• Flora (2000) 195, 339-348 http://www.urbanfischer.de/joumaJslflora © by Urban & Fischer Verlag Studies on the breeding systems of understorey species of a Chaco woodland in NE Argentina MARTA B. BIANCHIl, PETER E. GIBBS2, DARIEN E. PRAD0 3 and JosE L. VESPRINI 3 1 Catedra de Botanica, Facultad de Ciencias Bioqufmicas y Farmaceuticas, Suipacha 531, Universidad Nacional de Rosario, Argentina, e-mail: ucha@cablenet.com.ar 2 Plant Science Laboratories, Institute of Environmental and Evolutionary Biology, The University, St Andrews KYI69AL, Scotland U.K. 3 Facultad de Ciencias Agrarias, Campo Experimental Villarino, Zavalla, Universidad Nacional de Rosario, Argentina Accepted: September 7, 1999 Summary The breeding systems of seven understorey species in a Chaco woodland in NE Argentina were studied by means of hand pollinations and fluorescence microscopy observations of post-pollination events. Six species showed self-incompatibility (SI), with heteromorphic SI in Erythroxylum microphyllum, and probable homomorphic gametophytic SI (stylar inhibition of self- pollen tubes) in Aechmea distichantha, Bromelia serra, Cleistocactus baumannii, Dyckia ferox and Grabowskia duplicata. Opuntia retrorsa was self-compatible. A notable feature of inter-morph cross-compatibility in E. microphyllum was differential fruiting success using pollen from different stamen whorls of the long-style morpho The high incidence of self-incompatibility in the understorey component of this Chaco woodland is similar to that found in some South American montane forests, and is in contrast to the widespread self-compatibility reported for this stratum in some neotropical forests. It is proposed that these breeding system differences are probably linked to the different taxonomic families represented in the understorey strata of these communities and are not a consequence of adaptations to pollination biology or other ecological factors. Key words: Bromeliaceae, Cactaceae, Erythroxylaceae, Chaco, breeding systems, self-incompatibility Introduction Studies on the breeding systems of neotropical forests indicate that whilst self incompatibility (SI) or dioecy predominate in tree species, the shrubs and perennial herbs of the understorey are more likely to be self-com- patible (SC) taxa (BAWA 1974; RUIz & ARROYO 1978, BAWA et al. 1985, BULLOCK 1985, KRESS & BEACH 1994). Moreover, whilst some of these SC species may have mixed mating systems with variable degrees of outcrossing (PASCARELLA 1997), a number of studies indicate that self-compatible understorey plants do have lower levels of genetic variation than SI taxa (LOISELLE et al.1995). For humid forest at La Selva, Costa Rica, KRESS & BEACH (1994) cite 88% of (n = 17) species of the upper stratum of the forest with SI, compared with 66% (n = 25) SC species in the lower stratum. In the Brazi- lian cerrados the same tree vs. understorey distinction seems to apply, since OLIVEIRA (1991) cited 86% of 29 0367 -2530/00/195/04-339 $ 15.00/0 studied cerrado trees as SI, whilst BARBOSA (1997) reported 66.7% of (n = 56) shrubs, vines and herbaceous species as SC. However, SOBREVILA & ARROYO (1982) found some 42% of understorey plants (n = 39) to be SI in a montane tropical cloud forest in Venezuela, and ARROYO & USLAR (1993) reported 56% SI species in a sample of 25 shrubs and perennial herbs in a montane sclerophyllous forest in Central Chile. Chaco is the name applied to the vegetation covering the vast plains of north-central Argentina, W Paraguay and SE Bolivia between latitudes 15-35° S. Although there is a large floristic variation in the Chaco as a whole which follows climatic gradients, in general this vegeta- tion is divided into an eastern, more humid sector and a western, drier sector, whilst the submeridionallowlands include a large depression between these two areas (LEWIS 1991). Forest and savanna vegetation occupy the E- W sectors whilst the submeridionallowlands are dominated by Spartina argentinensis grasslands. To our knowledge there has been only one reproductive biol- FLORA (2000) 195 339