Disease-fighting broccoli nutrients thrive under alternate nitrogen source

Results presented here open a promising avenue for broccoli to improve both their defensive capacity and nutritional value by controlling the nitrogen source. ©iStock

Controlling the nitrogen source used by plants can increase the nutrient profile of commercially valuable vegetables like broccoli, reducing the use of pesticides in the process. 

That’s according to a new study that uses ammonia in combination with nitrification inhibitors resulting in broccoli (Brassica oleracea) with a greater concentration of antioxidants and glucosinolates.

“In general, glucosinolates have two properties: they are natural insecticides and one of them in particular, glucoraphanin, has anticarcinogenic properties,” explained Daniel Marino, one of the study researchers, based at the University of the Basque Country in Spain.

“Although we did not manage to study the glucosinolate content in the part of the broccoli of greatest food interest, which is the flower, we saw that the leaves of the young plants accumulated a greater quantity of glucoraphanin when we added the source of nitrogen by means of ammonium than when we did so using nitrate," he explained.

As well as a cancer-fighting element, glucosinolates are also thought to play a role in potassium channel regulation and salt-stress response.

Similarly, it has been suggested that they serve as a nutrient sink, absorbing nutrients like sulphur.

A deficiency in this element promotes glucosinolate degradation, which goes on to assist functions like protein synthesis.

Results in edible plants

Along with Marino, members of the research group NUMAPS (Nutrition Management in Plant and Soil), based at the University’s Faculty of Science and Technology began experimenting with broccoli after trials using the small flowering plant Arabidopsis thaliana.

Based on how the plant reacted to an ammonium source of nitrogen, the process was then refined for broccoli in order to maximise glucosinolate content.

Broccoli plants were grown for three weeks after which they were transported to a growth chamber and maintained for five weeks.

The plants were irrigated with nutrient solution along with either ammonium (5 millimolar (mM) NH 4 Cl) or nitrate nutrition (2.5mM Ca(NO3)).

Of the four glucosinolates quantified, glucoraphanin (4MSOB, 4-methylsulfinylbutyl), 4-methoxyglucobrassicin (4MO3IM, 4-methoxyindol-3-ylmethyl), and neoglucobrassicin (1MOI3M, 1-methoxyindol-3-ylmethyl) accumulated under ammonium nutrition, whereas the level of glucobrassicin (I3M, indol-3-ylmethyl) was similar for both nutrition regimes.

The use of nitrification inhibitors also caused this ammonia to be in the soil for longer helping to lessen nitrate leaching and also nitrous oxide emissions.

The team, led by Carmen González-Murua, also carried out field experiments where factors, such as the microorganism content in the soil, had to be taken into account.

These organisms also use ammonium as a source of nitrogen.

“So in the field experiments we will also be interested in analysing the glucosinolate content in the broccoli inflorescence, the part of the plant that is consumed most,” said Marino.

"On the other hand, we are also interested in knowing the effect that the glucosinolates could have on the ammonium tolerance of the plant itself," he added.

Reduced pesticide use

Treating plants with ammonium nutrition is not a new concept. It has been previously observed that the plant’s metabolic adaptation to this nitrogen source may also have positive effects.

In terms of food quality, a frequent characteristic associated with ammonium-fed plants is an increase in protein content.

It has been shown to increase the level of reserve proteins in wheat grain (Triticum aestivum ) while enhancing its bread-making qualities.

Glucosinolates have also been associated with health-promoting activities. Sulforaphane, which is produced from glucoraphanin hydrolysis, is the main glucosinolate accumulated under ammonium nutrition in this study.

Sulforaphane is thought to contribute to a reduction in the risk of carcinogenesis and heart disease when consumed as part of the human diet.

As well as a nutritional benefit, the increase in broccoli’s glucosinate content may also increase its natural resistance to pests potentially reducing the use of pesticides.

Source: Journal of Experimental Botany

Published online ahead of print: doi.org/10.1093/jxb/erw147

“Quantitative proteomics reveals the importance of nitrogen source to control glucosinolate metabolism in Arabidopsis thaliana and Brassica oleracea.”

Authors: Daniel Marino, Idoia Ariz, Berta Lasa  Enrique Santamaría, Joaquín Fernández-Irigoyen, Carmen González-Murua, Pedro M. Aparicio Tejo.

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