C. integerrimum is a functionally dioecious epiphytic orchid pollinated by a single species of euglossine bee during the raining season in the study population located in Central Veracruz, Mexico.
The C. integerrimum floral longevity concurred with the reported floral longevity of C. uncatum, falling both in the category of long-lived flowers (Milet-Pinheiro et al. 2015). A commonly pattern observed in long-lived flowers is that pollination may reduce the floral lifespan (Vale et al. 2011; Huda and Wilcock 2012; Vega and Marques 2014). Floral senescence stimulated by removal and/or deposition of pollinaria have been well documented in a number of orchid species (Chloraea alpine, Clayton and Aizen 1996; Acampe ochracea, A. papillosa, Eria pubescens, Rhyncostylis retusa, Dendrobium aphyllum, D. fimbriatum, D. hybrid, D. palpebrae, and Pelatantheria insectifera, Huda and Wilcock 2012; Epidendrum calanthum, E. cochlidium, and E. schistochilum; Vega and Marques 2014; Broughtonia lindenii, Vale et al. 2011). In C. integerrimum, the reduced life-time of staminate flowers after removing pollinaria were similar to the observed in C. uncatum (77% in C. integerrimum and 70% in C. uncatum). Nonetheless, pistillate flowers of C. uncatum lasted longer time without wilting than the pistillate flowers of C. integerrimum after pollination (68% in C. integerrimum and 83% in C. uncatum). Furthermore, the floral longevity of pistillate flowers was reduced to two days after them being visited and non-pollinated by bees.
Early flower senescence induced by pollination may be associated with the reallocation of resources to fruit production by the plant. Clayton and Aizen (1996) mentioned that reduced floral longevity in manual pollinated flowers was associated with a resource reallocation mechanism to developing fruits in Chloraea alpina. In Broughtonia lindenii was suggested that staminate flower wilting caused a reallocation of resources to fruit production, whereas pistillate flower wilting avoided re-visitation of already pollinated flowers (Vale et al. 2011). Vega and Marques (2014) recognized that floral longevity changes were determined by a trade-off between maintenance costs of floral attractiveness and allocation of resources to fruit production in several Epindendrum species. In C. integerrimum, the floral senescence may be accounted for saving-costs mechanism associated with floral maintenance rather than avoidance of re-visitation of flowers. The above because floral visitors exhibited an immediate rejection to land on a flower that was already visited, so that such flower ended to wither in the following days. In C. uncatum was reported the preference of pollinators for visiting non-pollinated and unvisited flowers (Milet-Pinheiro et al. 2015). Thus, the use of short-lived scent marks by bees to avoid re-visitation of flowers that were already visited by themselves and/or other bees may be possible in the genus and requires future research.
Plants’ reproductive success are closely linked with the attractiveness of its floral display to attract effective pollinators. In this study, C. integerrimum displayed a staminate-biased ratio, attracting a single species of pollinator in the sampled population. The observed floral display size and the biased sex ratio concurred with the reported in other Catasetum species (C. ochraveum, Romero and Nelson 1986; C. viridiflavum, Zimmerman 1991; C. uncatum, Milet-Pinheiro et al. 2015; Catasetum arietinum, Brandt et al. 2020). Furthermore, male individuals of a euglossine bee were attracted to flowers of C. integerrimum as in other Catasetum species (Janzen 1981; Romero and Nelson 1986; Whitten et al. 1986; Milet-Pinheiro et al. 2015; Brandt et al. 2020). The above corroborates that C. integerrimum exhibits a specialized pollination system by bees.
In the study population, male individuals of Eulaema polychroma visited the pistillate and staminate flowers of the species. Nonetheless, bees visited earlier the staminate flowers but in low frequency than pistillate flowers. Del Mazo Cancino and Damon (2007) recognized that staminate flowers produced a more diverse fragrance chemistry composition than pistillate flowers (15 compounds of floral oils in pistillate flowers and 29 compounds of floral oils in staminate flowers). Thus, fragrances produced by staminate flowers may contribute to explain the early arrival of male bees to those flowers. Whilst, the forced launching of pollinaria towards bees during its floral visitation contributed to explain the early departure and low visitation frequency of bees to staminate plants. The aversive behavior of bees caused by the forced attachment of pollinaria is coincident with the reported for several Catasetum species (Janzen 1981; Romero and Nelson 1986; Milet-Pinheiro et al. 2015; Nunes et al. 2017; Brandt et al. 2020).
