Stevia+: How far can cultivation measures in greenhouses increase the concentrations of stevioside in the plant.

In the food and beverage sector using steviol glycosides (SGs) as natural sweeteners has become more popular over the last years. Stevioside and rebaudioside A are the two main SGs, accumulating in the leaf and responsible for the sweet character of the Stevia plant. In its common open-field cultivation the crop Stevia is still being optimized regarding cultivars and cultivation measures to obtain more yield with higher contents of SGs.

In this research the crop Stevia was cultivated in a greenhouse aiming to increase productivity, per area, in time and in the relative content of SGs, as well as investigating further the behavior of the Stevia plant in this environment. In this context an experiment with controllable conditions was designed to include the most promising and feasible cultivation factors, according to literature and technical possibilities. The trial lasted from June to October 2015 whereby Stevia was grown in pots with flood irrigation in two adjoining greenhouse compartments.
The plants consisted of two cultivars, one seed-grown, the other one grown from cuttings.
The factors were CO2-level, nutrient supply and red light night interruptions mainly targeting in the same order an increase in biomass, relative content of SGs and prolonging the cultivation season by keeping the plants in the vegetative growth state.
The CO2-levels were maintained at 400 ppm and 700 ppm, the nutrient supply per flood irrigation had the EC-levels 1.5 and 2.3 and the red light night interruption occurred or not for 2h in the middle of the night with an intensity of 20 μmol/cm2s.
Measurements included biomass and development of the plants, chlorophyll content as well as a photosynthesis CO2 response and light response curve that was obtained during the growth phase.
In a concluding laboratory analysis the relative contents of SGs were determined with a HPLC.

The main results of this research are that enriched CO2 increases the assimilation rate at 700 ppm under full sun light conditions up to 50% compared to 400 ppm. This could be the potential yield increase if no other factors are limiting for production.
In this experiment, CO2-enrichment has led to a biomass increase of 25%.

Red light night interruptions however, do not influence photosynthesis and have no influence on the synthesis of SGs. Flowering was successfully delayed with the red light night interruptions but appears to be dependent on the origin of the plant material, since the two cultivars reacted differently, and so did plants grown as cuttings from perennial Stevia.
The nutrient supply showed that the high EC-level of 2.3 in the flood irrigation has no beneficial influence on biomass and on the relative content of SGs compared to the EC-level of 1.5 whereby in general salt accumulation in the pots happened easily with this irrigation method for Stevia.

Regarding the relative content of SGs it can be pointed out that the variation within plants from the same cultivar, grown in the same conditions was large and determined the final content rather than the applied cultivation measures. The two cultivars differed significantly from each other in relative content of SGs, which underlines the importance of cultivar selection for an industrial production scenario.

Towards further research in greenhouses, the next step is to cultivate a commonly open-field grown cultivar combined with different light conditions to investigate the interaction of light and relative content of SGs. This would reveal energy input and yield output in amount and quality, to give a direct comparison to the outside production and would lead to an indication of economic feasibility.