N11 2015 Northeastern Naturalist Notes Vol. 22, No. 3 B. Heinrich Rapid Flower-Opening in Iris pseudacorus Bernd Heinrich* Abstract. Iris pseudacorus (Yellow Flag Iris), normally produces 1 flower, or exceptionally, it can produce more than 1 flower per stalk at any one time. Flower buds and opened flowers are available at all times every day, but the transition stages between buds and expanded flowers are not. I here report that the flowers open explosively and discuss the proximate cause and the potential significance of this behavior. Iris (iris) flowers consist of 3 basic parts: 3 large, drooping leafy sepals; 3 vertical, often nearly equally showy petals (the banner); and a leafy style. The sepals have a greatly enlarged hanging lip with markings that serve as a nectar guide that help orient pollinators into a chamber under the style that serves as the roof of a tube formed by the style of the flower, with its stigma at the tube entrance, and the anther with its support closely adjoined. Pollen is both received and delivered as the pollinator enters the flower at one of 3 similar points. In most flowers, the style is a simple rod, but in irises it is flattened and flanged at the sides. The flower bud is round and spike-like, with the petals tightly wrapped around the other parts and each other. I observed a small Iris pseudacorus L. (Yellow Flag Iris, hereafter Yellow Flag), a nonnative plant, growing directly along the Atlantic shoreline of Star Island, near the border of Maine and New Hampshire. Replanted, it eventually flowered next to my doorstep in Maine. I watched it daily throughout June 2014, observing how the conical flower buds metamorphosed into resplendent flowers. Here, I reconstruct that transition based on observations from photographs and sketches. Early in the flower-opening cycle, the stem holding the Yellow Flag flower bud extended in length so that the bud reached above the 2 leaf bracts that had surrounded it the day before. Subsequently, the flower bud expanded near the base several hours before opening. A top view showed the 3 sepals curled into a swirl near their tips (Fig. 1). The opening of all sepals to nearly full extension occurred literally from one glance to the next in ~1 second. However, as in other irises, the 3 (tiny in this species) banner petals stayed upright. The flower remained fresh for 2 days, after which the petals again coalesced into a coil around each other and then shrank. The ovary grew, and the remains of the petals dried and dehisced. Flower opening and closing, as well as some other movement in plants, (e.g., Mimosa leaf movements for predator defense; Volkov et al. 2010), involve volume changes in different structures that expand or shrink as water is shifted in or out of expanding tissue. Water shifts by osmosis result in part from uptake of sugars and conversion from polysacharides to monosacharides (van Doorn and Kamdee 2014, van Doorn and van Meeteren 2003). Iris flower opening has been reported to require elongation of the flower pedicel and ovary, which is largely controlled by plant-growth hormones (Celikel and van Doorn 2012, van Doorn et al. 2013). Growth and osmotic pressure are involved in iris-flower expansion, but such gradual processes might not explain the mechanics of the sudden movement I observed in Yellow Flag. The growth of the pedicel beneath the flower is rapid and apparently allows the *PO Box 153, Weld, ME 04285; bheinrich153@gmail.com. Manuscript Editor: Greg Spyreas Notes of the Northeastern Naturalist, Issue 22/3, 2015 2015 Northeastern Naturalist Notes Vol. 22, No. 3 N12 B. Heinrich flowers to open very quickly. But the snap of petal extension, as observed here, would require prior storage of energy, followed by a trigger mechanism for sudden release, perhaps following the same pathway that causes the forcible ejection of seeds by a number of plant species (Stamp and Lucas 1983), including the common Impatiens capensis Meerb. (Orange Jewelweed) that can throw its seeds several meters from the fruit (Smitt et al. 1985). Sudden movements resulting from release of stored energy are common in arthropods (Heinrich 1993) and involve a variety of physical arrangements in which muscles may contract slowly, while the energy of their contraction is stored by a hold-fast. Release of the stored energy occurs in a burst, but only after a specific threshold of force is reached that overwhelms the hold-fast. Presumably, the iris’s stored energy would derive from increased water pressures in the flower tissues which would cause them to bend under tension; the banner tissue might become inflated relative to the sepal tissue that covers it in bud. This scenario would require a holding mechanism. In this case, just prior to opening, the petals, which are wrapped around each other in the unfolding bud, appear to be held in place by their contact near the sepal tips (Fig.1). This stable position might then persist until sufficient outward force builds up to release the hold of the petal s around each other. The functional significance of the iris flower’s structure, flowering schedule, and rapidopening mechanism, may have evolved under the selective pressures of