The Five Keys to Photosynthesis: Caring For Your Plants

There is more to growing than just the basics of sunlight and water. In fact, there are five important factors in determining how well your plants or crops will do: sunlight, carbon dioxide, water, soil organisms, and nutrients. Luckily it is easy to maintain healthy plants, healthy food, and healthy living soil.

The following is an excerpt from The Living Soil Handbook by Jesse Frost. It has been adapted for the web.

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The Five Keys to Photosynthesis

And so we return to the beginning of this discussion: Without effective photosynthesis, we cannot grow healthy plants and healthy food, nor maintain healthy living soil. Let’s examine the practical steps growers can take to maximize the photosynthetic process in the market garden.

Five key factors determine a plant’s ability to photosynthesize: sunlight, carbon dioxide, water, soil organisms, and nutrients.

The good news is that growers have some amount of control over each one. Though all of these factors work in concert, it’s important to keep each one individually in mind as you read the rest of this book.

Certainly all of these factors can be controlled to the nth degree in an automated greenhouse, and to a lesser extent under high tunnels or caterpillar tunnels. Indeed, this ability to control conditions is why more growers are incorporating protected culture into their farming systems— the plants perform well when they have optimum conditions for photosynthesis and are protected from extreme weather events. Even so, I’m not advocating for growing all of your crops in a completely controlled environment such as a greenhouse.

There are many ways to cater to a plant’s photosynthetic activity or protect it from weather extremes that don’t involve plastic or petroleum products, which may not be as environmentally friendly. One common way to mitigate excessive sunlight, for instance, is to shield plants using shade cloth or high-tunnel plastic. Growers can also strategically plant trees and shrubs to provide afternoon shade for growing areas, which additionally provides the benefits of more photosynthesis. The goal doesn’t have to be perfect control of the environment. Instead, focus on making conditions for photosynthetic activity as ideal as possible in your context. Keep that in mind as you read about the five key factors and consider what you can do to best cater to the needs of photosynthesis on your farm.

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Sunlight

In order for plants to photosynthesize, they need access to sunlight and they need that sunlight in the proper amounts. When plants receive either excessive or inadequate sunlight, photosynthetic activity slows down or ceases altogether.

An example of insufficient light is when young seedlings are germinated in low light conditions and stretch out in search of sunlight. This elongation— referred to as legginess—results in a weakly structured plant that may not be able to hold itself up. This can lead to problems such as foliar diseases, because drooping stems and leaves that touch the soil can be infected by soilborne pathogens. Of course, too little sunlight can also slow down the growth and production of maturing crops, making them weak, low yielding, and slow to mature. Growers can address low light levels by opening up tree canopies where needed or by providing supplemental lighting in greenhouses.

Plants can protect themselves from excessive sunlight, but there is a limit to their defenses. When exposed to excessive radiation from sunlight, plant tissues (including fruits) can incur physical damage in the form of sunburn, and dehydration and oxidative stress can occur as well. Generally speaking, oxidative stress is not a bad thing, and plants produce antioxidant compounds to combat oxidation. At a certain point, though, oxidative stress can become excessive, causing photosynthesis to cease.⁷

As mentioned earlier in this chapter, methods to mitigate damage from excessive sunlight include using shade cloth or planting trees and shrubs around gardens. We’ve found that some tender crops such as lettuce and arugula benefit from shade for a couple of weeks in the summer while they’re getting established in the field after transplant. Mobile structures like caterpillar tunnels can be excellent tools for adding sun protection where needed. Beets in particular flourish under the small amount of shade provided by high tunnel plastic in the summer. In several studies, using shade has shown to decrease the percentage of unmarketable (split or sunburned) fruit on crops like peppers and tomatoes.⁸ Too much shade is not a good thing, though. A study on purple pak choi found that photosynthesis improved after 5 days of lowered light levels, but dramatically decreased after 10 and 15 days.⁹ Interplanting, which is described in chapter eight, is another way to provide sun protection. For example, taller crops such as corn can be used to provide some shade for more sensitive crops like lettuce.

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Carbon Dioxide

No discussion of carbon dioxide management is complete without mentioning the enzyme called RuBisCo.

Essentially, RuBisCo (ribulose-1,5-bisphosphate carboxylase-oxygenase) is responsible for the first step in fixing carbon dioxide from the atmosphere. But it’s not particularly good at its job. Dr. Robert L. Houtz, a horticulturalist from the University of Kentucky who studies this enzyme, told me that RuBisCo fixes carbon dioxide “at a rate that is basically half of what it could be.”

Carbon fixation is a general term for the process of converting inorganic carbon into organic compounds; photosynthesis is a prominent example of carbon fixation.

