Despite the general belief that the interaction between extrafloral nectaries (EFNs) and ants is mutualistic, the defensive function of EFNs has been poorly documented in South American savannas. In this article, we evaluate the potential impact of EFNs (benefits and costs) on two species of plants from the dry areas of Central Brazil, Anemopaegma album and Anemopaegma scabriusculum (Bignoniaceae). In particular, we characterize the composition of substances secreted by the EFNs, test whether EFNs attract ants, and whether ants actually present a defensive role, leading to reduced herbivory and increased plant fitness. Histochemical analyses indicated that EFNs from both species of Anemopaegma secrete an exudate that is composed of sugars, and potentially lipids and proteins. Furthermore, EFNs from both species were shown to present a significant role in ant attraction. However, contrary to common expectations, ants were not found to protect plants against herbivore attack. No effect was found between ant visitation and flower or fruit production in A. album, while the presence of ants led to a significant decrease in flower production in A. scabriusculum. These results suggest that EFNs might present a similar cost and benefit in A. album, and a higher cost than benefit in A. scabriusculum. Since the ancestor of Anemopaegma occupied humid forests and already presented EFNs that were maintained in subsequent lineages that occupied savannas, we suggest that phylogenetic inertia might explain the presence of EFNs in the species of Anemopaegma in which EFNs lack a defensive function.
Leaf gas exchange characteristics were measured in twenty woody species that differ in succession status ranging from pioneer species (PS) to late succession species (LS) in a Brazilian rain-reforestation ecosystem. Photon-saturated photosynthetic rate, calculated per either a leaf area (P NA ) or a dry mass (P NM ) basis, differed among species. P NA and P NM were highest in PS and lowest in LS. Variation among species was 3-fold (from 7 to 23 µmol m -2 s -1 ) for P NA, and 5-fold (from 50 to 275 µmol kg -2 s -1 ) for P NM . The highest P NA (23 µmol m -2 s -1 ) and P NM (275 µmol kg -2 s -1 ) values were recorded in PS Croton urucurana, while the lowest P NA (7 µmol m -2 s -1 ) and P NM (50 µmol kg -2 s -1 ) values were recorded in LS Aspidosperma cylindrocarpon. A considerable overlap was recorded between PS and LS in values of stomatal conductance (g s ), transpiration rate (E), and leaf mass to area ratio (ALM). However, C. urucurana also showed highest g s and E. P NM was highly correlated with ALM in both PS and LS (r = -0.75 and -0.90, respectively). The high values of instantaneous transpiration efficiency (ITE) and intrinsic water use efficiency (WUE i ) were also observed in the PS when compared with the LS.
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