Impact of nutrient concentration and pH on vegetative growth and leaf metabolite composition of Vanilla planifolia in soilless culture

Abstract

Despite its status as one of the most expensive spices, the morphological and physiological responses of Vanilla planifolia (V. planifolia) to nutrient solution electrical conductivity (EC) and pH remain uncharacterised. This study quantified the effects of three nutrient solution EC levels (0.6, 1.2, 1.8 mS cm−1) and three pH levels (5, 6, 7) on early vegetative growth, biomass partitioning, and leaf metabolite composition of V. planifolia plants in a 103-day factorial soilless culture experiment. The root system showed the strongest treatment responses: root dry mass declined by 10.2% and root area by 37.4% from EC 0.6 to EC 1.8, while total plant dry mass showed a non-significant downward trend with increasing EC. Plants treated periodically with nutrient solutions of lower pH allocated a greater share of biomass to the roots, with the root mass fraction increasing by up to 9.5%. Root-zone acidification occurred across all treatments, with leachate pH dropping substantially below the applied values for every nutrient solution tested. Complementary untargeted metabolomics (UHPLC-HRMS; GC×GC-MS) of leaves revealed a significant increase of asparagine and γ-aminobutyric acid (GABA) at EC 1.8, constituting an early osmotic stress signature, and of secondary metabolites including vanillin and 4-hydroxybenzaldehyde. These results identify V. planifolia as a species sensitive to elevated EC that performs best at comparatively low nutrient-solution concentrations and provide baseline response data for soilless vanilla fertigation.