Reproductive Biology in Cytisus Multiflorus (Fabaceae)
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Ann. Bot. Fennici 41: 179–188 ISSN 0003-3847 Helsinki 23 June 2004 © Finnish Zoological and Botanical Publishing Board 2004 Reproductive biology in Cytisus multiflorus (Fabaceae) Tomás Rodríguez-Riaño*, Ana Ortega-Olivencia & Juan A. Devesa Departamento de Biología y Producción Vegetal (Botánica), Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain (*e-mail: [email protected]) Received 23 Sep. 2003, revised version received 15 Jan. 2003, accepted 20 Jan. 2003 Rodríguez-Riaño, T., Ortega-Olivencia, A. & Devesa, J. A. 2004: Reproductive biology in Cytisus multiflorus (Fabaceae). — Ann. Bot. Fennici 41: 179–188. We studied the reproductive biology of two morphs (LF, large flowers; and SF, small flowers) of Cytisus multiflorus, an endemic legume from SW Europe. Visits by pol- linators are indispensable for fruit-set to take place. After hand self-pollination, all the individuals have a uniform behaviour with null fruit, in spite of self-penetration of the ovules. This and the fact that the abscission of initiated fruit occurs en masse at the same time seems to indicate the possibility of a late-acting self-incompatibility in this species. Intramorph crosses showed that the SF morph has a significantly greater reproductive success than the LF morph. Intermorph crosses were quite ineffective due to the lesser viability of the LF morph pollen, although we cannot discard the possibil- ity that its ovules are also less viable. The situation is suggestive of the presence of a partial reproductive barrier between the two morphs that operates both before and after crossing. Key words: breeding system, Cytisus multiflorus, Fabaceae, pollination, self-incom- patibility Introduction investigated in any depth since the review by Arroyo (1981). We can only cite the case of In the family Fabaceae, Arroyo (1981) found Chamaecytisus palmensis (endemic to the Island that only 22.1% of the species studied of Papilio- of La Palma, Canary Islands), which behaves as noideae were self-incompatible. This proportion self-compatible (SC) in naturalized populations was very low in comparison with that of Caes- of New Zealand (Webb & Shand 1985), and that alpinioideae (62.3%) and Mimosoideae (66.7%). of Cytisus striatus (endemic to the W of Iberian She noted that self-incompatibility (SI) largely peninsula) and Retama sphaerocarpa (endemic correlates with the woody habit in this family, so of the W of the Mediterranean Region) which it would be reasonable to expect SI to be present behave as SI (Rodríguez-Riaño et al. 1999a). in the tribe Genisteae, with a majority of woody Most of the woody representatives of the genera. However, only three of its constituent Genisteae have yellow flowers, a papiliona- genera had been studied for SI (Argyrolobium, ceous corolla, and a monadelphous androecium Lupinus, and Ulex), and it was detected in only with dimorphic anthers (Polhill 1976, López et one species of the last genus (see Arroyo 1981). al. 1999, Rodríguez-Riaño et al. 1999b). Their Very few woody species of this tribe have been flowers have a great production of pollen (López 180 Rodríguez-Riaño et al. • ANN. BOT. FENNICI Vol. 41 et al. 1999, Rodríguez-Riaño et al. 1999c) and in LAS (Seavey & Bawa 1986, Seavey & Carter are clearly adapted to bee pollination. Also the 1994, Gibbs & Sassaki 1998). most widespread pollen presentation mechanism In this paper we study the reproductive biol- in this tribe is of the valvular type, in which ogy of Cytisus multiflorus, an endemic shrubby the reproductive column (androecium and gyn- Genisteae from the W of the Iberian peninsula, oecium) is released from the keel, and a small which is a good representative in Mediterra- cloud of pollen adheres to the ventral part of nean woodlands or in the scrub substituting the insect. The column comes back to its ini- these woodlands. A knowledge of their breed- tial position upon release from pressure by the ing system is indispensable for decision-making insect (López et al. 1999, Rodríguez-Riaño et al. relative to flora and vegetation conservation, to 1999b). deforested zone recovery, the sustainable use of The scarcity of reproductive studies, both certain zones (e.g. for apiculture), or even for for this tribe and for the subfamily in general, their practical use as ornamentals not requir- may be due to the difficulty in handling the ing irrigation, especially in Mediterranean type flowers, since anther dehiscence occurs very climates where water is the limiting resource. early. In some Leguminosae pollen is usually Cytisus multiflorus is a winter-flowering spe- released while the flower is in the bud stage, cies in the study population (Rodríguez-Riaño and in this stage is viable (Asmussen 1993, et al. 1999d) and produces a great number of Rodríguez-Riaño et al. 1999a, Rodríguez-Riaño white flowers per plant (range = 8259–15 228; et al. 2001). Nonetheless, although the stigma Rodríguez-Riaño et al. 1999d). The flowers is receptive in