fig. 21.20) or 10 (¼ 300/30) culture panels with 30 holes are necessary for each mat having 300 cubes. The culture panel with 25e30 seedlings is placed in a hydroponic system with a culture bed and nutrient solution circulation unit to grow the seedlings further at a PPFD of 100e150 mmol m2 s1 for about 10e15 days to obtain larger seedlings ready for the second transplanting (Fig. 21.21), when the ratio of projected leaf area to the culture panel area exceeds 0.9. The second transplanting is conducted using a culture panel (formed, foamed urethane board) with six to eight holes (Fig. 21.22). The larger the seedlings, the harder it is to pick them up and transplant them. However, the total cultivation area required for seeding, seedling production, and cultivation after the second transplanting can be reduced if larger seedlings are used for the second transplanting. If the hypocotyl of a transplant is too elongated, the transplant tends to lie down on the culture panel and its growth is delayed for a few days. The growth after the second transplanting is enhanced when the CO2 concentration is increased up to around 1000 ppm.
Seeding, seedling production and transplanting
21
Osamu Nunomura, Toyoki Kozai, Kimiko Shinozaki, Takahiro Oshio Japan Plant Factory Association, c/o Center for Environment, Health and Field Sciences, Chiba University, Kashiwa, Chiba, Japan
21.1 Introduction A standard procedure for producing seedlings in a plant factory with artificial lighting (PFAL), consisting of preparation, seeding, seedling production, and transplanting, is described in this chapter. The procedure described here needs to be modified depending on the plant species, culture system, daily production capacity of the PFAL, etc. The numerical values given in the description are merely examples and should be modified to suit the particular situation. Leaf lettuce (Lactuca sativa L. var. crispa) is chosen as the plant material in this chapter. The objective of this chapter is to provide the standard procedure for achieving a percent seed germination of 99% or higher (the percentage achieved by beginners would be around 90%), and percent transplantable seedlings from germinated seeds exceeding 99%. Accordingly, the percent transplantable seedlings from seeds sown exceeds 98% (¼ 99 99/100) or higher. The percent salable harvests of seeds sown and of transplanted seedlings should exceed, respectively, 98% or higher and 99%.
21.2 Preparation (1) Choose either untreated or treated seeds (Fig. 21.1). Treated seeds include naked (shell-less) and coated seeds. Treated seeds for enhancing rapid and uniform germination are preferable to untreated seeds. (2) Caution is necessary regarding whether or not to treat the seeds with a fungicide. The choice depends on the purpose of seedling production and the biological characteristics of the seeds. (3) Eliminate low-quality seeds based on their size, shape, color, and (specific) weight through visual checking or by using an automatic grading/sorting machine. (4) Prepare a sponge-like or foamed urethane seeding mat (called “mat” hereafter) (28 58 2.8 cm) (Fig. 21.2) consisting of 300 cubes or cuboids (2.3 2.3 2.8 cm) each with a small hole (7e10 mm in diameter, 5e10 mm in depth) on the upper surface of the cube. Each cube can be separated easily by hand from the mat (Fig. 21.3). Plant Factory. https://doi.org/10.1016/B978-0-12-816691-8.00021-2 Copyright © 2020 Elsevier Inc. All rights reserved.
285
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FIGURE 21.1 Lettuce seeds untreated (left) and coated (right). Treated seeds are classified into naked (shell-less) seeds, coated seeds, and coated naked seeds.
FIGURE 21.2 White-colored seeding mat (sponge-like foamed urethane mat) (28 cm wide, 58 cm long, and 2.8 cm high) consisting of 300 cuboids (2.3 2.3 2.8 cm).
Crossing slit Crossing slit
FIGURE 21.3 The cuboids each with a hollow at the center of the upper surface. There is a crossing slit at the center of each hollow to encourage the radicle of the germinated seed to grow downward smoothly.
21.2 Preparation
287
(5) At the center of each hole, there is a crossing slit (each 10 mm long) reaching the bottom of the cube to encourage the radicle (the youngest primary root) of a germinated seed to grow downward easily. (6) Prepare a foamed plastic tray (outside dimensions: 30 60 4.0 cm) to hold the mat (Fig. 21.4). Measure the weight of the empty tray prior to putting the mat in the tray. (7) Prepare a predetermined volume (3 L per tray) of nutrient solution, the strength of which is 1/4e1/8th that of the nutrient solution to be used after the second transplanting. (8) Press down the mat surface uniformly using a flat plate (30 60 cm) with many small holes or a press machine (Fig. 21.5) with a stacking and automatic feeding unit to drive out all the air from the mat. This step can be conducted by hand if only a few mats are to be pressed.
FIGURE 21.4 Formed, foamed polystyrene tray (relatively hard) to store the seeding mat (outer dimensions: 30 cm wide, 60 cm long, and 4 cm high; inner dimensions: 28 cm wide, 58 cm long, and 2.8 cm high).