Dr. Houtz as well as many evolutionary biologists believe this inefficiency is largely because RuBisCo evolved at a time when the atmosphere had very high levels of CO2 and only trace amounts of oxygen. When oxygen levels rose (because of photosynthesis), RuBisCo did not evolve to adapt to the changing atmosphere (though other mechanisms within plants did). So in order to make up for the enzyme’s inefficiency, a plant just creates a lot of RuBisCo in its leaves. The only way to increase RuBisCo’s rate of carbon fixation, says Dr. Houtz, is to increase the level of carbon dioxide around the plant.

The key to generating high CO2 levels naturally is fairly simple. In healthy soil that is high in organic matter, soil microbes respire quite a lot of carbon dioxide. This is the natural carbon cycle as described previously. Indeed, showing you how to boost that natural cycle in your gardens is one of the important messages of this book. Recall that plants push carbon into the soil through root exudates, microbes consume that carbon and organic matter, then respire much of it out in the form of CO2 (not dissimilar to what we do when we eat food). The plant then fixes that CO2 again and restarts the cycle. Fungi, for their part, are particularly adroit at respiring carbon dioxide, so soils higher in fungi—or garden walkways full of decomposing wood chips—assist in providing CO2 for RuBisCo to fix. Adding mushroom production to a closed greenhouse may also help replenish CO2 levels. They say the farmer’s footprint is the best fertilizer, and that’s sort of true in a greenhouse, too, because humans exhale carbon dioxide—so next time you’re pruning tomatoes in a hot greenhouse, remind yourself that you’re also helping to fuel the carbon cycle! To be sure, natural CO2 sources like these may not compete quantitatively with artificial methods of producing carbon dioxide.

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Water

As I described earlier, one stage of photosynthesis splits water molecules apart, yielding the oxygen we breathe. Water is also the transport system for nutrients and a fundamental part of the cooling mechanism for plants. Too much or too little water can have grave effects on a plant’s ability to carry out the rest of the photosynthetic process.

For starters, when water levels are low—such as in a drought or because of inadequate irrigation—specialized cells called guard cells that surround the stomata do not allow those pores to open. You’ll remember that stomata are the tiny pores in leaf surfaces that allow carbon dioxide into the plant and water to escape. These “valves” in the leaves help to create a negative pressure, which siphons water up from the roots (if you’ve ever wondered how plants get water to defy gravity, now you know—the stomata do much of that literal heavy lifting). This movement of water also helps to cool the plant. When water levels are low at the roots, however, the plant closes the stomata to regulate water loss.¹² Generally speaking, the more severe the drought, the more severe the closure. And if stomata are not open, photosynthesis will cease and the plant can overheat.

Alternatively, an oversaturation of water at the roots—caused by flooding or poor drainage—can likewise lead to closure of the stomata and produce other photosynthesis-limiting factors including poor soil respiration, inadequate mineral absorption, and more.¹³ This puts a heavy importance on decompacting soils and maintaining proper soil drainage and soil aeration to avoid soggy soils. (For more about soil compaction, see “Starting from Scratch” on page 35.)

Soil-enhancing organisms such as worms also use moisture to help breathe through their skin. Microbes utilize water for transportation, for the enzymatic activity required to extract minerals from rock particles, and more. Floods, stagnant water, or droughts can devastate beneficial soil fauna populations that contribute to plant health.¹⁴ Living soil does not exist without water, so an effective irrigation system coupled with good drainage, high organic matter, and sufficient soil coverage are essential to maximizing photosynthetic activity.

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Soil Organisms

As discussed previously, plants release various root exudates to attract certain microorganisms and manipulate the pH of the rhizosphere. However, while the plant has some say in the microbes they attract, the microbes also have some say in the plant.

Certain soil microorganisms release specific hormones that can alter the physical structure of a plant and regulate its growth.¹⁵ This means that the microbes assist with designing a plant’s architecture, from the roots to the leaves and fruit. Below ground microbes also play a role in aboveground plant defense or susceptibility.¹⁶ Soil organisms can affect the traits of flowers, making them more attractive to pollinators by manipulating the shape and size of the blossoms and also the composition of the nectar.¹⁷

The important takeaway here for growers is that microbes matter to photosynthetic activity throughout a plant’s life—they help keep the plant healthy, productive, pollinated, and in many cases, protected. Regular additions of composts, compost teas, or compost extracts that are rich with beneficial microbes is one way to ensure those populations are retained and grow. For example, we soak every tray of transplants we grow in compost extract or compost tea before transplanting to ensure the presence of diverse microbial populations in the root zone at the time plants are moved into garden beds. (For more on transplanting see chapter eight.)

