Maximizing crop photosynthesis across the entire canopy requires the optimization of many environmental factors

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.


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.

17 thoughts on “Maximizing crop photosynthesis across the entire canopy requires the optimization of many environmental factors

  1. This study was performed under naturally variable solar radiation. What effects, if any, do you think artificial sole source lighting would have had on the results of this experiment? As sole source lighting in controlled environments becomes more common, how do you think future studies investigating photosynthesis optimization should be approached?

    • The spectrum of solar radiation can vary significantly from sole source artificial lighting. There are numerous benefits to this high level of lighting control, and the ability to apply certain wavelengths is useful in many scenarios. But, there are drawbacks as well, specifically with reduced far red light and other natural spectra that occur.
      As sole source lighting becomes more common, photosynthesis optimization needs to take into account what wavelengths are being utilized and how those factors might affect plant growth. Plants respond to different spectra differently, and the responses seen can vary across species. More comprehensive analyses need to be done to flesh out the intricacies of wavelength-specific and crop-specific responses to lighting strategies.
      As far as the effect on this experiment: the results may change if sole source artificial lighting was used. For example, if the natural lighting was replaced with only LED lights that omitted green wavelengths, the plants may have been more productive across temperatures and [CO2]. The increased availability of red/blue wavelengths may increase productivity as more photons are absorbed and not reflected off of the plants. It would be interested to see a publication that considered so many factors of PPFD, [CO2], and temperature to determine optimum conditions for plant growth with different types of sole source lighting.

    • If artificial sole source lighting was used for this experiment, it can be expected that the results would surely change, however, I would expect to see a similar trend. Although, one factor that may change the trend is infrared radiation, which is absent in sole source artificial lighting. Infrared radiation from the sun will cause leaf/canopy temperatures to increase, which surely has an impact of photosynthesis. It would be interesting to determine how photosynthesis would change in the absence of infrared radiation as is the case in sole source artificial lighting. My thought is that photosynthesis would be maximized at a higher temperature since there will be no infrared radiation heating the actual plant. Thus, at the same aerial temperature, leaf/plant temperature of a plant grown under sole source artificial lighting should be less than the leaf/plant temperature a plant grown under sunlight.

    • First, you would see a change in the results since certain wavelengths from solar radiation would not be expressed. Secondly, you would also reduce the variance due to environment which would allow you to have a more accurate/predictive model. I think the most important consideration in investigating photosynthesis optimization is reproducibility, which will be confounded by the type of controlled environmental facility.

    • I wonder how difficult it would be to get even light uniformity using sole source lighting in an experiment large enough to make canopy-level generalizations. I would expect that light uniformity should be addressed in future experiments to avoid adding an additional factor contributing to variation.

    • I think that adding over replicated controls throughout the greenhouse design could allow for some environmental and spatial corrections in the greenhouse. Those spatial differences may be interpreted through a random-effects statistical framework to examine the genetic and environmental contributions to photosynthesis optimization.

  2. Different photosynthetic responses to environmental variability were observed in each crop. Why do you think chrysanthemum had a seemingly more plastic optimum temperature based on the results reported? Additionally, the economically important products are different in chrysanthemum (flower) and tomato (fruit). Is optimizing the environment for maximum photosynthesis always the best strategy to maximize the value of the marketable end product? Do other factors need to be considered? Would the environment need to be different to optimize these other factors?

    • Optimizing the environment for maximum photosynthesis in all cases is likely not the best strategy to maximize a marketable end product.
      For example: in the floriculture industry, the product is a flowering plant. In order to obtain this product, light control comes into play specifically with regard to photoperiod. Short-day and long-day plants need to have specific lighting strategies in order to induce flowering. Therefore, simply targeting the maximum lighting regime is not a good method if it affects the flowering behavior in a non-ideal way.
      One way to increase growth before flowering is optimizing CO2 in a controlled environment in order to maximize vegetative growth before flowering is induced with a specific lighting strategy. This is one possible way to target ideal vegetative and floral development to make the whole plant more appealing to a consumer market.
      Similar methods can be applied to the vegetable crop and cannabis industries, as these controlled environments are able to be manipulated to a high degree for specific goals.

