Controlling root zone temperature improves plant growth and pigments in hydroponic lettuce

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 limited. In this study, we carefully observed the effects of RZT in red leaf lettuce to understand its effect on lettuce growth and pigment content.

Lettuce (Lactuca sativa, red leaf cultivar 'Red Fire') was grown hydroponically in a plant factory with artificial light under three RZT treatments (15, 25, or 35 °C) for 13 days. We investigated the comprehensive effects of RZT on the production of red leaf lettuce by metabolome and ionome analyses.

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, a RZT heating treatment during plant cultivation that changed from 25 to 35 °C RZT for 8 days before harvest significantly increased shoot dry weight compared with the 35 °C RZT and significantly increased pigments compared with the 25 °C RZT. The 15 °C RZT resulted in significantly less pigment content 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.

This 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. This suggests that controlling RZT could be a viable method to improve lettuce quality via enhancement of pigment content quality while maintaining acceptable yields.