Some effects of the nitrogen supply on growth and development of Cirsium arvense

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Abstract
The influence of N nutrition on the growth and development of Canada thistle, C. arvense (L.) Scop., was investigated by growing seedlings in sand culture under controlled conditions. A preliminary experiment showed that seedlings at the cotyledon stage were very susceptible to injury by high salt concentration; a 10 times dilution of Hoagland''s solution was required for successful establishment. NO3-N at 210 ppm caused severe leaf necrosis and inhibition of shoot and root bud growth, but provision of NH4-N eliminated these effects. When seedlings were supplied with NH4NO3 at levels that ranged from 5.25-420 ppm N, shoot dry weight and root bud growth increased up to 210 ppm but decreased at 420 ppm, probably as a result of NH4 toxicity. By clearing roots with lactic acid it was shown that while growth of the root buds increased with the N level, the number of buds initiated was significantly reduced. When plants were grown initially at a low N level, a subsequent increase in the N supply permitted the root buds to escape from inhibition and develop as leafy shoots. This response suggested that under the experimental conditions, bud inhibition was due to internal competition for a limiting N supply.
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