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Changhoo Chun and Cary A. Mitchell

`Waldmann's Green' leaf lettuce (Lactuca sativa L.) is being used as a model leafy vegetable crop to develop a protocol for variable control of photosynthetic photon flux (PPF) during crop production. Feedback from real-time photosynthetic gas exchange rates by lettuce canopies is used to modulate electronic dimming ballasts of lamp banks. Algorithms within process-control software are being fine tuned to maximize increments of photosynthetic output relative to increments of photon input. Dynamic optimization of PPF was 21% more efficient than constant high PPF saturating photosynthesis with respect to biomass accumulated per photons absorbed. Dynamic optimization also is being combined with principles of phasic control, in which environmental resources such as photosynthetically active radiation (PAR) and carbon dioxide (CO2) are deliberatively limited in input during specific phases of crop development when plants are less sensitive to inputs (e.g., lag, plateau, and senescence phases) but optimized for the responsive exponential phase. Preliminary results indicate that leaf lettuce growth benefits from optimizing environments for no more than 4 or 5 days during a 20-day production cycle. Dynamic optimization of CO2 level is achieved by controlling the injection of CO2 into the inlet air stream of Minitron II crop canopy cuvette/growth chambers. Algorithms are being modified to simultaneously vary PPF and CO2 for optimum photosynthesis.

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Changhoo Chun, Machiko Tominaga and Toyoki Kozai

We recently showed that spinach (Spinacia oleracea L.) transplants produced under a short photoperiod and low air temperature were characterized by a delay of bolting and short flower-stalk length at harvest (Chun et al., 2000a). The present study was conducted to determine whether these changes are caused by the short photoperiod itself or by the lower integrated photosynthetic photon flux (IPPF). Shoot and root dry weights of transplants increased significantly with increasing IPPF, but were not affected by a change in the photoperiod. However, the floral development indices of transplants were significantly greater under a 16-than under a 10- or 13-hours/day photoperiod, but were not affected by a change in IPPF. The percentage of bolted plants 3 days after transplanting (DAT) increased significantly with increasing photoperiod (from 0% at 10 hours/day to more than 85% at 16 hours/day). Flower-stalk length increased with increasing photoperiod (e.g., at 14 DAT, from 15 mm at the shorter photoperiods to 80 mm at 16 hours/day), but was not affected by a change in IPPF. These results show that the delay of bolting that occurs when the photoperiod is reduced during transplant production is due to the delay of floral development and not to retarded vegetative growth as a result of reduced IPPF.

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Changhoo Chun, Robert J. Joly and Cary A. Mitchell

The planophile (horizontal) leaf presentation of closed cowpea (Vigna unguiculata Walp.) leaf canopies limits PAR absorption from overhead lamps to the top layer of overlapping leaves, resulting in suboptimal canopy photosynthesis and premature senescence and abscission of lower, shaded leaves. Very low crop yield rates have been obtained in growth chamber studies using dense cowpea stands compared to greenhouse and field studies using more widely spaced plants. Nine separate growth compartments were constructed in each of two growth rooms. Eight or sixteen 15-W fluorescent lamps were mounted horizontally or vertically in tiers within each compartment, remote from their ballasts, and which can be switched on or off separately according to different lighting strategies. Mylar sleeves around each tube prevents contacting leaves from overheating. Intracanopy lighting arrangements draw from 0.27 to 0.54 kW of power/m3 of growth volume, compared to 1.18 kW·m–3 for traditional overhead lighting. PPF within compartments varies from 80 to 280 mmol·m–2·s–1, depending on sensor location, lamp arrangement, and lamp number. Each compartment is equipped with a recirculating hydroponic system. One room is operated with overhead plus intracanopy lighting, whereas the other utilizes intracanopy lighting only. Cowpea canopies are being grown under different lighting strategies and compared for growth, yield, productivity, leaf orientation, and individual leaf gas-exchange rates. Electrical power draw and total electrical energy consumption are being compared among treatments.

