Influence of Propazine and Chlorthal Dimethyl on Mycorrhizal Development in Pinus radiata Seedlings

Abstract

In an investigation into the side effects of herbicides on mycorrhizal formation in Pinus radiata nurseries, propazine and chlorthal dimethyl were added at concentrations that approximated to normal (2.20 and 1.88 g a.I-1L) and half-normal (1.10 and 0.94 g a.i.-1L) field application rates respectively to P. Radiata seedlings growing in terra cotta pots. A humus-rich, surface soil from a 40-year-old P. radiata plantation was used as mycorrhizal inoculum. Root length, mycorrhizal numbers and type, and seedling growth were assessed and a histological examination of bulk mycorrhizal samples was carried out. Propazine applied at both dosage rates and chlorthal dimethyl at the higher rate significantly sup- pressed root growth during the first 3 months and mycorrhizal formation during both the early and later stages of growth. However, neither herbicide affected shoot growth after 3 months and both increased shoot growth at the end of 6 months. Two mycorrhizal types were identified, a white-coloured, heavily dichotomised ectomycorrhiza with a thick mantle and a reddish-coloured, weakly dichotomous ectendomycorrhiza. The roots of chlorthal dimethyl treated seedlings strongly suppressed intracellular penetration in the ectendomycorrhiza, producing an unusual association with only a Hartig net, which was similar to that seen in senescing associations and some long roots of conifers. The ratio of white to red mycorrhizal numbers was highest in the untreated controls and was reduced by both herbicides. Both herbicides appeared to effect mycorrhizal formation through their influence on root morpho- genesis and host physiology.