The effect of phosphite on meiosis and sexual reproduction

The fungicide phosphite was found to reduce pollen fertility in Australian and exotic species. In the perennial Dryandra sessilis the reduction was evident for a year after treatment with 2.5-10 gL,1 phosphite sprayed to run off. Pterochaeta paniculata, an annual, showed reduced pollen fertility in flowers that opened 16-30 days after spraying. The horticultural species Pelunia hybrida and Tradescantia virginiana also displayed reduced pollen fertility after phosphite treatment. Pollen mother cells of Tradescantia had a significant percentage of abnormal first and second divisions and micronuclei in the microspores for up to one month after spraying. There was evidence that phosphite induced premature tapetum breakdown in Pelunia but not Tradescantia. The percentage of abnormal meiotic cells and the frequency of premature tapetum breakdown appeared insufficient to account for the high levels of pollen infertility observed after phosphite treatment. Introduction Phosphite is the most cost effective, widely used fungicide for control of Phylaphlhara in natural ecosystems and horticultural crops (Coffey and Bower 1984; Wicks and Hall 1988; Ouimette and Coffey 1989; Guest and Grant 1991; Shearer 1994). Apart from providing relatively long-tenn control of the pathogen (Shearer el ai, 1991) it is noted for its generally low phytotoxicity on vegetative parts of plants (Barrett and Grant 1997). However, little is known about its effect on sexual reproduction, though several other fungicides, including the well known Benlate (active ingredient Methyl (butylcarbamoyl)-2benzimidazole carbamate), have been shown to cause aberrant pollen development, reduced pollen germination and altered patterns of synthesis of tapetal proteins (Church and Williams 1977; Ries 1978; Gentile et al. 1978; He and Wetzstein 1994; He el al. \996). It is necessary to understand the effects of phosphite on sexual reproduction as the compound is an important tool in the fight to conserve rare plant species threatened with Phylaphlhara. Consequently, we have exam ined the effect of phosphite on a number of aunual and perennial species in jarrah (Eucalyplus marginata) forest and from northern sandplains Kwongan vegetation (Fairbanks el al. 2001, 2002a). Detail will only be given of the effect of phosphite on pollen of a typical arulOal species Plerachaela paniculata and a perennial Dryandra sessilis 167 and in the model species Petunia hybrida and Tradescantia virginiana. Metbods Experimental design Plants species utilised in the field were in the jarrah forest at Jarrahdale 48 km SE of Perth, Western Australia, from the northern sandplain at Eneahba 250 km north of Perth, or, in the case of the horticultural species, plants were grown in the glasshouse at Murdoch University. Phosphite treatments involved spraying foliage to run off with the stated concentrations of phosphite (Foli-R-Fos 400. Unitec, Australia, mono-di-potassium phosphite) mixed with 0.25% synertol oil (Organic Crop Protectants, NSW, Australia) to enhance adhesion of drop lets. Pollen fertility was assessed by exposure of pollen grains to fluorescein diacetate (Widhlom 1972) and pollen germination by sowing pollen on nutrient medium (Alexander and Ganeshan 1989), As both sets of data gave similar results, only pollen fertility will be presented here. Pollen assessment of native species Dryandra sessilis is a member of the Proteaceae, a tall sclerophyllous shrub with sessile flowers in terminal clusters. Plants were sprayed Witll 0, 2.5, 5 and 10gL" phosphite in winter when they were in flower. There were 5