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Chieri Kubota* and Mark Kroggel

Increasing numbers of vegetable growers purchase their transplants from specialized transplant producers. Possible deterioration of transplants during transportation limits the market size as well as the potential sources of high quality transplants. To determine best conditions for transportation of seedlings, tomato (Lycopersicon esculentum; `Durinta') seedlings with visible flower buds were placed for 4 days under varied air temperature (6, 12, or the conventional transportation temperature of 18 °C) and light intensity 0 (conventional darkness) or dim light at 12 μmol·m-2·s-1 PAR). Plants were evaluated for visual quality, photosynthetic capacity, growth and ultimately fruit yield. Lower temperatures and illumination significantly maintained visual quality of the seedlings. Lower temperature maintained high photosynthetic capacity of the seedlings during transportation. Growth and development of the seedlings were significantly affected by higher temperature resulting in significantly delayed growth and development. Number of fruits set on the first truss was significantly reduced when seedlings were at 18 °C during transportation. Overall, simulated transport at 6 °C under light showed the best transportability without experiencing negative impact for the 4-day simulated transportation. Seedlings at 6 °C in darkness and at 12 °C under light and in darkness also showed satisfactory transportability. Seedlings at 18 °C exhibited serious quality deterioration of seedlings, delay in early growth and development, loss of flower buds on the first truss and yield reduction, which agrees with the fact that conventional transportation is currently able to be no longer than 3 days in duration.

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Min Wu and Chieri Kubota

Manipulation of the electrical conductivity (EC) of the hydroponic nutrient solution has been studied as an effective method to enhance flavor and nutritional value of tomato fruit. The objective of this research was to quantitatively understand the accumulation of lycopene, soluble sugars, and the degradation of chlorophyll in fruits as affected by EC and EC application timing relative to fruit ripeness stages. `Durinta' tomato was grown hydroponically inside the greenhouse under two EC (2.3 and 4.5 dS·m-1). The high EC treatment began immediately after anthesis (HEC treatment) or 4 weeks later (DHEC treatment), when fruits had reached maximum size, but still were green. Fruits were harvested weekly beginning 2 weeks after anthesis, until they reached red ripe stage. The chlorophyll concentration in tomato fruits showed no difference between treatments when compared at the same ripeness stages. The lycopene concentration of red ripe tomato fruits in HEC and DHEC treatments was 29% greater than that in low EC control (LEC treatment). However, there was no significant difference in lycopene concentration between HEC and DHEC. Both DHEC and HEC increased total soluble solid concentration (TSS) of red ripe tomato fruits compared with those grown in LEC; while the DHEC showed an increase of fruit TSS of 12%, the HEC had a greater enhancement of TSS of 19%. In addition, the fruit ripeness was accelerated under high EC, regardless of the timing of treatment. High EC treatment at early and mature green fruit developmental stages enhanced both fruit TSS and lycopene concentration; however, the nutrient solution EC effect on lycopene concentration was not dependent on the time of application during fruit development.

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Hans Spalholz and Chieri Kubota

Low-temperature storage is a technique to hold seedlings for a short period of time to adjust the production schedule of young seedlings. Labor-intensive grafting propagation can potentially benefit from the effective use of this technique to minimize peak labor inputs. Watermelon (Citrullus lanatus) seedlings are generally chilling sensitive and therefore difficult to store at low temperatures. However, the rootstocks used for watermelon grafting, interspecific squash (Cucurbita maxima × Cucurbita moschata) and bottle gourd (Lagenaria siceraria) are known to be chilling tolerant. To examine the influence of rootstocks on storability of watermelon seedlings, young seedlings of ‘Tri-X-313’ seedless watermelon grafted onto ‘Strong Tosa’ interspecific squash, ‘Emphasis’ bottle gourd, and ‘Tri-X-313’ watermelon as rootstock were placed for 2 or 4 weeks under 12 °C air temperature and 12 μmol·m−2·s−1 photosynthetic photon flux (PPF). Nongrafted watermelon seedlings were also treated in these same conditions. In addition, nonstored (grafted and nongrafted) seedlings were prepared for comparison. Regardless of seedling type (nongrafted or grafted with different rootstocks), all seedlings stored for 2 weeks had lower dry weight, comparable or greater number of leaves and stem length, when compared with their respective nonstored control groups after 2 weeks in the greenhouse. Seedlings stored for 4 weeks had lower number of leaves and stem length after 2 weeks in the greenhouse, except for those grafted onto the interspecific squash rootstock. Nongrafted and grafted watermelon seedlings with the same watermelon cultivar as rootstock showed significantly lower leaf net photosynthetic rates after 2 weeks in the greenhouse after the 2-week storage than those of nonstored control groups. In contrast, when grafted onto interspecific squash and bottle gourd rootstocks, seedlings showed comparable net photosynthetic rate to the control group. For all seedling types, 20% to 35% of seedlings died during 4-week storage or poststorage in the greenhouse, whereas all seedlings survived for the 2-week storage, except when grafted onto watermelon as rootstock. Therefore, chilling-tolerant rootstocks ‘Strong Tosa’ interspecific squash and ‘Emphasis’ bottle gourd improved storability of grafted ‘Tri-X-313’ watermelon seedlings but could not extend the storability beyond 2 weeks.

