The present study characterised chlorophyll fluorescence transients significantly altered in Arabidopsis leaves upon exposure to gamma radiation for 7.5 days at a low-dose rate of 0.8 Gy day−1 in a gamma phytotron. The JIP transients of Kautsky fluorescence induction were substantially affected by the consecutive exposure to the low-dose-rate gamma radiation. The performance index of photosynthesis (PI), which is based on the JIP transients, was decreased by up to about 40% in irradiated leaves. However, the maximum photochemical efficiency (Fv/Fm) and phenotypic traits were almost the same between the control and irradiated leaves. Based on these results, the PI is suggested to be a sensitive and reliable parameter for physiological evaluation of irradiated plants after exposure to low-dose gamma radiation.