Toshiki Asao

In recycled hydroponics, successive crop cultivation by maintaining electrical conductivity (EC) suffers lower growth performance due to accumulating autotoxic root exudates. In this study, the efficiency of alternate current electro degradation (AC-ED) was evaluated for degrading allelochemicals and recovering retarded lettuce yield cultivated in EC-adjusted repeatedly used nutrient solutions. From benzoic acid (BA)-added nutrient solution, BA was completely degraded after 24 h by applying AC-ED at 551 and 940 Hz frequency with 50 and 80% electrical duty. In lettuce bioassay, fresh mass was negatively affected without the AC-ED-treated solution. Finally, lettuce seedlings were hydroponically grown in a plant factory using a half-strength Enshi nutrient solution. Culture solutions were unchanged in non-renewed solutions. Nutrient elements were supplied based on the EC (1.42 dS m(-1)) of culture solutions. The fresh weight of lettuce was gradually decreased in subsequent cultures. Nutrient absorption rate was reduced in non-renewed solutions though enough of all nutrient elements were available in the solution. In the final culture, the highest shoot fresh weight (SFW) was recorded in the renewed (83.0 g plant(-1)) solution which was similar to the AC-ED-treated solution (81.0 g plant(-1)) and the lowest (58.0 g plant(-1)) was in the non-renewed solution. By applying AC-ED, 40% lettuce yield was recovered in the EC-adjusted solution without renewing. Therefore, it is recommended that the continuous application of AC-ED with the capacity of 551 Hz and 50% duty would be applied for recovering the retarded lettuce yield cultivated with repeatedly used culture solutions in recycled hydroponics. [GRAPHICS]

Recycled hydroponic solutions used for growing crops can accumulate allelochemicals that inhibit plant growth. We applied alternating current electrodegradation (AC‐ED) to fresh nonrenewed nutrient solutions (i.e. solutions remaining unchanged throughout the culture period) and once‐used nonrenewed solutions (i.e. solutions that had been used for a previous culture) for detoxifying autotoxic chemicals. Four experiments were conducted in which lettuce plants were grown in different nonrenewed solutions treated with AC‐ED and in renewed solution that was not treated with AC‐ED. Renewed solution (50% ‘Enshi’ solution) was changed at 14‐day intervals. In fresh starting solutions, no substantial difference was found in shoot fresh weight (SFW) between renewed and AC‐ED‐treated nonrenewed solutions at different intervals (experiment I) and frequencies (experiment III), but notably, the lowest yield was recorded in non‐renewed solution. In contrast, in the case of once‐used nonrenewed solution, weekly (experiment II) and thrice‐weekly and continuous (experiment IV) AC‐ED application showed significantly higher SFW compared to values for other solutions, and the lowest value was also found in nonrenewed once‐used culture solution. Therefore, we recommend the application of AC‐ED to non‐renewed solution either thrice weekly or continuously for efficient detoxification of accumulated allelochemicals to improve the growth, yield and quality of lettuce under two or more successive cultivations in recycled hydroponics.

イチゴ高設ベンチ栽培において，育苗の省力化および育苗専用施設の削減が可能な栽培方法について検討した．まず，苗養成施設での育苗作業を省力化するために採苗施設由来の子株を本圃に直接定植する方法を検討したところ，冷蔵処理を行うことで活着率が向上し，慣行育苗した場合と同様な総収量になった．次に，採苗施設での苗採り作業を省力化するために収穫株から発生したランナー子株を本圃直接定植した結果，慣行育苗や採苗施設由来の苗と同程度の生産性が認められた．しかし，これらの方法では親株から切り離した子株を本圃直接定植した際の枯死株および生育不良株の発生を回避することはできなかった．そこで，収穫株由来のランナー子株を収穫株と接続した状態で直接定植する「無育苗栽培法」を検討したところ，活着率の向上と慣行育苗法と同程度の収量性が認められ，育苗の省力化と育苗施設の削減が期待できることが示唆された．

Open access peer-reviewed chapter

Chronic kidney disease patients are restricted to foods with high potassium content but our daily diets including melon are rich in potassium. Therefore, we investigated the production of low-potassium melon through hydroponic nutrient management in soilless culture using perlite substrate during autumn season of 2012, 2014 and spring season of 2016. In the first study, melon plants were supplied with 50% standard ‘Enshi’ nutrient solution until first 2 weeks of culture. In 3rd and 4th week, amount of applied potassium was 50, 75, 100, and 125% of required potassium nitrate for each plant per week (based on our previous study). It was found that, melon plants grown with 50% of its required potassium nitrate produced fruits with about 53% low-potassium compared to control. In the following study, four cultivars viz. Panna, Miyabi shunjuukei, Miyabi akifuyu412, and Miyabi soushun banshun309 were evaluated for their relative suitability of low-potassium melon production. Results showed insignificant difference in fruit potassium content among the cultivars used. Source of potassium fertilizer as potassium nitrate and potassium sulfate and their restriction (from 1 or 2 weeks after anthesis) were also studied. There were no influences on fruit potassium content and yield due to sources of potassium fertilizer and restriction timings. In our previous studies, it was evident that potassium can be translocated from leaves to fruits at maturity when it was supplied nutrient without potassium. Thus, we also studied total number of leaves per plant (23, 24, 25, 26, and 27 leaves per plant). It was evident that fruit potassium, yield, and quality were not influenced significantly due to differences in number of leaves per plant. These studies showed that restriction of potassium nitrate in the culture solution from anthesis to harvest could produce melon fruits with low-potassium (>20%) content compared to potassium content of greenhouse grown melon (340 mg/100 g FW). Quality testing and clinical validation of low-potassium melon also showed positive responses compared to greenhouse grown melon.

Open access peer-reviewed chapter

Open access peer-reviewed chapter

Japan has a very mountainous topography, steep landforms, and varied climate making it less suitable than other countries for growing horticultural crops. However, good crop production technologies make it possible to grow high quality produce in Japan. The major horticultural crops are mainly produced in greenhouses and high tunnels however, a vast majority of greenhouse systems are outdated. Horticultural production practices could be more sustainable through the use of hydroponics in the greenhouse. Commercial hydroponic systems have proved more productive (20-25% higher) than conventional systems of agriculture. A number of research projects have been conducted to better understand production constraints in hydroponics, such as autotoxicity, and to identify suitable cultivars and supplementation techniques. This review discusses greenhouse production techniques for vegetables and ornamentals in hydroponics, as well as constraints and means for sustainable production.