Cultivar-Specific Responses of Spinach to Root-Zone Cooling in Hydroponic Systems in a Greenhouse Under Warm Climates

Growing spinach year-round via greenhouse hydroponics in warm climates can be challenging because of the intolerance of many spinach cultivars to heat. Root-zone cooling in hydroponic systems in warm climates may be a promising cooling method to alleviate heat stress; however, its effectiveness is still unknown in spinach plants. This study aimed to investigate the impact of root-zone cooling on the growth and physiological responses of four spinach cultivars (‘Lakeside’, ‘Hammerhead’, ‘Mandolin’, and ‘SV2157’) grown in deep water culture hydroponic systems in a greenhouse during the summer season in two growing cycles. The experiment consisted of the following three root-zone temperatures (RZTs): Control (ambient water temperature), RZT24 (24 °C), and RZT21 (21 °C). Among the four cultivars, ‘SV2157’ performed equally regardless of the treatment, demonstrating superior heat tolerance versus the other three cultivars. ‘Mandolin’ exhibited the greatest benefit from root-zone cooling, with increases in shoot dry weights of 87% and 94% under RZT24 and RZT21, respectively, compared to those under control treatment. Additionally, total leaf areas significantly increased under the two root-zone cooling treatments. ‘Lakeside’ and ‘Hammerhead’ generally benefited from root-zone cooling, although the magnitude of growth increases was small or statistically insignificant. However, ‘Lakeside’ and ‘Hammerhead’ were highly responsive to lower ambient air temperatures, as evidenced by increases of 121% and 90%, respectively, in shoot fresh weights across the treatments in Cycle 2 (average air temperature of 24.7 °C) compared to those in Cycle 1 (29.3 °C). Physiological responses to root-zone cooling varied among cultivars, with ‘SV2157’ exhibiting the highest chlorophyll, carotenoid, and anthocyanin levels. Higher total phenolic contents under control treatment in Cycle 1 in all three cultivars except for ‘SV2157’ suggested greater reactive oxygen species production, indicating oxidative stress. Root-zone cooling reduced oxidative stress indicators, including mortality (%), hydrogen peroxide content, and malondialdehyde content, and minimized cell leakage. Based on plant growth, physiological and biochemical traits, and electricity consumption, cooling the root zone to 24 °C rather than 21 °C is recommended for hot summers with high air temperatures.