Effect of Potassium and Boron Foliar application on some of the Qualitative and Quantitative Characteristics of Stevia (Stevia rebaudiana L.)

In recent years, in order to increase public awareness about maintaining a healthy body, a lot of research has been done on the use of sweeteners to find ingrained types of them. Accordingly, new sources have been proposed for the preparation of these food additives one of the most appropriate plant of which is Stevia. In the leaves of Stevia, different varieties of steviol glicosides are produced which are much sweeter than normal sugar. The success percentage of the uptake of food elements through leaf feeding is about 95%, and in the way of attraction through the root is about 10%. In fact, foliar spraying is a shortcut for plant nutrition. Although potassium is one of the essential macro elements in the enzymatic activities of the plant, there is little recognition about its impact on the plant. Also, boron is an essential micro element for vascular plants and is involved in the transport of carbohydrate, cellular differentiation, cell wall synthesis and membrane health. The aim of this study was to consider the effect of potassium and boron foliar spraying on qualitative and quantitative yield of Stevia.

This research was conducted as a factorial experiment based on a randomized complete block design with four replications in experimental field of Khoozestan Agricultural and Natural Resources Sciences University, during 2016-2017 cropping season. Experimental factors were included: boron foliar spraying from boric acid source at four levels (0, 25, 50 & 75 ppm) and potassium foliar spraying from potassium sulfate source at three levels (0, 10000, 15000 ppm). 

The results revealed that the interaction of potassium and boron foliar spraying resulted in significant difference dry matter yield, leaf yield, plant height, leaf area index, percent of leaf stevioside, percent of leaf nitrogen, leaf potassium contentnt and leaf boron content. The treatments combination of 15000 ppm potassium and 50 ppm boron produced the highest dry matter yield (1495/8 kg/h), but created no significant difference with treatments combination of 15000 ppm potassium and 75 ppm boron (1490/2 kg/h). Also, treatments combination of 15000 ppm potassium and 75 ppm boron had the highest leaf yield (1134/68 kg/h) and percent of leaf stevioside (10/34 %). The treatments combination of no potassium foliar spraying and 25 ppm boron had the lowest dry matter yield (1407/73 kg/h) and leaf yield (725/6 kg/h). The results of this study indicate that potassium and boron foliar spraying had positive effects on dry matter yield, leaf yield and percent of stevioside. Potassi increased enzyme activity through its role in photosynthesis improving synthesis of protein and carbohydrates, translocation of photosynthetic and boron through its role in improving root deployment, carbohydrates translocation. In addition, cell wall synthesis and structure, had significant effect on dry matter yield and leaf yield of stevia.

Due to its more effective and faster effect of foliar sprying than nutrition (through the root, Salim et al., 2009), the compilation of macro element (potassium) and micro element (boron) foliar sprying did better to achieve the high qualitative and quantitative yield of stevia.