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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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Tomomi Eguchi and Chieri Kubota

In tomato (Solanum lycopersicum), grafting position is recommended to be below rootstock cotyledons to avoid undesirable axillary shoots growing out from the cotyledons. In contrast, grafting above the rootstock cotyledons is desired to assure adequate distance between grafted union and soil line, only if there is no potential grow-out of axillary shoots from rootstock cotyledons. The objective of this preliminary study was to examine fatty alcohol application on cotyledonary axils of tomato seedlings to control undesirable axillary shoot extension from rootstock in tomato grafting. Solution containing various concentrations of a commercial fatty alcohol compound was applied to different growing stages of cotyledonary axillary shoots (nonextended buds or extended shoots) of tomato seedlings grown in a greenhouse. When fatty alcohol was applied directly to cotyledonary axillary buds, the seedlings were then pinched to force-induce the axillary shoot extension to assure the efficacy of the fatty alcohol treatment. High concentrations (10% and 15%) of fatty alcohol suppressed incidence of axillary shoot extension to less than 7% by killing buds. However, when applied to extended axillary shoots, application with 2% or higher concentrations of fatty alcohol caused plant collapse because excess fatty alcohol flowed down the stem and presumably damaged the root system. Therefore, we concluded that application of fatty alcohol to control cotyledonary axillary shoots of tomato rootstock could be possible only if fatty alcohol at effective concentration (10% to 15%) is applied exclusively to the target buds.

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Min Wu and Chieri Kubota

Five cultivars (Blitz, Mariachi, Quest, Rapsodie, and Trust) of tomato (Solanum lycopersicum) were grown hydroponically in a greenhouse to determine photosynthetic and transpirational responses to three electrical conductivities (EC) [2.3 (control), 4.8, and 8.4 dS·m−1] of inflow nutrient solution. Leaf photosynthetic light response curves were measured during the early vegetative growth stage for cv Mariachi and Rapsodie and during the reproductive growth stage for all five cultivars. Leaf transpiration rate and leaf conductance were measured for all five cultivars in both stages. During the vegetative growth stage, high EC treatment of 8.4/14.3 dS·m−1 inflow/efflux solution reduced leaf conductance and transpiration rate by 28% and 29%, respectively, compared with low EC treatment (2.3/5.9 dS·m−1), regardless of cultivar. Effects of EC treatments on leaf photosynthetic light response curves were cultivar specific. For ‘Mariachi’, moderate EC (4.8/8.7 dS·m−1) and high EC treatments in the vegetative growth stage reduced the maximum photosynthetic rate by 49% compared with the low EC treatment. However, for ‘Rapsodie’, the moderate EC treatment increased the maximum photosynthetic rate during the vegetative stage by 8% and 47% compared with low and high EC treatments, respectively. During reproductive growth stage, EC treatment did not significantly affect the transpiration rate, but high EC treatment reduced the leaf conductance by 15%, regardless of cultivar. Parameters of leaf photosynthetic response curves were affected by cultivar and EC treatment. Compared with the low EC treatment, the moderate EC treatment did not significantly affect the maximum photosynthetic rate of any cultivar except ‘Rapsodie’, which showed the greatest maximum photosynthetic rate in the moderate EC treatment. The results showed that the plant physiological response under elevated EC was cultivar and growth-stage specific, and increasing the inflow EC to the moderate level of around 4.8 dS·m−1 during the reproductive growth stage would not negatively impact photosynthesis, transpiration, and leaf conductance of tomato plants, for all cultivars tested in the present experiment.

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Ricardo Hernández and Chieri Kubota

To increase the available photosynthetic photon flux (PPF) for plant growth, greenhouse growers sometimes use electric lighting to supplement solar light. The conventional lighting technology used to increase PPF in the greenhouse is high-pressure sodium lamps (HPS). A potential alternative to HPS is high-intensity light-emitting diodes (LEDs). The objective of this study is to compare supplemental LED lighting with supplemental HPS lighting in terms of plant growth and morphology as well as discuss the energy use efficiencies of the fixtures. There were three light treatments: 1) blue LED (peak wavelength 443 nm); 2) red LED (peak wavelength 633 nm); and 3) HPS, to provide 3.7 ± 0.2 mol·m−2·d−1 (background solar radiation of 6.3 ± 0.9 mol·m−2·d−1). Cucumber (Cucumis sativus) plants at the transplanting stage (26 to 37 days) under HPS had 28% greater dry mass than did plants under the LED treatments. This can be attributed to the higher leaf temperature under the HPS treatment. No differences were observed in growth parameters (dry mass, fresh weight, or number of leaves) between the blue and red LED treatments. Plants under the blue LED treatment had greater net photosynthetic rate and stomatal conductance (g S) than those under the red LED and HPS treatments. Plants under the blue LED and HPS treatments had 46% and 61% greater hypocotyl length than those under the red LED, respectively. The fixture PPF efficiencies used in the experiment were 1.9, 1.7, and 1.64 μmol·J−1 for the blue LED, red LED, and HPS treatments, respectively; however, the fixture growing efficiency (g·kWh−1) of HPS was 6% and 17% greater than the blue LED and red LED treatment, respectively. In summary, supplemental red LED produced desirable plant compactness and HPS had greater fixture growing efficiency than LEDs.