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Chieri Kubota and Mark Kroggel

Increasing numbers of greenhouse vegetable growers purchase transplants from specialized transplant propagators. Possible deterioration of transplants during transportation limits the market size as well as the potential sources of high-quality transplants. To determine the best conditions for transportation of seedlings, tomato (Lycopersicon esculentum Mill., cv. Durinta) seedlings with visible flower trusses were placed for 4 days inside growth chambers to evaluate the effects of short-term exposure to different air temperatures (6, 13, or a conventional transportation temperature of 19 °C) under darkness or illumination at 12 μmol·m–2·s–1 PPF. Plants were evaluated for visual quality, photosynthetic ability, growth, and fruit yield. Lower temperatures and illumination significantly maintained visual quality of the seedlings. Lower temperature maintained high photosynthetic ability of seedlings during the 4-day treatment. After transplanting in the greenhouse, a significant number of trusses exhibited flower abortion or delayed fruit development when seedlings were treated at 19 °C regardless of light intensity. Results suggested that 6 to 13 °C was the best transportation temperature for up to 4 days, which was later validated by an actual transportation trial between British Columbia and Arizona.

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Po-Lung Chia and Chieri Kubota

Plant morphology control is a critical technique in commercial greenhouse transplant production. Light treatment at the end of the day affects a phytochrome-regulated response affecting plant height among other characteristics and has been studied by biologists for many years. Recognizing the need to produce long hypocotyls in vegetable grafting, effects of end-of-day far-red (EOD-FR) light on tomato rootstock hypocotyl elongation were investigated. Two commercial rootstock cultivars, Aloha (Solanum lycopersicum) and Maxifort (S. lycopersicum × S. habrochaites), were used for the experiments examining responses to EOD-FR light quality [red to far-red ratio (R/FR)] and EOD-FR light dose in a greenhouse environment. In the EOD-FR light quality experiment using ‘Aloha’ seedlings, incandescent light (R/FR = 0.47) induced significant hypocotyl elongation (20%) compared with the untreated control. Incandescent light with a spectral cut filter (reducing R/FR to 0.05) induced a greater hypocotyl elongation (44%) than unfiltered light, confirming the importance of use of light with a lower R/FR (or purer FR) light source in EOD-FR treatments. In the experiment on EOD-FR light dose–response, hypocotyl elongation of both ‘Aloha’ and ‘Maxifort’ was increased by increasing FR intensity as well as FR treatment duration at a lower dose range. The dose saturation curve of hypocotyl length was well described using a Michaelis-Menten-type model with FR dose (in mmol·m−2·d−1) as an independent variable. The model-based estimation of 90% saturating FR light dose for ‘Aloha’ and ‘Maxifort’ was 5 to 14 mmol·m−2·d−1 and 8 to 15 mmol·m−2·d−1, respectively, although practical near saturation dose seems to be 2 to 4 mmol·m−2·d−1 for both cultivars. None of the EOD-FR treatments affected plant dry weight, stem diameter, or plastochron index. Hence, elongation was achieved without compromising growth and development. EOD-FR was shown to be an effective non-chemical means allowing transplant propagation industry to produce long hypocotyls for grafting use.

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Chieri Kubota, Chalermpol Kirdmanee and Toyoki Kozai

Cymbidium (cv. Lisa rose) PLB (protocorm-like bodies) were cultured in liquid 1/2 MS medium with/without 20 mg g-1 sucrose under continuous lighting conditions. The vessels were shaken at 100 rpm under PPF (photosynthetic photon flux) of 20 and 140 μmol m-2 s-1 and CO2 concentrations outside the vessel (Cout) of 450 and 2000 μmol mol-1 conditions. Photoautotrophic growth was obtained at high PPF and high Cout. The chlorophyll content of the PLB in the medium without sucrose at high PPF and high Cout was almost 3 times that with sucrose at low PPF and low Cout. The number of newly developed PLB with sucrose at low PPF and low Cout was 1.6 times that without sucrose at high PPF and high Cout; the dry weight per unit PLB with sucrose at low PPF and low Cout was almost 3 times that without sucrose at high PPF and high Cout. Photoautotrophic growth of the PLB might be further promoted at higher CO2 concentration (> 1%).

