About 10 years ago, I experienced that the black leaf clips used with Pocket-PEA fluorometer (firmly suggested by Hansatech vs. the white version used for Handy-PEA), when used under high irradiance such as in hot summer of mediterranean area, by artefact, strongly underestimate Fv/Fm.

This is mainly due to increased leaf temperature (caused by warming-up of the black locating ring of clip) which increases Fo.

I published two papers confirming previous works by other authors (I also measured leaf temperature underneath the clip).

1) PHOTOSYNTHETICA 49 (3): 371-379, 2011

2) PHOTOSYNTHETICA 50 (3): 467-471, 2012

I am experiencing again same troubles with grapevine leaves.

I really would like to know if other researchers have ever got same results, and why it seems that Hansatech really does not care about it. It would be a pleasure to realise I just do something wrong.

The attached file shoes Fo, Fm and Fv/Fm of leaves of grapevine "Greco di Tufo" under rainfed conditions. In my opinion The very low or nil values fo Fv/Fm are caused by increased leaf temperature caused by the black leaves, but I hope I am wrong and some body can explain me how to solve the problem.