Does Ethylene Treatment Mimic the Effects of Pollination on Floral Lifespan and Attractiveness?

Annals of Botany 89: 375±383, 2002 doi:10.1093/aob/mcf053, available online at www.aob.oupjournals.org

WOUTER G. VAN DOORN1 1Agrotechnological Research Institute (ATO), Wageningen University and Research Centre, PO Box 17, 6700 AA Wageningen, The Netherlands

Received: 21 June 2001 Returned for revision: 10 November 2001 Accepted: 15 December 2001

In some species pollination may result in rapid changes in perianth colour and form (petal senescence and abscission, ¯ower closure), rendering the ¯owers less attractive to pollinators. It has been suggested that this effect is mediated by ethylene. Flowers from about 200 species and 50 families were exposed to ethylene (3 ppm for 24 h at 20 °C). The effects on petal senescence and abscission have been described previously. Flower closure and perianth colour changes were generally ethylene-sensitive, but responses showed no consistency within families. Several ¯owers known to respond to pollination by rapid cessation of attractiveness were also exposed to ethylene: this produced the same effect as pollination, both on ¯ower colour and form. Species that respond to pollination by changing ¯ower form or colour were found exclusively in families in which the species are generally ethylene-sensitive (with regard to changes in perianth form and colour). However, several families are generally ethylene-sensitive but contain no species reported to respond to pollination. ã 2002 Annals of Botany Company

Key words: Ethylene sensitivity, ¯ower closure, ¯ower longevity, petal abscission, petal colour, petal wilting, petal withering, petal senescence, pollination.

