Effect of Jasmonic and Salicylic Acid on Antioxidant Enzymes, PAL and Phenolic Compounds in Stevia rebaudiana bertoni

Salicylic acid and jasmonates (chemical elicitors) are considered as key signaling compounds in induction process, which leads to accumulate of secondary metabolites. External uses of these compounds cause to induce pseudo stress in plants and excites defensive replies in plants, in response to induction of oxidative stress, the plant increases amount of antioxidant genes expression and increase enzymatic activity and non-enzymatic anti oxidants concentration (they often have a medicinal aspect).

The present study investigated the effects of jasmonic acid and salicylic acid on hydrogen peroxide content, electrical lockage, GPX, GST and PAL activity, total phenol, flavonoid and anthocyanin changing in Stevia rebaudiana bertoni under field conditions. The experiment arranged as a randomized complete block design with 12 treatments and three replications in 2015-2016 at Amol city in Mazandaran Province in Iran. Experimental treatments were spraying by different concentration of jasmonic acid, salicylic acid and components of jasmonic acid- salicylic acid. Foliar application started after plant establishment in vegetative phase. Each experimental plot was 2 m× 3 m consisting 10 rows with 30 cm row spaces and seedling transplanted on 15 May. At the end of foliar application, sampling was done for the measuring. Sampling for biochemical analyses from second fully developed leaf was done and freezed in liquid nitrogen, then quickly carried out to laboratory.

The analysis of variance showed that different concentration of jasmonic acid and salicylic acid and spraying both of them in 7-day intervals appeared to be effective (with 99% confidence) on studied traits (data not shown). Spraying with jasmonic and salicylic acid increased hydrogen peroxide content in 9 treatments compared to the control. The highest amount of hydrogen peroxide content in compounds treatment 20 ????M JA -1 mM SA with mean of 5.46 ????mol in fresh weight observed. Plant in response to jasmonic and salicylic acid spraying increased GPX activity in 10 treatments and GST activity in 7 treatments compared to the control and follow them electrical lockage in most treatments (10) was lower than control. The highest amount of GPX activity in treatments 5 ????MJA and 5 ????MJA with average of 0.11 ????molmin-1g-1 FW observed. The highest amount of GST activity was obtained from treatments of 0.5 mM SA -5 ????M JA and 1 mM SA with average of 0.35 ????molmin-1g-1 FW. The PAL enzyme activity (the first enzyme in phenyl proponed compounds biosynthesis pathway) in 10 treatment increased. The highest amount of PAL activity was in compounds treatment of 50 ????M JA -1 mM SA with average of 1140 ????mol cynamic acid mg-1 protein min-1. The PAL enzyme activity had correlation with hydrogen peroxide concentration and GST activity.  The PAL enzyme initiates a phenylpropanoid route that converts L-phenylalanine to trans-cyanamide acid deamination. The PAL enzyme can consider as an antioxidant enzyme because it has the role of depositing oxygen radicals through phenolic compounds. Spraying increased total phenol content in 8 treatments compared to the control. The highest amount of total phenol content was observed in compounds treatment of 20 ????M JA -1 mM SA with mean of 423.7 mgg-1 FW. Spraying with jasmonic acid and salicylic acid increased flavonoid concentration in 9 treatments compared to the control. The highest amount of flavonoid content was in treatment of 20 ????M JA -1 mM SA with mean of 110 mgg-1 FW. Spraying increased anthocyanin concentration in 6 treatments compared to the control. Anthocyanins are the most important group of natural pigments after chlorophyll that are involved in light protection. The noticeable point is that in most treatment PAL enzyme activity, total phenol, flavonoid and anthocyanin content in compound treatment increased.

The highest amount of total phenol and flavonoid content observed in compounds treatment of 20 ????M JA -1 mM SA and anthocyanin increased in total compounds treatment and using PAL enzyme activity had correlation and significant effect (data not shown). Stevia has anti–cancer effect, anti-blood glucose effect and anti-cardiovascular effect. This effects for the existence phenolic compounds in stevia such that had the ability to remove ROS, so increasing phenolic component by jasmonic acid and salicylic acid spraying in stevia was useful.