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Hae Young Na*, Dong Jin Shin and Changhoo Chun

Pimpinella brachicarpa (Chamnamul in Korean) is an indigenous plant that grows in Korean mountain areas. It has not been cultivated yet but is gathered to use as a vegetable. Its difficulty of propagation by seeds is one of the major reasons not to be cultivated as a horticultural crop despite its demand. As a promising propagation method for the Chamnamul, we have developed a micropropagation system using somatic embryogenesis. In the present study, induction of embryogenic callus of the Chamnamul affected by part of explants (leaf and stem) and concentration (0, 0.1, 0.5, 1.0, 1.5, and 2.0 mg·L-1) of growth regulators (2.4-D, IAA, IBA, and NAA) was investigated to find the best conditions for embryogenic callus induction. A full strength of MS medium was used for a 50-day culture for all the treatments. The embryogenic callus was firm and light yellow in color and was distinct from the non-embryogenic callus that was friable and semitransparent. More embryogenic callus was induced in the treatments that the stem was used as an explant comparing with the treatments that the leaf was used. The 2.4-D treatments resulted in the better induction of embryogenic callus than other growth regulator treatments, and 1.5 mg·L-1 was the most effective among all the 2,4-D concentration treatments. Addition of 0.1 mg·L-1 BA to 2.4-D treatments retarded the induction of embryogenic callus of the Chamnamul, while the promotion of induction and multiplication of embryogenic callus was reported in many plant species by adding BA with low concentration to an auxin-base medium. The better induction was found in the treatments of darkness and dim lighting (10 μmol·m-2·s-1 of PPF) than in treatments of the higher PPF.

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Hyeon-Hye Kim, Changhoo Chun, Toyoki Kozai and Junya Fuse

Spinach (Spinacia oleracea L.) was chosen to demonstrate that value-added transplant can be relatively easily produced under artificial light in a closed system. Transplant production under artificial light was divided into three periods, and the photoperiod during each period was varied. It was found that the rate of floral development could be controlled by photoperiod treatments, although floral initiation itself could not be manipulated. Short photoperiod treatments retarded floral development and stem elongation. This occurred even when the transplants were transferred for transplanting to natural conditions with long days and high temperatures. In conclusion, by providing the short photoperiod during the transplant production process, marketable plants with negligible bolting can be produced under natural long-day conditions. Moreover, the production cost per transplant in summer could be reduced by using a combination of natural and artificial lighting during the transplant production process. These results open the possibility to produce value-added transplants of different species under artificial lighting conditions and control their floral development and/or stem elongation for a timely and profitable harvest.

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Hyeon-Hye Kim, Changhoo Chun, Toyoki Kozai and Junya Fuse

Spinach (Spinacia oleracea L.) was chosen to demonstrate that the respective vegetative or reproductive conditions of transplants can be controlled in their early stages of development under artificial light in a closed system. Transplant production under artificial light was divided into three growth phases and the photoperiod during each of these phases was varied. The rate of floral development was controlled by photoperiod, but floral initiation itself was not affected. Short photoperiod treatments (8 or 12 hours/day) retarded floral development and stem elongation (bolting). This delay continued even after the transplants were transferred to natural long-day (15.5 hours/day on average) conditions with high temperatures (17 and 37 °C minimum and maximum). We concluded that by using short photoperiods during transplant production, marketable plants with reduced bolting could be produced under natural long-day conditions. In Japan, spinach with this rosetting capacity would be of greater value. Further, this concept opens the possibility of producing better quality transplants of several species under artificial lighting conditions of appropriate length, and thereby controlling their floral development and/or bolting.

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Tae-Cheol Seo, Changhoo Chun, Hyung-Kweon Yun and Han-Cheol Rhee

Edible chrysanthemum, pak-choi, endive, chicory, and lettuce were hydroponically cultured under root-restricted conditions in DFT systems and their growth and nutritional values were investigated. Cylindrical plastic tubes 100 mm tall and 20, 25, and 30 mm in diameter were used for root restriction. Growth of all the species was retarded, as the roots were restricted. Pak-choi and edible chrysanthemum showed the greater reduction in growth compared with chicory and endive. Percentage of dry matter, C:N ratio, and ascorbic acid and anthocyanin contents increased in the root-restricted treatments. Changes in mineral contents as affected by root restriction were not consistent among tested species. Optimized root volumes to improve the nutritional values and to reduce the retarding of growth varied according to species of leafy vegetables. Tubes of Φ25mm × 10cm and Φ30mm × 10 cm gave the best results in chicory, endive, and lettuce, and edible chrysanthemum and pak-choi, respectively. Results indicate that nutritional values of hydroponically cultured leafy vegetables can be improved by root restriction using plastic tubes.

