Black leaf-clips increased minimum fluorescence emission in clipped leaves exposed to high solar radiation during dark adaptation

Tomato and pepper leaves were clipped with black leaf clips for dark adaptation under solar radiation in the late spring or early summer 2010 in southern Italy. The leaves showed highly variable maximum PSII quantum yield (F-v/F-m = 0.026-0.802) using a continuous-excitation fluorometer Pocket PEA. These results were confirmed using the modulated fluorometer FMS1 on tomato leaves in mid summer, with F-v/F-m as low as 0.222 +/- 0.277 due to nearly equal minimum (F-o) and maximum (F-m) fluorescence emission. A significant clip effect on F-v/F-m occurred after only 12 (tomato) or 25 (pepper) min. Increasing the leaf temperature from 25 to 50 degrees C reportedly induced an F-o increase and F-m decrease so that F-v/F-m approached zero. The hypothesis that black leaf clips overheated under intense solar irradiance was verified by shrouding the clipped leaves with aluminum foil. In clipped leaves of pepper, F-v/F-m with the black clip/Pocket-PEA was 0.769 +/- 0.025 (shrouded) and as low as 0.271 +/- 0.163 (nonshrouded), the latter showing a double F-o and 32% lower F-m. An 8% clip effect on F-v/F-m was observed with the white clip/FMS1. To avoid the clip effect in high irradiance environments, F-v/F-m measurements with black clip/Pocket PEA system required leaf dark adaptation with radiation-reflecting shrouds. It would be useful if manufacturing companies could develop better radiation-reflecting leaf clips for the Pocket PEA fluorometer.