Interlinked Temperature and Light Effects on Lettuce Photosynthesis and Transpiration: Insights from a Dynamic Whole-Plant Gas Exchange System

Environmental control in closed environment agricultural systems (CEA) is challenging due to the high energy demand and the dynamic interactions between plants and their heterogeneous phylloclimate. Optimization of crop production in CEA systems therefore requires a thorough understanding of whole-plant functioning and the interconnected plant-climate interactions. Such optimization is limited by an incomplete knowledge of how leaf-level measurements of gas exchange relate to whole-plant processes and how to scale-up point measurements of the heterogeneous environment to inform plant-level decisions. To address both, a dynamic whole-plant gas exchange system was developed to quantify the effect of temperature, relative humidity and light intensity on whole-plant photosynthetic and transpiration rates in lettuce (Lactuca sativa L.). Results showed that light intensity was the primary driver for whole-plant photosynthesis, with temperature optima increasing from 5 °C at a photosynthetic photon flux density (PPFD) of 150 µmol·m−2·s−1 to 13 °C at 400 µmolm−2·s−1. These optima for lettuce plants were 10 to 20 °C lower than those observed at leaf level due to a shifted balance between respiration and photosynthesis within the complex habitus of lettuce. The results showed a decoupling of transpiration and photosynthesis under high relative humidity, with vapour pressure deficit (VPD) values of 0.5 kPa or lower, which physically limited transpiration. The newly developed dynamic gas exchange system has proven to be a helpful tool for examining the relative importance and combined effects of environmental factors on whole-plant photosynthesis and transpiration. Potential future applications of this system include research on phylloclimate, implementation in production facilities, and validation of crop models.