The pollinator efficiency of E. polychroma in C. integerrimum flowers depends on matching the morphology and behavior of bees to the floral morphology and flower opening in C. integerrimum. In this sense, bees responded to floral morphology in the species, contacting the sexually structures of flowers. Together with floral morphology, C. integerrimum flowers opened in acropetalous sequence, matching with the bees’ activity pattern within the inflorescence in pistillate plants (i.e., successive visits to flowers from the base to the top). In staminate plants, the aversive behavior of bees associated with the forced attachment of pollinaria avoided successive visitation of flowers, mitigating pollen mass wastage within an inflorescence. Thus, bees carrying pollinaria were potentially able to pollinated a distant plant. Overall, there is a matching between C. integerrimum floral traits and behavior of E. polychroma as pollinator. Furthermore, the observed high pollinaria deposition efficiency by euglossine bees in C. integerrimum concurs with the reported by Janzen (1981), who observed that the two bees bearing pollinaria were able to deposited them on the stigmas of C. maculatum flowers. In the same study, Janzen (1981) mentioned that only 9% of bees were returned to the same plant, suggesting that bees move over long distances. Johnson et al. (2005) observed a trend accumulation of pollinaria on a moth’s proboscis by multiple visits to flowers, causing a reduction in pollinaria export, as well as obstruction of pollinaria deposition on stigmas of Disa copperi flowers. To my knowledge, Catasetum plants do no experience reproductive costs caused by an excess of pollinaria accumulation on bees during floral visitation. Thus, forced attachment of pollinaria to pollinators’ body by catapult mechanism seems to play an important role in reducing reproductive costs caused by pollinators return to the same staminate plant.
Tremblay (1992) suggested that low taxonomic number of efficient pollinators per orchid species could be related to its role in diminishing the costs of failure pollen transfer. In this regard, the suitability of E. polychroma as pollinator of C. integerrimum seems to be mediated by floral trait dimorphism and dioecious nature of C. integerrimum. In other words, the floral trait associated with attachment of polliaria to pollinators by catapult mechanism avoided reproductive costs associated with pollen mass wastage by interrupting stereotyped behavior of bees during its foraging within a staminate plant. In pistillate plants, bees bearing pollinaria were able to deposit them on the stigmas of flowers in every observed case.
Regardless of the efficiency in depositing pollinaria by bees, C. integerrimum exhibited a low fruit production by pollen limitation that was able to be released through manual pollinations. Nonetheless, the gained advantage in the fecundity by pollen limitation release seemed to lead to an over allocation of resources for fruiting that constrained the next reproductive event by reducing the production of reproductive structures. These results are consistent with the reported by several authors, who have documented that alleviating pollen limitation caused a high fruit production but it constrained future growth and/or reproduction in some tropical orchids (Tipularia discolor, Snow and Whigham 1989; Epidendrum ciliare, Ackerman and Montalvo 1990; Dendrobium monophylum, Bartareau 1995).
While pollen limitation has been a topic of major research interest, little is known about ecological and evolutionary consequences of a potential maximum fruit production threshold in angiosperms. Specifically, sexual reproduction may confer fitness benefits and opportunities of adaptation, but it also may incur fitness costs for reproductive plants, as well as a low quality of produced offspring. Hence, this study highlights the importance to understand that the current pollen-limitation concept may be an oversimplification of the optimal threshold of sexual reproduction in animal-pollinated plants, so that it will require a concept re-evaluation at theoretical and procedural level.
The findings of this study showed that floral senescence and fruit production were resulted from pollen vector efficiency in picking up and depositing male gametes on conspecific stigmas, as well as in avoiding male gametes wastage mediated by floral traits in C. integerrimum. Although C. integerrimum exhibited a fruit production increase after alleviated pollen limitation, a reduction in both flower number and fruit set production were observed in the subsequent reproductive event. Together, these results were consistent with the predicted hypothesis that C. integerrimum would exhibit traits and cost-saving mechanism associated with its main pollinators’ identity and behavior. The prevalent ecological conditions seem to favor the maintenance of low fruit production in C. integerrimum plants, so they may be able to adjust its fitness function across entire lifespan.
The unparallel and extraordinary floral adaptations in orchids were a source of inspiration to Charles Darwin, who devoting an entire volume to “The Various Contrivances by Which Orchids are Fertilized by Insects”. Since Darwin’s time to date, the pollination ecology of orchids does not stop surprising us, displaying study cases of beautiful life forms on earth.