Stigma Closure Research Articles

Effects of heterospecific pollen on stigma behavior in Campsis radicans: Causes and consequences.

Pollinator sharing of co-flowering plants may result in interspecific pollen receipt with a fitness cost. However, the underlying factors that determine the effects of heterospecific pollen (HP) are not fully understood. Moreover, the cost of stigma closure induced by HP may be more severe for plants with special touch-sensitive stigmas than for plants with non-touch-sensitive stigmas. Very few studies have assessed HP effects on stigma behavior. We conducted hand-pollination experiments with 10 HP donors to estimate HP effects on stigma behavior and stigmatic pollen germination in Campsis radicans (Bignoniaceae) at low and high pollen loads. We assessed the role of phylogenetic distance between donor and recipient, pollen size, and pollen aperture number in mediating HP effects. Additionally, we observed pollen tube growth to determine the conspecific pollen-tube-growth advantage. Stigma behavior differed significantly with HP of different species. Pollen load increased, while pollen size decreased, the percentage of permanent closure and stigmatic germination of HP. Stigmatic HP germination increased with increasing aperture number. However, HP effects did not depend on phylogenetic distance. In addition, conspecific pollen had a pollen-tube-growth advantage over HP. Our results provide a good basis for understanding the stigma-pollen recognition process of plant taxa with touch-sensitive stigmas. We concluded that certain flowering traits drive the HP effects on the post-pollination period. To better understand the impact of pollinator sharing and interspecific pollen transfer on plant evolution, we highlight the importance of evaluating more factors that determine HP effects at the community level.

Stigma Sensitivity and the Duration of Temporary Closure Are Affected by Pollinator Identity in Mazus miquelii (Phrymaceae), a Species with Bilobed Stigma.

A sensitive bilobed stigma is thought to assure reproduction, avoid selfing and promote outcrossing. In addition, it may also play a role in pollinator selection since only pollinators with the appropriate body size can trigger this mechanism. However, no experimental study has investigated how the sensitive stigma responds to different pollinators and its potential effects on pollination. Mazus miquelii (Phrymaceae), a plant with a bilobed stigma was studied to investigate the relationship between stigma behaviors and its multiple insect pollinators. The reaction time of stigma closure after touched, duration of temporary closure, and factors determining permanent closure of the stigma were studied when flowers were exposed to different visitors and conducted with hand pollination. Manual stimulation was also used to detect the potential differences in stigmas when touched with different degrees of external forces. Results indicated that, compared to pollinators with a small body size, larger pollinators transferred more pollen grains to the stigma, causing a rapid stigma response and resulting in a higher percentage of permanent closures. Duration of temporary closure was negatively correlated with the speed of stigma closure; a stigma that closed more rapidly reopened more slowly. Manual stimulation showed that reaction time of stigma closure was likely a response to external mechanical forces. Hand pollination treatments revealed that the permanent closure of a stigma was determined by the size of stigmatic pollen load. For large pollinators, the speedy reaction of the stigma might help to reduce pollen loss, enhance pollen germination and avoid obstructing pollen export. Stigmas showed low sensitivity when touched by inferior pollinators, which may have increased the possibility of pollen deposition by subsequent visits. Therefore, the stigma behavior in M. miquelii is likely a mechanism of pollinator selection to maximize pollination success.

Losing one's touch: Evolution of the touch‐sensitive stigma in the Mimulus guttatus species complex

The stigmas of several species are touch sensitive and respond to pressure by closing. Previous research suggests that stigma closure could prevent self pollination within a flower during a pollinator's visit or enhance male function by increasing pollen export. Both factors could be favored in outcrossers, and neither would be beneficial in selfers. We investigated variation in stigma-closing and the duration of closure in annual and perennial populations of the variable species Mimulus guttatus and whether four closely related selfing species (M. cupriphilus, M. laciniatus, M. nasutus, and M. pardalis) have lost their touch sensitivity. We grew plants in a controlled environment and performed experiments with and without the addition of pollen to the stigma. In M. guttatus, the speed of stigma-closing was rapid and unaffected by the deposition of pollen. Populations varied significantly in closing speed, which may reflect their geographic location. For annual populations only, anther-stigma separation significantly affected closing speed. Also, stigmas that closed quickly stayed closed longer, and stigmas that received pollen remained closed longer. Finally, in the selfing species, stigma-closing was more variable; some populations have entirely lost the ability to respond to touch. We discuss our results in the context of traits that promote outcrossing and traits that are under selection during the evolution of selfing. This is the first characterization of variation in touch responses across multiple populations within a species and the first to demonstrate the loss of touch sensitivity in selfing lineages.

Florivory and pollinator visitation: a cautionary tale.