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Johann S. Buck, Chieri Kubota and Merle Jensen

Cherry tomato (Solanum lycopersicum var. cerasiforme) plants were grown hydroponically with three different regimes of electrical conductivity (EC) of the nutrient solution to develop an effective EC management method to enhance the fruit quality. The EC treatments examined were 1) continuous high EC [4.7 dS·m−1 (HE)], 2) continuous low EC [2.8 dS·m−1 (LE)], and 3) high EC combined with midday (1030–1530 hr) low EC [midday reduction of high EC (MDR)]. The research was conducted to obtain preliminary information on the effect of EC treatments on the yield and fruit quality for 15 weeks of harvest under semiarid greenhouse conditions. Harvested fruit were sorted to several quality grades, including the “premium” grade based on fruit size, color, and total soluble solids. The number of fruit per truss was significantly higher in cultivar L308 than in cultivar L907 and in the LE treatment than in the HE or MDR treatment. The fruit size decreased over time regardless of EC treatment and cultivar. Cumulative yield of 15 weeks was greater in the LE treatment (26.3 kg·m−2) than in the HE treatment (22.1 kg·m−2) for ‘L907’, and there were no significant differences between the three EC treatments for ‘L308’ (24.1–28.1 kg·m−2). The cumulative yield in the MDR treatment was similar to that in the LE treatment regardless of cultivar. When quality attributes such as total soluble solids concentration measured for randomly sampled fruit were considered, cumulative premium-grade yield was the greatest for the HE treatment (12.9 or 17.6 kg·m−2) and was the smallest for the LE treatment (1.4 or 12.1 kg·m−2), regardless of cultivar. The cumulative yield of premium-grade cherry tomatoes in the MDR treatment was not significantly different from that in the HE treatment for ‘L308’ but was 11% less than that in the HE treatment for ‘L907’. Therefore, together with cultivar selection, the MDR treatment may be a potential alternative to a more commonly practiced continuously high EC treatment in semiarid greenhouses with limited environmental control capacity in which increasing the nutrient EC to increase quality is desired without significantly decreasing yield.

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Chieri Kubota, Haruna Maruko and Toyoki Kozai

For vegetative propagation of sweetpotato, single or multi-node leafy cuttings are used as propagules. A quantitative understanding of leaf development is important for predicting the number of propagules produced after a given production period under various environmental conditions. For plant production in a relatively closed structure, controlling CO2 concentration is necessary, but effects of CO2 concentration on cutting production rates of sweetpotato are not well-investigated. Single-node cuttings each with a fully expanded leaf (the initial leaf blade length was 66 mm) were grown under one of three levels of CO2 concentration (400, 800, and 1200 μmol·mol-1), 250 μmol·m-2·s-1 PPF, 16 h/day photoperiod, and 29 °C air temperature. The plant dry weight increased faster in the higher CO2 concentrations. Changes in the number of harvestable cuttings during the production period was defined by changes in the number of leaves reaching a leaf blade length (LBL) longer than a given standard length (LS). The number of harvestable cuttings increased almost linearly with time after the LBL of the first leaf reached the LS, regardless of CO2 concentration. The effect of CO2 concentration on cutting production rate (number of harvestable cuttings per day) was varied with different LS. For example, at LS = 20, 30, and 40 mm, the cutting production rate increased slightly at higher CO2 concentrations, while at LS = 60 mm, it decreased significantly at higher CO2 concentrations. This indicates that, under the present experimental conditions, increasing CO2 concentration increased the number of small leaves that might not be usable as cuttings (propagules). Environmental control is necessary in vegetative propagation to increase the number of propagules and the biomass usable as propagules, thereby minimizing energy and resources needed for the propagule/transplant production process.

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Chieri Kubota, Ayami Yamaguchi and Toyoki Kozai

For vegetative propagation of sweetpotato, single or multi-node leafy cuttings are used as propagules. A quantitative understanding of leaf development and the effects of environment conditions on leaf emergence and expansion rates is important for predicting the number of propagules produced after a given production period. Single-node cuttings each with a fully expanded leaf were grown under two levels of photosynthetic photon flux (PPF, 160 and 250 μmol·m–2·s–1) and photoperiod (10 and 16 h/day). The time courses of the number of leaves larger than the standard leaf area (As) were obtained by analyzing the time courses of leaf blade length recorded every day on each leaf. The number of leaves larger than a given As increased almost linearly after the first leaf reached to the As. PPF and photoperiod affected both the duration until the appearance of the first leaf with As and the leaf development rates (leaves per day). The effects of PPF were more pronounced than photoperiod for the development rate of the leaves regardless of As. Results obtained in these experiments were incorporated into our previously developed model, and the number of propagules produced under different environment conditions was predicted. Such techniques need to be used effectively for planning and environment control of vegetative propagation.