INTRODUCTION Woltering et al., 1995, 1997; Hilioti et al., 2000; Llop- Flowers are attractive to pollinators because they provide a Tous et al., 2000). Many ¯owers change colour when their stigmas become source of food, and they advertise this by their form and colour. Petals of ¯owers that remain unpollinated eventually non-receptive. This change coincides with the cessation of nectar production or pollen availability. Pollinators avoid wither or abscise; this may be preceded by a change in ¯owers that no longer produce a reward, and this results in colour or by ¯ower closure. In some species pollination increased pollinator ef®ciency. Retention of the perianth reportedly advances these changes in ¯ower colour or form, beyond stigma receptivity (and pollen availability) is but in other species it does not (Motten, 1986). Where suggested to increase the plant's longer-distance attractive- pollination has an effect, the time course of the symptoms ness to pollinators, whereas at close range the pollinators may depend on the pollen load (Stead and Moore, 1983). It has been suggested that advanced petal wilting and will discriminate between the ¯oral colour phases. Although changes in petal colour occur towards the end of ¯ower life abscission, following pollination, is mediated by endogenous ethylene, although experimental evidence has been in at least 74 plant families (Weiss, 1991), an effect of pollination on petal colour has been described in only a reported for a few species only. Inhibitors of ethylene few species (Gori, 1983). Similarly, only a few studies synthesis or ethylene action prevented the effect of pollin- mention an effect of ethylene on ¯ower colour, for example ation on petal wilting in carnation (Nichols et al., 1983; in Petunia (Solanaceae; Gilissen, 1977), Cymbidium Larsen et al., 1995), orchids (O'Neill et al., 1993; Porat (Orchidaceae; Woltering and Somhorst, 1990) and et al., 1995) and Petunia (Hoekstra and Weges, 1986). In Lupinus (Fabaceae; Stead and Reid, 1990). In Cymbidium Digitalis (Stead and Moore, 1983) and Pelargonium (Hilioti and Lupinus, inhibitors of ethylene synthesis and ethylene et al., 2000), anti-ethylene compounds prevented the action prevented the colour change, indicating that the pollination effect on petal fall. Thus, although ethylene changes are produced by endogenous ethylene. seems to be involved in the production and/or the effect of Several ¯owers close permanently at the end of their the pollination signal, the nature of the agent that is lifespan. Pollination may hasten closure, e.g. in several transported from the stigma to the petals is still unclear, and orchids (Fitting, 1909). Earlier ¯ower closure in may differ between species. It has been suggested that the Phalaenopsis, following pollination, was prevented by signal transported is electrical in nature (Spanjers, 1977), or inhibitors of ethylene action and ethylene synthesis, that it is ACC, ethylene or other factors affecting ethylene indicating a role of endogenous ethylene (Porat et al., production or ethylene sensitivity (Porat et al., 1995; 1994). For correspondence. Fax: +31±317±475347, e-mail: w.g.vandoorn@ Ethylene treatment of ¯owers results in rapid petal ato.wag-ur.nl wilting or abscission, depending on the species. Petal ã 2002 Annals of Botany Company 376 van Doorn Ð Comparing the Effects of Ethylene Treatment and Pollination on Petals wilting has been found to be ethylene-sensitive or insensi- potted plants or at least ®ve ¯owers were used per chamber, tive, and these two categories were consistent within and two replicate chambers were used. Each species was families or subfamilies. Species in which pollination tested at least twice. Data on ¯ower closure and ¯ower advances petal wilting tended to belong to families in colour were collected from experiments performed on which most species showed ethylene-sensitive wilting. approx. 200 species (van Doorn, 2001). The present data Many other ¯owers exhibit petal fall, which is generally have thus been collected from the same plants as previously highly ethylene-sensitive. Petal fall is also generally con- published data on petal senescence and abscission. When sistent within families or subfamilies (Woltering and van comparing data on commercial species from different Doorn, 1988; van Doorn, 2001). These data, and reports on experiments, the cultivars used may have differed as the the effects of ethylene on petal colour and ¯ower closure, cultivar names were not always known. have led to speculation that the effects of pollination on ¯ower form and colour are generally mediated by endogenous ethylene (van Doorn, 1997). This suggestion Classi®cation of ethylene sensitivity has now been investigated further. A number of species that After ethylene treatment, the potted plants and ¯owers are known to terminate pollinator attractiveness rapidly were placed under controlled environmental conditions of following pollination were treated, whilst unpollinated, with 12 h ¯uorescent white light (15 mmol m±2 s±1) and 12 h exogenous ethylene. If the effects of pollination on ¯ower darkness, 60 % relative humidity and 20 6 1 °C. Changes in form and colour are indeed mediated by endogenous the perianth were determined daily. Ethylene sensitivity was ethylene then each of these ¯owers should be sensitive expressed as described previously (van Doorn, 2001). to exogenous ethylene and the symptoms of ethylene Effects were expressed as the percentage of the time taken treatment should be similar to the changes observed after for the symptoms to occur, compared with the controls. For pollination. example, a 100 % response indicates that clear symptoms A detailed comparison between the effects of ethylene occurred in all plants within 1 d of treatment. Similarly, a and pollination requires data on ¯ower colour and closure. 50 % response indicates that the symptoms occurred within Since there are limited data in the literature, ¯ower closure half the time taken by the controls. These percentages were and petal colour were studied here in approx. 200 species then grouped into ®ve classes as follows: class 0, no from 50 families. response (not sensitive); class 1, up to 33 % effect (low sensitivity); class 2, between 33 and 66 % effect (inter- mediate sensitivity); class 3, 66±99 % effect (high sensi- MATERIALS AND METHODS tivity); class 4, ethylene response already dramatic at the end of treatment (very high sensitivity). Plant material Potted plants were bought at a ¯ower auction at Aalsmeer (The Netherlands) or at retail outlets in Wageningen (The Taxonomic classi®cation Netherlands). The plants were well watered, and used for Plants were grouped into families according to the experiments on the day of purchase. Several other species classi®cation of the Angiosperm Phylogeny Group (APG, were obtained as cut parts, either from the Botanical Garden 1998), which is partially based on molecular techniques. of the Agricultural University in Wageningen, or from the Compared with older classi®cations such as that of ®eld. After severing, the ¯owering stems were immediately Heywood (1978), APG (1998) subtracts the basal-most placed in water. Stems were transported to the laboratory angiosperms from the dicotyledons, and calls the remaining within an hour of cutting, where they were used immedi- dicots `eudicotyledons'. Compared with the system of ately. Cut ¯owers from a few other species were bought at Heywood (1978), Dahlgren et al. (1985) separate the the Aalsmeer ¯ower auction. These were taken to the Alstroemeriaceae from the Amaryllidaceae and group laboratory (without water) within 3 h of purchase. The stems the Liliaceae in several families. APG (1998) largely of these ¯owers were recut under water and used for con®rmed the classi®cation of the monocotyledons pro- experimentation the same day. posed by Dahlgren et al. (1985). In the eudicotyledons, APG (1998) include the former Lobeliaceae in the Campanulaceae, and show that Saxifragaceae is a highly Ethylene treatment polyphyletic group. APG place the Ribesioideae subfamily Treatments were carried out as described previously (van of the Saxifragaceae (sensu Heywood) in a separate family, Doorn, 2001). Brie¯y, ethylene was injected into closed 70 l the Grossulariaceae. APG uses the synonyms Asteraceae for stainless steel chambers held at 20 6 1 °C, in darkness, Compositae, Fabaceae for Leguminosae and Lamiaceae for exposing the plants or ¯owers to 3 (2´8±3´3) ppm for 24 h. Labiatae. Excess carbon dioxide was absorbed by calcium hydroxide in the chamber. Partial pressures of ethylene and carbon dioxide were checked regularly during the treatments. RESULTS Control ¯owers were stored under identical conditions except that ethylene was removed using Ethysorb Effects of ethylene on ¯ower closure (aluminium oxide and potassium permanganate; Stay Of the 200 species tested, ethylene treatment resulted in Fresh Ltd, London, UK). In each experiment at least three rapid ¯ower closure in a few monocotyledonous species and van Doorn Ð Comparing the Effects of Ethylene Treatment and Pollination on Petals 377

