Female-biased nectar production in the protandrous, hermaphroditic shrub Salvia hierosolymitana (Lamiaceae)

Flowering progresses upward along vertical inflorescences in the protandrous dichogamous shrub Salvia hierosolymitana (Boiss.). Flowers' gender nectar production rates and their vertical distribution were recorded in two populations (northern and central Israel) over 3 years. Female-phase flowers produced significantly more nectar than male-phase flowers and were more abundant at the inflorescences' base. Thus, nectar availability gradually decreases along inflorescences. Female-biased nectar production can benefit plants by increasing pollinator visits to female-phase flowers, enhancing pollination success when pollen is scarce. In congruence with this hypothesis, the following observations suggest that pollen in S. hierosolymitana may be in short supply: (1) freshly dehisced anthers contained 40% of sterile pollen; (2) pollen counts on female-phase stigmas were low (mean±s.e. 11.6±1.56); and (3) counts of germinated pollen tubes at the pistils' base were even lower (5.02±0.54). The nectar gradient along the inflorescence may also be adaptively beneficial in other aspects. Foraging insects that follow this gradient are expected to move from female-phase flowers near the inflorescences' base, to male-phase flowers closer to the top. Thus, reducing the risk of geitonogamy and promoting outcrossing while moving from male-phase flowers of one individual to female-phase flowers of another.