Abstract
Greenhouse production of baby leaf vegetables grown in hydroponic floating trays has become extremely popular in recent years. Rocket (Eruca sativa Mill.) can grow in temperatures varying between 10 and 20 °C; nevertheless, a root-zone temperature (RZT) range of 18–23 °C is considered optimal for high productivity, photosynthesis, and production of metabolites. Maintaining such temperatures in winter raises production costs and prevents sustainability. In this study, we tested the impact of lower RZT on plants’ status and recorded their responses while providing energy for heating using photovoltaic solar panels. We used three hydroponic tanks for cultivation; a non-heated (control) tank (12 °C) and two heated tanks; a solar panel-powered one (16 °C) and a public grid-powered one (22 °C). Methylation-sensitive amplified polymorphisms (MSAP) analysis of global methylation profiles and chlorophyll fluorescence analysis were employed to assess methylation and physiology levels of rocket leaves. We found that there is demethylation at 16 °C RZT in comparison to 22 °C RZT. Reduction of temperature at 12 °C did not reduce methylation levels further but rather increased them. Furthermore, at 16 °C, the effective quantum yield of photosystem II (PSII) photochemistry (ΦPSII) was significantly higher, with a higher PSII electron transport rate (ETR) and a significantly decreased non-regulated energy loss (ΦΝO), suggesting a better light energy use by rocket plants with higher photosynthetic performance. ΦPSII was significantly negatively correlated with DNA methylation levels. Our results show that at 16 °C RZT, where plants grow efficiently without being affected by the cold, DNA methylation and photosynthesis apparatus systems are altered. These findings corroborate previous results where hydroponic production of rocket at RZT of 16 °C is accompanied by sufficient yield showing that rocket can effectively grow in suboptimal yet sustainable root-zone temperatures.
Highlights
Rocket plants were grown in heated hydroponic tanks powered by electricity provided by the public grid and solar panels, while control rocket plants were grown in tanks that were not heated
To document the extent to which rocket plants experienced cold in their root zones when grown under suboptimal conditions inside heated and non-heated hydroponic tanks, and whether this affects their performance, we used different kinds of analysis
It was found that greenhouse rocket plants with an root-zone temperature (RZT) of 16 ◦ C, as controlled by PV panels, demethylate their DNA compared to plants grown with an RZT of 22 ◦ C
Summary
In a floating hydroponic system, where plants develop on floating polystyrene trays inside tanks with a recirculating nutrient solution [2], water waste is minimal and nutrients do not leak, preventing environmental pollution [3]. Baby leaf vegetables such as rocket (E. sativa Mill.) are suitable for cultivation in floating disks. Cultivation of baby leaf vegetables in hydroponic floating systems is simple and cheap It provides plants with uniformity in their development, growth, and harvesting times, plus better control over growing conditions
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