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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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Chieri Kubota and Toyoki Kozai

Growth and net photosynthetic rate of potato (Solanum tuberosum L.) `Benimaru' plantlet in vitro were studied under a conventional photomixotrophic condition [with 20 g sucrose/liter in the medium and under 70 μmol·m-2·s-1 photosynthetic photon flux (PPF)] with minimal ventilation (MV) and under photoautotrophic conditions (without sugar in the medium and under 190 μmol·m-2·s-l PPF) with enhanced natural ventilation using an air diffusive filter (DV) or with forced ventilation (FV). Fresh weight of the plantlets cultured in the FV and DV treatments was 2.4 times that of the plantlets cultured in the MV treatment. Net photosynthetic rate and dry weight per plantlet were the highest in FV followed by DV. For photoautotrophic micropropagation, FV was superior to DV.

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Ryo Matsuda and Chieri Kubota

Tomato has been considered as a potential biological production platform for plant-made pharmaceuticals (PMPs). However, information is limited for protein productivity and dynamics in tomato fruit. As part of our PMP production project, total soluble protein (TSP) content of green and red fruits was analyzed for selected six greenhouse cultivars. The six cultivars consisted of various fruit types (including cluster, cherry, mini plum, and grape types) with sizes ranging from 10 to 150 and from 14 to 188 g fresh weight for green and red fruits, respectively. In fruit TSP content per unit dry weight (DW), approximately seven- to eightfold variation was observed among cultivars for both green and red fruits. There was no significant correlation between fruit TSP content per DW and fruit DW or fruit type over the cultivars irrespective of fruit ripening stage, indicating that fruit weight or fruit type of a cultivar cannot be an indicator of the TSP content per DW. There was also an inconsistent trend in differences in fruit TSP content per whole fruit between green and red fruit among cultivars. Our results suggest that a low-yielding cultivar can produce high TSP content per unit ground area resulting from high TSP content per DW in fruit. A background tomato cultivar or genotype candidate for commercial PMP production should thus be selected from among various fruit types considering both yield potential and fruit TSP content.

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Chieri Kubota and Toyoki Kozai

A storage method of transplants in vitro was developed using light compensation points in conjunction with low temperatures. Broccoli (cv. Ryokurei) plantlets, aseptically germinated and cultured for three weeks in vitro, were used as model transplants. Culture conditions were: 23C air temperature, 160 μmol m-2s-1 PPF, and 3.6 air exchanges per hour of the vessel. Prior to storage, light compensation points were determined at 3, 5, 10, and 15C for the plantlets cultured with or without 20 g liter-1 sugar in the medium. Plantlets were stored for six weeks at 5, 10, and 15C under either 0 or 2 μmol m-2s-1 continuous PPF. The light compensation points varied with air temperature and with medium sugar level. Plantlet dry weight during storage was best maintained by keeping CO2 exchange rate of the plantlets close to zero throughout the storage period. High transplant qualities were successfully preserved at light compensation points: 2 μmol m-2s-1 PPF at 5-10C without sugar, and at 5C with sugar in the medium. This method may be applicable for storage of other crop transplants, plug seedlings and cuttings as well.

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Chieri Kubota and Toyoki Kozai

Broccoli (Brassica oleracea L. Botrytis Group `Ryokurei') plantlets, cultured photoautotrophically (without sugar in the medium) in vitro for 3 weeks at 23C and 160 μmol·m–2·s–1 photosynthetic photon flux (PPF), were stored for 6 weeks at 5, 10, or 15C under 0 (darkness) or 2 μmol·m–2·s–1 PPF (continuous lighting) supplied by fluorescent lamps (white light). Dry weight of the plantlets stored for 6 weeks at 5 or 10C in light was not significantly different from that of the plantlets before storage. Dry weight of the plantlets decreased as temperature increased and was maintained at higher levels in light than in darkness. Chlorophyll concentrations of the plantlets were higher at the lower temperatures. Chlorophyll fluorescence kinetics indicated higher activities of chlorophyll of the plantlets stored in light than in darkness. Lighting at as low as 2 μmol·m–2·s–1 PPF was important to preserve photosynthetic and regrowth abilities and dry weight of the plantlets during low-temperature storage.