Epigenetic and Physiological Responses to Varying Root-Zone Temperatures in Greenhouse Rocket

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 degrees C; nevertheless, a root-zone temperature (RZT) range of 18-23 degrees 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 nonheated (control) tank (12 degrees C) and two heated tanks; a solar panel-powered one (16 degrees C) and a public grid-powered one (22 degrees 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 degrees C RZT in comparison to 22 degrees C RZT. Reduction of temperature at 12 degrees C did not reduce methylation levels further but rather increased them. Furthermore, at 16 degrees C, the effective quantum yield of photosystem II (PSII) photochemistry (Phi PSII) was significantly higher, with a higher PSII electron transport rate (ETR) and a significantly decreased non-regulated energy loss (Phi NO), suggesting a better light energy use by rocket plants with higher photosynthetic performance. FPSII was significantly negatively correlated with DNA methylation levels. Our results show that at 16 degrees 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 degrees C is accompanied by sufficient yield showing that rocket can effectively grow in suboptimal yet sustainable root-zone temperatures.