light intensity for Paph

[naoki]

I came across this paper:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1066343/

It basically measured the net photosynthetic rate of Paph. insigne for different amount of light. I think it is an open access paper. There are lots of jargons, but Fig. 1 is pretty obvious.

The best amount of light (photosynthetically active radiation; PAR) was around 100 micro mol/m^2/s. The photosynthetic rate gradually start to decrease above this point (this happens with most plants).

Now, most people don't have the expensive PAR quantum meter, but you can use cheap light meters (in foot-candles or lux) to get approx. PAR. Here is an approx. conversion between lux and PPFD (photosynthetic photon flux density) for different types of light sources:

http://cpl.usu.edu/files/publications/poster/pub__6740181.pdf

You can use the column "PPF/klux" column (under "Ratios important for Photosynthesis"). For example, with cool white CFL, the table list the conversion factor of 12.4. So if your light meter says 8064 lux (=8.064klux = 749 foot-candles), the CFL is giving 100 micro mol/m^2/s (= 8.064klux * 12.4).

For greenhouse people, the conversion factor is 16.2. So dividing 100 micro-mol/m^2/s by the conversion factor 16.2, you get 6.173 klux = 6173 lux =573 foot-candles, which will give 100 micro-mol/m^2/s of PAR.

To put this in context, the summer sun around noon has about 2000 micro-mol/m^2/s of PAR and 10000 foot-candles. So the saturation point is about 1/20 of full sun.

If you don't have a light meter, and if you are using 6500K T5HO, you can check out my measurement here. With 4 bulb T5HOs, you can put top of the leaves within slightly less than 1-foot (595 foot-candles @ 1 foot).

I'm posting this because I was a bit surprised how low the light saturation point of Paph is. I didn't realize that I was giving a bit more light than they need with my artificial lights.

Photosynthetic rate depends on many environmental variables (and age of leaves etc), so one should think that the study is giving rough estimates, though. For example, the article also points out photosynthetic rate is dramatically reduced with lower RH (Fig 2). Also the article points out how low the photosynthetic rate of Paphs (and other orchids like Cymb. and Phrag.) are in comparison against other shade plants like Viola.

Obviously, different species of paphs have different saturation points.

lux and foot-candle can be easily converted each other with this web page

 

[TyroneGenade]

Read the methods and materials sections. Humidity was not kept constant. The stomal closing could have been a consequence of lower humidity. With the stomata closed, the supply of CO2 would be cut off. In these experiments photosynthesis was measured by CO2 fixation. The other issue is that the plants were grown under low light. Under high light they may display a greater photosynthesis rate. "10% shade" isn't a very useful measurement. That would be 2000 micromol/m2/s in the Amazon but only 50 on a sunny winter's day in California. Grown under brighter light, and with constant humidity, the plants may perform better.

But, an interesting paper all the same.

 

[naoki]

Tyrone, they have controlled the RH, so VPD is set to be 0.5kPa. In the 2nd experiment, they varied the VPD to see the effect you are talking about.

Stomata opening is measured in stomata conductance. Paphs are unique because the guard cells of the stomata don't have chloroplasts (Phrags and Cyps do have them), and measuring the stomata response was probably one of the motivation of the study.

You are right some plants show plasticity (e.g. in chloroplast concentration), and we don't know the condition under which the leaves were formed. But I assume that nursery would be using typical "paph condition".

Also, even with CA in winter, you do get around 1000 micromol/m^2/s (not 50) at noon.

http://clearskycalculator.com/quantumsensor.htm

Here is a handy calculator to get approx PPFD from the sun for different location and time.

 

[TyroneGenade]

Hi Naoki,

Yes, they seemed more interesting in the stomata than photosynthetic response. The RH was constant in the green house but they say it was changed in the experimental chamber. They don't declare what it was and how it changed (very sloppy reporting but quite typical in the scientific literature). As you say, the VPD was 0.5 but plants do best at 0.8 to 0.95 so the plant wasn't happy... We shouldn't take the photosynthetic response too seriously...

10% shade would reduce the 1000 micromol/m2/s to 100. The plant is being grown in low light to start with. Higher light would quickly saturate the photosystems.

Still, as I said, this is interesting data all the same. It is a good reminder that light isn't everything. We need to maintain proper humidity.

