| | Introduction | Objectives | Studies | Conclusion | Acknowledgements
.
Introduction
Glucosinolates are secondary plant products present in several dicotyledonous plants, predominantly in the members of Brassicaceae family. Broccoli sprouts and florets contain the compound glucoraphanin, a member of the glucosinolate group. Glucosinolates exhibit no biological activity, however, upon cell damage they undergo hydrolysis by myrosinase (thioglucoside glucohydrolase) to yield glucose and unstable product, which undergoes a spontaneous Lossen-type rearrangement. Eventually glucoraphanin is converted by myrosinase in plants and by gut microbes to sulforaphane, a potent inducer of mammalian detoxication enzyme activity. Several studies have indicated that consumption of Brussels sprouts reduced colorectal cancer and enhanced the synthesis of the detoxification enzyme glutathione S-transferase.
Recent studies by Professor Bernhard Juurlink and co-workers at the University of Saskatchewan have shown a reduction in blood pressure, atherosclesrotic-like changes and a reduction in stroke and heart disease in a model rodent system, following the ingestion of broccoli sprouts with high levels of glucoraphanin. They also have reported that among with Brassica species, field pepperweed (Lepidium campestre L.), a winter annual weed native to Europe, grown on disturbed sites, waste grounds, roadsides on Canadian prairies, accumulates significant levels of glucoraphanin in the leaves. The glucoraphanin content generally varies among accessions of field pepperweed. Since field pepperweed is currently considered a weed species that has no food or pharmaceutical value, the growing requirements and the optimum plant growth stage for harvest of this species that grown under greenhouse conditions for maximum glucoraphanin yield is not known. Several studies were conducted under different growing conditions in greenhouses and growth chambers to achieve the following objectives.
Lepidium campestre
Objectives
- To evaluate and select the most promising accessions of field pepperweed with higher productivity and glucoraphanin content under greenhouse conditions.
- To determine the best growing conditions (day length and diurnal temperature) for higher glucoraphanin production.
- To examine glucoraphanin content at various growth stages to determine the best crop growth stage of harvest for higher glucoraphanin yield.
- To examine the impact of pre- and post-harvest stress on glucoraphanin content in field pepperweed.
Studies
A greenhouse study was conducted using seven accessions and a local collection, to evaluate for glucoraphanin content and productivity. Results indicated that above ground biomass production of different accessions varied from 5.2 to 35.6 g/plant, where the accession Ames 13180 produced the highest biomass (35.6 g/plant) and the local collection produced the lowest biomass (5.2 g/plant). Glucoraphanin content of accessions varied from 3.1 to 180.8 μm/FW g). The accession Ames 15718 had the highest and the local collection had the lowest glucoraphanin content among the accessions. However, highest glucoraphanin yield/plant was observed from Ames 13179, mainly due to higher biomass production. The accessions Ames 15718 and Ames 13180 were found to be most promise accessions in terms of glucoraphanin production under greenhouse conditions.
Lepidium campestre greenhouse trial
A growth chamber study was conducted to determine the impact of growing conditions (12/12h day/night, 24/6°C; 12/12h day/night, 24/12°C; 16/8h day/night, 24/6°C and 16/8h day/night, 24/12°C) on plant growth and glucoraphanin content. On average, under short day conditions (12/12h day/night), field pepperweed produced a significantly higher number of leaves/plant, and significantly lower above ground biomass when plants were grown under warmer nights (12°C) than those grown under cooler nights (6°C). Under long day conditions, cooler nights (6°C) were favorable for leaf production compared to warmer nights (12°C), but night temperatures had no impact on above ground biomass production. On average, plants growing under short day conditions, had significantly higher glucoraphanin content when grown under cooler nights than those under warmer nights. However, under long day conditions, night temperatures had no significant impact on glucoraphanin content. On average, accession Ames 15718 was superior in terms of glucoraphanin production compared to other accessions, and for higher glucoraphanin production, Ames 15718 should be grown under short day with cooler night conditions.
A study was conducted to examine the impact of crop growth stage on glucoraphanin content using Ames 13180. Results indicated that young seedlings at 1-2 true leaf stage did not contain glucoraphanin in leaves. However, as the plant growth progresses (40 days after transplanting), the leaf glucoraphanin content had reached 874 μm/DW, but as seedling gets older (50 days after transplanting), the glucoraphanin content was reduced by 44%, indicating that early rosette seedling would be the ideal growth stage of harvest for the highest glucoraphanin content.
Two separate studies were conducted to determine the impact of pre- and post-harvest stresses imposed on plants, on glucoraphanin content of field pepperweed. The pre-harvest stress study was conducted using Ames 13179 and post-harvest stress study was conducted using 3 field pepperweed accessions Ames 13179, Ames 13180 and Ames 15718. The pre-harvest stress treatment imposed at full-rosette stage by maintaining water content at 50% field capacity for 3 or 6 days, or irrigating plants with 150 mM NaCl for 3 or 6 days, prior to harvest. The post-harvest stress was imposed by drying the harvested leaves at room temperatures for 4 days, prior to extract glucoraphanin. The salt treatment applied for 3 and 6 days enhanced the leaf glucoraphanin content by 39% and 66%, respectively over the water control. Conversely, water stress treatment applied for 3 and 6 days reduced glucoraphanin content by 15.5% and 18%, respectively over the water control. On average, desiccation treatment enhanced glucoraphanin content by over 3.4 times compared to that of fresh leaves. The response to the desiccation treatment, however, was accession specific. The treatment enhanced the glucoraphanin content by 6, 3 and 2 times in Ames 13180 and Ames 13179, respectively, compared to the corresponding controls. Based on results, it was concluded that salt stress imposed through NaCl treatment for 6 days prior to harvest the crop could be used as a means of enhancing the glucoraphanin content in field pepperweed. Moreover, desiccation of leaves at room temperatures prior to process can enhance the glucoraphanin content. Ames 15718 was the most promise accession in terms of glucoraphanin productivity, thus this accession may be useful for further evaluation.
Conclusion
Our study indicates that field pepperweed can successfully be grown under greenhouse conditions as a source of glucoraphanin production.
Among the tested accessions, three accessions namely Ames 15718 and Ames 13180, have the most promise in terms of for glucoraphanin productivity.
For maximum production of glucoraphanin field pepperweed:
- should be grown under short day (12 h day length at 24°C) and cooler night (12 h night at 6°C) conditions.
- may be subjected to moderate salt stress through NaCl 150 mM treatment for 6 days prior to harvest leaves.
- should be harvested at early rosette stage (about 40 days after transplanting).
- harvested leaves should be desiccated at room temperatures (20 ± 2°C) for 4 days prior to glucoraphanin extraction.
Further studies are required:
- to evaluate the interaction effect of above promising treatments to select the most effective treatment combination/s for glucoraphanin production using field pepperweed.
- to evaluate effect of different nutrients on biomass production and glucoraphanin level in field pepperweed.
- to identify other compounds containing in leaf extraction.
- to develop method/s for purification of glucoraphanin.
Acknowledgements
We greatly appreciate the financial support through the New Initiatives Fund of Alberta Agriculture, Food and Rural Development and technical support from Judy Webber, Andrew Fox, Patricia Coté, Jo-Ann Hughes and Dale Terry at CDCS, Brooks. We thank Shelley Barkley at CDCS, Brooks for reviewing this report. |
|