TAB LE 1. Plants that show ¯ower closure at the end of ¯oral life, and the effect of ethylene treatment (3 ppm in darkness, for 24 h at 20 °C) on closure

Family Species Flower closure, response class Other changes, response class

Monocotyledons Amaryllidaceae Cyrtanthus purpureus 2 w 2 Alliaceae Bloomeria aurantiaca 3 w 3 Convallariaceae Liriope koreana 3 w 0 Hemerocallidaceae Hemerocallis lilio-asphodelus 0 w 0 H. `Invictus' 0 w 0 H. `Mabel Fuller' 0 w 0 H. `Mrs. J. Tigert' 0 w 0 H. `Black Prince' 0 w 0 Hyacinthaceae Galtonia candicans 3 w 0 G. sp. 3 w 0 Lachenalia sp. 0 w 0 Liliaceae Erythronium americanum 3 wa 3 Orchidaceae Phalaenopsis sp. 4 w 4 Eudicotyledons Aizoaceae Delospermum cooperi 3 w 3 D. lyndenburgensis 3 w 4 Dorotheanthus bellidiformis 3 w 4 Carpathea pomeridiana 4 w 4 Lithops dorothea 3 w 3 Mesembryanthemum productum 3 w 3 Cactaceae Mammillaria sp. 3 w 3 Campanulaceae Campanula pyramidalis (r) 3 w 4 C. garganica 3 w 4 C. glomerata 3±4 w 3 Edraianthus graminifolius 3 w 3 Caryophyllaceae Dianthus caryophyllus 4 w 4 Convolvulaceae Ipomoea alba 4 w 4 Convolvulus arvensis 4 w 4 Quamoclit coccinea 4 w 4 Crassulaceae Crassula falcata 2 w 0 Echeveria setosa 4 colour 4, w 0 KalanchoeÈ blossfeldiana 4 colour 3, w 2 Sempervivum sp. 4 w 0 Gentianaceae Exacum af®ne 0 w 0 Gentiana dahurica 4 w 0 G. kochiana 2 w 0 G. sino-ornata 3 w 0 Sabatia sp. 2 w 0 Malvaceae Malva alcea 4 w 4 M. sylvestris 4 w 4 Plumbaginaceae Armeria maritima 4 w 3 A. pseudoarmeria 4 w 4 Ceratostigma plumbaginoides 4 w 4 Limonium latifolium 3 w 3 Plumbago auriculata 4 w 4 Portulacaceae Lewisia cotyledon 4 w 4 Portulaca grandi¯ora 4 w 4 P. umbraticola 4 w 4 Solanaceae Solanum dulcamara 3 w 3

Floral closure and effects of ethylene are compared with other changes that occur in petal colour or petal form at the end of ¯oral life and after exposure to ethylene. Changes in form are wilting (w), sometimes occurring almost concomitantly with petal abscission (wa). Response classi®cation as described in the text. Taxonomic classi®cation according to APG (1998). in several eudicotyledonous plants (Table 1). In the Orchidaceae, ¯ower closure was clearly advanced by monocotyledonous ¯owers tested, closure apparently did ethylene. In eudicotyledonous ¯owers, closure generally not occur throughout any of the families. Flower closure in responded to ethylene. Rapid ¯ower closure after Hemerocallis and Lachenalia occurred at the end of ¯ower ethylene treatment was observed in all species tested in life (concomitant with wilting) and closure did not respond the Aizoaceae, Convolvulaceae, Plumbaginaceae and to ethylene, and in Cyrtanthus (Amaryllidaceae) ethylene Portulaceae. Rapid closure also occurred in a few species had only a small effect. In several other monocotyledonous in the Campanulaceae, Crassulaceae and Gentianaceae. In ¯owers, such as Bloomeria, Galtonia and some contrast, no response was observed in Exacum af®ne 378 van Doorn Ð Comparing the Effects of Ethylene Treatment and Pollination on Petals

TAB LE 2. Plants that show a clear change in petal colour at the end of ¯oral life, and effect of ethylene treatment (3 ppm in darkness, for 24 h at 20 °C) on petal colour

Family Species Changes in petal colour, response class Other changes, response class

Monocotyledons Orchidaceae Cattleya sp. 4 w 4 Cymbidium sp. 4 w 4 Eudicotyledons Caprifoliaceae Weigela ¯orida 4 w 0 W. `Gustave Mallet' 4 w 0 Crassulaceae Echeveria setosa 4 w 0 KalanchoeÈ blossfeldiana 3 w 2 Sedum hybrid 0 w 0 S. spectabile 0 w 0 S. spurium 0 w 1 Fabaceae Lotus corniculatus 4 w 1 Solanaceae Petunia hybrid 3 wa 4

Changes in petal colour and ethylene effects are compared with changes that occur in petal form at the end of ¯oral life and after exposure to ethylene (petal wilting or withering, w; petal wilting or withering occurring concomitantly with abscission, wa). Response classi®cation as described in the text. Taxonomic classi®cation according to APG (1998).

