Controlling Root Zone Temperature Improves Plant Growth and Pigments in Hydroponic Lettuce.

BACKGROUND AND AIMS: Air and root zone temperatures are important environmental factors affecting plant growth and yield. Numerous studies have demonstrated that air temperature strongly affects plant growth and development. Despite the extensive literature on air temperature, comprehensive studies on the effects of root zone temperature (RZT) on plant growth, elemental composition, and pigments are still limited. This study carefully observed the effects of RZT in red leaf lettuce to understand its effect on lettuce growth and pigment content.

METHODS: Lettuce (Lactuca sativa, cv. red-leaf 'Red Fire') were hydroponically grown in a plant factory with artificial light under 3 RZT treatments (15, 25, and 35 °C) for 13 days. The present study investigated the comprehensive effects that RZT had on the production of red leaf lettuce by metabolome and ionome analyses.

KEY RESULTS: The 25°C RZT treatment achieved maximum shoot and root dry weight. The 35°C RZT decreased plant growth but significantly increased pigment contents (e.g., anthocyanins, carotenoids). In addition, the RZT heating treatment during plant cultivation, which changed from 25°C RZT to 35°C RZT for 8 days before harvest, significantly increased shoot dry weight compared to the 35°C RZT, and significantly increased pigments compared to the 25°C RZT. The 15°C RZT resulted in significantly less pigment contents relative to the 35 °C RZT. The 15°C RZT also resulted in shoot and root dry weights greater than the 35°C RZT but less than the 25 °C RZT.

CONCLUSIONS: The present study demonstrated that plant growth and pigments can be enhanced by adjusting RZT during different stages of plant growth to attain enhanced pigment contents while minimizing yield loss. The end result from this study suggests that controlling RZT could be a viable method to improve lettuce quality via pigment content quality enhancement while maintaining acceptable yields.