We studied the apparent quantum yield of photosynthesis in the leaves of cucumber (Cucumis sativus L.) plants exposed to daily short-term (2 h) temperature drops to 12°C in the darkness (in the end of the night) or light (at the beginning of the day). In order to assess the effect of leaf age on their response to DROP, plants were exposed to DROP treatments when the first leaf was non-mature (in the exponential growth phase) or mature, or during the entire period of leaf expansion. It was shown that DROP causes a decrease in the efficiency with which light energy is utilized in photosynthesis regardless of leaf age. It is suggested that changes in the apparent quantum yield under the influence of DROP treatments are associated with changes in the pigment composition. Unlike leaf age, the presence of light during DROP treatments significantly affected the degree of changes in apparent quantum yield of photosynthesis. In particular, DROP in the light caused significantly greater reduction in the apparent quantum yield than DROP in the darkness. Moreover, DROP in the light reduced not only the apparent quantum yield, but also the rate of photosynthesis. DROP in the darkness did not affect the rate of photosynthesis. It is assumed that the decrease in the light use efficiency in DROPtreated plants results from the initiation of plant defense response to low temperatures that allow avoiding photosynthetic apparatus damage.