(Gentianaceae), whilst in Sabatia (Gentianaceae) and the species level. Table 4 shows that, even at the genus Crassula (Crassulaceae) the response was only slightly level, high consistency was found between the symptoms ethylene-sensitive. observed following pollination and the effects of ethylene treatment. Again, the symptoms were highly ethylene- sensitive. One exception was noted: in Pulmonaria Effects of ethylene on petal colour of®cinalis a colour change has been observed following Following exposure of approx. 200 species to exogenous pollination (SuÈssenguth, 1936), but in the present experi- ethylene, colour changes in the perianth were observed in ments petal abscission following ethylene exposure was the Orchidaceae, but not in any other monocotyledonous very rapid, apparently precluding a colour change. ¯owers studied. Colour changes following exposure to ethylene were observed in a few eudicotyledonous families, such as the Boraginaceae (all generea tested), Taxonomic comparison of the effects of ethylene treatment Caprifoliaceae (one genus) and Crassulaceae (two genera) and pollination (Table 2). A change in colour was usually followed by Table 5 compares the families in which we investigated wilting/withering of the petals. Colour changes that responses of petals to ethylene treatment with data regarding occurred concomitantly with petal wilting have not been effects of pollination on ¯oral attractiveness. Species known noted separately, as they may be due to the changes to respond to pollination by a change in ¯ower form or accompanying wilting, such as water loss. colour are restricted to families that show similar changes Colour changes (prior to petal wilting or petal abscission) after ethylene treatment. Table 5 also shows several families and ¯ower closure generally occurred in different species, in which the ¯owers generally exhibit an ethylene-sensitive but two species tested, both in the Crassulaceae, showed change in petal form or colour, whereas no incidence has both a colour change and ¯ower closure. A similar ®nding been reported for similar pollination effects. was reported in some orchid species (Fitting, 1909). DISCUSSION Comparison of pollination and ethylene effects In many species, ¯ower closure was sensitive to ethylene Table 3 lists a number of species that are reported to show (Table 2). However, in some species ethylene had only a advanced petal wilting, petal abscission, a rapid colour slightÐor noÐeffect on closure. Interestingly, no effect of change or ¯ower closure following pollination. After pollination on ¯ower closure has been reported in these treatment with ethylene the observed symptoms were the ethylene-insensitive species. A pollination effect on closure same as those following pollination. In all species studied may therefore be absent in this group of species. the responses were highly ethylene-sensitive. In the present experiments, ¯ower closure was not In Table 4 a comparison is made between reports in the observed in any of the families in which petals abscise. literature on the effects of pollination in a genus, and the This is apparently due both to the experimental conditions effects of ethylene treatment on species in the same genus. and to the high sensitivity of petal abscission to exogenous Several reports on the effects of pollination date from the ethylene: in many species the petals had already dropped by 19th century (or even before) and species' names may have the end of the ethylene treatment, and in others the petals changed since. Nomenclature has not been investigated in shattered within a day of treatment. Flower closure was detail. In other cases the plants have not been identi®ed at observed in several families in which ethylene-sensitive van Doorn Ð Comparing the Effects of Ethylene Treatment and Pollination on Petals 379

TAB LE 3. Comparison of the effects of pollination and exogenous ethylene on ¯oral attractiveness

Perianth change(s) following ethylene Family Species Perianth change(s) following pollination treatment and response class

Monocotyledons Liliaceae Erythronium americanum closure, w closure 3, wa 3 Lilium martagon a a 4 Eudicotyledons Boraginaceae Borago of®cinalis a a 4 Pulmonaria of®cinalis a* a 4 Campanulaceae Lobelia cardinalis w w 3 Caryophyllaceae Dianthus caryophyllus closure, w closure 4, w 4 Lychnis ¯os-cuculi w w 3 Ericaceae Vaccinium macrocarpon a a 4 Fabaceae Lotus corniculatus colour colour 4, w 1 Grossulariaceae Ribes aureum a a 3 Portulacaceae Portulaca umbraticola closure, w² closure 4, w 4 Solanaceae Nicotiana tabacum w wa 4 Lycopersicon esculentum wa wa 4