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Nutrients

At the time of this writing, scientists have identified 17 macro- and micronutrients that plants require for survival (see “The 17 Essential Nutrients” on page 22). All of those nutrients play a role in photosynthesis and plant growth. For example, the chlorophyll molecule, which is an essential component of photosynthesis, contains magnesium atoms. So if magnesium is lacking in the soil, plants may not be able to make enough chlorophyll to carry on optimal levels of photosynthesis. Potassium, another essential macronutrient, facilitates the diffusion of CO2 into chloroplasts.¹⁸ Nitrogen, for its part, plays numerous roles in photosynthesis and is even embedded in our DNA and RNA—no life exists without it. Micronutrients have important roles as well in photosynthesis; manganese, for example, helps the reactions powered by solar energy that result in the splitting of water molecules.¹⁹

As discussed previously, plant roots absorb the nutrients that microbes harvest. However, it is important that all 17 nutrients are present within the top six or so inches of soil for plants to readily access them, especially at the critical early stages of growth. Some agronomists hypothesize that every nutrient that plants need is already present in every soil. Whether or not you agree with that conceptual framework, the results of a soil test can help to confirm the nutritional status of your soil. Any grave deficiencies in the first six inches of soil can be addressed by adding the missing nutrients under the guidance of an agronomist. Cover crops—which are crops grown to enrich the soil—can also help to bring up nutrients that are present in deep layers of the soil. (I cover soil testing, fertilization practices that supply minerals, and cover cropping in more detail in later chapters.)

No matter what you apply to the soil, always be aware that it is soil life that feeds plants, not farmers. Your goal is not to feed the plants directly but to feed the microbial populations in the soil. Synthetic chemical fertilizers formulated as “plant food” can have grave negative effects on soil life, altering or even killing biology in the soil and nearby waterways.

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Notes

7. Paula Mu oz and Sergi Munn -Bosch, “Photo-Oxidative Stress during Leaf, Flower and Fruit Development,” Plant Physiology 176 (February 2018): 1004–14, https://doi.org/10.1104/pp.17.01127.

8. Joseph Masabni et al., “Shade Effect on Growth and Productivity of Tomato and Chili Pepper,” HortTechnology 26, no. 3 (June 2016): 344–50, https://doi.org/10.21273/HORTTECH.26.3.344.

9. Hongfang Zhu et al., “Effects of Low Light on Photosynthetic Properties, Antioxidant Enzyme Activity, and Anthocyanin Accumulation in Purple Pak-Choi (Brassica campestris ssp. Chinensis Makino),” PLoS ONE 12, no. 6 (2017): e0179305, https://doi.org/10.1371/journal.pone.0179305.

10. Leiv M. Mortensen, “Review: CO2 Enrichment in Greenhouses. Crop Responses,” Scientia Horticulturae 33, nos. 1–2 (August 1987): 1–25, https://doi.org/10.1016/0304-4238(87)90028-8.

11. Megha Poudel and Bruce Dunn, “Greenhouse Carbon Dioxide Supplementation,” Oklahoma State University, March 2017, https://extension.okstate.edu/fact-sheets/greenhouse-carbon-dioxide-supplementation.html.

12. Hadi Pirasteh–Anosheh et al., “Stomatal Responses to Drought Stress,” in Water Stress and Crop Plants: A Sustainable Approach, ed. Parvaiz Ahmad (Hoboken, NJ: John Wiley & Sons, 2016).

13. T. T. Kozlowski, “Plant Responses to Flooding of Soil,” BioScience 34, no. 3 (March 1984): 162–67, https://doi.org/10.2307/1309751.

14. Kyaw Aung, Yanjuan Jiang, and Sheng Yang He, “The Role of Water in Plant–Microbe Interactions,” Plant Journal 93, no. 4 (February 2018): 771–80, https://doi.org/10.1111/tpj.13795.

15. Randy Ort z-Castro et al., “The Role of Microbial Signals in Plant Growth and Development,” Plant Signaling and Behavior 4, no. 8 (August 2009): 701–12, https://doi.org/10.4161/psb.4.8.9047.

16. Robin Heinen et al., “Effects of Soil Organisms on Aboveground Plant-Insect Interactions in the Field: Patterns, Mechanisms and the Role of Methodology,” Frontiers in Ecology and Evolution 24, (July 2018): article 106, https://doi.org/10.3389/fevo.2018.00106.

17. Nicholas A. Barber and Nicole L. Soper Gorden, “How Do Belowground Organisms Influence Plant-Pollinator Interactions?” Journal of Plant Ecology 8, no. 1 (February 2015): 1–11, https://doi.org/10.1093/jpe/rtu012.

18. Merle Tr nkner, Ershad Tavakol, and B lint J kli, “Functioning of Potassium and Magnesium in Photosynthesis, Photosynthate Translocation and Photoprotection,” Physiologia Plantarum 163, no. 3 (2018): 414–31, https://doi.org/10.1111/ppl.12747.

19. Taka-aki Ono et al., “X-ray Detection of the Period-Four Cycling of the Manganese Cluster in Photosynthetic Water Oxidizing Enzyme,” Science 258, no. 5086 (November 1992): 1335–37, https://doi.org/10.1126/science.258.5086.1335.

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