Characterising the C2 phenotype and nutrient quality in wild rocket (Diplotaxis tenuifolia)

Walsh, Catherine and Lundgren, Marjorie and Roberts, Mike (2025) Characterising the C2 phenotype and nutrient quality in wild rocket (Diplotaxis tenuifolia). PhD thesis, Lancaster University.

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Abstract

Climate change is placing immense pressure on agricultural systems to ensure sufficient production for an ever-growing global population. Producers must not only increase yields to satisfy these demands, but also safeguard crop nutrition to prevent nutrient deficiencies and consequent malnourishment. Many C3 crop species already experience nutritive decline under ambient atmospheric CO2 concentrations, compared to previous levels. With elevated CO2 not occurring in isolation, the problem will become further exasperated by interactions with other environmental factors such as high temperatures which induce photorespiratory conditions. The extent of nutritional demise, however, may be determined by the photosynthetic type of the crop. Currently, the only commercial food crop to use the rare C2 mode of photosynthesis is wild rocket (Diplotaxis tenuifolia), a nutritionally dense salad crop which optimises photorespiration through separating the pathway across mesophyll and bundle sheath cells. This functions to effectively concentrate and reassimilate carbon released from photorespiration to improve photosynthetic efficiency under warm and dry conditions. Firstly, I examined the extent of intraspecific diversity of C2 photosynthesis across 15 cultivars of D. tenuifolia, which all proved to be of a C2 phenotype with no evidence of C3 or C4 physiologies within the species. I then characterised the photosynthetic type of the closely related brassica, E. sativa, known for its impressive stress resilience and nutrient density. These results indicated that E. sativa was a proto-Kranz phenotype, a C3 photosynthesis subtype along the C4 evolutionary continuum, prior to establishment of the C2 photorespiratory pump. Thirdly, I took a subset of D. tenuifolia cultivars with high, medium, and low CO2 compensation points, to investigate the mineral nutrient and glucosinolate profile changes across a range of growth temperatures and an elevated CO2 concentration to understand the influence of C2 metabolism on nutrient leaf content under environmental changes. These findings suggested that D. tenuifolia will retain mineral nutrients more consistently than C3 crops under climate change owing to its C2 metabolism. However, the glucosinolate content did not reflect this, leaving the crop profile vulnerable to variation under elevated CO2. Finally, I theorized the mechanisms behind these findings surrounding the integration of C, N and S in C2 species, explaining the improbability of C4 photosynthesis evolving in Brassicaceae. This work has commercial application for current and future agricultural systems to maintain nutrient consistency under climate change and contributes towards improving understanding of C2 evolution in Brassicaceae.