To elucidate the effect of ionizing radiation on the non-photochemical quenching (NPQ) of chlorophyll fluorescence, we analyzed the buildup and release of NPQ in Arabidopsis wild-type (WT) and npql-2 mutant plants after gamma-irradiation. The npq1-2 mutant cannot normally induce the buildup of NPQ by a mutation in the violaxanthin de-epoxidase gene. A dose of $50;Gy;h^{-1}$ for 4 h significantly suppressed such buildup in the mutant and, more noticeably, in the WT. Both the initial rise and maximum level of NPQ were gradually inhibited after gamma-irradiation. In contrast, the release of NPQ and the maximum photochemical efficiency (Fv/Fm) of Photosystem II were largely unaffected in either genotype. This inhibition of NPQ buildup could be partly attributable to a significant decrease in the content of carotenoids, including xanthophyll pigments. Moreover, inhibition that was dependent on the xanthophyll cycle substantially enhanced the sensitivity of irradiated leaves to a photoinhibitory illumination of $800;{mu}mol$ photons $m^{-2};S^{-1}$. The difference in Fv/Fm values between the WT and npq1-2 under that photoinhibitory level of illumination was much smaller in the irradiated leaves than in the control. However, NPQ inhibition did not cause a significant difference in efficiency between WT and mutant when both were treated with UV-B irradiance of $2.4;W;m^{-2}$. Therefore, we suggest that a significant decrease in carotenoid content after gamma-irradiation should partially contribute to the enhanced sensitivity of irradiated plants, at least to high-light photoinhibition. This is accomplished by suppressing the thermal dissipation of excess light absorbed by photosynthetic pigments.