I have measured midday leaf water potential and stomatal conductance of tree seedlings during summer drought in a region with a mediterranean climate, under 1) dense canopy cover; 2) intermediate canopy cover; 3) open conditions. As expected, leaf water potential measured in the open was lower than in the other environments, likely as a response to lower soil moisture. However, stomatal conductance in the open remained at high levels even in seedlings that died shortly after the measurements, I presume due to cavitation. What could explain that these seedlings kept their stomata open, despite their low water potential?
How high did the air temperature get? It is possible that the plants were attempting to cool their leaves through evaporative cooling. What was transpiration doing when you made these measurements?
Thanks a lot Maegan. Actually I think I found the answer to my question in this article by Urban et al. (doi:10.1093/jxb/erx052), which apparently describes such a response in poplar and loblolly pine. I did not measure air temperature precisely, but at least near the floor it reached above 40ºC in some sites, so I think think stomatal opening for cooling despite low water potentials might have well been a possibility. The mean stomatal conductance measured in the open was 332 ± 203 µmol m-2 s-1.
Klaus, the first thing that comes to my mind is that the control of stomata opening in your species might have a strong influence of light and either the total amount or the spectra available in the open plots may be causing the conductance to be higher than you expected.