Nitrate-to-ammonium ratio shapes vegetative growth, nutrient status, and physiology of Vanilla planifolia in soilless culture

Abstract

Vanilla is a high-value crop whose demand exceeds supply, yet evidence-based nutrient management remains unexplored. This study investigated the effect of nitrogen form on vegetative growth, biomass allocation, and physiology of Vanilla planifolia in soilless culture. Ninety-five plants were assigned to five NO₃⁻-N: NH₄⁺-N ratios (0:100 to 100:0; n = 17 each) at 47 mg nitrogen L⁻¹ and a nitrogen-free solution (n = 10), over 140 days under controlled conditions. Total dry mass declined linearly with increasing ammonium proportion (p < 0.001), with relative growth rate the most responsive parameter: ammonium nutrition reduced total dry mass by 28% and relative growth rate by 36% relative to purely nitrate-fed plants. Increasing ammonium supply acidified the rhizosphere (substrate pH declined by 1.2 units, from 5.93 to 4.72) and tended to reduce leaf biomass fraction and total root length; tissue nitrogen concentration increased linearly with increasing ammonium proportion across all organs. A treatment × shoot-position interaction revealed temporally compounding growth advantages under nitrate nutrition. These results provide first evidence that nitrogen form, together with the accompanying shift in rhizosphere pH, shapes early vegetative growth in vanilla, with nitrate‑dominated supply producing the highest vegetative growth under the controlled soilless conditions tested.