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I'm curious to know if the process of photosynthesis has a "ramp-up" time to become useful to a plant, or if it begins instantaneously when the plant is exposed to light after a period of darkness.

For example, if I'm growing a plant indoors using artificial light, does the plant start converting CO2 the very moment I switch on the light, or does it take a certain period of time for the reaction to reach a critical momentum for the plant to be able to produce sugars?

Or, as a thought experiment, if I turned on the light for 10 minutes, would the plant produce roughly the same amount of sugars as if I toggled the light on for 1 minute, then off for 10 minutes, 10 times in a row?

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This is a really interesting question! It gets to the heart of how photosynthesis works.

First, note that whether or not the plant will immediately start producing glucose depends on a variety of factors (what type of plant, what type of light). For example, many succulents undergo CAM photosynthesis (Crassulacean Acid Metabolism) in which light capture and CO2 fixation are temporally separated (the former during daylight hours and the latter at night). Additionally, artificial light comes with different wavelength compositions (ratio of red to far-red light can differ) and photosynthesis rates will change accordingly read more here.

But I haven't yet addressed the heart of your question: how long does it take for a plant to start photosynthesizing once it is exposed to light? To think about this, consider what is actually happening when light hits a leaf. Light energy is absorbed by pigments in the chloroplasts, which transfer energy to neighboring pigments until an electron is transferred to a reaction center. From here, "light reactions" of photosynthesis continue and usable chemical energy is obtained. From there, the CBB cycle (sometimes called the Calvin Cycle, Calvin-Benson, or "dark" reaction) continues, which is where the CO2 from your question comes into play, and 6-carbon sugars are produced. It isn't a matter of "ramping up", so much as completing this series of chemical reactions.

So now, it is time to consider your thought experiment. Let's say that you have a bean plant in your kitchen (C3 photosynthesizer), and you conduct your experiment shining a white-light lamp on the plant with the experimental conditions as described. I believe that you would produce roughly the same amount of sugars in each because glucose synthesis occurs on the order of seconds (~30s). If you exposed your bean plant to white light for only 5-second increments, there would not be time to produce the energy needed to complete the CBB cycle.

Aside: the experiments that first elucidated the CBB cycle were conducted by exposing cyanobacteria (a photosynthetic bacteria) to labeled (heavy) carbon, and exposing it to light for a few seconds, and characterizing which compounds were produced from that heavy carbon within that few-second time frame. Read more about that experiment in the previous link. It sounds like Calvin himself wondered the same thing you did!

For further reading on the basics of plant physiology, including photosynthesis, I always recommend the plant bio "bible": Plant Physiology and Development by Taiz L., Zeiger E., 2010. I still consult it often.

Hope this helps. Stay inquisitive!

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    $\begingroup$ This is a fantastic answer! Thanks so much for taking the time to write it. Very interesting. $\endgroup$
    – Bungle
    May 9 '20 at 5:39

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