Effect of different levels of hormonal priming on germination characteristics, time to different percentages of germination, mean germination time, germination rate and seed vigor of Stevia (Stevia rebaudiana Bertoni)

Seed germination is the first plant developmental stage and one of the most important stages in the life cycle of plants and plays an important role in crop establishment. This stage of development is strongly influenced by environmental factors. At between seed improvement techniques, Seed priming is a promising technique for overcoming on associated problems with poor germination and then irregular establishment of crop under normal and stressful conditions. So that, primed seeds tend to show better germination and growth even when imposed to stressful conditions. There are several different methods for seed priming includes hydro-priming and hormone priming. According to the researchers, Stevia rebaudiana is a slender herb. Short day, perennial native to Paraguay and South-West Brazil. It is widely distributed in USA, Brazil, Japan, Korea, Taiwan and South-East Asia. It belongs to the family Asteraceae. Reproduction in the wild is mainly through seed, but seed viability is poor and highly variable. Therefore, in order to investigate the effects of priming on seed germination characteristics of Stevia, this plant selected and in experiment was studied.

In order to investigate the effect of two auxin (indole-3-acetic acid) and cytokinin (Tetra hydro pyranyl benzyl adenine) hormones on germination characteristics of Stevia seeds, experiment was used. The experiment was a conducted in factorial completely randomized design with 4 replications in seed technology laboratory of Shahed University in 2015. The seeds by auxin (IAA) at concentrations of zero (hydropriming with distilled water), 0.1, 0.5, 1 and 1.5 ppm and cytokinin hormone (PBA) in concentrations of zero (hydro-priming with distilled water), 0.5, 1, 1.5 and 2 ppm in combination were primed. Then Petri dishes prepared on contain treated seed, were transferred into the germinator with temperature 25±1 degree centigrade, Relative humidity 70%, 16 hours of light and 8hours of darkness. Daily counting of germinated seeds was performed from second day until germination fixed at a specified time and during the counting, a seed were considered germinated when its radical length 2 mm or exceeds 2 mm. Thus Traits include mean germination time, Germination rate, Germination Percentage, seed vigor and time to different percentages of germination, was measured.

In study effect of auxin (IAA) on germination characteristics of Stevia seeds, the results showed that highest germination rate and lowest mean germination time (3.07 day) in seeds were observed when treated with 0.1 ppm auxin. Lowest time to10, 25, 50, 75and 90 germination percent were recorded with seeds hydro-priming with distilled water. Therefore, hydro-primed seeds than primed seeds, arrived earlier to 10% germination. Also, significant differences not were observed between different concentrations of auxin for percentage and uniformity of germination. In study effect of Cytokinin (PBA), lowest mean germination time in seeds was observed when treated with hydro-priming and 1ppm of PBA hormone. Also Seeds treatment with 0.5 ppm and 1.5 ppm PBA Showed highest germination rate. In study effects of interaction of auxin and cytokinin on germination characteristics of Stevia seeds, Composition IAA+PBA(1.5+1ppm), led to largest germination percentage of seed (66.66%) and thus lowest number of seeds soft. Highest length vigor (with mean 8.13) were obtained in seeds treatment with IAA+PBA (0.1+0 ppm) and lowest length vigor (with mean 2.68) in seeds treatment with high concentrations from hormone of cytokinin (1.5 ppm). on the other hand highest weight vigor (with mean 9.17) were observed in seeds treatment with 0.1 ppm IAA. In general, according to the results, the impact of IAA compared to PBA more was on studied traits in this research. However, hydro-priming also showed a positive effect on the majority of traits. Thus for the production of Stevia seed with germination rate, Speed of germination and high vigor use of auxin in low concentrations is recommended. For achieve more germination percentage combination treatment IAA + PBA (1.5+1 ppm) can be suggested.