Effects of foliar biostimulants and two substrates (organic and conventional) on the aerial biomass and the major steviol glycosides (stevioside and rebaudioside A) of two Stevia rebaudiana (Bertoni) Bertoni cultivars in a pot experiment in Greece.

Abstract

Background: Steviol glycosides (SGs) from Stevia rebaudiana (Bertoni) Bertoni have garnered global interest due to their intense sweetness (over 300 times sweeter than sucrose) with negligible calories and potential health benefits. This study aims to address the low productivity of Greek stevia crops in terms of dry leaf yield (1235 kg/ha) despite favorable Greek agro-climatic conditions. Aim: To investigate the agronomic reasons behind the low productivity of stevia cultivation in Greece, with final target to enhance the yield of dry leaves and major steviol glycosides (SGs) by exploring new cultivar and nutrition management strategies. Approach: A pot experiment assessed the impact of three foliar plant biostimulants (calcite, seaweed extract, and amino acids) and two soil fertilizers (organic and conventional) on aerial biomass productivity and major steviol glycosides (stevioside and rebaudioside A) of two new stevia cultivars (cvs Olga and Ambrosia). Key growth and productivity traits such as height, covered area, leaf chlorophyll concentration, yield in dry leaves, leaf SGs concentration, and rebaudioside A/stevioside ratio were measured. Main results: Under optimal Greek agro-climatic conditions (day length > 13 h, average temperature 23.4°C, soil moisture 41 - 43%, harvest time ≥ 110 DAT), an organic substrate enriched with N rate 1.2 g per plant from high-quality organic fertilizer (C/N < 20), along with a combination of foliar plant biostimulants (calcite 0.3%, seaweed extract 0.02%, amino acids 0.2% at low dose rates) facilitated significant improvements in dry leaf yield (87 g per plant) and major steviol glycosides concentration (17.6 g per plant), with major SGs concentration reaching 20.28% (11.09% stevioside + 9.22% rebaudioside A), and the highest rebaudioside A/stevioside ratio (0.83) in the tested stevia cultivar (cv. Olga). Conclusion: The study suggests that the current substrate (N rate 0.88 g/plant) and cultivar (cv. SRB-128) contribute to the low productivity of stevia cultivation in Greece. It highlights the potential of suitable cultivars treated with appropriate substrate and foliar plant biostimulants to bridge the productivity gap between conventional and organic farming in the Mediterranean region. However, further research on biostimulant mechanisms, cultivar selection, and nutrition management is necessary to optimize leaf biomass, steviol glycosides concentration, and rebaudioside A/stevioside ratio across diverse local environments.