 

[consettbay2003]

My reading at one foot below a 4 bulb T5HO fixture is well above 595 fc.

 

[naoki]

Tyrone, you have the direction of VPD screwed up a bit. oke:

VPD of 0.5kPa means RH of 78.6% (@20C). 1kPa and 1.5kPa corresponds to RH of 57.2% and 35%, respectively. If you look at the figure 2, you can see photosynthetic (PS) rate seems to saturate around 78.6%. When RH drops to 57%, PS rate drops by 25%. When RH drops to 35%, PS rate is about half of the maximum rate. These drops are obviously coming from closed stomata (lower stomata conductance). So if you meant that plants are not happy when RH (not VPD) is 0.5, the data is supporting it. But the irradiance curve is measured at high RH.

They are probably using open-system CO2 gas analyzer (Li-cor LI-6400XT is one of the standard, but they used something older than this). Here is some info from wikipedia. When Polyantha was measuring photosynthesis under different florescent light in this thread, he was using a closed-system. One issue of closed system is that RH of the chamber changes (leaf releases H2O through stomata). With open-system, you are flowing air around the leaves with known amount of CO2 and H2O. Then you measure the CO2 and H2O of the air coming out from the chamber. The difference in CO2 concentration between incoming and outgoing air gives "net" photosynthetic rate. The difference in H2O concentration gives the stomata conductance. Therefore, the RH is well controlled by the open-system machine. This is how they can change VPD.

You said that the plant is being grown in low light to start with. Where did you get that? It says "the plants were shaded to avoid high irradiances, which were observed to cause leaf damage". There is no more detail, but that's how we grow most paphs, right? (BTW, I'm not trying to be argumentative. Just trying to get things straightened up, so it's not misleading.)

Consettbay, I don't know what is causing the difference. What fixture, bulbs, and light meter did you use? And what was your measurement?

I used hydrofarm 2-feet fixture with 24W, 6500K bulbs (about 6 months old at that time). The quality of the reflector would influence, and also the distance between bulbs could be different (if your bulbs are closer, it will give higher fc).

It could be the difference in the light meter. Here, they talk about the difference in the spectral response/sensitivity:
http://www.otc.co.uk/Problem_With_Lux_Meters.php
With florescent light, which has only 3 major peaks, the difference in the spectral response could make a big difference. I didn't know this until you pointed this out.

My light meter is an old one (Gossen Ultra Pro, which was really expensive at that time). The spectral response of my meter doesn't seem to be the best; this person measured it with a meter which has the identical sensor to mine:
http://faculty.mssm.edu/brodis01/profile_files/flash_photometry.html
Mine responds to blue and red light more than it should be. But this bias of Gossen will cause a higher value of fc than it should be with florescent light.

I do tested my meter against Nikon D300 and Sony NEX cameras with a 18% gray card etc., and they give approximately similar values. But I can't test 10-20% differences in this comparison.

Sorry, it's probably too much details. But I'm interested in hearing your measurement and set-up (I might need a new light meter).

 

[Ray]

I used a $40 Hydrofarm light meter under a single one of their 24" 24W "high performance" T5HO lamps (specular aluminum reflector), and saw this:

 

[naoki]

Thanks, Ray. So my meter seems to be 20-30% lower.

I spent a bit of time reading about photometric light meter. This is a pretty interesting page:
http://www.olino.org/us/articles/2009/12/07/measuring-illuminance-correctly

It basically shows that consumer grade light meter could be really off with some types of light (e.g. florescent and LED). This is basically coming from the deviation in the spectrum sensitivity. There is a standard curve, V(lambda), to define luminosity (fc and lux). However, only high-end ones (Konica-Minolta T-10, Li-cor LI-210, etc.) have the sensitivity which is close to the ideal V(lambda), and these cost around $1000. Some of the consumer grade light meters are tweaked to suit their purpose (e.g. to match the film emulsion response). Mine is for photographic purpose, so it is probably calibrated to a different way. I also learned Quality Light Metric in Hollywood is a good place to get calibration. I thought that the modern Silicon photodiode based light meter doesn't need calibration much, but this may not be the case.

I've thought that a photometric light meter is pretty straight forward, but it's not the case. It's similar to the case with PAR Quantum meter, where the cheaper ones are ok for general use, but it is problematic for certain types of light.