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We investigated the effects of microbubbles, generated by a swivelling microbubble generator in hydroponics nutrient solution, on the growth of leaf lettuce (Lactuca sativa). Twenty-four lettuce seedlings at the four- to five-leaf stage each were transplanted into two culture containers at 21 ± 1 °C (day) and 18 ± 1 °C (night) under fluorescent lamps that provided a photosynthetic photon flux of 173 ± 18 and 171 ± 16 μmol·m−2·s−1 averaged at eight points at the canopy level for micro- and macrobubbles conditions, respectively, during a photoperiod of 16 h per day. Seedlings were cultivated for 2 weeks in two deep flow technique (DFT) hydroponics culture systems in which micro- or macrobubbles were produced, respectively, by a microbubble aerator and aquarium aeration stones. The nutrient solution was maintained at a temperature of 22 ± 1 °C during the experiment. Fresh and dry weights of the microbubble-treated lettuce were 2.1 and 1.7 times larger, respectively, than those of the macrobubble-treated lettuce. Although the reasons for growth promotion by microbubbles are still under investigation, we speculate that the larger specific surface area of the microbubbles and negative electronic charges on the microbubbles surfaces may promote growth because microbubbles can attract positively charged ions that are dissolved in the nutrient solution. These results indicate that microbubbles generated in a DFT hydroponics culture system can remarkably promote plant growth.
A light-emitting diode (LED)-low light irradiation (LLI) storage system was developed for suppressing the change in dry weight and maintaining the quality of green plants during long-term storage. In this system, the carbon dioxide (CO2) exchange rate was maintained at zero by automatically adjusting the photosynthetic photon flux density (PPFD) with a proportional-integralderivative (PID) controller. The voltage supplied to the LEDs was controlled by the difference between the inflow (400 μmol·mol-1) and outflow CO2 concentrations in the storage case. Grafted tomato (Lycopersicon esculentum; scion = `House Momotaro'; rootstock = `Anchor T') plug seedlings were stored at 10 °C for 35 days under four different LLI conditions as a system operating test: fixed red light irradiation at 2 μmol·m-2·s-1, PID-controlled red light irradiation with no blue light, and PID-controlled red light irradiation with blue light at 0.2 or 1.0 μmol·m-2·s-1. The results showed that the automatic PPFD control during LED-LLI helped suppress changes in dry weight during storage as expected. Furthermore, it was found that addition of a low percentage of blue light improved the morphological appearance of the seedlings and reduced the PPFD required to suppress the change in dry weight.