# Rubisco concentration light dependent?



## JoshP12 (23 Jun 2020)

Hi all,

So, I've been thinking . I recall reading that the argument for stable CO2 during photoperiod was because of the Rubisco concentration in a plant. In other words, getting a stable CO2 is the key to match your light.

If light drives CO2 uptake, and stability of CO2 is needed for Rubisco (so we don't waste energy making it ... resulting in a confused unhealthy plant), then how does light and Rubisco fit into the equation together?

Let us use the following example to illustrate:

Lights on with a ramp:



TimeCO2Lights13 ppm (turn on CO2)0%210 ppm0% ramp begins315 ppm20%418 ppm30%523 ppm40%625 ppm (gas off)50% ramp down begins720 ppm30%818 ppm15%910 ppm0%

Ok, if you have intense gaseous exchange this makes no sense as you will off gas CO2 way quicker - so it's not perfect. However, if the CO2 demand = the lights, then why does it need to be stable? If you prefer 50 ppm, or 30 ppm, of CO2, just create a nice curve to 50, or 30.

Does CO2 concentration dictate Rubisco concentration?
Does light intensity dictate CO2 uptake rate which in turn dictates Rubisco concentration - as a result ... Does light dictates Rubisco concentration?

I have no idea - I just remember hearing mention that you just match your CO2 to your light.

Please help!

Josh


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## JoshP12 (23 Jun 2020)

I found it! https://www.ukaps.org/forum/threads...r-if-patience-is-the-virtue.60454/post-594876

It is CO2 concentration. 

Thread closed .


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## Simon Cole (23 Jun 2020)

RuBisCO is very abundant in plant leaves. But both carbon dioxide and light are interdependent variables in catalysis. You can say that one dictates the other because without either the plant cannot grow, but the relationship is mutually inclusive. It just depends how you choose to view the system. The same applies to light. If you wanted maximum growth then you would need maximum light matched to RuBisCO concentration, but not exceeding it's point of fixation. When certain plants receive too much light then carbon fixation switches to oxygen fixation, and the process stops. So to achieve maximum growth under your schema, you would apply both to any given growth rate. Because carbon dioxide concentrations tend to be greatest constraint, you can choose to reduce lighting to match net growth. However, plants will tend towards their genetic disposition, and RuBisCO is usually produced far in excess of how much carbon dioxide you can provide, unless the species in question has different evolutionary traits. For example, plants tend to moderate the number of chloroplasts so that they do not waste too much energy producing photosynthetic enzymes. The problem of stunted or poor growth tends to be either too much light and not enough carbon dioxide (oxygen fixation, and light inhibition) or not enough growth due to limitation of photosynthesis (in which all variables are at play). I frequently see too high light intensity, resulting in poor growth, that many people subscribe to nutrients, when actually it is a plant starving from lack of carbon fixation photosynthesis, ergo no sugars. I remember my science teacher did an entire phD on this topic. The process is very complex. But in my mind your plan is rational.


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## JoshP12 (23 Jun 2020)

Simon Cole said:


> RuBisCO is very abundant in plant leaves. But both carbon dioxide and light are interdependent variables in catalysis. You can say that one dictates the other because without either the plant cannot grow, but the relationship is mutually inclusive. It just depends how you choose to view the system. The same applies to light. If you wanted maximum growth then you would need maximum light matched to RuBisCO concentration, but not exceeding it's point of fixation. When certain plants receive too much light then carbon fixation switches to oxygen fixation, and the process stops. So to achieve maximum growth under your schema, you would apply both to any given growth rate. Because carbon dioxide concentrations tend to be greatest constraint, you can choose to reduce lighting to match net growth. However, plants will tend towards their genetic disposition, and RuBisCO is usually produced far in excess of how much carbon dioxide you can provide, unless the species in question has different evolutionary traits. For example, plants tend to moderate the number of chloroplasts so that they do not waste too much energy producing photosynthetic enzymes. The problem of stunted or poor growth tends to be either too much light and not enough carbon dioxide (oxygen fixation, and light inhibition) or not enough growth due to limitation of photosynthesis (in which all variables are at play). I frequently see too high light intensity, resulting in poor growth, that many people subscribe to nutrients, when actually it is a plant starving from lack of carbon fixation photosynthesis, ergo no sugars. I remember my science teacher did an entire phD on this topic. The process is very complex. But in my mind your plan is rational.



