The chlorophyll a fluorescence transient measured under high light shows a typical O-J-I-P polyphasic rise. However, under certain stress situations such as heat or drought stress, a rapid phase with a maximum around 300 µs has been observed and called K (Guissé et al. (1995a) Arch Sci Genève 48: 147–160). Here, we show that under various conditions, the appearance of the K-step and the following dip, as well as the lowered maximum fluorescence level (FM) attainable, can be explained by an imbalance between the electron flow leaving the RC to the acceptor side and the electron flow coming to the RC from the donor side. This leads to a stable oxidation of the secondary electron donor, the tyrosine Z (YZ), and possibly to the accumulation of P680+. In the case of heat stress, we confirm that this situation is caused by an inhibition of electron donation to YZ, which is due to a damaged oxygen evolving complex (OEC). Finally, we present a model which includes the OEC, YZ, P680, QA and QB which is in good agreement with the experimental data. The appearance of the K-step, under natural conditions, can now be used as a convenient stress indicator and specifically attributed to a damage on the electron donor side.