Jenny Jobling

Factors Affecting Growth of Perennial Wall Rocket and Annual Garden Rocket

Popularity of baby leaf salad crops has increased and is likely to continue. Despite their commercial importance, cultivation of these crops is limited by a lack of appropriate production systems. It is important to establish abiotic constraints for efficient production of these crops to improve supply and quality. In separate experiments, responses of perennial wall rocket (Diplotaxis tenuifolia [L.] DC.) and annual garden rocket (Eruca sativa Mill.) to harvest number, seasonal conditions, and fertilizer level were measured. The yield of perennial wall rocket was influenced by the interaction between the cultivar, harvest number, and season. No clear response between these factors was identified, with the highest yields for the cvs. European wild rocket [DT1] and Apollo [DT2] for the first harvest summer crop (2.7 and 2.8 kg/m2, respectively) and the cv. Nature [DT3] for the second harvest summer and spring crops (2.5 kg/m2, respectively). For annual garden rocket, there was an interaction between the yield and season with the highest yield during summer (3.3 kg/m2), followed by winter (2.7 kg/m2) and spring (1.9 kg/m2). Nitrogen rate did not affect yield or dry weight for either crop, meaning that the species require only little additional nitrogen for efficient growth, and crops may currently be overfertilized. Optimal production is achieved during spring and summer for perennial wall rocket and summer and winter for annual garden rocket, allowing for the interchangeable use of species throughout the growing seasons. The growth of perennial wall rocket improves from first to second harvest but decreases for annual garden rocket, meaning that the perennial species is better suited to at least two harvests.

Variations in the most abundant types of glucosinolates found in the leaves of baby leaf rocket under typical commercial conditions

BACKGROUND Changes in the concentration of the three most abundant glucosinolates were measured in the leaves of perennial wall rocket [Diplotaxis tenuifolia (L.) DC.], and annual garden rocket (Eruca sativa Mill.). HPLC-MS was used to identify glucoraphanin, 4-hydroxyglucobrassin and glucoerucin from perennial wall rocket, and glucoraphanin, glucobrassicin and 4-methoxyglucobrassicin from annual garden rocket. In separate experiments the responses of glucosinolates to harvest number, seasonal conditions, nitrogen supply and post-harvest storage conditions were measured. RESULTS For perennial wall rocket, season influenced the concentration of glucoraphanin, which were highest for the spring [379 µg kg(-1) fresh weight (FW)] and summer (317 µg kg(-1) FW) plantings. The concentration of 4-hydroxyglucobrassin was higher in the leaves of first harvest crops. This response was due to this glucosinolate not being detected in the leaves of second harvest crops. Thus, the parent glucosinolate was altered between the first and second harvests in response to the abiotic stresses caused by harvesting. For annual garden rocket, there was an interaction between the harvest number and season for all glucosinolates measured. However, no clear response was observed between these factors. Higher concentrations of glucobrassicin and 4-methoxyglucobrassicin were measured for first harvest leaves when compared to the second harvest. This was due to the absence of detection of these glucosinolates in the leaves of second harvested plants; consequently higher total glucosinolate concentrations were measured for the first harvest winter (1224 µg kg(-1) FW) and summer (864 µg kg(-1) FW) crops. CONCLUSION The concentrations of individual glucosinolates vary greatly over typical pre- and post-harvest commercial conditions. The absence of 4-hydroxyglucobrassin for perennial wall rocket, and glucobrassicin and 4-methoxyglucobrassicin for annual garden rocket between harvests, illustrates that abiotic stress from harvesting has the capacity to alter the types of glucosinolates in leaves. Concentrations do not generally decline during a typical storage period, indicating that the potential benefits of these compounds are not lost during post-harvest storage.

Effect of Nitrogen Supply and Storage Temperature on Vitamin C in Two Species of Baby Leaf Rocket, and the Potential of These Crops for a Nutrient Claim in Australia

Baby leaf crops such as rocket are popular in mixed salads and add flavor and diversity to these dishes. Rocket leaves contain many beneficial compounds to human health, including vitamin C. It is important to determine abiotic factors that influence the concentration of this compound; in order to improve the supply of quality produce with a high nutritional value. The possibility of a nutrient claim for leaves of perennial wall rocket [Diplotaxis tenuifolia (L.) DC.] and annual garden rocket (Eruca sativa Mill.) was examined under the Food Standards Australia New Zealand (FSANZ) framework. Crops were supplied with 0, 100, 200, and 300 kg · ha−1 of nitrogen (N); leaves were stored at 0, 4, and 7°C for 15 days. The cultivar and amount of nitrogen supplied to perennial wall rocket did not affect the concentration of vitamin C in leaves at harvest, with a mean of 79.6 mg 100 g−1. For annual garden rocket there was an interaction between cultivar and nitrogen supply, with higher levels of supply generally resulting in lower vitamin C concentrations at harvest. The postharvest stability of vitamin C in both crops was best maintained at the lowest storage temperature. The results of this study illustrate that if stored at 0°C, both species contain vitamin C in sufficient quantities 15 days after harvest, to result in a “good source” health claim [>25% recommended daily intake (RDI)]. However, the storage temperature in the supply chain is typically ∼7°C, still permitting a “source” health claim (>10% RDI) for both species.