Temperature and light are important factors determining growth and development of plants. The difference between day and night temperature (DIF – defined as day temperature minus night temperature) influences plant morphology in a wide range of species. In this study, the effect of photoperiod and DIF on stem elongation and growth of dill plants was investigated. The first experiment included a comparison of different temperatures between day and night (DIF-5 – 20/25°C, DIF0 – 20°C and 25°C, DIF+5 – 25/20°C) for a 16-h photoperiod. The second experiment comprised the photoperiod of 16 and 24 hours for two constant temperatures – 20 and 25°C. Significant differences between DIF+5 and DIF-5 were observed in the case of plant height. Elongation growth was enhanced by positive DIF and was inhibited by negative DIF, although the strongest stem elongation inhibition was recorded at the temperature of 25°C and a 16-h photoperiod. Leaf area was greatly affected by the temperature and DIF, but only slightly influenced by the photoperiod. Leaf area in DIF-5 was smaller than that in DIF+5 or 0 (20°C). The highest essential oil content in the herbage was recorded for a constant temperature of 25°C as well as 20°C and a 24-hour photoperiod.

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