Comparative Phytochemical Profiles Across Edible Leaves of Brassica spp.

Thesis Abstract

Brassica vegetables are widely consumed worldwide, yet comparative phytochemical profiling across edible leaves remains fragmented, hindering evidence-based selection for nutrition-focused breeding and processing. This study addresses the gap by examining inter- and intra-species variation in phytochemical constituents and their implications for health-promoting properties. The aim is to systematically compare phytochemical profiles of edible leaves from Brassica oleracea var. capitata (cabbage), Brassica rapa (pak choi), Brassica napus (kale), and Brassica juncea (mustard greens) grown under uniform agronomic practices. Specific objectives are to (i) quantify baseline levels of key phytochemicals including glucosinolates, phenolic acids, flavonoids, carotenoids, and goitrogenic compounds; (ii) evaluate seasonal and ontogenetic effects on phytochemical concentrations; (iii) assess anti-nutritional and antioxidant activity correlations with total phenolics and carotenoids; and (iv) identify marker compounds that robustly discriminate species and harvest stages using multivariate approaches. A cross-sectional experimental design will be employed, with three cultivars per species grown in a controlled field in the temperate region of Southern Germany during two growing seasons. A total of 360 leaf samples will be collected at three developmental stages (45, 65, and 85 days post-sowing) with 10 biological replicates per cultivar per stage per season. Analytical quantification will be performed using high-performance liquid chromatography–mass spectrometry (HPLC-MS) for glucosinolates and phenolics, gas chromatography–mass spectrometry (GC-MS) for volatile and non-volatile components, and UV-Vis spectrophotometry for total carotenoids and total phenolics. Antioxidant capacity will be assessed via DPPH, ABTS, and ferric reducing antioxidant power (FRAP) assays, while anti-nutritional glucosinolate hydrolysis products will be monitored by LC-MS/MS. Data will be analyzed using multivariate statistics, including principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) to identify discriminant phytochemical profiles, and multiple linear regression to explore relationships between phytochemicals and antioxidant activity. ANOVA will test differences among species, cultivars, harvest stages, and season, with post hoc Tukey tests for pairwise comparisons. Regression models will be informed by the Theory of Planned Behavior and the Nutritional Epidemiology framework to interpret implications for dietary uptake, while the concept of phytochemical synergy will guide interpretation of interaction effects among compound classes. Expected findings include significant interspecific and intraspecific variation in glucosinolate profiles (e.g., aliphatic vs. indole glucosinolates) and phenolic content (including chlorogenic acid, caffeic acid derivatives, and quercetin glycosides), with kale and mustard greens showing higher total glucosinolates and antioxidant capacity than cabbage and pak choi at later harvests. Ontogenetic increases in carotenoids and certain flavonoids are anticipated, while seasonal variation may modulate hydrolysis products, altering potential health benefits. The study is expected to identify a concise set of marker compounds that differentiate Brassica species and developmental stages with high sensitivity and specificity, offering a basis for targeted breeding and post-harvest processing to optimize nutritional quality. The study contributes to knowledge by providing a unified, cross-species phytochemical atlas for Brassica leafy greens, linking compositional profiles to developmental timing and seasonality under uniform agronomic conditions, and informing breeding strategies for enhanced nutraceutical value. The findings will inform dietary recommendations and processing guidelines to preserve or enhance beneficial phytochemicals. Conclusions will emphasize the importance of harvest timing and cultivar selection in maximizing health-promoting constituents, with recommendations for breeding programs to prioritize marker compounds identified as robust discriminators and correlates of antioxidant activity.