Factors affecting nitrate variability in lettuce

Greenway, C., 2001. Factors affecting nitrate variability in lettuce. MPhil, Nottingham Trent University.

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Investigations were carried out in order to determine plant nitrate variability within lettuce and in addition to determine a suitable technique for rapid, on-field nitrate testing of lettuce. Hydroponic and soil studies were conducted at Horticulture Research International (HRI), Stockbridge House, Selby, North Yorkshire, in order to investigate the factors associated with nitrate variability in lettuce. Three hydroponic trials were conducted during an 18 month period of research, which included a study to determine inherent nitrate variability in lettuce, a study to determine the influence of different nutrient rates on plant nitrate concentration and a study to determine nitrate variability among lettuce cultivars. In addition to determining plant nitrate concentration, fresh weights of the lettuce were also assessed in all of the hydroponic studies. The influence of varying light intensity and soil nitrogen concentrations on plant nitrate concentration and head weight of lettuce was investigated during two soil studies. Soil trials were conducted during a winter and summer growing period in order to compare the influence of each treatment on nitrate variability and head weight within a winter and summer lettuce crop. A third soil trial involved raising eight lettuce varieties, which consisted of both butterhead and continental types, in order to determine cultivar variability in nitrate content and head weight.

An initial hydroponic investigation illustrated the relatively controlled growing conditions and resulting crop uniformity that are achievable within an NFT system. The level of nitrate variability within the hydroponic lettuce crop was limited and was not found to be significant between individual heads. Furthermore, the head weights of the hydroponic lettuce were generally consistent. In the second hydroponic trial there was significant nitrate variability between lettuce raised at different nutrient rates. An approximate increase of 25% plant nitrate concentration was observed in hydroponic lettuce when the nutrient supply was increased from 100ppm to 150ppm, and a further 20% increase was observed when the nutrient supply was increased from 150ppm to 200ppm nitrate-nitrogen. The head weight of lettuce was also significantly influenced by an increase in the nutrient rate from 100ppm to 150ppm, and from 100ppm to 200ppm. There was however, no significant increase in the mean head weight of lettuce when the nutrient solution was increased from 150ppm to 200ppm nitrate. Significant nitrate variability was found among the three lettuce cultivars during the third hydroponic trial, with the continental variety Miami exhibiting the highest nitrate concentrations and lowest head weight of the cultivars grown. The results from the two soil trials showed reduced nitrate variability within the winter crop compared to a significant level of nitrate variability within the summer lettuce crop, which was attributed to the differences in soil nitrogen concentration and light intensity. The results of the soil variety trial were in agreement with previous investigations, and showed significant nitrate variability between continental and butterhead lettuce varieties, with the highest nitrate concentrations observed in continental types. In addition, the head weights of continental varieties grown in soil were significantly lower compared to the head weights of butterhead lettuce. A negative correlation between head weight and nitrate concentration was observed in the butterhead cultivars, but a similar relationship was not observed within the continental varieties.

The Nitrachek meter was found to be a reliable and accurate method for use as a rapid nitrate testing technique despite the observance, during the later part of the investigations, of some degree of underestimation in the nitrate readings obtained with this method. The Horiba Cardy meter showed promise for use as a rapid nitrate testing method, with recommendations for further investigations in order to establish the repeatability of this technique. The Ion Selective Electrode, Orion model, once again proved to be the most reliable method for determining sap nitrate concentrations from lettuce samples.

The identification of a reliable extraction method was deemed to be an essential component in the determination of a rapid nitrate testing technique. A standard, established technique for the extraction of nitrate from vegetables was not found to be in use amongst UK or European analytical laboratories. Indeed, the range of extraction methods currently used throughout the UK alone was found to be extensive. A great deal of variability was observed amongst the nitrate values obtained with different methods of nitrate extraction using lettuce samples that were obtained from the same crop. In particular, it was found that hot water nitrate extraction could significantly influence the nitrate concentration of extracts from lettuce used for analysis. Therefore, during the final stages of the investigation a series of small scale studies were conducted to determine the effect of nitrate extraction procedure on the mean nitrate concentration of extracts from lettuce. It was concluded that further studies would be necessary in order to validate a suitable nitrate extraction procedure for use along side the chosen technique for nitrate determination.

Item Type: Thesis
Creators: Greenway, C.
Date: 2001
ISBN: 9781369316940
Rights: This copy of the thesis has been supplied so that anyone who consults it is understood to recognise that its copyright rests with the author and that no quotation from the thesis and no information derived from it may be published without the authors prior written consent.
Divisions: Professional Services > Libraries and Learning Resources
Record created by: Linda Sullivan
Date Added: 30 Sep 2020 15:19
Last Modified: 20 Sep 2023 09:03
URI: https://irep.ntu.ac.uk/id/eprint/41047

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