The species indicated, known to have a shorter period of ¯oral attraction following pollination (references in van Doorn, 1997), were exposed to ethylene (3 ppm in darkness, for 24 h at 20 °C). Responses were: petal abscission (a); petal wilting or withering (w); concomitant abscission and wilting/withering (wa); ¯ower closure; or a change in colour. Response classi®cation as described in the text. Taxonomic classi®cation according to APG (1998). * SuÈssenguth (1936) reports that in two Pulmonaria species tested (P. angustifolia and P. montana) petals change colour irrespective of pollination. Similarly, Oberrath et al. (1995) found that the colour change in Pulmonaria sp. occurs independently of pollination. Ethylene does not effect the colour change (van Doorn, 2001). ² Petal abscission is reported in this species in the literature, but in our tests wilting preceded abscission. petal wilting is found, and was absent from a number of by a change in the ultraviolet range (Silberglied, 1979; Gori, (sub)families which show ethylene-insensitive petal wilting 1983). Changes in the UV range and subtle colour changes (e.g. Dipsacaceae, the Ericoideae subfamily in the in the visible part of the spectrum may also occur following Ericaceae, Fumariaceae, Primulaceae and Saxifragaceae). ethylene treatment; these were not investigated in the However, in some species in which petal wilting is ethylene- present study. insensitive (such as Galtonia and Sempervivum), ethylene In species where pollination has an effect on ¯ower rapidly induced ¯ower closure. colour and form (time to petal wilting, petal abscission or A colour change, after ethylene treatment, was generally ¯oral closure), ethylene treatment produced the same visible ethylene-sensitive, but in one genus tested (Sedum) it was symptoms as pollination, and induced these symptoms not. In Pulmonaria species the change from red to blue also rapidly. Although only a limited number of species were occurs irrespective of pollination (SuÈssenguth, 1936; tested, the results are consistent with the idea that the effect Oberrath et al., 1995), and ethylene was found to have no of pollination on perianth form and colour is generally effect on colour (van Doorn, 2001). Thus, when it comes to mediated by endogenous ethylene (Table 3). This sugges- petal colour, a relationship between the effects of ethylene tion requires further testing, using ethylene inhibitors. The and pollination seems possible, as is the case for ¯ower responses have also been tested on some representative closure. When the petals are insensitive to ethylene, species within the same genus (Table 4). The results show a pollination also seems to have no effect. good ®t between pollination and ethylene effects and, In our tests, colour changes preceded petal wilting (or although this evidence is weaker than that from a compari- wilting that occurred concomitantly with abscission), but son of species, it does not contradict the hypothesis. did not precede petal abscission. This may be due to the The literature shows some examples in which the experimental conditions, as discussed above for ¯ower responses found after pollination are somewhat different, closure. Colour changes, after ethylene treatment, were at least in time, from the response found in unpollinated found in the Orchidaceae, but were not observed in any ¯owers. One example is Cyclamen, where the petals wilt other monocotyledon family tested. The species in these and desiccate in unpollinated ¯owers, but following pollin- monocotyledonous families are ethylene-insensitive with ation the petals show rapid fall while still turgid. An regard to petal wilting. Only a few species in the abscission zone is apparently activated by ethylene, as a eudicotyledons tested showed a colour change, although result of pollination, whereas in unpollinated ¯owers the the eudicotyledons comprise many families in which petal abscission zone is not activated (Halevy et al., 1984). The wilting is generally ethylene-sensitive. In these tests we eventual wilting of unpollinated Cyclamen ¯owers may be observed changes that were clear to the naked eye. Some similar to that occurring in ¯owers in which petal wilting is species exhibit a small colour change in the nectar guides ethylene-insensitive. Other examples are Potentilla argen- following pollination, which is often adequate to inform the tea and P. nepalensis. Here the petals of unpollinated pollinating insects (Gori, 1983). Other ¯owers may respond ¯owers reportedly remain turgid and ®nally fall, whereas 380 van Doorn Ð Comparing the Effects of Ethylene Treatment and Pollination on Petals

TA BL E 4. Comparison of the effects of pollination and exogenous ethylene on ¯oral attraction

Species in which pollination Perianth Species tested for Response, and Family response was tested change(s) ethylene effects response class

Monocotyledons Orchidaceae Cattleya bowringiana closure, w Cattleya sp. closure 4, w 4 Dendrobium superbum closure, w Dendrobium sp. closure 4, w 4 Phalaenopsis amabilis closure, w Phalaenopsis sp closure 4, w 4 P. violacea closure, w Eudicotyledons Cactaceae Mammillaria glochidiata closure, w Mammillaria spp. closure 3, w 3 Campanulaceae Campanula rapunculoides w Campanula garganica w 4 C. glomerata w 3 C. pyramidalis w 4 Cruciferae Cardamine angustata a Cardamine pratensis a 4 Ericaceae Rhododendron spp a Rhododendron brachycarpum* a 4 Gentianaceae Eustoma grandi¯orum² w Eustoma sp. w 4 Gentiana saxosa closure³ Gentiana dahurica closure 4, w 0 G. serotina closure G. kochiana closure 2, w 0 G. sino-ornata closure 3, w 0 Geraniaceae Geranium pyrenaicum a Geranium gracile a 4 G. pusillum a G. nodosum a 4 G. sanguineum a 4 Malvaceae Malva sp. closure, w Malva alcea closure 4, w 4 M. silvestris closure 4, w 4 Primulaceae Anagallis philippi a Anagallis arvensis a 3 Primula sp. a Primula denticulata wa 2±3 P. rosea `Grandi¯ora' wa 2±3 P. vialii w 2 Ranunculaceae Delphinium consolida a Delphinium ajacis a 4 Nigella arvensis a Nigella damascena a 4 Scrophulariaceae Digitalis sp. a Digitalis sp. a 3 Solanaceae Petunia hybrid colour, wa Petunia hybrid wa 4 (in some hybrids also colour 3)