Thank you for this reply .


Hmmm ... so in theory with a low enough ramp and a high enough injection rate, you could turn the gas on when the lights come on and then turn them off an hour (or whatever you want) before lights off?

Does anyone do this?

Josh


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## Zeus. (23 Jun 2020)

JoshP12 said:


> Hmmm ... so in theory with a low enough ramp and a high enough injection rate, you could turn the gas on when the lights come on and then turn them off an hour (or whatever you want) before lights off?



No IMO, 

My understanding is the plants basically measures the [CO2] when the lights come on as photosynthesis starts to 'boot up'. So from the [CO2] the plant may adjust the  RuBisCO  production to suit the [CO2] when the light initiates photosynthesis. So 'ideally' if the [CO2] is stable from lights on till CO2 off, the [RuBisCO] needed in the plant remains constant in the plant and CO2 uptake in the plant is non limited by an inadequate  [RuBisCO] in the plant.

Plants main nutrient is Carbon for Carbon uptake to be non limiting the [RuBisCO] in the plant needs to suit the [CO2] , so fluctuating [CO2] means the plant is constantly changing is RuBisCO production and resultant [RuBisCO] in the plant and the plant can become carbon deficient.

Bit like being in a small boat in middle of the ocean and no fresh water -"Water, water everywhere but not a drop to drink"


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## JoshP12 (23 Jun 2020)

Zeus. said:


> No IMO,
> 
> My understanding is the plants basically measures the [CO2] when the lights come on as photosynthesis starts to 'boot up'.



Does the intensity of the light matter? Or is it any non-zero amount of light? If so, does ambient light play into this? 

Josh


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## Zeus. (23 Jun 2020)

JoshP12 said:


> Does the intensity of the light matter? Or is it any non-zero amount of light? If so, does ambient light play into this?
> 
> Josh



@ceg4048 may be able to provide a better answer than myself. I would off said low intensity makes like to no difference esp if its below the light compensation point for the plant, but I would also be guessing at the same time.

Some scapers do there tanks in cellars with no ambient light so gaining full control of any photons at any time. I do have some carboard a shield my tanks from direct sunlight for when it is low at certain times of year.


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## JoshP12 (1 Jan 2021)

This paper suggests that CO2 concentration and RuBisCo concentration are not dependent:

http://www.plantphysiol.org/content/plantphysiol/88/4/1310.full.pdf

Enzyme activity was found to be independent of growth CO2 concentration, and ranged between 0.91 and 1.10I mol CO2 min-' mg-' protein (mean activity of 1.01 mol CO2 min-' mg-' protein).

Further, a minor dip in CO2 or increase in CO2 will not affect RuBisCo (it will however impact photosynthetic rates):

As shown in Table 1, rubisco activity was not affected by short-term exposures (- 1 h) to external CO2 concentrations ranging from 1 10 to 880, ML L-', in plants grown in 330 (mean activity of 96 ,umol CO2 m 2 s-') or 660 uL CO2 L' (mean activity of 98, Mmol CO2 m-2 s-1). I

Paper is old and done on Soybeans but still valuable!


Also, @Simon Cole mentioned this here:


Simon Cole said:


> RuBisCO is very abundant in plant leaves....RuBisCO is usually produced far in excess of how much carbon dioxide you can provide, unless the species in question has different evolutionary traits.



I think the plant just makes the RuBisCo and doesn't care about anything else ... just whether it is starving or not.

Josh


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