**3. Major constraints in crop production**

Zaman and co-workers found that different soils types have significantly affected the growth and leaf yield. Maximum plant height, leaf area, fresh and dry weight, branch and leaf number were observed in the plants grown on non-calcareous soil and were similar to the plants grown on acidic soil. Leaf biomass yield was found to be maximum in non-calcareous soil. Díaz and co-workers [112] studied the effect of altitude and fertilizers on biomass production of stevia and found that the organomineral fertilizer decreased the differences in nutrient uptake between different altitudinal gradients than the mineral fertilizers and increased biomass production by 49%. The highest stevioside yield of 30 g/m<sup>2</sup> was gained before flowering during the starting of September with an increased yield of leaf biomass when the crop was raised during the optimal planting season of onset of spring [113]. Micropropagated shoots were exposed to drought stress by using polyethylene glycol (PEG) 6000 for four weeks and it was observed that 4% PEG 6000 concentration enhanced the growth dynamics and pharmaceutical compounds as a defensive response against reactive oxygen species produced in water deficit conditions [114]. Weeds are a common problem during agricultural operations which hinders crop production and requires a large number of synthetic chemicals for controlling the weed population.

Taak and co-workers [115] reported that the use of herbicides controlled the weed growth but the use of mulches like rice straw and eucalyptus leaves increased the plant growth characteristics as well. However, mulching not only control the weed proliferation in stevia but also have a significant effect on the dry weight and leaf biomass. The effect of different concentrations of titanium dioxide nanoparticles on physiological and phytochemical properties was investigated by Rezaizad and co-workers [116] and found that 400 mg/l significantly affected the rebaudioside-A and B content while 80 mg/l had maximum effect on rebaudioside-C and F content. Nitrogen content is negatively correlated with steviol glycoside in leaves while the increased steviol glycoside is correlated with a decreased ratio of rebaudioside A over stevioside [117]. Stevia cultivation in temperate climatic conditions depends mainly on the genotype's ability to withstand the overwinter and is found that there is yield loss if crop is harvested in the first year where first-year harvest modality impacts the SG yield for three years [118]. Light favors germination and at least 20°C temperature are required. Germination can be accelerated at increased temperature for 24 h but it reduces the total germination [119].
