Biochemical Systematics and Ecology, Vol. 14, No. 5, pp. 507-514, 1986. 0305-1978/86 $3.00+O.00 Printed in GreatBritain. Pergamon Journals Ltd. Phytoalexin Induction in Plants of Tropical Environment MARCIA R. BRAGA, M. CLAUDIA M. YOUNG, J. VALTER A. PONTE, SONIA M. C. DIETRICH*, VICENTE DE P. EMERENCIANO and OTTO R. GOTTLIEBt *Se(~o de Fisiologia e Bioquimica de Plantas, Instituto de Bot~nica, C.P.4005, 01000 S~o Paulo, SP, Brazil; tlnstituto de Quimica, Universidade de S~o Paulo, C.P. 20780, 01498 S&o Paulo, SP, Brazil Key Word Index--Phytoalexins; dicotyledons; tropical plants; cultivated plants. Abstract--Phytoalexin induction, so far reported mainly for cultivated plants, is applied to wild species of tropical environ- ment. The phenomenon is strongly influenced by the experimental techniques. Application of the more common drop diffusate procedure results in the predominance of positive phytoalexin responses in cultivated (rather than in wild) species, while the facilitated diffusion procedure leads to comparable results for cultivated and wild species. Positive phytoalexin responses by both procedures are more common in the rainy (spring and summer) than in the dry (autumn and winter) season. Introduction Production of phytoalexins has been reported for a restricted number of species belonging to a few families, such as mainly Leguminosae [1, 2] and Solanaceae [3]. Most of these species are cultivated in temperate regions [4] and have been submitted to cultivation and thus most probably also to selection processes. Further- more the mechanical permeability of leaf surfaces to fungi and to chemical compounds may be quite diverse and the phytoalexin response must hence depend also on the analytical test procedure. Finally the chemical composition of a plant species may vary within little known limits according to ambient factors and the distinction among constitutional (endogenous) compounds and phytoalexins may be difficult. In the present work we addressed ourselves to the preliminary investigation of these problems; i.e. the ubiquity of phytoalexin formation throughout the plant kingdom [5]; the comparison of methods for the detection of phytoalexins; and the effects of environmental conditions on phytoalexin formation. In quest of answers Trichoderma pseudokoningii Rifai was selected as inducer of phytoalexin response. This fungus is a saprophyte [6] of rapid growth rate and abundant sporulation. The spores are (Received 14 May 1986) green and thus easily distinguished from the dark gray spores of Cladosporium cladospori- oides (Fresen) de Vries selected as detector of fungitoxins. The drop-diffusate [7] (designated dd) and the facilitated diffusion [8] (fd) tech- niques were used (in modified forms, see Experimental) in the assays of phytoalexin induction. Results The 169 analysed species are listed in Table 1 according to a modern classificatory system [10]. Whenever possible the species were assayed by both the dd and the fd methods. In some cases, however, only one or the other was employed due either to the experimental difficulty in the application of droplets to a pillous, a glanduliferous or a slippery epidermis [11] (fd technique preferable) or necrosis of leaf discs (dd technique preferable). The number of positive responses by the dd and fd techniques for the entire collection of 169 species (14 and 35.%, respectively) and for the 100 species assayed by both methods (19 and 37%, respectively) were very close. The means of the diameters of inhibition halos produced by both techniques were also nearly identical (dd 22 mm, fd 25 mm). The data of Table 1 were used to draw diagrams for the correlation of positive phyto- 507