İt is generally between 11:00 - 14:00 except special characters. You can find detailed information in "Physiological Breeding II: A Field Guide to Wheat Genotyping - CIMMYT Center"

Hi,

it depends on the species and on the hypothesis you are working on. Are you interested in maximum gas exchange levels? Are you looking into stress responses? I would suggest following the daily pattern of the species you are studying first, with measurements every two hours between 7 am and 6 pm approximately, so you can see when maximum levels occur.

Best regards,

Elisabetta

Depend by solar zenith angle (in the summer after midday)

Best regards

Emanuele

In my expérience with orchidaceae, it is after 16-17h

It depends on your objectives, Also if your interests are focused on maximal or critical values, In some environments did not match maximum response of each response (i.e. On the other hand, the response are variable according the species due mainly stomatal control.

Agree with Andrea. Your objective is depended. If you want to get the maximal photosynthetic rate, the measure should be taken around 10:00 am. If you want to get the diurnal variation curve, the measure should be taken from sunrise to sunret, maybe 6 am to 6 pm, with 2 hours as an interval. Transpirate rate should be measured after sunset at least 0.5 hour. we can get both photosynthetic rate data and stomatal conductance data using photosynthesis system from Licor or PP Systems in a single measurement.

Its better to measure at 09:00 am to 11:00 am.

Several studies recommended to conducted gas exchange measurements using Licore photosynthesis system between 11:00 am and 1:00 pm

http://journal.ashspublications.org/content/139/5/537.full.pdf+html

good luck!!

Basically agree with Hua, we have made our mesaurements with the Licor photosynthesis system in the morning between 7.00 and 10. 00 am in springtime and summer (during growthing period of cereals). We investigated Photosynthesis and so on in diseased and healthy leaves to assess the influences of a disease on it. Thus, the comparison between healthy and diseased leaves was our aim. But , what ever you will do, you should undertake your measurement always at the same period of the day! Best regards Petra

I agree with Elisabetta, Andrea and Hua. Make a diurnal measurement on a clear day and then see which part of the day you would like to study in your project.

I agree with Elisabetta, Andres, Hua and Jose, its awesome to indicate the best time by takeing samples with intercals for maximum and best time for sampling according the study goal.

Hi,

you can measure at 3 to 4 AM, each :Photosynthetic rate, transpiration rate. because, we used it for Medicago sativa and Festuca arundinacea from 5 to 40°C in the green house. Unfortunately about ;stomatal conductanc.

Thank you.

with pleasure if you want we have one pulications about it

Yes, if possible.

I concur with majority "IT DEPENDS" responses. Two cautionary points though:

1) it is very tedious trying to do such measurements from dawn to dusk with instruments like the one you are using. If you want to go the 'whole day' route, opt for a simpler and faster hand-held instrument; some of these will give you more data than you need (which is a good thing), including instantaneous/ intrinsic transpiration rate.

2) taking physiological measurements over extended periods as suggested by some above, is wrought with massive errors, especially due to weather, and to a lesser but critical extent, fatigue of both the instrument and the data collector. From my experience, I would suggest you redifine your objective to confine your study with narrow time windows, consistently over many days. If you have staggered planting dates/ phenological phases, and about 4 replicates and data taken in a completely random manner, each day, the better. Data collection method is the most critical point of all physiological processes I have undertaken with any photosystem instruments.

the measure should be taken around 10:00 am.

Between 8-10 am most stomatas will be open, and this also minimizes high afternoon temperatures.

When the sun reaches the zenith (directly overhead) at midday

Best regards

9-11 am is the best time for measurements

9 to 11:30 AM you can measure the physiological parameters but depends on sunshine and weather condition.

Dear Benhmimou,

It totally depends on your hypothesis species and the region's climate to decide when to take the physiological readings. This is because the all the physiological parameters you have mentioned are highly sensitive to environmental condition, and may instantly change with the changing environment. So "when to take readings" may be best defined by what you actually want to record.

For example if you are working on alpine sub alpine or temperate region of mountains, peak time of available sunlight would be be between 12-2pm, you may record the reading either diurnally or mid day depending upon your your objective. While at subtropical or tropical it is experienced the peak time for optimum available sunlight would be start even at 9 or 10 pm only.