Plants tested for ethylene effects were in the same genus as those reportedly responding to pollination (references in van Doorn, 1997, or, in the case of an additional species, below). Responses were: petal abscission (a); petal wilting or withering (w); concomitant abscission and wilting/ withering (wa); ¯ower closure; or a change in colour. Response classi®cation as described in the text. Taxonomic classi®cation according to APG (1998). * The species tested for ethylene effects also included several commercial cultivars. ² Reported by Ichimura and Goto (2000). ³ There are reports of earlier petal wilting following pollination in the literature but this is apparently the result of ovary growth (Webb and Littleton, 1987) those of pollinated ¯owers wilt rapidly and abscise imme- the Fumariaceae; Schemske et al., 1978). However, the diately thereafter (GaÈrtner, 1844). Petal fall in unpollinated effects of pollination reported to date, and those of ethylene, ¯owers may be due not to a well-regulated abscission do not coincide, as there are many (sub)families in which process, but simply to tearing at the (usually small) petal changes in ¯oral form and colour are ethylene-sensitive, base. This tearing occurs as a result of growth of the while there are no reports of pollination affecting the timing subtending tissue (Reiche, 1885; van Doorn and Stead, of these ¯oral changes in these groups. There may be several 1997). This process may also occur both in pollinated and reasons for this discrepancy. First, the number of species in unpollinated ¯owers. One would expect more instances of a which pollination reportedly has an effect on ¯ower form or such a discrepancy between these symptoms in pollinated colour is small compared with the number of species in and unpollinated ¯owers, but these have apparently not been which the petals are known to respond to ethylene. Thus, the reported by others, nor have they been found in our effects of pollination may have received less attention. experiments (Woltering and van Doorn, 1988; van Doorn, Secondly, there are biological reasons for the absence of a 2001). Similarly, in some species ¯ower closure or a change pollination effect. Generally, pollination does not affect in colour may occur in pollinated ¯owers, due to increased ¯ower form or colour in species in which the female phase ethylene production, whereas no such changes occur in precedes the male phase (protogynous ¯owers), as is the unpollinated ¯owers. case in many families. Additionally, pollination tends not to Table 5 shows that species known to respond to shorten ¯oral attractiveness in ¯owers that are short-lived, pollination by a change in ¯oral form or colour are restricted or in which the female phase is very short. Modulation of to families that exhibit similar changes after ethylene ¯oral attractiveness apparently confers little bene®t in these treatment. This relationship is substantiated by reports of a species. Many ¯owers, perhaps up to one-third of all lack of a pollination effect on ¯oral lifespan (for example in conspicuous ones, last only a day or less (Ashman and van Doorn Ð Comparing the Effects of Ethylene Treatment and Pollination on Petals 381

TAB LE 5. Comparison of the effects of exogenous ethylene and those of pollination, on ¯ower form and colour, in a number of plant families

Family Ethylene response and sensitivity class Effects of pollination reported Result other than predicted

Monocotyledons Agavaceae w 0 none Alismataceae w 4 none xx Alliaceae w 0 (exceptionally w 3) none Amaryllidaceae w 0 (exceptionally 2±3) none Asphodelaceae w 0±1 none Cannaceae w 0 none Colchicaceae w 0 none Commelinaceae w 3±4 none xx Convallariaceae w 0 none Haemodoraceae w 0 none Hemerocallidaceae w 0, closure 0 none Hyacinthaceae w 0 (exceptionally w 1±2), closure 0 or 3 none Iridaceae w 0 (exceptionally w 4) none Liliaceae wa 1±2 or 3; w 1±2; a 0 w, a, closure Orchidaceae w 4, colour 4 w, colour, closure Eudicotyledons Acanthaceae a 4 none xx Aizoaceae w 3, closure 3±4 none xx Asteraceae w 0±1 none Boraginaceae a 4 a, colour Cactaceae w 4 w Campanulaceae w 3±4 (some species w 2) w Caprifoliaceae a 4 (some species w 0) none xx Caryophyllaceae w 4, closure 4 w, closure Convolvulaceae w 4, closure 4 none xx Crassulaceae w 0 (some species w 1±2), closure 2±4, colour 0 ± 4 none xx Cruciferae a 4 a Dipsacaceae w or wa 2±3 none Ericaceae (Ericoideae) w 0 none (Rhododendroideae) a 3±4 a (Vaccinioideae) a 3±4 a Fabaceae w 0 and w3; some species colour 4 colour, closure, some species w Fumariaceae wa 2 none Gentianaceae w 0, closure 0±4 closure Geraniaceae a 4 a Gesneriaceae a 4 none xx Grossulariaceae a 3 a Lamiaceae a 4 a Malvaceae w 3±4 w, colour, closure Oleaceae a 4 none xx Papaveraceae a 4 none xx Plumbaginaceae w 3±4, closure 3±4 none xx Polemoniaceae wa 2±4 wa, colour Portulacaceae w 3±4, closure 4 a, closure Primulaceae a 4, wa 2±3, w 2 a Ranunculaceae a 3±4 a Rosaceae a 3±4 a Rubiaceae a 3±4 none xx Saxifragaceae w or wa, 0 none Scrophulariaceae a 3±4 a Solanaceae wa or w, 3±4; some species colour 4 wa or w, colour Valerianaceae a 4 none xx

