Original paper: Körner, O., Heuvelink, E., and Niu, Q. 2009. Quantification of temperature, CO2, and light effects on crop photosynthesis as a basis for model-based greenhouse climate control. The Journal of Horticultural Science and Biotechnology. 84:233-239. https://doi.org/10.1080/14620316.2009.11512510

Photosynthesis is impacted by multiple environmental factors including temperature, light intensity, and carbon dioxide (CO₂) concentration. If optimal environmental conditions that maximize photosynthesis are quantified, they can be employed in controlled environments to increase crop productivity. Attempts to measure such optimal conditions have been undertaken in the past. Environmental setpoint measurements from these studies have even been compiled and implemented into various mathematical models known as “crop photosynthesis models” (CPMs) that can predict potential photosynthetic activity based on a plant’s environment. However, many of the environmental setpoints used in CPMs have relied on leaf-level photosynthesis measurements and optimization which are not always compatible with canopy-wide photosynthesis optimization. This potential incompatibility is caused by differences in the microclimate between the various levels in a crop’s canopy. For example, light intensity generally decreases as you move from the top of the canopy down to lower leaves. Also, there can be large variations in individual leaf temperature and humidity throughout the canopy which will affect photosynthesis. Other studies have investigated canopy-wide photosynthesis, but many were performed in poorly-sealed greenhouses where conditions could potentially fluctuate. Oliver Körner and his colleagues sought to more accurately quantify optimal environmental conditions for canopy-wide photosynthesis by using well-sealed greenhouses equipped with air conditioning and CO₂ supplementation. These environmental control measures allowed for experiments in which temperature and CO₂ concentration could be effectively manipulated and accurately maintained. The ability to control CO₂ concentration and measure CO₂ consumption in the greenhouse system was critical to this study. Photosynthesis was quantified by monitoring the amount of CO₂ consumed by the plants in the greenhouse. Minimizing any gas exchange with the natural environment was crucial to ensure any measured CO₂ change was a result of photosynthesis.

The photosynthetic responses of two different crops (cut-chrysanthemum and tomato) were quantified under different temperatures and CO₂ concentrations. ‘Reagan Improved’ chrysanthemum plants were exposed to different combinations of three temperature setpoints (23, 28, and 33 °C) and three CO₂ concentrations (400, 700, and 1000 µmol CO₂ mol^-1) under natural light levels. Similarly, CO₂ consumption was measured in ‘Moneymaker’ tomatoes under different combinations of three temperature setpoints (20, 26, and 32 °C) and two CO₂ concentrations (400 and 1000 µmol CO₂ mol^-1). Increasing CO₂ concentration raised the maximum potential photosynthetic rate in both crops across all tested temperature setpoints, and this effect was greater in chrysanthemum than tomato. Additionally, higher CO₂ levels led to a higher photochemical efficiency (µmol CO2 µmol photons^-1) in both chrysanthemum and tomato. Temperature effect on photosynthetic rate was more complicated although photochemical efficiency in both crops consistently decreased as temperature increased. In chrysanthemum and tomato, both light intensity and CO₂ concentration affected how temperature affected maximum photosynthetic rate. Using discrete light intensities (600, 900, and 1200 µmol m^-2 s^-1), optimum temperatures for maximum photosynthesis at 400 and 1000 µmol CO₂ mol^-1 were calculated. In chrysanthemum, the optimum temperature at all three light intensities was below 23 °C at 400 µmol CO₂ mol^-1 so a trend was not clear. At the same CO₂ concentration in tomatoes, optimum temperature for tomato photosynthesis increased with higher light levels, and the largest increase in optimum temperature occurred between 900 and 1200 µmol m^-2 s^-1 (25.3 to 27.1 °C). Optimum temperature for chrysanthemum and tomato photosynthesis at 1000 µmol CO₂ mol^-1 both increased when light intensities increased. At this CO₂ concentration, optimum temperature changed the most in both crops when light intensity was changed from 600 to 900 µmol m^-2 s^-1. Specifically, chrysanthemum optimum temperature changed from 23.5 °C to 26.9 °C while tomato optimum temperature increased from 26.6 °C 28.4 °C.

Körner and his colleagues sought to quantify the optimum environmental conditions (temperature, CO₂ concentration, and light intensity) for canopy-level photosynthesis in two crops (cut-chrysanthemum and tomato). Higher CO₂ levels increased maximum photosynthesis and photochemical efficiency in both crops with this effect being greater at higher temperatures. Similarly, higher CO₂ concentration led to an increased optimum temperature for photosynthesis, and this occurred at the largest level when light intensity was high. Variability in the canopy microclimate (most notably temperature and light intensity) resulted in different environmental factor effects than those observed in leaf-level photosynthesis models. In general, environmental conditions caused smaller changes in canopy-level photosynthesis when compared to leaf-level photosynthesis. While basic trends were similar in both chrysanthemum and tomato, the results indicate that optimum environmental conditions for photosynthesis must be quantified for individual crops. Differences between crops including leaf area and canopy architecture must be accounted for to create accurate CPMs. In conclusion, this study indicates that crop-specific responses to interactions between multiple environmental factors must be accounted for in CPMs to accurately quantify canopy-level photosynthesis.