Effect of gibberellin and cytokinin growth regulators on some biochemical characteristics and dry weight of stevia plant (Stevia rebaudiana Bertoni)

Communication between cells in organic plants for growth and development is done by chemical messengers called hormones. Meanwhile, GA3 is one of the most well-known plant hormones that cause various growth reactions in the plant, and most of their activity is the longitudinal growth between the nodes. Cytokinin is another type of plant phytohormones whose most important effect is in the process of cell division. In this research, the effect of cytokinin and gibberellin hormones on some chemical compounds and the dry weight of the Stevia plant shoot was studied under controlled conditions. A factorial experiment was conducted in the form of a completely randomized design in the greenhouse of Razavi Khorasan Agricultural Research and Training Center in three replications in 2017. Factors included gibberellin (0, 50 and 100 ppm) and cytokinin (0, 50 and 100 ppm). The average temperature of the greenhouse was approximately 25 to 30 degrees Celsius and its humidity was 65%. Irrigation was done regularly every three days. Before treatment, the potted plants were adapted to the greenhouse environment for two weeks. The treatment was carried out as foliar spraying, in this way, for six weeks and before watering at 10 am for each plant, 20 cc at the beginning and 50 cc at the end of the growth were sprayed with the desired solutions so that the leaves were completely wet. At the end of the experiment, the aerial parts were removed from the ground and placed in paper envelopes inside a ventilated oven with a temperature of 70°C for 48 hours. The samples were weighed immediately after leaving the oven with a scale of 0.001. The results showed that the highest amount of soluble sugars was obtained from the application of 50 ppm of gibberellin along with 100 ppm of cytokinin (1.61 mg/g dry weight of leaves), which showed an increase of about 25% compared to the control. The amount of leaf protein was significant only in the presence of different concentrations of gibberellin hormone. The highest amount of proline was obtained from the combined application of 100 ppm gibberellin and 0 ppm cytokinin (1.31 mg/g fresh weight of leaf), which showed an increase of about 63% compared to the control. Foliar spraying with gibberellin hormone decreased the amount of chlorophyll a. The highest amount of chlorophyll b was obtained from simultaneous application of 50 ppm levels of cytokinin and gibberellin. The lowest dry weight of the shoot was shown in the presence of the combined treatment of two hormones at the level of 100 ppm, and the highest value was obtained at the level of 100 ppm of gibberellin hormone. The results of the correlation coefficients showed that the dry weight of the shoot had a positive and significant correlation with the amount of proline (r=0.63**). It seems that the hormone auxin (acidification and loosening of the cell wall) and gibberellin enable elongation and growth by changing the characteristics of the cell wall. Therefore, based on the results of this experiment, the increase in the dry weight of aerial parts can be attributed to the effect of gibberellin and different ratios of hormones inside the plant. In general, the results showed that gibberellin levels were more effective than cytokinin hormone on the biochemical and morphological indicators of stevia plant; So that the maximum dry weight of shoot was obtained from the effect of 100 ppm gibberellin hormone. Photosynthetic pigments showed different reactions to different concentrations of studied hormones. The effect of 100 ppm concentration of gibberellin on the amount of protein, soluble sugars and proline was significant compared to other levels.