The role of light in cucumber plant response to a diurnal short-term temperature drop

In greenhouse production a temperature drop concept has become an important tool to control stem elongation of a number of ornamental plant species, bedding plants and vegetable transplants. This concept takes advantage of the fact that, in many species, plant stem elongation is sensitive to a short diurnal temperature drop in the morning or in the end of the night. Here, the role of light in cucumber plant ( Cucumis sativus L.) response to a diurnal temperature drop has been investigated. Plants were grown under relatively “low” (120 µmol m-2 s-1 PPFD) or “high” (320 µmol m-2 s-1 PPFD) light intensity. The effect of a 2 h temperature drop from 23° to 9°C, either in the end of the night (in darkness) or in the beginning of the day (in light) on plant growth, photosynthetic and respiratory activity was studied. Light-demanding and shade-tolerant cucumber hybrids were used in experiments. The results showed differential temperature regulation of plant morphology and photosynthetic activity in light and darkness. A temperature drop in the light led to more pronounced decrease in biometric and photosynthetic parameters of plants than a temperature drop in darkness. Light intensity during plant growth also modified plant response to a temperature drop. In particular, a temperature drop in the night led to a decrease in photosynthetic activity only in plants grown under "low" light. The respiration of the leaves, on the contrary, was intensified as a result of a temperature drop in plants grown under “high" light. Light-demanding and shade-tolerant hybrids had generally similar response to a temperature drop, but some differences were also revealed. Thus, the degree of light inhibition of mitochondrial respiration decreased due to a temperature drop only in the leaves of light-demanding hybrids. They also demonstrated greater reduction of photosynthetic pigment content in response to a temperature drop compared to shade-tolerant plants. This suggests that plant response to a diurnal short-term temperature drop depends to large extend on the timing of the drop treatment (in light or darkness) and light growing conditions. The degree of plant light demand may also be important.