Nitrogen Rate Optimization Improves Nitrogen Partitioning, Chlorophyll Status, and Vegetative Growth in Vanilla × tahitensis

Nitrogen (N) management is a critical factor influencing growth and physiological performance of vanilla. However, quantitative information on N requirements for Vanilla × tahitensis remains limited. This study evaluated six N application rates (0, 2, 4, 8, 16, and 32 g N plant−1 yr−1) on tissue N partitioning, leaf chlorophyll index (LCI), vegetative growth, and biomass under controlled container conditions. Plants were arranged in a randomized complete block design (RCBD), and final analyses were conducted on destructively sampled plants (three plants per treatment; total n = 18). N concentration increased in all tissues with increasing N supply, but responses differed among tissues. Stem N concentration exhibited the greatest proportional increase at high N rates, whereas leaf N increased more gradually. Growth traits and LCI followed curvilinear patterns, with intermediate N rates generally associated with improved vegetative performance. Quadratic models estimated optimal N rates between 13 and 21 g N plant−1 yr−1, with predicted leaf N concentrations of 1.5–2.2%. However, confidence intervals indicated substantial uncertainty for variables with low model fit. LCI was strongly correlated with leaf N concentration, supporting its use as a nondestructive indicator of plant N status. Because the study was conducted under controlled conditions, field validation is required before broader recommendations are made.