Bract size affects resource availability and fruit set in a hummingbird-pollinated plant with distyly polymorphism

Abstract

Plants can use non-floral signals to advertise the presence of resources to pollinators. The distylous Psychotria poeppigiana (Rubiaceae) has reddish bracts with small yellow flowers. Similar to other bracted plants with inconspicuous flowers, such bracts may signal the presence of nectar to pollinators. We investigated whether bracts act as honest signals in P. poeppigiana and how variation in bract traits affects floral reward and fructification. We asked: (1) Is there a relation between attractive traits (i.e. bracts and flowers) and the availability (quantity) and quality of the resource offered? (2) Do bract traits influence fructification rate? We hypothesized that bracts act as an honest signal to pollinators, being positively associated with nectar quantity and quality. If the signal is honest, we also expected that bracts with more resources could potentially attract more visits and result in a higher fruit set. We tested: (1) bract and flower trait differences between long-styled (L-styled) and short-styled (S-styled) morphs; (2) the relationship between bract (size, shape, asymmetry, and colour) and flower (length and diameter) traits and floral resource availability and quality; (3) bract trait effect on fruit set; and (4) whether bract size influences the total number of flowers and bract mortality. Larger bracts were positively associated with nectar volume, number of flowers, and increased bract mortality. In contrast, smaller bracts were linked to higher fruit set. Bract colour and asymmetry had no significant effect on resource production or fruit set. We additionally found differences between morphs: the S-morph had larger bracts (10.37%), longer flowers (13.67%), and more flowers per bract (22%) than the L-morph. We conclude that bracts in P. poeppigiana act as an honest signal to pollinators, as larger bracts produced more nectar. The higher fruit set in smaller bracts and increased mortality in larger ones suggest a potential division of reproductive roles, with S-styled (larger bracts) flowers contributing to male reproductive function and L-styled (smaller bracts) to female function.

Keywords: floral polymorphism; floral reward; honest signalling; pollination.