Analysis of light-induced reduction of the photochemical capacity in field-grown plants. Evidence for photoinhibition?

It was tested whether field-grown plants (Phaseolus vulgaris, Zea mays and Helianthus annuus) reflect photoinhibitory effects under natural conditions. Attached leaves were used for determination of the photochemical capacity of Photosystem II (Fv/Fm) by means of a portable fluorimeter (PSM, BioMonitor, S.). For a more qualitative description of Fv/Fm, the modifications of the absolute values F0, Fm as well as of the half-rise time of Fm (T/2) were also considered. By comparing artificially shaded and 'sun exposed' plants, the direct influence of light on the photochemical capacity was investigated. Under low natural light conditions the differences of the photochemical capacity between shaded and 'sun exposed' leaves were negligible in all three species. On a day with full sunlight a decline of Fv/Fm was observable at noon-time in the 'sun exposed' leaves of all three species, although the absolute values differed between the species compared. Additionally, the extent of the recovery of Fv/Fm was varying. Both phenomena could be due to differences in the photosynthetic apparatus (e.g., C3-C4, ontogenetic stage, sun-shade type), to self-shading phenomena (comparing leaf layers of Zea and Helianthus) or to differences in the activity of repair mechanisms possibly caused by other environmental factors (vapour pressure deficit = VPD, drought and temperature phenomena).Nevertheless, the results of the shading experiments and the comparison of species lead to the conclusion that primarily light-induced reduction of the photochemical capacity appears at noon in leaves exposed to full sunlight, a partial restoration of Fv/Fm takes place till the evening. Artifically shaded plants show only a slight alteration of the photochemical capacity.