COMPARATIVE EFFECTIVENESS OF SHORT-TERM DAILY TEMPERATURE DROP AND PERIODIC DROUGH AS METHODS TO REGULATE ELONGATION OF CUCUMBER (Cucumis sativus L.)

Daily short-term temperature drop (DROP) and “periodic drought” (non-lethal water defi- cit) are used for plant height control as techniques inhibiting plant growth as an alternative to the use of retardants (chemical growth control). However, it is not known which of these two techniques is more effective and whether their combined effect can be stronger. In this paper taking cucumber as an example we have shown for the first time that a temperature drop technique is more effective than “periodic drought”. Temperature drops combined with “periodic drought” retard plant growth and enhance plant tolerance, but depending on the relative air humidity may decrease values of some physiological and morphological parameters. The aim of this work was: (a) a comparative as-sessment of the effectiveness of DROP treatments and “periodic drought”, and (b) the study of the combined effects of these two techniques on plant growth and tolerance to chilling temperature and water stress. Cucumber plants ( Cucumis sativus L.) were exposed daily to a temperature of 10  С for 2 hours at the end of the night (DROP treatment) for 6 days. The plants were watered daily or watered after the drying of the substrate (once every 2-3 days) creating “periodic drought” (drought treatment). Control plants were watered daily and not exposed to low-temperature treatments. All experiments were carried out at a low (30 %) or high (80 %) relative air humidity (RH). After the termination of the DROP treatments, plants from each treatment (control, DROP, drought, DROP + drought) were subjected to a cold test in the darkness for 1 day at a temperature of 4  С. The plant height, length of leaf petioles, the area and number of leaves and plant dry mass were determined. The compactness of the plants was determined as the plant dry weight or leaf area per unit stem length (in mg/cm or cm 2 /cm). Plant tolerance to low tempera- ture and water stress was estimated by relative electrolyte leakage from leaf tissues and the intensity of lipid peroxidation, as assessed by the content of malonic dialdehyde. Differences between the treatments means were tested with one-way ANOVA followed the least significance difference (LSD) test with p < 0.05 level of significance. The obtained results indicate that DROP-treated plants had more dry mass and leaf area per unit length of the stem compared to those treated by “periodic drought”. However, DROP treatments were effective in increasing plant compactness only under high (80 %) RH, while low (30 %) RH leveled out the effects of a temperature drop. “Periodic drought” can produce small, but not truly compact plants due to a more significant decrease in the leaf area and plant biomass compared to plant size. Thus, a temperature drop is a more effective technique compared to “periodic drought” that can be used to control plant growth and obtain compact plants. The combination of DROP treatments with “periodic drought” also increases plant compactness and besides enhances plant tolerance to water stress induced by low temperature. However, for a number of parameters (number of leaves, compactness of plants at low RH), the combination of DROP treatments and “drought” led to a worse result than the ap- plication of only the first of these two agro-practices.