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Accumulation of nitrate in silybum marianum

A.R. Harradine

Department of Agriculture, New Town Research Laboratories, New Town, Tas 7008

Concentrations of nitrate toxic to grazing stock may accumulate in plants under high soil nitrogen level, low light intensity, low soil water availability and high temperatures or after application of phenoxyacetic herbicides (I). Variegated thistle (Salybum marianum) is one of several common plants in Tasmania capable of accumulating toxic levels of nitrate (1). No quantitative data is available on either the etiology of, or the effect of phenoxyacetic herbicides on, nitrate accumulation in S. marianum. The experiment reported is the first in a series designed to investigate these features of S. marianum biology.

Methods

S. marianum plants were grown individually in 20 cm diameter pots in 50/50 peat/sand mix (pH adjusted to 6.0) in a glasshouse from seed sown on 9.10.83. Plants were grown under either 100, 75, or 50% of full incident light for the whole trial period or under 100% light until 7 days prior to harvest when they were placed under 75 or 50%. Pots received 250 ml of Long Ashton solution (2) two times per week until 9.11.83 and four times per week thereafter. Ky03 and Ca(NO3)2 were varied to provide nitrate levels of 9, 15 or 21 mequiv L . Top growth was harvested on 13.12.83, 8 weeks after emergence, dried at 80C and analysed for NO3 (3). The experiment was a factorial with 3 replicates.

Results and Discussion

Table 1 Concentration of nitrate in S. marianumplants (ppm N0 dry wt. basis)

Plant NO-3 level increased with increasing applied NO-3 and in shade (Table ]). Plants under 50% light for the whole trial period had significantly higher NO-3 levels than those under 75% light at the lower applied NO; levels. Where light was reduced by 50% for 7 days prior to harvest, plant NO3 increased significantly in the 15 and 21 mequiv L-1 applied NO3 treatments but a 25% light reduction for the same period had no significant effect.

A plant concentration of 9200 ppm NO3 on a dry weight basis is considered the toxicity threshold level for ruminants (1). The above data show that this level can be readily exceeded by S. marianum plants growing in shade and/or in medium to high fertility soils. Prolonged heavy cloud cover could also increase NOT content to highly toxic levels.

1. McBarron, E.J. 1977. "Medical and Veterinary Aspects of Plant Poisons in New South Wales". Dept. Agric., NSW.

2. Hewitt, E.J. 1966. Tech. Comm. No. 22. Comm. Bur. Hort. F. Plant. Crops, Comm. Agric. Bur.

3. Terrey, D.R. 1965. Anal. Chim. Acta. 34, 41-45.

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