Position‐dependent reproductive success of flowers in Dactylorhiza maculata (Orchidaceae)

Abstract

Summary1. This study explores the importance of flower position for reproductive success of individual flowers of the deceptive terrestrial orchid, Dactylorhiza maculata (L.) Soó. 2. Upper flowers had smaller lips and produced lighter pollinia than those in other parts of the inflorescence, probably due to architectural effects. 3. Dry weight of seed capsules decreased from bottom to top in the inflorescence in both open-pollinated and hand-pollinated plants. However, removal of flowers from the middle and lowest parts of the inflorescence considerably increased seed production of the upper flowers. Decreased seed set was due to decreased resource availability because of the long maintenance time of opened but not pollinated lower flowers. 4. Experimental self-pollination resulted in reduced seed set compared to that following pollination with cross-pollen, but decreased seed set was independent of flower position within an inflorescence. 5. Dactylorhiza maculata with large inflorescences did not have better pollination success. The production of more flowers than the plant is able to fully support may have evolved as an ovary reserve in case of flower herbivory, or as a 'bet-hedging' strategy through which the plant can increase seed production in years with good pollinator availability.

Pollination success in monochromic yellow populations of the rewardless orchid Dactylorhiza sambucina

“…Indeed, when Darwin (1885: p. 41-42) tested the expectation of seasonally changing fruit set in Dactylorhiza maculata (L.) Soo´, albeit using only six plants, he found that the flowers in the lower halves of the spikes did not consistently have fewer capsules than those in the upper halves. Using samples of 19, 14 and 15 plants, Vallius (2000) found that in open-pollinated Finnish populations of D. maculata, capsule set was greatest in the middle part of the spikes in one year, while there was no difference in capsule production among different spike sections in the other two years. More experiments with individual bees of known foraging history are therefore needed to resolve what tips the balance between bees investing in learning to avoid rewardless plant individuals vs. following other behavioral programs, such as periodically (re-)sampling new and old food sources.…”

Modularity and intra-floral integration in metameric organisms: plants are more than the sum of their parts

“…Corolla lip length of the orchid Dactulorhiza declines with flower position but spur length does not [82]. In flowers of Aquilegia canadensis, nectar spur, gynoecium and stamen lengths are invariant with position while the size of the showy sepals declines [83].…”

Fruit size decline from the margin to the center of capitula is the result of resource competition and architectural constraints

“…Although fruit size and/or fruit set declines have been shown in several species in earlier studies (reviewed in Diggle 2003), these were attributed either to competition for resources (Medrano et al 2000;Vallius 2000;Kliber and Eckert 2004) or to architectural constraints (Byrne and Mazer 1990;Wolfe and Denton 2001;Guitián et al 2001). Very likely, the relative importance of architectural constraints and resource competition is species-or even trait-speciWc (Ashman 1992;Wolfe 1992;Diggle 1997;Ashman and Hitchens 2000;Kliber and Eckert 2004).…”

“…The investment decline in Xowers and fruits has most often been described for linear inXorescences, such as racemes (Byrne and Mazer 1990;Susko and Lovett-Doust 1999;Vallius 2000;Kudo et al 2001;Wolfe and Denton 2001;Hiraga and Sakai 2007;Zeng et al 2008). Although position eVects are also known in plants having other inXorescence architectures, such as umbels (Wyatt 1980) and capitula (Zohary 1950;Burtt 1977), these structures have been studied much less.…”