This sounds like an issue with your calibration. A bit more information would be helpful - what system was this obtained with? Li-6400?

Zeroing the water (calibration) is a time consuming process...it could also be a problem with the seal of the cuvette, or an issue with the leaves being sampled. Were they damp at all?

Do you mean negative H2O content in the air or negative transpiration rate?

In either case, I will suspect that there is a problem with your measurements. Repeat the measurements and make sure that the device is properly calibrated for H2O (air with absolutely no water should give a zero measurement). Also make sure that the leaf or any part in the equipment is NOT under the dew point temperature.

This sounds like an issue with your calibration. A bit more information would be helpful - what system was this obtained with? Li-6400?

Zeroing the water (calibration) is a time consuming process...it could also be a problem with the seal of the cuvette, or an issue with the leaves being sampled. Were they damp at all?

Yes, we are using Li-6400. As I been informed, we send the equipment to US for factory calibration every 2 years. We did the standard check up before the measurement (standard procedures). The leave sample was dry and no indication of moisture on the surface. Ambient shown negative H2O and repetition of the measurement also recorded the same results (not all but majority) .

I still do not understand which parameter is appearing as negative. Can you tell us the name and/or abbreviation of the negative parameter as it is described in the manual?

This raw data probably describe my question. Please see the attachment. Thanks.

The negative number highlighted looks to be the photosynthetic rate - your plant is respiring. This is not a water parameter (your conductance is positive).

Perhaps a silly question - did you turn the light on?

Yes, Light Source=6400-02 or -02B LED Source.

Anyway the measurement was from Sweet Potato leave. (Day measurement).

But do you have the PARin values in your file?

Please find the attachment.

Hi there again,

I had a look to the file. As Weston Testo suspected, the lamps are OFF. You can see that the "PARi" measurements are virtually zero in your file. If you did order the Li-6400 to turn ON the lamp, then I will suspect that there is a problem with the configuration. I also see that you are using what it looks like a custom configuration named "LED2x3". Make sure that the light source in such configuration is properly set.

One more thing, you are using a flow of 500. Depending on the photosynthesis rate at the lower CO2 concentrations, that might be too high. For best results, you should always adapt your flow rate to your photosynthesis rate at each CO2 and light levels. If you photosynthesis rate is low, it is better to use a lower flow rate. Just to keep a good signal to noise ratio. Of course, do not go too low neither.

Success!

Thank you so much for the answer and it sounds logic! I will inform my friend about this, probably should check setting correctly. Thanks again!

Just to confirm what Aaron said, the problem is simply that the plants weren't getting any light. Make sure that the lamp is on BEFORE you clamp the leaf in the cuvette. IF you did that, as Aaron said, you may have a configuration problem, but I find that it is easy to forget to turn the light on. An easy fix.

The flow is potentially, but not necessarily high. I tend to think that your sweet potato plants will have a pretty high photosynthetic rate and you probably are fine at 500, but maybe 400 is better.

Good luck!

Okay, thank you friends. I will inform his technician about this.

It is difficulty to comment on data logger outputs without knowing exactly whether the settings were optimal or not. However, given the air and leaf temperature outputs I am of the opinion that these were too high for a C3 plant to photosynthesize efficiently. Remember also that the time was between 13 and 13:30, usually the rate of photosynthesis could be limited by high air and leaf temperatures because of increased stomata resistance and hence, low CO2 uptake. At this particular time and temperature, respiration rate was higher than the rate of photosynthesis. This information is coming from observations I made on your data, so I cannot be so sure about that. I hope this information will give some insight into your observation.

I will take your comment for my recommendation for him as his technician will sample it again with settings checked. Probably, with the next measurements we can conclude all the possibilities. Thanks. (Next sample will be in the middle of January 2014)

When you work with plants under effects of combined drought stress and cloudy day,  you can see - (minus) transpiration. With sun, stomata will open and increase the transpiration rate.  I agree with above the researher, it can also be needded calibration in time.