Floral morphometric analysis of Prosopis affinis spreng. (Fabaceae) suggests flexibility of the reproductive system in isolated populations within the Brazilian pampa

Abstract

Prosopis affinis is native tree of the Brazilian Pampa, recognized as an important species to local culture and economy. Despite the ecological and the economic potential of this tree, studies in Brazil are limited to descriptions of the occurrence sites of P. affinis. Aiming to determine the existence of diversification at floral and/or at population levels for this species, morphological characteristics of inflorescences and flowers of Prosopis affinis growing in the Pampa biome were measured, while the pollen/ ovule ratio and the outcrossing index were determined as an indirect estimate of the species breeding system. No clear reproductive specialization related to flower position in the inflorescence was observed, although basal flowers in the inflorescence presented reduction of the androecium. The results also showed that P. affinis presents a breeding system intermediary between facultative autogamy and xenogamy, with upper stamens supporting self-pollination, reduced stamens supporting outcrossing and the likely occurrence of an evolutionary shift at geographical level. These findings highlight the importance of conserving the remnants of P. affinis in southern Brazil and further investigate the pollination biology, the breeding system and the genetic diversity of this species, promoting its conservation and better understanding the effect of forest fragmentation in the Pampa biome. Further analyzes comparing isolated and larger continuous populations, as well as estimations of the real outcrossing rate of P. affinis through genetic studies and controlled experiments to evaluate auto-compatibility are recommended in order to generate more information about this issue.