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With how much light intensity does the plant start photosynthesis?

Cornelius

Cornelius

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Joined
3 Nov 2015
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Location
The Netherlands
Food for thought;

Before the light goes on it is never 100% dark in the tank because there is always natural light.
To what extent does this natural light influence the assimilation of the plant?
When does the plant start assimilating? Light intensity 5%, at 10% at 20%?

And suppose, hypothetically, if the plant assimilates at 25% light, the CO2 values must also be 25% of the recommended dose of 30ppm..? I'm aware that photosynthesis mechanisms are disabled at night an it takes about 1/2 hour or so for the systems to be enabled and functioning again.
But what light intensity do we need need to start assimilation?

Comments, thoughts please
 
Well we can no use percent as it's light unit dependant.
Different plants do have a minimum PAR they need to stay alive and a Max PAR they can handle before they hole/melt which is dependant on the [CO2] and localised flow.
Some of my lights start at 5% but I have them maxing out at 65% whilst my T5s are on or off.
I take about 40mins to reach 65% with it ramping slowly except when the T5s come on when it jumps OFC

Very low light intensity is good for two things
1. Extended viewing
2. Algea - they will use very low PAR
 
Search the forum for "light compensation point", especially posts by CEG4048 for discussion of minimum light levels necessary for plants to start photosynthesising.

It actually quite high, as seen by the human eye, before even "low light" plants start synthesising.
 
slide_32.jpg
LCPSun_ShadePlants.jpg

http://i878.photobucket.com/albums/ab345/joyban70/LCPSun_ShadePlants.jpg (nice link, worth a read)

Can't find actual numbers on fresh water plants this fast.
This is about coral:
image017.png
 
Cor said:
When does the plant start assimilating? Light intensity 5%, at 10% at 20%?
Click to expand...
Percentage is not a relevant term in this context. In absolute terms, the value of the incident light intensity must be weighed against the value of the sugar produced by that intensity, measured in Photosyntheically Active Radiation (PAR) measured in micromoles per second per square meter.

When the value of the sugar produce by the incident light is equal to the value of the sugar being burned by the plant in order to feed it's cells then that light intensity is called Light Compensation Point (LCP).

LCP for typical aquatic plants are at a PAR of about 10-20 micromoles.

Cheers,
 
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Hi Marcel,
The references you list are more focused toward CO2 Compensation Point as opposed to LCP, but it demonstrates the fact that one cannot easily discuss light without also discussing CO2. That is specifically because the energy derived from light is used to remove the Carbon from CO2 in order to fabricate the Phosphate sugar (G3P) which ultimately is converted to glucose which feeds the plant cells.

A more relevant example of LCP experiments, and a slightly easier read, can be found here: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-89132003000200011
In this study, the species Egeria najas was used to determine it's LCP.
E. najas was collected from the Itaipu Reservoir located at the southern border of Brazil and Paraguay.

Given that the response of growth and photosynthesis to light depends also on temperature and CO2 (Barko and Smart, 1981; Madsen and Sand Jensen, 1994), experiments were carried out under conditions approaching those of the locations where E. najas was collected.
Click to expand...

You can see (in plain English as opposed to techno-babble) in the results section that LCP for this plant was measured to be 6 micromoles and that light saturation occurs at 50 micromoles. So even though they could keep the plats at 120 micromoles, as far as growth rates, it was more or less a matter of diminishing returns.

The author goes on to concluded that these numbers would be typical of any of the species native to that particular collection site.

Certainly these values cannot be automatically extrapolate to all the 400 or so species available to us, but I have seen other studies of more familiar species where the numbers are not radically higher.
This also illustrates the hobbyists fret too much over "not having enough light".
Beginners often are convinced that they need to "upgrade" their light, and the results are often catastrophic.

Cheers,
 
Superb links and discussion everyone. Thanks!!
 
As an add on, if you have access to a PAR meter, then you can measure the levels at various light 'levels' and see.

I have a tank in my office and had to reduce photoperiod to 4 hours to avoid algae. It looks super dim to me because the office is quite bright. It's very difficult to judge levels of light.
 
Hi Ceg,

Thanks for the link and explaiination.. I was actualy searching for articles that i know are posted before in a simular discussion a few years back.
One had a list with several plant species and their LCP among them Elodea. I thought with lcp and plantname i might find it back and stumbled upon a lot others instead. But i know the links to these papers dwel somewhere in the ukaps annals. Unfortunately i didn't bookmark that thread. The member posting it was MarcelG, He studied the mater (aquatic plants and algae) at a university i believe, he had a very interesting website about the subject back then, with articles, data and microscopic pictures etc... I believe it was in Chez language.and never translated..

Dunno what happened but all of a sudden he was gone, he changed his nickname or his account got deleted. Also his personal website is no longer active and off line for several years now. Well i remember him being quite sensitive and fiercely defensive and got involved in some flaming discussions. That might be the reason for him leaving. But that i do not know.

I only remember he's gone but posted quite some intersting scientific articles and datasheets about the mater.. And i can not find it back.. This thread contains a plant list and LCP data.

@LondonDragon might be able to find back what he changed his nickname into, it was MarcelG.. But this member name no longer excists. 🙂

I can't be the only one that remembers this thread? I remember more vividly because we share first name i guess..
 
@zozo , there was a fellow who was studying plants at home (with reasonable methodology) but kept the results behind a paywall. He was getting funding from some local hobbyists and argued his case but didn't want to show us his results.

Is he the one who you were talking about?
 
rebel said:
@zozo , there was a fellow who was studying plants at home (with reasonable methodology) but kept the results behind a paywall. He was getting funding from some local hobbyists and argued his case but didn't want to show us his results.

Is he the one who you were talking about?
Click to expand...

No idea, remebering his website and all content it was rather very professional looking. It didn't look as a one mans job. WIth a lot of data and microscopic pictures. As said it was in Chez language or affiliated langauge. He indeed did a lot of test setups monitoring light and CO², probably also at home.. He was also the one claiming to have excact same positive result on using long term 15ppm CO² instead. And i remember him trying fiercly to debunk a lot of theories. A bit too fiercly sometimes and threads sometimes got a bit flamming unfortunately.. And than he dissapeared after changing his member name first...

And trying to find back threads from a changed name? I run into a wall.. But dispite the sometimes flaming nature, he posted some very interesting scientific datasheets and articles, fieldstudies etc.
 
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