Florivory, or damage to flowers by herbivores, can make flowers less attractive to pollinators, potentially resulting in reduced plant fitness. However, not many studies have combined observations with experiments to assess the causal link between florivory and pollination. We conducted field observations at eight sites in northern California, combined with field experiments that involved artificial floral damage, to study the effect of florivory on pollination in the hummingbird-pollinated sticky monkeyflower, Mimulus aurantiacus We used two indicators of pollinator visitation, stigma closure and the presence of microorganisms in floral nectar. The field observations revealed that stigma closure was less frequent in damaged flowers than in intact flowers. In the experiments, however, floral damage did not decrease stigma closure or microbial detection in nectar. Instead, neighbouring flowers were similar for both indicators. These results suggest that the observed negative association between florivory and pollination is not causal and that the location of flowers is more important to pollinator visitation than florivory in these populations of M. aurantiacus.

BREEDING BARRIERS IN RED PHALAENOPSIS ORCHIDS

Breeding barriers in producing hybrids by intergeneric, inter- and intraspecific hybridization in Phalaenopsis have been observed. In this study, four cultivars of red Phalaenopsis which are known to be difficult in hybridization, including Doritaenopsis Ever Spring Prince '#1' (ESP), Dtps. Queen Beer 'KHM 159' (QB), Dtps. Taida Salu `KHM 101' (TS), and Phalaenopsis Brother Girl 'B' (BG) were investigated for the mechanisms underlying the crossing barriers. Root tip squash observation showed that all of these selected cultivars are triploids with 2n = 3x = 57 chromosomes. To test their crossing behaviors, these four cultivars were either self-crossed, or used as the pollen and pistillate parents in reciprocal crossing with three cultivars with normal behavior in crossing, including Phal. Brother Irene 'Feng Fong', Phal. Sogo Yukidian 'V3', and Phal. Tai Lin Redangel 'V31'. These normal cultivars are tetraploids with 2n = 4x = 76 chromosomes. The crossing results showed that the closure of stigma cavities occurred in TS and BG, while such stigma closure was never observed when QB or ESP was used as the pollen parent in crosses. However, hybrid progenies were obtained only from the crosses between ESP or TS (pistillate parent) with the normal cultivars. Taken together, these results suggest that crossing difficulties in these four selected cultivars may be caused by odd chromosome set and some other factors of male or female parents.

Stigma closure and re‐opening in Oroxylum indicum (Bignoniaceae): Causes and consequences

The study of plant responses to touch, particularly the responses of leaves, stems, and roots, has a long history. By contrast, floral responses are relatively unexplored. Stigma closure is common in the Bignoniaceae, but the factors influencing it are not well understood. We investigated factors influencing stigma closure and reopening and its effects on pollen receipt in seven Oroxylum indicum trees near Hat Yai, Thailand. The effects of pressure, conspecific and heterospecific pollen, and pollen load (the amount of pollen deposited) on stigma behavior were examined in 270 flowers (of the total 430 flowers evaluated in the entire study). Pressure alone resulted in faster closure than did conspecific pollination and faster reopening than did heterospecific pollination. Stigmas never reopened after conspecific pollination. Pollen load had no effect on stigma behavior. Stigmas discriminated between conspecific and foreign pollen; they reopened only after pollination with the latter. A manipulative experiment revealed that stigma closure did not affect the number of conspecific pollen grains received. We also counted pollen tubes in styles that were either hand-supplemented with outcross conspecific pollen or open-pollinated. Pollen tube numbers were highest after light pollination (∼900 grains), indicating that interference among pollen grains may occur after pollination with very heavy loads (>6000 grains). Possible fitness consequences of these responses are discussed.

Permanent stigma closure in Bignoniaceae: Mechanism and implications for fruit set in self-incompatible species

The Bignoniaceae possess sensitive bilobate stigmas that close after stimulation. We determined factors involved in stigma closure and reopening in four species of Bignoniaceae from the National Parks of Catimbau, Northeastern Brazil ( Anemopaegma laeve, Arrabidaea limae, Jacaranda rugosa and Tabebuia impetiginosa). The study showed that any mechanical touch promoted the stigmatic closure. Only the deposition of viable con-specific pollen (self- and cross-pollen), however, maintained stigmas permanently closed. In Arrabidaea limae part of the stigmas reopened after self- and cross-pollination. After mechanical stimulation, deposition of pollen from other plant species, of dead con-specific pollen or of sand, the stigma lobes opened again after 38 to 68 min. The definitive closure may have a negative effect on the reproductive success of the involved plants, because the stigmas are permanently unavailable for pollen deposition. In self-incompatible species with mass-flowering blooming pattern, where pollinators promote high levels of geitonogamy, definitive stigma closure causes low fruit set.

