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Only a few studies on terrestrial toxicity are available. These studies, however, point on a low toxicity to terrestrial organisms. Moreover, L-tryptophan is considered as an important precursor of plant growth regulator.

L-Tryptophan is not to be classified as dangerous/hazardous for the environment according to Directive 67/548/EEC and according to the Regulation (EC) 1272/2008 on classification, labeling and packaging of substances and mixtures (CLP Regulation) based on the eco-toxicological data on aquatic organisms, the low bioaccumulation potential, and the proven ready biodegradability.

Taking into account, the aforementioned aspects, L-tryptophan toxicity to terrestrial organisms seems to be of low concern and further testing is not necessary.

In Annex X of Regulation (EC) No 1907/2006, it is laid down that toxicity tests with terrestrial organisms (soil macro- and microorganisms, arthropods, plants, and birds) shall be proposed by the registrant if the chemical safety assessment indicates the need to investigate further the effects on terrestrial organisms. According to Annex I of this regulation, the chemical safety assessment triggers further action when the substance or the preparation meets the criteria for classification as dangerous according to Directive 67/548/EEC or Directive 1999/45/EC or is assessed to be a PBT or vPvB. The hazard assessment of L-tryptophan reveals neither a need to classify the substance as dangerous to the environment, nor is it a PBT or vPvB substance, nor are there any further indications that the substance may be hazardous to the environment. According to reliable study results, L-tryptophan is considered to be rapidly degraded in the environment and the bioaccumulation potential is regarded to be insignificant and therefore secondary poisoning is unlikely to be a relevant exposure route.

The available data point at a low toxicity to terrestrial organisms. In a screening test (filter paper contact test) conducted similar to OECD Guideline 207, a very low toxicity of L-tryptophan to Eisenia foetida was observed (48 h LC50>1000 µg/cm²); 1000 µg/cm² is the highest concentration to be tested according to guideline. In two studies it is reported that germination of lettuce and mung bean were not affected by L-tryptophan concentrations up to 0.2 g/L (lettuce: 72 h EC50>2 g/L; mung bean: NOEC>0.2 g/L). Based on these findings a low toxicity of L-tryptophan to terrestrial plants is to be expected. L-Tryptophan significantly induced the respiration rate in the arable soil as well as in the forest soil. The mean SIR (substrate-induced respiration) of three replicates was 45.5 nmol CO2 per g soil per hour (SD 7.19 nmol CO2 per g soil per hour) for arable soil and 42.4 nmol CO2 per g soil per hour (SD 1.53 nmol CO2 per g soil per hour) for forest soil. This indicates that L-tryptophan is metabolised and used for primary metabolism (via the kynurenine pathway) and it is very unlikly that the substance possess any advers effect for soil microorganisms. This is further supported by a study, in which the growth and yield of mung bean in the presence of Rhizobium phaseoli was investigated.

Moreover, L-tryptophan is considered as an important precursor of plant growth regulator. In one study, the growth and yield of mung bean in the presence of Rhizobium phaseoli and L-tryptophan was investigated. The presence of L-tryptophan at a concentration of 0.2 g/L did not alter the growth and yield promoting activity of the soil microorganisms (NOEC>=0.2 g/L). Therefore, the performance of further studies with terrestrial organisms is assumed to be not justifiable.