FIGURE 21.5 A press machine for foamed mats to drive out all the air from the mat and fill the pores with water. The machine can be connected to a stacking and automatic feeding unit. Sasinami Seisakujo Corporation, Aichi Prefecture, Japan.
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Chapter 21 Seeding, seedling production and transplanting
(9) Concurrently, soak the mat in nutrient solution so that all the capillary tubes and/or pores in the mat are filled with nutrient solution, resulting in no air bubbles in the mat. (10) Weigh the tray containing the mat and nutrient solution, and add or remove a small amount (ca. 0.2 L) of nutrient solution to obtain the predetermined target weight of 3280 g (e.g., 215 g for tray, 3000 g for nutrient solution, and 65 g for the mat) common to all the trays. (11) Check that the surface of the horizontal mat is wet evenly and that the level of free nutrient solution is just at the bottom of each hole of the mat. An evenly wet mat surface and free nutrient solution at the bottom of holes are important to achieve uniform seed germination over the mat. If the wetness at the mat surface is uneven, often due to insufficient capillary suction within the mat, the mat needs to be replaced with a new one.
21.3 Seeding (12) Place one seed in the wet hole of each cube (Fig. 21.6) using tweezers, a seeding plate, a semiautomatic seeder (Fig. 21.7), or an automatic seeding machine (Fig. 21.8). Confirm that the seed touches the wet center of the hole. (13) Cover the mat surface with thin plastic film (0.02 mm thick) to keep its surface constantly moist during the germination stage (Fig. 21.9). (14) Move the trays to a germination space. Ensure that the trays are always horizontal to prevent the seeds and nutrient solution from moving in the tray during transportation. (15) The trays are stacked in case the seeds are negatively photoblastic (dim light is unnecessary for germination of photoblastic seeds). Apply a slight downward pressure on the mat surface of the uppermost tray using an additional thin plastic plate (0.5e1.0 mm thick). (16) If the seeds are positively photoblastic, the trays are placed in a germination rack with the vertical distance between the tiers being 7e10 cm. Dim light is applied to the surface of each mat from the side using vertically placed fluorescent tubes or string-type LED lamps.
FIGURE 21.6 Lettuce seeds sown on the mat. The seeds touch the wet center of the hole to germinate smoothly.
21.3 Seeding
289
FIGURE 21.7 Tweezers (upper left), seeding plate (lower left), and semiautomatic seeding tool (right: Minoru Industrial Co., Ltd., Okayama Prefecture, Japan). The seeding plate has two transparent plastic plates with small holes in a grid-like fashion. A handle is connected to the right-hand side of the upper plate. When one seed has been placed in each hole of the upper plate, the seeding plate is placed above the mat and the handle is pulled to the right-hand side. Then, each seed falls down at each center of the hole. The seeds are stacked in the upper container of the semiautomatic seeding tool. The machine is moved to the left-hand side manually. Then, one seed drops down from a hole at the bottom of the rotary container. The distance between the holes can be adjusted manually to suit different mats.
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Chapter 21 Seeding, seedling production and transplanting
FIGURE 21.8 Automatic seeding machine for sponge-like seeding mat. Nansei-Kobashi Co., Ltd., Mie Prefecture, Japan.
FIGURE 21.9 Formed polystyrene tray with seeds covered with thin plastic film (0.02 mm thick) (left) and covered with an empty foamed polystyrene tray (right) to keep the mat surface wet.
(17) Microorganisms grow easily in the germination space, so periodic cleaning and/or sterilization are required. (18) The set point of air temperature varies between 15 C and 30 C with the plant species and cultivar (15e22 C for lettuce plants). (19) Two to 3 days after sowing, remove the plastic film from the tray. (20) At seed germination, a radicle comes out first (Fig. 21.10). Then, the hypocotyl with unfolded cotyledon leaves covered with seed coat appears (Fig. 21.11). (21) Confirm that all the radicles are growing downward into the mat. By applying slight downward pressure to the germinating seeds, the radicles grow downward smoothly through the crossing slit into the bottom of the cube. If the downward pressure is too low, the radicle will not penetrate the mat (Fig. 21.12).
21.3 Seeding
291
FIGURE 21.10 Germinated lettuce seed with radicle. Hypocotyl and cotyledon are still in the seed coat.
Seed coat
Hypocotyl
Hypocotyl
Radicle
Unfolded cotyledon Radicle
FIGURE 21.11 Lettuce (Lactuca sativa L. var. crispa) seed 22 h after seeding (left) and 42 h after seeding (right). Cotyledon is still folded and partly in the seed coat.
(22) At this stage, percent germination is expected to be 99% or higher. If not, it is necessary to analyze the cause to improve the percent germination. (23) Move the trays from the germination space to a seedling production space. Hereafter, the seedlings grow by photosynthesis. (24) Grow the germinated seeds to seedlings with unfolded green cotyledons at a photosynthetic photon flux density (PPFD) of about 50e100 mmol m2 s1. Green cotyledons usually expand fully in 4e7 days after seeding (4 days in the case of lettuce seeds) (Figs. 21.13 and 21.14).
