PLANT RESPONSES TO A DAILY TEMPERATURE DROP OF DIFFERENT INTENSITY AND DURATION
The review presents own and published data on the effects of a daily temperature drop (DROP) of different intensity and duration on plants. It is concluded that the effect of a temperature drop on morphological, physiological and biochemical characteristics of chilling-sensitive and chilling-tolerant plants depends to a large extent on the absolute value of the lower temperature used for DROP treatments. The morphogenetic effect of DROP treatments, measured by changes in linear growth parameters, increases with the lowering of the temperature used for DROP treatments, while the plant response, estimated by other physiological and biochemical parameters (biomass accumulation, chlorophyll content, photosynthetic activity, water relations indices, chilling tolerance) depends more on whether the temperature falls below the critical level, resulting in a plant stress response. The response of plants to DROP treatments depends more on the absolute value to which the temperature is dropped and on the duration of the chill (since in this case the stress response is triggered in the plant) than on the temperature gradient. The duration of the temperature drop becomes more important when the temperature is lowered to values beyond the optimum range. The optimum temperature ranges for different processes (characteristics) (for example, plant height or time until flowering) can vary significantly, determining the nature and effectiveness of DROP treatments with certain parameters in relation to various processes (characteristics). In most cases, the differences in plant response to DROP treatments of different duration are not determined by the usual dose-effect relationship, since beside the duration, the mode of temperature exposure (single long-term or daily short-term) plays a significant role. In addition, the effectiveness of the DROP treatments of a given intensity and duration for practical purposes in horticulture and floriculture is largely species- (variety-) specific.