Multivariate Analysis of Soil Chemical Properties Associated with TSS/TA Ratio and Betalain Concentration in White fleshed Dragon Fruit (Selenicereus undatus)
Nangsel Tshomo and Kuenga Penjor Gyeltshen
https://doi.org/10.55925/btagr.26.9106
ABSTRACT
White fleshed Dragon fruit (Selenicereus undatus) quality is primarily assessed using two key traits: sweetness, estimated by the total soluble solids to titratable acidity (TSS/TA) ratio, and pigmentation, measured as betalain concentration. This study examined the association between soil chemical properties and fruit quality (TSS/TA ratio and betalain concentration) in white fleshed dragon fruit across three production sites representing natural soil variability in Bhutan. Thirty fruits were analyzed for TSS, TA, and TSS/TA ratio, and composite soil samples were analyzed for organic matter (OM), nitrogen (N), phosphorus (P), potassium (K), micronutrients (Fe, Mn, Zn, Cu, B), sulphur (S), electrical conductivity (EC), and pH. Correlation and principal component analyses revealed strong interrelationships among soil parameters, indicating multicollinearity and underlying soil chemical gradients. Sweetness (TSS/TA ratio) was strongly promoted by available phosphorus and potassium but reduced by higher soil EC and pH. Betalain concentration declined with increasing nitrogen and organic matter, while copper showed a positive association. Ridge regression confirmed phosphorus and potassium as the strongest drivers of sweetness, whereas nitrogen and organic matter consistently suppressed both sweetness and pigmentation. Threshold analysis demonstrated nonlinear soil–fruit quality responses, including a decline in predicted betalain concentration at higher soil organic matter levels. Multi-response optimization through joint optimal analysis predicted single point optimum soil condition and their optimal range that balance sweetness and pigmentation, characterized by approximate soil chemical values such as pH of 6.9 (range: 6.7–7.0), organic matter of 0.96% (range: 0.90–1.05%), EC of 1.24 dS/m (range: 1.20–1.30 dS/m), phosphorus of 52 mg/kg (range: 50–60 mg/kg), and potassium of 18 mg/kg (range: 16–21 mg/kg). For sweetness-focused production, higher phosphorus (~86 mg/kg; range: 80–90 mg/kg) and potassium (21 mg/kg; range: 20–22 mg/kg) levels were most favorable. In contrast, pigmentation was favored under lower nitrogen and organic matter (0.84%; range: 0.8–0.9%) and slightly alkaline soil conditions (pH 7.6; range: 7.5–7.7). These findings provide soil-based guidance for precision nutrient management, enabling growers to target either sweetness, pigmentation, or balanced fruit quality under naturally variable field conditions.
Keywords: Dragon Fruit; TSS/TA Ratio; Betalain, Principal Component Analysis; Ridge Regression; Joint Optimal Analysis