Plants can synthesize some antioxidants, including L-ascorbic acid (AsA) and polyphenol, in response to environmental stresses. Antioxidants detoxify reactive oxygen species in plants and also aid in human health. In this study, we demonstrate that a novel hydroponic treatment can increase leafy vegetable nutritional quality without retarding growth. Leaf lettuce (Lactuca sativa) was grown hydroponically and subjected to rhizosphere drought stress by lowering the water level in the solution tub before harvesting. Appropriate drought stress using this method could increase AsA, polyphenol, and sugar content by 24%, 50%, and 17%, respectively, and decrease nitrate nitrogen content by 18% without reducing yield. Similar effects of drought stress on AsA content were observed in four other plant species. This hydroponic method has a universal potential to increase leafy vegetable quality without reducing yield in controlled environments such as plant factories.

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An in vitro regeneration and transient expression systems were developed for the halophyte sea aster (Aster tripolium L.), an important genetic resource for salt tolerance. Adventitious shoots were formed from both leaf explants and suspension-cultured cells in a Murashige and Skoog (MS) (Physiol Plant 15:473-497, 1962) basal salts containing 500 mg l -1 casamino acids, and supplemented with 5.4 lM a-naphthaleneacetic acid (NAA) and 4.7 lM kinetin to the culture medium. Hyperhydricity of shoots was avoided by increasing the ventilation of the culture vessel. Root formation from shoots was promoted in the presence of 26.9 lM NAA. A high yield of protoplasts was isolated using 1% cellulase and 0.25% pectinase from both leaf mesophyll and suspension-cultured cells, and these were used for transient expression. The highest level of transient expression of the green fluorescent protein was obtained with 1 9 10 5 protoplasts ml -1 , 25 lg batch -1 of plasmid vector, and 30% polyethylene glycol 4,000.

Lettuce tipburn is an irreversible physiological disorder caused by calcium deficiency that decreases the crop value. Breeding a tipburn-resistant cultivar is the only causal therapy in many cases. In this study, we investigated an efficient method to evaluate lettuce resistance to tipburn in vitro. Seedlings of 19 lettuce cultivars representing three head types were cultured on agar medium containing EGTA, which chelates Ca 2? . The percentage of tipburned leaves decreased proportionally with EGTA concentration. Susceptible cultivars were distinguished at 0.01 mM EGTA, whereas resistant cultivars were classified at 1.0 mM EGTA. Based on mean values of tipburn measurements, tipburn susceptibility was highest for 'Leaf Lettuce', followed by 'Butterhead Lettuce', and then 'Crisphead Lettuce'. Two cultivars were selected for further tests using hydroponic and pot culture. The rank order of susceptibility to tipburn in these experiments was consistent with that of the in vitro assay. The in vitro evaluation of lettuce susceptibility to calcium deficiency is useful for initial screening of lettuce cultivars against tipburn incidence. Resistant cultivars identified in this study are practical candidates for cultivation in controlled environments, such as a plant factory, while sensitive cultivars are also useful as indicator plants to monitor environmental conditions.

The strawberry fruit contains abundant polyphenols, such as anthocyanins, flavan-3-ol, and ellagitannin. Polyphenol enrichment improves the quality of strawberries and leads to a better understanding of the polyphenol induction process. We measured the total polyphenol content of strawberry fruits under different growth conditions, developmental stages, and treatment conditions during pre-harvest and post-harvest periods. High fruit polyphenol content was observed in cold treatment, which was selected for further analysis and optimization. A transcriptome analysis of cold-treated fruits suggested that the candidate components of polyphenols may exist in the phenylpropanoid pathway. Coverage with a porous film bag excluded the effects of drought stress and produced polyphenol-rich strawberry fruits without affecting quality or quantity. The degree of stress was assessed using known stress indicators. A rapid accumulation of abscisic acid was followed by an increase in superoxide dismutase and DPPH (2,2-Diphenyl-1-picrylhydrazyl) activity, suggesting that the strawberry fruits responded to cold stress immediately, reaching the climax at around 6 days, a trend consistent with that of polyphenol content. These findings enhance our understanding of the mechanism of post-harvest polyphenol accumulation and the value of strawberries as a functional food.