pollinator behavior. Figure 1. Sketches from life and photographs of the stages in the cycle of a flower. Numbers 1–4 show flower-bud development. Number 4, with side and top views, show the bud before opening. Number 5 shows side and top views of the flower 1–2 seconds after openi ng. N13 2015 Northeastern Naturalist Notes Vol. 22, No. 3 B. Heinrich Although I observed Archilocus colubris L. (Ruby-throated Hummingbird) visiting the iris, these flowers likely evolved under the selective pressure of large bees that visit the flowers in search of nectar. Naïve bees travel and sample widely, but eventually tend to frequent flowers that provide sufficient rewards (Heinrich 1976). Thus, if a bee keeps encountering empty flowers that she identifies by their location and floral signals, she will learn to avoid such flowers. Each iris flower’s flag-like sepals have 2 nectaries at their base. I used 20-μL disposable glass pipettes to extract the nectar from individual nectaries in the Yellow Flag flowers I studied and found that they yielded up to 2 μL. This nectar would not be enough to satiate a large bee, but it is probably enough to stimulate her to search for another flower of the same kind (Heinrich 1975b). My isolated Yellow Flag plant produced seeds. The plant was growing in a forest clearing, with no other plants of its kind; thus, these seeds were unlikely to have been the product of cross-pollination (except for perhaps some hybridization with other species of iris in the area, though that is thought to be uncommon; G. Spyreas, Illinois Natural Hitory Survey, Champaign, IL, pers. comm.). The fruits of this plant eventually dehisced, and the seeds were large enough (7–8 mm in diameter) that they likely would have dropped straight down. Yellow Flag also spreads by vegetative growth. Taken together, these observations suggest that clumped or neighboring Yellow Flag might reflect self-pollination or vegetative reproduction. In order for this plant to achieve cross-pollination, pollinators may need to travel far. Plants using nectar as a reward must offer an adequate amount of food energy to retain flower-constant bees (Heinrich 1975a, b). But, a bee that repeatedly visits the same plant (or clone) might not outcross it unless it also visits other (perhaps isolated) individuals of the same species. With its relatively large seeds, which might limit dispersal potential, neighboring Yellow Flag plants could become genetically inbred without longer distance outcrossing events. Have these irises evolved mechanisms to encourage cross-pollination through increased inter-plant pollinator travel, despite the potential costs? Yellow Flag flowers must be visible from afar to attract naïve bees, and at the same time its large yellow sign must be an honest advertisement of reward. The complex flower, however, could be a misleading signal if nectar availability was delayed by slow, gradual flower expansion. Instant flower opening would preserve honesty of advertising because naïve bees will not be fruitlessly attempting to enter it before nectar is present. Furthermore, the briefer the duration of the flower’s availability for fertilization, the less likely it will be revisited by the same bee. Comparative data with other species’ natural history might help to bracket potential selective pressures for the observed explosive flower opening, and differentiate it as either an adaptation for increasing the percentage of out-crossed versus selfed flowers, or as an incidental by-product of chance. Acknowledgments. I greatly appreciate the critical reviews and helpful suggestions by John Alcock and 2 anonymous reviewers. Literature Cited Celikel, F.G., and W.G. van Doorn. 2012. Endogenous ethylene does not regulate opening of unstressed iris flowers but strongly inhibits it in water-stressed flowers. Journal of Plant Physiology 169:1425–1429. Heinrich, B. 1975a. Bee flowers: A hypothesis on flower variety and blooming times. Evolution 29:325–334. Heinrich, B. 1975b. Energetics of pollination. Annual Review of Ecology and Systematics 6:139–170. Heinrich, B. 1976. Foraging specializations of individual bumblebees. Ecological Monographs 46:105–128. 2015 Northeastern Naturalist Notes Vol. 22, No. 3 N14 B. Heinrich Heinrich, B. 1993. The Hot-Blooded Insects. Harvard University Press, Cambridge, M A. 600 pp. Smitt, J., D. Ehrhardt, and D. Swartz. 1985. Differential dispersal of self-fertilized and outcrossed progeny in Jewelweed (Impatiens capensis). American Naturalist 126:570–575. Stamp, N.E., and J.R. Lucas. 1983. Ecological correlates of explosive seed dispersal. Oecologia 59:272–278. van Doorn, W.G., and C. Kamdee 2014. Flower opening and closure: An update. Journal of Experimental Botany 65:5749–5757. van Doorn, W.G., and U. van Meeteren. 2003. Flower opening and closure: A review. Journal of Experimantal Botany 54:1801–1812. van Doorn, W.G., I. Dole, F.G. Celikel, and H. Harkema. 2013. Opening of iris flowers is regulated by endogenous auxins. Journal of Plant Physiology 170:161–164. Volkov, A.G., J.C. Foster, K.D. Baker, and V.S. Markin. 2010. Mechanical and electrical anisotropy in Mimosa pudica pulvini. Plant Signaling and Behavior 5:1211–1221.