Leaf lettuce (Lactuca sativa L. ‘L-121’) growth in hydroponics with different nutrient solutions used to generate ultrafine bubbles

Abstract

Fine bubble (FB) and ultrafine bubble (UFB) generators have been installed in hydroponic systems to promote plant growth. However, their application is limited because the mechanism of the effect of UFBs is not well understood. We aimed to evaluate the effect of UFBs on promoting leaf lettuce growth in hydroponics using nutrient solutions prepared with tap water (Control) and two UFB water—tap water (standard, STD) and calcium nitrate solution (original, ORL). We then compared the differences among the three treatments under the same nutrient concentration and environmental conditions. The shoot fresh weights, dry weights, nutrient element (Zn, K, Mn, Fe, Cu, Ca, and Mg) concentrations, and total nutrient element quantity were measured after harvesting. The shoot fresh and dry weights were higher in the STD and ORL treatments than in the control, but no difference was noted between the STD and ORL treatments. Plant nutrient element uptake was higher in the STD and ORL treatments than in the Control treatment. Additionally, Mn uptake differed between the STD and ORL treatments. The results indicated that plant growth promotion in the STD and ORL treatments was affected by reactive oxygen species generated by UFBs. UFB properties also allowed for the supply of sufficient nutrients enabling rapid growth. Furthermore, we suggest that the type of solution used altered the properties of the FBs and UFBs, which in turn influenced the uptake of nutrients by the plants.