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Chapter 21 Seeding, seedling production and transplanting
(A)
(B)
Seed coat
Radicle Radicle
FIGURE 21.12 Germinated lettuce seed with radicle which did not penetrate into the substrate. The radicle was lifted up from the substrate, (A) 2 days after seeding, and (B) 7 days after seeding.
FIGURE 21.13 Lettuce (Lactuca sativa L. var. crispa) seeds with cotyledons 72 h after seeding. The cotyledon is nearly unfolded and starts photosynthetic activity at this growth stage.
(25) Algae grow quickly on the wet mat surface under light (Fig. 21.15). To prevent this, the level of free nutrient solution in the tray needs to be lowered by 10 mm to keep the mat surface dry, when the radicle with fine roots or rootlets comes out into the nutrient solution (Fig. 21.16). (26) A black-colored sponge-like, foamed urethane seeding mat is effective to suppress algae growth, although PPFD is reduced due to its low light reflectivity (Fig. 21.17).
21.3 Seeding
293
FIGURE 21.14 Ninety-six germinated lettuce seeds on the foamed urethane mat, 5 days after seeding. For commercial plant production, the germination percentage needs to be 98% or higher. In the photograph, the two seeds in the circle germinated 2 days later than the other seeds.
(B) (A)
(C)
FIGURE 21.15 Algae grow quickly in the presence of light and nutrient solution on the wet seeding mat surface. (A) Spotted algae growth on the mat; (B) spotted algae growth on the mat; (C) algae growth on the surface and within the mat.
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Chapter 21 Seeding, seedling production and transplanting
FIGURE 21.16 Seedling with roots which penetrated the cuboid. At this timing, the level of nutrient solution is lowered by 10 mm to allow the seeding mat surface to dry out and thus inhibit rapid growth of algae.
FIGURE 21.17 Black-colored seeding mat (sponge-like foamed urethane mat) which suppresses the growth of algae.
21.4 Seedling production and transplanting (27) Three to 7 days after seeding, cotyledons are unfolded and green in color. In the case of lettuce seeds, cotyledons are fully unfolded 72 h after seeding. Then, small true leaves can be observed by 5 days after seeding. Seven days after seeding, two true leaves are unfolded (Fig. 21.18). (28) The seedlings start growing quickly 10e12 days after seeding. The seedlings 14e16 days after seeding (Fig. 21.19) are suitable for the first transplanting to a culture panel (30 60 1 cm) having 24e30 holes (Fig. 21.20) (29) Transplant each seedling with the cube to the culture panel for further growth after the radicle with rootlets has come out from the bottom surface of the cube and the radicle is submerged
21.4 Seedling production and transplanting
295
FIGURE 21.18 Lettuce seedlings with unfolded cotyledon 3 days (upper left), 5 days (upper right), and 7 days (bottom) after seeding.
FIGURE 21.19 Lettuce (Lactuca sativa L. var. crispa) seedlings 14 days after seeding, ready for the first transplanting. Blackcolored foamed urethane cubes are used as substrate.
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FIGURE 21.20 Formed culture panel (relatively hard board) with 26 holes (2.2 cm in diameter) for the first transplanting (29.8 cm wide, 59.6 cm long, 1.4 cm thick).
FIGURE 21.21 Lettuce seedlings (Lactuca sativa L. var. crispa) 24 days after seeding, ready for the second transplanting. Fresh weight: 10e12 g/plant, root fresh weight ratio: 0.40e0.45.
FIGURE 21.22 Formed culture panel (relatively hard board) with six holes (2.2 cm in diameter) for the second transplanting (29.8 cm wide, 59.6 cm long, 1.4 cm thick).
21.4 Seedling production and transplanting
(30)
(31)
(32)
(33) (34)
297
about 30 mm beneath the bottom surface. Transplanting can be done manually or using a transplanting machine. Twelve (¼ 300/25) culture panels (formed, foamed urethane board) each with 26 holes (Fig. 21.20) or 10 (¼ 300/30) culture panels with 30 holes are necessary for each mat having 300 cubes. The culture panel with 25e30 seedlings is placed in a hydroponic system with a culture bed and nutrient solution circulation unit to grow the seedlings further at a PPFD of 100e150 mmol m2 s1 for about 10e15 days to obtain larger seedlings ready for the second transplanting (Fig. 21.21), when the ratio of projected leaf area to the culture panel area exceeds 0.9. The second transplanting is conducted using a culture panel (formed, foamed urethane board) with six to eight holes (Fig. 21.22). The larger the seedlings, the harder it is to pick them up and transplant them. However, the total cultivation area required for seeding, seedling production, and cultivation after the second transplanting can be reduced if larger seedlings are used for the second transplanting. If the hypocotyl of a transplant is too elongated, the transplant tends to lie down on the culture panel and its growth is delayed for a few days. The growth after the second transplanting is enhanced when the CO2 concentration is increased up to around 1000 ppm.