Hugo F. Gutiérrez

Limiting factors of reproductive success in Bromus auleticus (Poaceae). 1. Flowering phenology, sexual expression, and pollen production

Abstract The reproductive biology of Bromus auleticus was studied in three populations to identify factors related to flowering phenology and pollen production that could explain low fruit set. Flowering phenology was examined at the levels of population, plant, inflorescence, spikelet, and floret. Only one flowering event per year was recorded. Flowering proceeded downwards in the inflorescence and upwards within a single spikelet. Spikelets had on average 7.5 ± 1 chasmogamic, hermaphrodite florets and an apical, sterile or staminate floret. Each floret was open for 3 hours; pollen was shed over 10–75 minutes but the stigma remained receptive for 15–30 hours. There was ample overlap in flowering time among spikelets of the same inflorescence and among individuals of the same population; thus, both geitonogamy and cross‐pollination were possible. Hence, the limited female reproductive success of this species cannot be explained by its flowering phenology, as no mechanism exists (in space or time) that reduces or prevents the possibility of self‐fertilisation at the levels of the floret, spikelet, and/or inflorescence, or pollen interchange among individuals of the population. Neither the amount of pollen per flower or its fertility rate were limiting factors for adequate caryopsis setting.

Limiting factors of reproductive success in Bromus auleticus (Poaceae). 2. Fruit set under different pollination regimes, pollen viability, and incompatibility reactions

Abstract Several aspects related to the reproductive biology of the valuable Argentinian native forage grass Bromus auleticus were studied in three populations to identify its reproductive system and the factors that limit its reproductive success. In vitro pollen fertility and germinability were measured, and the behaviour of pollen tubes under self‐pollination was compared with that of cross‐pollen. Fruit set under different pollination conditions was also determined. Pollen amount and pollen fertility do not per se limit fruit set, and pollen fertility was not impaired by bagging; however, in vitro pollen germination was low (22.5%) which may limit reproduction. The differential behaviour of pollen tubes under selfing and crossing, as well as the high pollen:ovule ratio, indicate allogamy. Substantial fruit set (38.3%) occurred only when stigmas received nonself pollen in adequate amounts. Treatments providing only self‐or no pollination drastically reduced or suppressed fruit set, which suggests lack of apomixis and, at the same time, the operation of a self‐incompatibility mechanism. The limited reproductive success of a natural population of this species may be the consequence of self‐incompatibility coupled with reduced genetic variation for incompatibility alleles.