Data on ethylene effects on petal wilting and petal abscission from Woltering and van Doorn (1988) and van Doorn (2001). It was predicted that families in which a symptom is generally ethylene-sensitive would contain at least one species that reportedly has a shorter span of ¯oral attraction, following pollination. Taxonomic classi®cation according to APG (1998)

Schoen 1976). Families that contain many short-lived ethylene-sensitivity in the perianth may not be to modulate ¯owers include the Commelinaceae, Convolvulaceae and ¯oral attractiveness. Infection of petals by fungi, for Papaveraceae. Thirdly, the reason behind the evolution of example, generally results in ethylene production, which 382 van Doorn Ð Comparing the Effects of Ethylene Treatment and Pollination on Petals in turn leads to rapid petal abscission thus removing the monocotyledons: structure, evolution and taxonomy. Berlin: Springer infection. Verlag. Fitting H. 1909. Die Beein¯ussung der OrchideenbluÈten durch die The question as to the biological meaning of the lack BestaÈubung und andere UmstaÈnde. Zeitschrift fuÈr Botanik 1: 1±86. of ethylene-sensitivity also remains unanswered. Flowers Gilissen LJW. 1977. Style-controlled wilting of the ¯ower. Planta 133: that do not respond to ethylene/pollination by changes in 275±280. form and colour are relatively rare in the eudicotyle- Gori DF. 1983. Post-pollination phenomena and adaptive ¯oral changes. dons. In contrast, numerous families in the monocoty- In: Jones CE, Little RJ, eds. Handbook of experimental pollination biology. New York: Van Nostrand Reinhold, 31±49. ledons exhibit neither ethylene-sensitivity nor show an Halevy AH, Whitehead CS, Kofranek AM. 1984. Does pollination effect of pollination on ¯oral form and colour. Why this induce corolla abscission of Cyclamen ¯owers by promoting ethylene is so is unclear. Many aspects of ¯oral biology vary production? Plant Physiology 75: 1090±1093. among monocotyledons and eudicotyledons, such as the Heywood VH. 1978. Flowering plants of the world. Oxford: Oxford University Press. trait of petal abscission (almost exclusively ethylene- Hilioti Z, Richards C, Brown KM. 2000. Regulation of pollination- sensitive and almost exclusively found in eudicotyle- induced ethylene and its role in petal abscission of Pelargonium 3 dons) vs. petal wilting (in monocotyledons and eudico- hortorum. Physiologia Plantarum 109: 322±332. tyledons). The reproductive strategy may also have a Hoekstra FA, Weges R. 1986. Lack of control by early pistillate ethylene bearing on the difference in ethylene sensitivity of petals of the accelerated wilting of Petunia hydrida ¯owers. Plant Physiology 80: 403±408. between monocotyledons and eudicotyledons. The for- Ichimura K, Goto R. 2000. Acceleration of senescence by pollination of mer probably show more vegetative reproduction, by cut `Asuka-no-nami' Eustoma ¯owers. Journal of the Japanese rhizomes, corms and bulbs. Retention of (turgid) Society for Horticultural Science 69: 166±170. pollinated ¯owers in a group of ¯owering stems Larsen PB, Ashworth EN, Jones ML, Woodson WR. 1995. Pollination- enhances a plant's long-distance attractiveness to induced ethylene in carnation: role of pollen tube growth and sexual compatibility. Plant Physiology 108: 1405±1412. pollinators (Gori, 1983). This may be a partial explan- Llop-Tous I, Barry CS, Grierson D. 2000. Regulation of ethylene ation for the virtual absence of pollination-induced biosynthesis in response to pollination in tomato ¯owers. Plant changes in ¯oral form and colour in the monocyledons. Physiology 123: 971±978. Insofar as ethylene-sensitivity in petals relates to patho- Motten AF. 1986. Pollination ecology in the spring wild¯ower community of a temperate deciduous forest. Ecological Monographs 56: 21±42. gen removal or isolation, it is unclear why so many Nichols R, Bu¯er G, Mor Y, Fujino DW, Reid MS. 1983. Changes in species (including almost all monocotyledons) do not ethylene and 1-aminocyclopropane-1-carboxylic acid content of have this capacity. pollinated carnation ¯owers. Journal of Plant Growth Regulation 2: 1±8. Oberrath R, Zanke C, BoÈhning-Gaese K. 1995. Triggering