Biologia floral e polinização de Anemopaegma chamberlaynii Bur. & K. Schum. (Bignoniaceae).

The present work deals with the floral biology, reproductive system and phenological events of Anemopaegma chamberlaynii Bur. & K. Schum. (Bignoniaceae), in the coastal strand vegetation (restinga) of Maricá RJ., between 1997 and 2000. The flowers display daytime anthesis, are yellow, very attractive, tubulous, hermaphrodite, odoriferous and produce nectar as the floral reward. The nectar is secreted by a nectariferous disk and accumulated in a chamber. The stigmas are bifid, wet and papillous. The stigmatic lobes are receptive to pollen only on the inner surfaces and are sensitive to disturbance. If the stigma is touched but no pollen is deposited, the lobes close and soon reopen. The same occurs when self-pollen is deposited on the receptive surface. However, after manual pollination with cross-pollen the lobes close permanently, enabling to estimate levels of pollination by monitoring stigma closure. Euglossa cordata L., Epicharis dejeanii Lepeletier and E. semiflava Moure are potential pollinators. Xylocopa ordinaria Smith is the primary nectar robber. The species is self-incompatible and has a low fruit set (Fruit/Flower=4.86%). The flowering pattern is the “multiple bang”, showing five episodes per year, with the same intensity, all of them occurring in the most hot and rainy months.

Benefits and costs of floral visitors to Chilopsis linearis: pollen deposition and stigma closure

In pollination mutualisms, nectar‐robbers are usually considered antagonists; visitors that enter flowers (legitimate visitors) are usually considered mutualists. However, nectar‐robbers may provide some benefits to plants, whereas legitimate visitors may inflict some costs. The costs and benefits of floral visitors to Chilopsis linearis (desert willow) were compared by number of pollen grains deposited and their effect on stigmas. Because these plants are self‐incompatible and pollen‐limited, they depend on visitors for services by pollinators to reproduce. On a per‐visit basis, only one legitimate visitor, Bombus sonorus (bumblebees) generally benefited plants in terms of pollen deposition. However, no species of visitor was consistently beneficial; every one was at least sometimes ineffective in terms of pollen deposition. Chilopsis had sensitive stigmas that closed immediately upon touch and sometimes reopened later. Whether stigmas remained permanently closed or reopened depended on number of pollen grains deposited and tended also to be affected by source either, outcross or self. Legitimate visitors sometimes cost plants by causing stigma closure without depositing enough pollen to set a fruit. When abundant, a visitor such as Apis mellifera (honeybees) that was only occasionally beneficial on a per‐visit basis may have provided a greater benefit to plants as a population than a more effective, but rare visitor. In contrast, nectar‐robbers did not benefit plants by depositing pollen grains, but they also did not inflict costs on plants by causing stigmas to close without adequate pollen. There are two general implications of these results. First, individuals of species that appear to be mutualists can vary greatly in the benefits that they give to their partners. Second, apparent mutualists can inflict costs on plants that apparent exploiters (nectar‐robbers) do not.

Hummingbird foraging position is altered by the touch-sensitive stigma of bush monkeyflower.

The two stigma lobes of bush monkeyflower (Mimulus aurantiacus) close together rapidly in response to touch by a hummingbird pollinator and usually remain closed for the life of the flower, preventing further pollen receipt. Previous work showed that hummingbirds visiting bush monkeyflowers with closed stigmas export more than twice as much pollen to recipient flowers as birds visiting flowers with open stigmas. To investigate how stigma closure increases pollen export, we used videotape to examine the interaction between bird and flower. Hummingbirds altered their method of foraging in response to whether the stigma was open or closed. When stigmas were closed, birds most frequently entered flowers from above, relative to the long axis of the corolla tube, bringing the bill close to the ceiling of the corolla where anthers and stigma are positioned. However, birds were much more likely to enter corollas from below when stigmas were open, keeping the bill farther from the sex organs, and resulting in failure to contact open stigmas 33% of the time. Distance of the bill from the corolla-tube ceiling was associated with frequency of contact with anthers and, when contact was made, explained pollen export in a non-linear fashion. Morphometric analyses showed that smaller corolla openings promoted stigma contact, but corolla openings that were wide relative to their height produced increased contact with anthers. Changes in hummingbird foraging position and reduced pollen export when visiting flowers with open stigmas helps to explain the observation of permanent stigma closure, even when less than full seed set has been achieved.

Resolution of male-female conflict in an hermaphroditic flower.