    • I think if the goal is to maximize the quantity of the end product, optimizing the environment for maximum photosynthesis is likely the best strategy. However, if quality is prioritized over quantity, I do not think that optimizing the environment for maximum photosynthesis would be the best strategy to achieve the highest quality end product. For example, increasing temperature to increase photosynthesis will lead to increasing fruit temperature, which generally will decrease the fruit quality. Thus, if the goal is to maximize the quantity of the end product, optimizing the environment for maximum photosynthesis is likely the best strategy. But, optimizing the environment for maximum photosynthesis will likely not lead to the highest quality end product.

    • I am not an expert in plant physiology, but i would speculate that temperature optimization for crop photosynthesis varies because tomato has spend some energy on fruit production unlike chrysanthemum. I think environment plays an important role, but it has to be coupled with Maintenance of plants (e.g. pruning) and nutrient/substrate optimization to maximize the end product.

    • I think that considering the relationship between photosynthetic rate and growth rate is important when optimizing environmental conditions and it may be different between crops. Manipulating the plant’s maximum photosynthetic capability or the efficiency with which it translates photosynthetic energy into growth might lead to increased production/value later on. There also may be other factors that could influence growth rate (e.g. nutrient availability) that need to be optimized and may fluctuate based on the environment.

    • I think that the environmental optimization for maximum photosynthesis not always results in the maximization of profitable end products. Of course, maximum photosynthesis has a significant effect on total productivity (total dry matter yield). However, according to types of organ-specific target sinks for marketable end products, as an example of flower (chrysanthemum) and fruit (tomato) herein, the environmental factors should be finely investigated and adjusted to maximize not only photosynthesis but also source-sink transport. In particular, contrary to florescence, since fruition is more complicated due to the sequential involvement of flowering hormones such as florigen and phytochrome and various enzymes related to polymerization or conversion of assimilates, I think that further study is needed to optimize environmental factors for fruiting crops.

    • In regard to chrysanthemum’s more plastic range, I wonder about the past breeding history compared with tomato. In my mind, the inbreeding and loss of variation in the genetic material to produce a commercial tomato cultivar is greater than that of the chrysanthemum cut flowers. If chrysanthemum is genetically more similar to its wild Asteraceae relatives, it may still have some of the adaptability present in wildflowers to adapt to various weather conditions as they vary from year to year. In this case, it may be interesting to investigate the plasticity of a wild tomato versus commercial tomato in a similar experiment.

      • Tomato breeders have utilized wild relatives as a source of genetic diversity for biotic and abiotic stress mitigation since the early 20th century (Prescott-Allen and Prescott-Allen, 2013; Hajjar and Hodgkin, 2007). Many commercial cultivars are derived from introgression populations using wild tomato relatives. I think it would depend on the center of origin of the wild tomato accession that you wanted to measure and what wild relative genetics may be present in the commercial tomato cultivar. LA1141 is an accession of Solanum galapagense and was collected at a site where it was growing on the interior walls of a volcanic crater on Isla Santiago in the Galápagos Islands. Inbred backcross lines derived from this accession demonstrate a broad ability to mitigate environmental stress.

        Literature cited:

        Prescott-Allen, R., & Prescott-Allen, C. (2013). Genes from the wild: using wild genetic resources for food and raw materials. Routledge.

        Hajjar, R., & Hodgkin, T. (2007). The use of wild relatives in crop improvement: a survey of developments over the last 20 years. Euphytica : International Journal of Plant Breeding, 156, 1-13.

        • Thanks for that summary! Do you think, in general, that wild accessions would have more plasticity for optimum temperature than commercial tomato cultivars? My instinct would be yes. But possibly the breeding of commercial cultivars has incidentally kept genetic material that allows the most adaptable crop in this variable to be selected.

  3. I wonder is there is any current breeding effort in most common greenhouse crops to achieve a uniform leaf area index, spacing or optimizing canopy architecture since they are so important for an accurate CPMs. For example: I know most varieties of tomatoes growth in the greenhouse are indeterminant, so architecture is constantly changing, and light intensity generally decreases as you move from the top of the canopy down to lower leaves. Probably there is room for breeding efforts to improve photosynthetic responses and eventually more accurate CPMs.

    • Leaf area index is manageable by the practice called leaf pruning or deleafing. Typically, greenhouse tomato is maintained at leaf area index around 4.0 where maximum light interception occurs. However, this practice can be a bit more scientific (but currently not), based on understanding of light environment in the canopy.

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