Petunias were grown in compost-peat substrates with different Zn contents (basic load, 400, 800, and 1600 mg kg(-1)) in order to identify toxic levels of Zn in plant dry matter and of phytoavailable (CaCl(2)-extractable) Zn in compost-peat substrates. Yield reduction of petunias was only observed at an extremely high Zn content (635 mg Zn kg(-1) plant d.m.), although chlorosis was evident at lower Zn levels (160 mg Zn kg(-1) plant d.m.). Thus, the occurrence of Zn-induced chlorosis during the growth period was chosen as the toxicity parameter. Chlorosis and plant Zn content were reduced by additional Fe supply, although the Fe content of the plants was not affected. During a six week growth period, petunia Zn content and chlorosis increased in the first two and three weeks, respectively, and then decreased, whereas plant Fe content decreased sharply between the second and third week after the start of Zn treatment. Chlorosis first occurred with a plant Zn content of 160 mg kg(-1), corresponding to a CaCl(2)-extractable Zn content in the substrate of 6 mg l(-1), which was identified as the toxicity level for phytoavailable Zn in compost-peat substrates. Taking a safety factor into account, a critical level of 4.5 mg l(-1) was computed, which should not be exceeded in order to avoid Zn-induced chlorosis. (C) 2000 Elsevier Science B.V. All rights reserved.