and ecological signi®cance of ¯oral color change in lungwort CONCLUSIONS (Pulmonaria spec.) Flora 190: 155±159. The results lend support to the idea that pollination- O'Neill SD, Nadeau JA, Zhang XS, Bui AQ, Halevy AH. 1993. Interorgan regulation of ethylene biosynthetic genes by pollination. induced changes in ¯ower colour and ¯oral form (due to Plant Cell 5: 419±432. ¯ower closure, or to wilting or abscission of petals) are Porat R, Borochov A, Halevy AH, O'Neill SD. 1994. Pollination- generally regulated by endogenous ethylene. The results induced senescence of Phalaenopsis petals: The wilting process, also show a relationship between some physiological ethylene production and sensitivity to ethylene. Plant Growth traits and taxonomy. The effects of ethylene on ¯oral Regulation 15: 129±136. Porat R, Halevy AH, Serek M, Borochov A. 1995. An increase in colour and ¯oral closure are not consistent within ethylene sensitivity following pollination is the initial event families or subfamilies, unlike the previously reported triggering an increase in ethylene production and enhanced effects of ethylene on petal senescence and abscission. senescence of Phalaenopsis orchid ¯owers. Physiologia Plantarum In addition, the reported effects of ethylene and 93: 778±784. Reiche C. 1885. UÈ ber anatomische VeraÈnderungen, welche in den pollination on ¯ower form and colour were found to Perianthkreisen der BluÈten waÈhrend der Entwicklung der Frucht coincide in some families but not in others. vor sich gehen. JahrbuÈcher fuÈr wissenschaftliche Botanik 16: 638± 687. Schemske DW, Willson MF, Melampy MN, Miller LJ, Verner L, ACKNOWLEDGEMENT Schemske M, Best LB. 1978. Flowering ecology of some spring woodland herbs. Ecology 59: 351±366. I am grateful to Jan J. Bos (Department of Taxonomy, Silberglied RE. 1979. Communication in the ultraviolet. Annual Reviews Wageningen Agricultural University) for permission to use of Ecology and Systematics 10: 373±398. Spanjers AW. 1977. Bioelectric potential changes in the style of Lilium ¯owers from the botanical garden and for relevant taxo- longi¯orum Thurb. after self and crosspollination from the stigma. nomic literature. Planta 153: 1±5. Stead AD, Moore KG. 1983. Studies on ¯ower longevity in Digitalis. II. The role of ethylene in corolla abscission. Planta 157: 409±414. LITERATURE CITED Stead AD, Reid MS. 1990. The effect of pollination and ethylene on the colour change of the banner spot of Lupinus albifrons Bentham APG (The Angiosperm Phylogeny Group). 1998. An ordinal clas- ¯owers. Annals of Botany 66: 655±663. si®cation for the families of the ¯owering plants. Annals of the SuÈssenguth K. 1936. UÈ ber den Farbwechsel von BluÈten. Berichte der Missouri Botanical Garden 85: 531±553. Deutschen Botanischen Gesellschaft 54: 409±417. Ashman TL, Schoen DJ. 1996. Floral longevity: ®tness consequences and van Doorn WG. 1997. Effects of pollination on ¯oral attraction and resource costs. In: Lloyd DG, Barrett SCH, eds. Floral biology. New longevity. Journal of Experimental Botany 48: 1615±1622. York: Chapman and Hall, 112±139. van Doorn WG. 2001. Categories of petal senescence and abscission: a re- Dahlgren RMT, Clifford HT, Yeo PF. 1985. The families of the evaluation. Annals of Botany 87: 447±456. van Doorn Ð Comparing the Effects of Ethylene Treatment and Pollination on Petals 383 van Doorn WG, Stead A. 1997. Abscission of ¯owers and ¯oral parts. petals ± Morphological and taxonomical relationships. Journal of Journal of Experimental Botany 48: 821±837. Experimental Botany 39: 1605±1616. Webb CJ, Littleton J. 1987. Flower longevity and protandry in two Woltering EJ, Somhorst D. 1990. Regulation of anthocyanin synthesis in species of Gentiana (Gentianaceae). Annals of the Missouri Cymbidium ¯owers: effects of emasculation and ethylene. Journal of Botanical Garden 74: 51±57. Plant Physiology 136: 295±299. Weiss MR. 1991. Floral colour changes as cues for pollinators. Nature Woltering EJ, Somhorst D, van der Veer P. 1995. The role of ethylene 354: 227±229. in interorgan signalling during ¯ower senescence. Plant Physiology Woltering EJ, van Doorn WG. 1988. Role of ethylene in senescence of 109: 1219±1225.