In this study, phosphorus (P) strength [(full strength: 1×-P, half strength: 0.5×-P and zero strength: 0×-P)] and root zone temperature (10 and 15 °C RZT) were investigated in red romaine lettuce to determine their effects on anthocyanin content. This experiment was conducted using a deep-flow technique (DFT) hydroponic system in a greenhouse during the winter in Gangwon province in the Korea Republic. The treatments were 1×-P (15 and 10 °C RZT), 0.5×-P (15 and 10 °C RZT) and 0×-P (15 and 10 °C RZT). The results showed that the P (phosphorus), K (potassium) and Ca (calcium) accumulation contents were lower in the 0×-P -treated red romaine lettuce leaves than in the other treatments. The maximum red romaine lettuce yields per plant, leaf number and leaf length were exhibited during the 1×-P at 15 °C RZT treatment. In 0×-P at 10 °C, RZT treatment showed lower leaf color values of L*, b*, chrome, ºh and higher leaf color a* values. Moreover, in the 0×-P treatment at 10 °C RZT, the leaf chlorophyll content (SPAD), chlorophyll a, chlorophyll b and chlorophyll total were the lowest. Anthocyanin content was the highest in the 0×-P treatment, followed by 0.5×-P and 1×-P treatments at both 15 and 10 °C RZT. Regardless of the P intensity treatment, anthocyanin content was higher at 10 °C RZT than at 15 °C RZT. In summary, the 1×-P at 15 °C RZT treatment is the best for red romaine lettuce leaf yield, and the 0×-P at 10 °C RZT treatment is the best for lettuce leaf anthocyanin content.
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