this phase, its receptivity is inop- have a valvular type pollen presentation system erative due to the presence of a stigmatic surface (López et al. 1999), they produce no nectar that blocks the germination of the pollen grain. and its longevity is 9.4 ± 1.6 days (Rodríguez- Only the rupture of this surface by a pollinator Riaño et al. 1999d). In a previously studied will allow the pollen to germinate (Shivanna & population (see Study site) two floral morphs Owens 1989, Rodríguez-Riaño et al. 1999a). (SF, small-flowered morph; LF, large-flowered In Cytisus striatus and Retama sphaerocarpa, morph) were detected. These differed by the studies of pollen-pistil interaction have indicated size of the flowers (mean ± S.E., LF = 14.5 ± the existence of a certain degree of self ferti- 0.10 mm, range 13.0–16.0 mm; SF = 10.7 ± lization, confirming the low fruit-set obtained 0.17 mm, range 9.0–12.75 mm) and the repro- after self-pollination (2.4%–3.5% in Cytisus and ductive pieces (anther volume, size of the pollen 3.5%–4.3% in Retama). Also, the high level of grains, and size of the ovary), with significantly self-sterility in both species seemed to be due greater values in the LF morph than in the SF rather to an early action of inbreeding depression morph, but not in their morphology or colour. than to late acting self-incompatibility (= LAS; Also, the production of pollen per flower is sig- Rodríguez-Riaño et al. 1999a). It is really dif- nificantly greater in the LF morph than in the SF ficult, however, to distinguish between the two morph, but the percentage germination of pollen possibilities, and we must use clues such as: (1) in vitro is significantly greater in the SF morph in the case of the inbreeding depression, seed- than in the LF morph. With respect to female set varies widely between selfed individuals, function, the production of ovules per flower whereas in the LAS phenomenon the behaviour does not differ significantly between the two is uniform between individuals, with values of morphs (for further details see Rodríguez-Riaño selfed seeds either zero or close to zero; (2) the et al. 2001). blocking or rejection of embryos occurs over a The two morphs presented a phenologically wide range of stages (from the first cell divisions differentiated pattern, reaching their peak of to germination) in the inbreeding depression, and maximum flowering with a difference of ca. 4 more or less in the same state in the LAS; (3) weeks, with the flowering of the LF morph ear- the fall of flowers and young fruit after selfing lier than that of the SF morph, and although in occurs progressively over time in the inbreeding both cases the final crop of fruit and seeds was depression, and approximately at the same time sparse, it was much smaller in the LF morph ANN. BOT. FENNICI Vol. 41 • Reproductive biology in Cytisus multiflorus 181 (Rodríguez-Riaño et al. 1999d). The pollen– Pollination and breeding system pistil interaction studies showed that a percent- age of self pollen germinated on the stigma, and The pollen/ovule ratio (P/O) for each morph (10 that in the SF morph the percentages of ovules flower buds from 20 individuals in each morph) penetrated by pollen tubes after both hand self- was estimated according to Cruden (1977). In pollination (54.9%) and hand cross-pollination the field, insect censuses were made, record- (56.8%) were similar; in the LF morph the ing the number of visits received by a series percentage after hand cross-pollination (39.9%) of 20 selected plants in an area of some 20 m2. was significantly greater than after hand self- The census was performed over 18 h spread pollination (20.9%). This indicated that the out over 23 d throughout the blooming time rejection of the self pollen tubes was greater in (January, February, and March), at intervals of the LF than in the SF morph (Rodríguez-Riaño approximately three hours and for a period of et al. 2001). Also, in the intermorph crossing 10 min/observation. The duration of each visit experiments, the ovular penetration was found was recorded, as well as the type of pollinator, to be lower (although not significantly) when its behaviour on the flowers, the time of day, the LF morph was the pollen donor (SF ¥ LF) and the weather conditions at the study site. We than when the SF morph was the donor (LF ¥ considered those insects to be pollinators, which SF). This could be due to a lesser viability of made contact with the flowers’ sexual organs the male function in the LF morph (Rodríguez- during a visit. Riaño et al. 2001). To determine the breeding system, a series of The objectives of the present work were: plants was selected and subjected to the follow- (a) to determine the reproductive biology of the ing pollination treatments: (A) natural pollina- two morphs of Cytisus multiflorus; (b) to deter- tion (control); (B) spontaneous self-pollination mine the production levels of fruit and seeds (SSP, bagged plants); (C) hand self-pollination in both intermorph pollination experiments and (HSP); and (D) hand cross-pollination (HCP).