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Hee-Ju Lee, Sang-gyu Lee, Changhoo Chun and Jung-Myung Lee

Use of grafted seedlings is a practical method to overcome salt accumulation, deterioration of physicochemical properties of soil, and accumulation of soil-borne pathogen that farmers, as well as commercial plug seedling producers, in Korea mainly adapted. Graft-take, subsequent growth, and quality characteristics of grafted hot pepper (Capsicum annuum L.) seedlings composed of three scions and 10 rootstocks were investigated. `Manita', `Chungyang', and `Nokkwang' were cultivars of scions used—they are the major hot pepper cultivars in Korea. The ten rootstock cultivars can be categorized into three groups: cultivars specially bred for rootstocks (`Konesian Hot', `PR-380', `R-Safe', and `Tantan'); cultivars recently bred in NHRI, Korea with the potential to be rootstocks (`Wonkwang1' and `Wonkwang2'); and cultivars originally bred for fruit harvest, but used as rootstocks due to their tolerance to soil-borne pathogens (`Kataguruma', `PR-Data', `PR-Gangza', and `PR-Power'). All the plants were treated with 5 mg·L-1 diniconazole solution 2 weeks after grafting and were soaked into 1.4% salt solution for 48 hours about 5 weeks after grafting. All the grafted seedlings showed feasible growth, including normal flowering and fruit set, and any symptoms of phytophthora blight and anthracnose were not found during 17-day-long experiment. Seedlings grafted onto `Tantan' rootstock showed stronger tolerance to high salt concentration than those grafted onto other rootstocks. Use of some, such as `Wongang 1', `PR-Data' and `Kataguruma', was alleviated the salt-induced growth inhibition.

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Changhoo Chun, Ayumi Watanabe, Toyoki Kozai, Hyeon-Hye Kim and Junya Fuse

Spinach (Spinacia oleracea L. cv. Dimple) was chosen to determine whether bolting (i.e., elongation of flower stalks) could be controlled by manipulating the photoperiod during transplant production in a closed system using artificial light. Plants grown under various photoperiods during transplant production were transferred and cultured under natural short photoperiods and artificial long photoperiods. Vegetative growth at transplanting tended to be greater with the longer photoperiod because of the increased integrated photosynthetic photon flux. Bolting initiation reacted qualitatively to a long photoperiod, and the critical photoperiod for bolting initiation was longer than 13 h and shorter than 15 h. The plants grown under a longer photoperiod during transplant production had longer flower stalks at harvest. The long photoperiod and/or high temperature after transplanting therefore promoted flower stalk elongation. Growing plants under a photoperiod that was shorter than the critical photoperiod during transplant production reduced elongation of the flower stalks, thus there was no loss of market value even though the plants were cultured under a long photoperiod and high temperature for 2 weeks after transplanting.

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Sung Kyeom Kim, Duk Jun Yu, Ro Na Bae, Hee Jae Lee and Changhoo Chun

Grafted transplants are widely used for watermelon culture in Korea mainly to reduce the yield and quality losses caused by soil-borne diseases. It is normal practice to cure the grafted transplants under high relative humidity (RH) and low photosynthetic photon flux (PPF) conditions for a few days after grafting to prevent the wilting of the transplants. Transpiration rate (TR) and net photosynthetic rate (NPR), however, could be suppressed under those environmental conditions. In the present study, TR and NPR of the grafted watermelon transplants were compared during graft union formation under 18 environmental conditions combining two air temperatures (20 and 28 °C), three RHs (60%, 80%, and 100%), and three PPF s (0, 100, and 200 μmol·m-2·s-1). Percentages of graft union formation and survival were also evaluated. TR and NPR dramatically decreased just after grafting but slowly recovered 2 to 3 days after grafting at 28 °C. The recovery was clearer at higher PPF and lower RH. On the other hand, the recovery of TR and NPR was not observed in 7 days after grafting at 20 °C. Differences in TR and NPR affected by RH were nonsignificant. Percentage of graft union formation was 98% when air temperature, RH, and PPF were 28 °C, 100%, and 100 μmol·m-2·s-1, respectively, which was the highest among all the treatments. Percentage of survival was over 90% when air temperature was 28 °C and RH was higher than 80% (when vapor pressure deficit was lower than 0.76 kPa). In addition, higher PPF enhanced TR and NPR and promoted rooting and subsequent growth of grafted transplants. Results suggest that the acclimation process for grafted watermelon transplants can be omitted by properly manipulating environmental factors during graft union formation.