Interspecific competition between fat hen (Chenopodium album L.) and sunflower (Helianthus annuus L. NSH-33 hybrid) in pure and mixed stands of identical plant density (35 x 35 cm spacing) was studied in smáli plot field experiments under drought stress. Decrease in net photosynthetic rate (E^) due to interspecific competition was not statistically significant in either species in the first part of the growing season. During drought stress, however, significantly decreased in sunflower, while it hardly changed in C. album in the same (interspecific) competition situation. In pure stands, transpiration rate (E) was lower in C. album than in sunflower and this difference was more pronoímced in mixed stands. Consequently, C. album showed a very high water use efficiency (WUE) especially in the shade layer, which accounts for a larger part of the canopy in this species. By contrast, WUE in sunflower decreased, especially in the sun layer of the mixed stand. Interspecific competition reduced the total biomass more severely in sunflower than in C. album by the end of the growing season. The reduction was especially remarkable in the biomass of the reproductive organs. Reproductive effort expressed as reproductive allocation was higher in C. album than in sunflower. Hence the reproductive effort in sunflower and C. album in both intra- and interspecific competition seemed to be correlated with WUE, which is a prime characteristic of drought stress tolerance.
Genetic distances and phylogenetic clustering are calculated for sympatric populations of Rhipicephalus pusillus (Gil Collado) 1939 ticks, using cuticular hydrocarbon analysis and several statistical tools (stepwise discriminant analysis, Nei’s genetic distance using different sets of compounds, and Cavalli-Sforza’s Brownian motion model). Discriminant analysis on genic frequencies permits the detection of the most ancient geographic relationships between the populations, while the genetic distance method determines current evolutive affinities. However, methods based on quantitative estimation provide a poor resolution. Genetic distances are very high in the populations studied, suggesting a strong reproductive isolation of the specimens involved.