The physiology of photosynthesis: implications for supplemental lighting

Supplemental lighting is often required to produce high-quality plants in a timely manner. However, the costs associated with that supplemental lighting can be high. For the profitable production of high-quality crops, it is important to assure that the crops can use supplemental lighting efficiently. To develop to most cost-effective lighting methods possible, we decided to start with the basics: quantify how efficiently different crops can use light and use that knowledge to develop smarter lighting strategies. In this article, we will focus on perennials. There are of course many species of perennial plants, so we decided to focus on 10 popular species.

Light use efficiency of perennials
The first two questions we wanted to answer were simply: 1) how efficiently do different species use light? And 2) how does this depend on the light intensity (or photosynthetic photon flux density, PPFD)?  Most commonly, scientists measure how much CO2 is fixed by plants and converted into sugars. We wanted to start at a more basic level: the light reactions of photosynthesis. The light reactions of photosynthesis function much like solar panels: light energy (or photons) is absorbed and used to create a current (electron transport rate). The energy generated by that current is then used to convert CO2 into sugars. So, for rapid growth, a high electron-transport rate is required. These processes are easy to measure and we did so for 10 species at light levels ranging from 0 to 750 µmol·m-2·s-1. Two things are obvious (Figure 1):

The photosynthetic light use efficiency decreases with increasing PPFD. This is true for all species and unavoidable. There are differences in how efficiently different species use light, especially at higher light levels. Perhaps not surprisingly, species that grow well in the shade, like hosta and heuchera, use high light levels less efficiently than species that prefer full sun.

Since the electron transport rate is calculated from the light use efficiency and PPFD, species differences in electron transport rate are similar to those in light use efficiency. However, light use efficiency decreases, and electron transport rate increases with increasing PPFD. That creates a bit of a conundrum: since a high electron-transport rate is required for rapid growth, it is inherently coupled to a lower light use efficiency.


Figure 1. Using the physiological information presented in Figure 1, we can easily calculate the photosynthetic light use efficiency and electron transport rate of 10 popular perennials as a function of PPFD.

Developing better lighting strategies
Using the data in Figure 1, we can easily calculate the increase in electron transport rate (and presumably photosynthesis and growth) when we provide a certain amount of supplemental light. Figure 2 shows how much we can expect the electron transport rate to increase as we provide 100 µmol·m-2·s-1 of supplemental light. This helps us see important information for making better supplemental lighting decisions:

Electron transport rates increase more when supplemental lighting is provided when there is little sunlight. A low light crop like hosta does not benefit at all from supplemental lighting when there is ample sunlight, while a high light crop like Rudbeckia does (but not nearly as much as at low levels of sunlight!). This has direct implications for managing supplemental lighting:

Provide supplemental lighting preferentially when there is little sunlight. It will increase growth more than providing the same amount of supplemental light when there is ample sunlight. In many cases, supplemental lighting control is already controlled based on a certain sunlight threshold: the lighting system will come on only when the sunlight drops below that threshold.

For threshold control, sunlight thresholds should be species-specific, whenever possible. For low light species, using a lower lighting threshold will prevent supplemental lighting when those species cannot use the supplemental lighting effectively. As is clear from Figure 2, providing supplemental light when there is above 400 µmolm-2·s-1 will increase photosynthesis of Rudbeckia, but will have little effect on the photosynthesis of hosta.


Figure 2. The increase in electron transport rate (vertical arrows) as the result of 100 µmol·m-2·s-1 of supplemental lighting (horizontal arrows). The x-axis shows the amount of sunlight. Red symbols and arrows are for Rudbeckia, black/grey ones for hosta.

Based on this research, we have coined a new term: the daily photochemical integral. You are probably familiar with the daily light integral: a commonly used term that describes how much light a crop receives over the course of a day. The daily photochemical integral is somewhat similar: it describes the daily amount of photochemistry (the scientific term for electron transport) of a crop. The goal of an efficient supplemental lighting program should be to get the highest daily photochemical integral with the least amount of supplemental light. There are two ways to do this:

Since plants use light more efficiently when sunlight is low, provide supplemental light when there is little sunlight. Use longer photoperiods whenever possible. If you use longer photoperiods, but the same daily light integral, the instantaneous PPFD will be lower. And that allows the plants to use the light more efficiently, increasing the daily photochemical integral. Figure 3 shows this effect for a few different perennial crops. As you can see, the benefits of long photoperiods are especially pronounced for low-light species, like hosta. But all species will have a higher daily photochemical integral with longer photoperiods. An added benefit is that longer photoperiods require a lower PPFD and thus fewer light fixtures: the capital expense of the lighting system will be reduced. There is one important caveat: the flowering of many species is at least partly controlled by photoperiod, so before using longer photoperiods on a large scale, do trials on a smaller scale to assure that longer photoperiods do not trigger premature flowering of long-day crops or prevent flowering of short-day crops.


Figure 3. The estimated daily photochemical integral of four perennial species as a function of photoperiod. In all cases, the crops received a daily light integral of 15 mol·m-2·d-1. Using longer photoperiods, with the same daily light integral, increases the daily photochemical integral, a proxy for photosynthesis.

So far, all of this is based on basic physiology and theory. So do longer photoperiods, with the same daily light integral, really increase growth? Yes, as we reported last year in an AFE Research Update, the growth of Rudbeckia ‘Goldsturm’ seedlings increases when we use longer photoperiods, but with the same daily light integral (Figure 4). We have tested this for a number of leafy greens as well, and so far, all species grow faster with longer photoperiods, even if we do not increase the daily light integral.


Figure 4. Longer photoperiods result in better growth of Rudbeckia seedlings. The control plants on the left did not receive supplemental light (and an average DLI of about 5 mol/m2/d. The other plants all received a DLI of 12 mol/m2/d, but that light was spread out over photoperiods ranging from 12 to 21 hours. Both root and shoot growth increased with longer photoperiods.

What does this mean to the floriculture industry?
A basic understanding of photosynthetic physiology can help develop better lighting strategies. The three most important take-home messages:

Provide supplemental light preferentially when sunlight levels are low.
The optimal lighting strategy is species-dependent. Longer photoperiods can increase growth without increasing the amount of supplemental light that is provided.

What is next?
Industry support of the American Floral Endowment helped to make this research possible. And the financial support of AFE helped us to get a subsequent $5,000,000 grant from the USDA’s Specialty Crops Research Initiative. This project, titled Lighting Approaches to Maximize Profits, brings together scientists and engineers from around the country. Having a diverse team working on lighting issues in the controlled environment agriculture industry will help us integrate horticultural production, economics, and engineering. This will result in holistic approaches to optimize supplemental lighting strategies. 

For more information:
American Floral Endowment
+1 (703) 838-5211
www.endowment.org
 
 

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