Nanobubble- and Microbubble Aeration Affect Leaf Quality Without Changing Yield of Lettuce Grown in Floating Systems
https://www.mdpi.com/2311-7524/11/9/1141#
Dissolved oxygen (DO) concentration in nutrient solution is
critical for maximizing yield and optimizing quality traits of
lettuce plants grown in floating systems. This study evaluated
the effects of two aeration systems—a Venturi system (V) and
a Venturi system combined with a nanobubble generator using
electromagnetic waves (VN)—compared with a non-aerated
control (C), on quali-quantitative traits of lettuce plants grown in
a floating system over two consecutive harvests. Both aeration
treatments significantly increased DO levels in the nutrient solution
compared to C, with the VN treatment maintaining the highest value
throughout the crop cycle. Although no significant differences in lettuce
yield were observed, both aeration treatments enhanced the leaf concentration
of P, Mn, Zn, and Cu in the second harvest, and Mg in both harvests.
Moreover, the VN treatment lowered leaf nitrate concentration in both
harvests compared to the other treatments. The increase in DO in the
nutrient solution delayed leaf senescence, as evidenced by higher chlorophyll
index and lower anthocyanin levels in the lettuce leaves harvested at the end
of the trial for both aeration systems. These results suggest that aeration,
particularly with nanobubbles, can be an effective and sustainable strategy
to enhance the quality traits of lettuce grown in a floating system.
This study highlights the importance of oxygenation in nutrient solutions
for hydroponic lettuce production. The use of aeration systems, particularly
the Venturi system coupled with a nanobubble generator (VN), significantly
increased the dissolved oxygen (DO) levels in the nutrient solution. Both
aeration systems (VN and Venturi system, V) helped to mitigate the natural
decline of DO due to intensive root respiration.
Despite similar growth performance among treatments in terms of yield
parameters, the VN aeration system positively influenced several quality
traits of lettuce leaves, like the uptake of essential minerals (P, Mg, Mn, Zn, and Cu)
and reduced nitrate concentration in leaves. The increase in DO in the nutrient
solution also delayed leaf senescence, as evidenced by a higher chlorophyll
index and lower anthocyanin levels in the lettuce leaves harvested at the end of the trial.
Moreover, while a slight reduction in leaf Ca concentration was observed under
the VN treatment, no symptoms of tip burn occurred, indicating that the Ca levels
remained above the critical physiological threshold. Given the nutritional relevance
of minerals in the human diet, the use of nanobubbles may offer an effective
strategy for enhancing the nutritional quality of lettuce while maintaining production standards.
Future studies are necessary to explore the potential benefits of this innovative aeration
system under different growing seasons, especially under hot climate conditions
where oxygen solubility is reduced and root respiratory demand increases.
Because the temperature effect on the nanobubble stability is negligible in a
short time [44], the nanobubble system could provide positive results, especially
under a high temperature regime where oxygen solubility is reduced and oxygen demand by roots is enhanced.
