The effects of organic and conventional fertilisation on glucosinolates in broccoli

The world organic food, fibre and cosmetics market was reportedly worth AUD 147.2 billion in 2018, with the Australian market worth AUD 2.6 billion per annum. Organic farming practices include growing diverse legume pastures, long-term rotations, compost production, the use of naturally-derived fertilisers and the use of mixed crops and livestock. Meta-analysis data from research conducted primarily in the EU and the USA suggests that organic food has significantly higher antioxidant activity than conventional food, leading to health benefits for consumers. Broccoli (Brassica oleracea var. italica) is an important vegetable that contains several beneficial secondary metabolites including glucosinolates, which are linked to a reduction in certain cancers. While meta-analyses indicate secondary metabolite concentrations are often higher in organically grown crops, the role of organic farming practices, particularly nitrogen management, on glucosinolate levels in broccoli is unclear. Specifically, the interactions between total nitrogen applied, the release rate of the fertiliser nitrogen and concentrations of key glucosinolates in broccoli have not been resolved. This study tested the hypotheses that: i) organic nutrition (composted manure) will produce more glucosinolates in broccoli than mineral fertiliser when nutrient levels are matched; ii) slow release of nitrogen throughout the season is a major contributor to higher production of glucosinolates.The results of this study showed that when all other nutrients are present, aliphatic and indole glucosinolates increase in concentration in broccoli florets with increasing rates of N, regardless of the source (synthetic or organic). Additionally, organic N nutrition as a result of lower N release rates produced less glucoraphanin, glucoiberin, sinigrin and glucobrassicin than mineral N nutrition. The study also showed that there was no evidence that small, regular doses of N affect these glucosinolate concentrations compared to a large basal dose of N. Overall, this thesis found organic N nutrition did not increase secondary metabolites concentration in broccoli compared to synthetic N sources, rather, effects of organic vs mineral N may depend on the crop species, other nutrients availability, and the specific metabolites being investigated.