Morphological, anatomical and chemical studies on Stevia plant and its response to the growth regulators GA3 and IBA

A field experiment was carried out at Giza Agricultural Research ýStation, during the two successive seasons 2007-08 and 2008-09 aiming ýto investigate the effect of two growth regulators GA3 and IBA with ýthree concentrations (0, 250 and 500 ppm), applied in single or double ýfoliar sprays on the botanical, yield and chemical characteristics of ýstevia (Stevia rebaudiana (Bertoni) Hemsl.) plants. In addition, a ýgreenhouse experiment, was carried out to study the effect of the ýplanting date (spring, summer and fall), cutting position (tip, mid and ýbasal) and IBA concentration (0,500 and 1000 ppm) on the success of ýstevia propagation by stem cuttings. The field experiment showed that ýGA3 induced significant promotion effect on all morphological traits ýý(plant height, number of branches/plant, leaves fresh and dry ýweights/plant, stem fresh and dry weights/plant and total dry weight of ýleaves over the four cuts (the yield)), while decreased flowering ýbranches percentage comparing with control. The effect of ýconcentration regardless of type of growth regulators, 500 ppm induced ýsignificant effects on the previous morphological traits. Also, the result ýexplained the role of number of sprays on promoting the vegetative ýgrowth,as plants sprays twice showed increases in all morphological ýtraits compared with theþ þsingle spray. Studying the anatomy, GA3 ýinduced different effects on the anatomical structure of the main stem ýand leaf of stevia according to the used concentration. The maximum ýreduction in stem diameter was observed with 500 ppm GA3 ýtreatments. It is observed that, both cortex and vascular tissues in GA3 ýtreated plants showed relatively decrease in thickness as compared with ýtheir respective control. Treatment with IBA, induced thicker stem due ýto increasing the thickness of all included tissues, especially phloem and ýxylem tissues and pith diameter, also there are positive effects on ýanatomical structure of the leaf blade. The highest stevioside percentage ýý(8.85%) was in the 4thþ þý cut followed by the 1st cut of plants treated with ýdouble sprays of 500 ppm GA3. The interaction between the studied ýfactors proved that plants sprayed twice with 500 ppm IBA showed the ýhighest rebaudioside A (4.94%). Non treated plants showed the highest ýcarbohydrates percentage (57.2%) as compared with that of GA3 ýý(55.8%) and IBA treatments (55.9%). Maximum percentage of survival ýcuttings was obtained from basal cuttings treated with 1000 ppm IBA ýconcentration in spring (89.3%) while the lowest value was obtained ýfrom tip cuttings with 0 ppm IBA in fall (20.3%).ý