282 Table 2 Mean (SE) length <strong>and</strong> width <strong>of</strong> corolla tubes <strong>of</strong> ten <strong>flower</strong>s from each plant with smaller <strong>flower</strong>s in the summer. All summer measurements were significantly smaller than winter measurements (Paired t-test, P
Table 4 Percentage <strong>of</strong> fruit set in L. acaciae as a result <strong>of</strong> pollination by bees (bagged <strong>flower</strong>s) <strong>and</strong> combined pollination by birds <strong>and</strong> bees (unbagged <strong>flower</strong>s) in Summer 1993. The percentage <strong>of</strong> fruit set in unbagged <strong>flower</strong>s was significantly higher than in bagged <strong>flower</strong>s (Wilcoxon signed rank test, P=0.043)&/ tbl. c: &tbl. b: <strong>Loranthus</strong> acaciae Acacia raddiana Acacia tortilis Nectarinia osea osea Turdoides squamiseps Pycnonotus xanthopygos Migrating birds Apis mellifera Anthophora sp. Xylocopy pubescens Discussion Phenology <strong>of</strong> the <strong>flower</strong> The development <strong>of</strong> the <strong>flower</strong> is accompanied by changes in morphology <strong>and</strong> colour from totally green to totally red, which may directly affect the attractiveness <strong>of</strong> the <strong>flower</strong>s to <strong>flower</strong> visitors. A green mature <strong>flower</strong> bud is unrewarding <strong>and</strong>, therefore, unattractive to <strong>flower</strong> visitors. At the opened <strong>flower</strong> phase, the green rewarding <strong>flower</strong>s with red anthers <strong>and</strong> stigma are easily located by <strong>flower</strong> visitors. The red non-rewarding but most attractive <strong>flower</strong>s remain on the plant for several weeks, indicating that they may serve as long distance “flags” adding to the attractiveness <strong>of</strong> the whole plant (Schemske 1980) <strong>and</strong>, therefore, contribute to its success in competing for pollinators. The red corolla may also signal a non-rewarding <strong>flower</strong>, hence directing <strong>flower</strong> visitors to unpollinated <strong>flower</strong>s (Casper <strong>and</strong> Pine 1984; Eisikowitch <strong>and</strong> Rotem 1987). Phenology <strong>of</strong> the plant <strong>and</strong> the population Plants with an extended <strong>flower</strong>ing period may serve as a long-term resource (Bertin 1982; Dobkin 1984), which allows the presence <strong>of</strong> a constant population <strong>of</strong> pollinators (Stiles 1977; Waser <strong>and</strong> Real 1979). The <strong>flower</strong>ing Plant number Bagged <strong>flower</strong>s Unbagged <strong>flower</strong>s n Fruit set (%) n Fruit set (%) 35 105 1 90 8 36 143 0 140 4 37 183 7 93 12 38 272 0 120 0 39 192 1 96 1 40 196 0 113 1 Number <strong>of</strong> <strong>flower</strong>s 1091 652 Mean 1.5 4.3 &/ tbl. b: Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Month Fig. 4 <strong>Flowering</strong> periods <strong>of</strong> L. acaciae <strong>and</strong> its main hosts (bold lines) <strong>and</strong> activity periods <strong>of</strong> its <strong>flower</strong> visitors (thin lines)& ig. c: / f 283 period <strong>of</strong> L. acaciae is long (approximately 10 months) (Fig. 2). However, only a small number <strong>of</strong> <strong>flower</strong>s are produced each day, so <strong>flower</strong> visitors probably call on several plants before they are satiated. This <strong>flower</strong>ing strategy promotes cross-pollination (Augspurger 1979; Sazima 1977) <strong>and</strong> <strong>of</strong>fers plants the following possible advantages (Bawa 1983; de Jong et al 1992): (1) reduced risk <strong>of</strong> reproductive failure; (2) the possibility <strong>of</strong> mating with more individuals in the population <strong>and</strong> (3) better control over relative investment in <strong>flower</strong>s <strong>and</strong> fruit. Most L. acaciae plants had two <strong>flower</strong>ing peaks, one in the summer <strong>and</strong> one in the winter, with a strong decrease in <strong>flower</strong> number during the fall. Halevy <strong>and</strong> Orshan (1973) found similar <strong>flower</strong>ing patterns in Acaciae raddiana, a predominant host <strong>of</strong> L. acaciae, which had two <strong>flower</strong>ing peaks, one in June <strong>and</strong> one in November. In some A. raddiana plants the two <strong>flower</strong>ing peaks were separated by a short non-<strong>flower</strong>ing period. Halevy <strong>and</strong> Orshan (1973) pointed out that A. raddiana originated in Africa, <strong>and</strong> assumed that its phenological cycle in Israel may be the result <strong>of</strong> an endogenic rhythm which is a relict <strong>of</strong> its original phenology. This explanation may also apply to the the <strong>flower</strong>ing phenology <strong>of</strong> L. acaciae. Since both hosts show a relatively similar ratio <strong>of</strong> parasites with two <strong>flower</strong>ing periods a year, we can assume that the identity <strong>of</strong> the host is not the main factor influencing the polymorphism within the population, that other factors such as the physiological condition <strong>of</strong> the host may be involved. The <strong>flower</strong>ing periods <strong>of</strong> both Acaciae species overlapped a major part <strong>of</strong> the <strong>flower</strong>ing period <strong>of</strong> L. acaciae (Fig. 4). During this period, both host <strong>and</strong> parasite provide a rich source <strong>of</strong> nectar <strong>and</strong> pollen, attracting insects <strong>and</strong> birds, especially orange-tufted sunbirds which also feed on the nectar <strong>and</strong> insects provided by the hosts, thus contributing to their pollination as well. Plitman (1991) stated that, in the case <strong>of</strong> the parasite Cuscuta (Cuscutaceae), “the plants show phenological plasticity that corresponds with that <strong>of</strong> the main perennial hosts, usually a phase behind, thus reducing the deleterious effect on the host’s reproductivity”. The <strong>flower</strong>ing peaks <strong>of</strong> L. acaciae are also a phase behind the <strong>flower</strong>ing peaks <strong>of</strong> A. raddiana <strong>and</strong> the <strong>flower</strong>ing peak <strong>of</strong> A. tortilis (June), but further research is required to clarify the nature <strong>of</strong> this relationship. <strong>Flowering</strong> synchrony during the entire <strong>flower</strong>ing period was relatively low (Table 1). According to Bawa
Change language