The flowers of most angiosperm species are hermaphroditic. Spatial separation of male and female organs within a flower (hercogamy) is a common character traditionally interpreted as an adaptation to reduce intrafloral self-fertilization, one potential cost of hermaphroditism. Another possible cost that may lead to selection for hercogamy is physical interference between male and female floral functions. Here, I present evidence demonstrating the role of a floral character in reducing female interference with male function. The bi-lobed stigma of the bush monkeyflower closes after receiving pollen, causing increased spatial separation of the anthers and stigma ('movement' hercogamy). Experimental manipulations show that flowers with closed stigmas export more than twice as much pollen to other flowers as those in which the stigma is prevented from closing. However, stigma closure only minimally reduces the potential for intrafloral self-pollination. This study provides the first experimental evidence that selection to reduce intrafloral male female interference can be a strong selective force and can drive the evolution of floral characters usually interpreted as mechanisms to reduce self-fertilization.

Interspecific pollen transfer as a mechanism of competition: effect of Castilleja linariaefolia pollen on seed set of Ipomopsis aggregata

We investigated whether the deposition of Castilleja linariaefolia (Pursh) V. Grant pollen on flowers of Ipomopsis aggregata (Benth.) in DC. reduced I. aggregata's seed set. Ipomopsis flowers were hand-pollinated with either pure conspecific pollen (the control) or with C. linariaefolia pollen applied prior to, simultaneously with, or following conspecific pollen. Flowers that were simultaneously given C. linariaefolia and I. aggregata pollen received 39% less conspecific pollen than flowers in the control, but did not set fewer seeds per fruit. Flowers that were given C. linariaefolia pollen first received 30% less conspecific pollen and set 38% fewer seeds per fruit than the control. These results suggest that there is a priority effect; deposition of C. linariaefolia pollen on I. aggregata flowers results in competition, but only when C. linariaefolia pollen is deposited first. Unlike pollen of another competitor of I. aggregata (Delphinium nelsonii Greene. Per.), C. linariaefolia pollen caused competition without inducing an increased rate of stigma closure. Only 12% of I. aggregata's stigmas closed when C. linariaefolia pollen was deposited, and seed set of I. aggregata was reduced relative to the control whether their stigmas closed or not. The low rate at which C. linariaefolia pollen induces closure of I. aggregata's stigmas could be an adaptation to reduce the negative effects of competition.Key words: Castilleja linariaefolia, competition, hand pollination, hummingbirds, Ipomopsis aggregata, pollination.

Stigma behavior in Mimulus aurantiacus (Scrophulariaceae)

The bilobed stigma of many species in the order Scrophulariales closes in response to touch by an animal pollinator. In hummingbird-pollinated bush monkey flower, Mimulus aurantiacus (Scrophulariaceae), closure is rapid, occurring within seconds of tactile stimulus. We investigated the proximate causes of stigma closure and subsequent reopening in M. aurantiacus, as well as potential costs and benefits of stigma closure for female fitness. Stigma closure is elicited by both touch and pollen, but closure in response to pollen is much slower, requiring 0.5-1.5 h. Stigmata reopen within 2.5-4.5 h if touch, but no pollen, is applied. Upon receipt of pollen, most stigmata remain closed for the remaining lifetime of the flower, even if less pollen is received than is needed for full seed set. Those stigmata that do reopen after pollination generally require between 20 and 28 h to do so, much longer than for unpollinated stigmata. Reopening after pollination appears to be a response to low seed set rather than to low pollen load. Natural pollination of stigmata manipulated to prevent closure shows that closure does not increase capture of pollen or seed set. In fact, closure reduces the average pollen load received by flowers. Despite this, there is no evidence that stigma closure has any negative effect on female fitness in terms of seed set or germinability. Hypotheses for the adaptive significance of stigma closure are discussed. Understanding proximate causes of stigma closure and reopening is essential in the evaluation of these hypotheses.

Pollen precedence and stigma closure: a mechanism of competition for pollination between Delphinium nelsonii and Ipomopsis aggregata.

Previous experiments showed that the sympatric herbs Delphinium nelsonii and Ipomopsis aggregata compete for hummingbird pollination and that deleterious effects of the former species on seed set of the latter involve interspecific pollen transfer. However, seed set was not reduced when pollen of both species was applied simultaneously to I. aggregata stigmas. Hence a competitive effect may require arrival of foreign pollen before conspecific pollen. To explore this possibility we subjected I. aggregata flowers to a "competition" treatment in which they received D. nelsonii pollen 6 h before I. aggregata pollen, or to a "control" in which they received only the conspecific pollen. Foreign pollen precedence decreased mean seed set by almost 50%, which is consistent with effects observed in previous experiments. Reduced seed set can be explained by the fact that foreign pollen often caused stigma lobes to close together within 1.5-6 h, reducing subsequent receptivity. Stigma closure was also elicited by conspecific pollen, but not by mechanical stimulation, and was influenced by size of the pollen load and identity of the plant being pollinated.