Controlled root-zone temperature effect on baby leaf vegetables yield and quality in a floating system under mild and extreme weather conditions

Optimized Design of Irrigation Water-Heating System and Its Effect on Lettuce Cultivation in a Chinese Solar Greenhouse

In cold regions, the low irrigation water temperature is an important factor of low-temperature stress for greenhouse crops. In this paper, an irrigation water-heating system (IWHS) is proposed to increase the water temperature by utilizing the excess heat in the solar greenhouse. The heat-collection capacity of the system was analyzed by screening the IWHS process parameters in a Chinese solar greenhouse, and a warm-water irrigation experiment for lettuce was conducted. The results demonstrated that the water temperature increased with the increase in wind speed, and the increase in daily average water temperature reached the maximum value of 8.6 °C at 4.5 m/s wind speed. When the heat exchanger was installed at a height of 3.0 m, the collector capacity increased by 17.8% and 6.0% compared with the heating capacity at 0 m and 1.5 m, respectively, and the operation termination water temperature was 22.0-32.2 °C and its coefficient of performance (COP) was optimal. Surface darkening of the heat exchanger did not affect the heat-collection capacity of the system. Using the IWHS effectively improved the temperature of lettuce irrigation water in the Chinese solar greenhouse. The increased frequency of warm-water irrigation significantly promoted lettuce growth and increased the average yield per plant by 15.9%. Therefore, IWHS effectively increased the irrigation water temperature in a Chinese solar greenhouse in winter. Improving the system would enhance its economic and application value.

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 °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.

Environmental and Cultivation Factors Affect the Morphology, Architecture and Performance of Root Systems in Soilless Grown Plants

Soilless culture systems are currently one of the fastest-growing sectors in horticulture. The plant roots are confined into a specific rootzone and are exposed to environmental changes and cultivation factors. The recent scientific evidence regarding the effects of several environmental and cultivation factors on the morphology, architecture, and performance of the root system of plants grown in SCS are the objectives of this study. The effect of root restriction, nutrient solution, irrigation frequency, rootzone temperature, oxygenation, vapour pressure deficit, lighting, rootzone pH, root exudates, CO2, and beneficiary microorganisms on the functionality and performance of the root system are discussed. Overall, the main results of this review demonstrate that researchers have carried out great efforts in innovation to optimize SCS water and nutrients supply, proper temperature, and oxygen levels at the rootzone and effective plant–beneficiary microorganisms, while contributing to plant yields. Finally, this review analyses the new trends based on emerging technologies and various tools that might be exploited in a smart agriculture approach to improve root management in soilless cropping while procuring a deeper understanding of plant root–shoot communication.