Evaluation of Cardinal Temperatures for Seed Germination of Nepeta glomerulosa Boiss., a Native Medicinal Plant of Iran

The genus Nepeta with the common Persian name “Pune-Sa” is one of the most important genera of Lamiaceae family. It has aromatic plants which are endemic of Iran. Its different species are distributed almost in all parts of Iran. Some of these species are valuable in medicine and used for medicinal purposes. Nepeta glomerulosa Boiss. is one of the most medicinal species of this family. This study was conducted to examine seed germination characteristics and to evaluate cardinal temperature of seed germination of N. glomerulosa.
Seeds of N. glomerulosa were collected from Abade region of Fars province in the middle of September 2014. Treatments were including four levels of cold period (1, 2, 3 and 4 weeks at5ºC) and four levels of potassium nitrate (0, 0.1, 0.2 and 0.3 %). Factorial experiment with a completely randomized design in three replications was done, to evaluate seed germination percentage, germination rate, T50, mean germination time, root and shoot length, root and shoot fresh and dry weight. Also, the effects of 8 constant temperatures including 5, 10, 15, 20, 25, 30, 35 and 40 ˚C and 3 variable temperatures including 5/15, 10/20 and 20/30 ˚C (12h light/12h dark), was investigated to evaluate germination percentage, germination rate, mean germination time and cardinal temperatures of seed germination.
The results showed that cold period and potassium nitrate interaction was significant only on root length. Potassium nitrate had no significant effect on germination rate, but the germination rate was increased by increasing the duration of being at5 ˚C, so that the highest germination rate was observed in 3 and 4 weeks at5 ˚C (1.7 and 1.2 /day respectively). The lowest germination rate (1.0 /day) was observed in 2 weeks at5 ˚C treatment. The maximum shoot length (51.5 mm), shoot fresh weight (30 mg), shoot dry weight (1.3 mg) and root dry weight (0.6 mg) was obtained in 2 weeks at 5 ˚C treatments andpotassium nitrate 0.1% was also the best treatment for abovementioned traits. Among the treatments, 3 weeks cold period and 0.1 % potassium nitrate showed the most positive effect on seed germination characteristics of N. glomerulosa, although, had no significant effect on germination percentage.
Based on the results of the second experiment, temperature had significant effect (P≤0.01) on germination percentage, germination rate and mean germination time. The highest amount of germination percentage (76 and 77.3 %), germination rate (4.4 and 5 /day) and mean germination time (5.1 and 5.4 /day) were recorded in the range of 20 to 25 ˚C. Germination rate and germination percentage were substantially reduced by increasing temperature above 25 ˚C. The day and night variable temperatures had significant effect on seed germination percentage and germination rate of N. glomerulosa seeds. The highest amount of germination percentage (72 and 69.3 %) and germination rate (4.6 and 5.7 /day) were observed at10/20 and 20/30 ˚C. The results of the relationships between temperature and germination rate showed that base, optimum and maximum temperatures for seeds of N. glomerulosa were 15 ˚C, 22 ˚C and 46 ˚C, respectively.
The overall results showed that most chilling occurred under natural conditions during winter and early spring which caused increasing inseed germination of lots of plants. This also found that increasingin chilling period significantly improved the seed germination characteristics of N. glomerulosa. Also, treating with potassium nitrate had significant effects on germination properties of N. glomerulosa. Although applying potassium nitrate and cold period did not show interaction effects on seed germination.
The effects of different day and night temperatures on seed germination rate and percentage have been reported in many medicinal and aromatic plants as may be due to the similarity of these conditions to the natural conditions. Comparing cardinal temperatures of N. glomerulosa with the other species of this genus indicated that they have different cardinal temperatures. Many of researchers reported that germination cardinal temperatures are significantly different amongspecies and different genotypes of species. Determining of cardinal temperatures helps us to estimate temperature range of wild species in natural areas, planting time and also suitable areas for cultivation.