Flowering seasonality and flower characteristics of Loranthus ...

Sex Plant Reprod (1996) 9:279–285 © Springer-Verlag 1996 

ORIGINAL PAPER 

&roles:Yiftach 

Vaknin · Yoram Yom Tov · Dan Eisikowitch 

Flowering seasonality and flower characteristics 

of Loranthus acaciae Zucc. (Loranthaceae): 

implications for advertisement and bird-pollination 

&misc:Received: 

29 January 1996 / Revision accepted: 13 June 1996 

&p. 

1:Abstract 

The flowering biology and pollination ecology 

of Loranthus acaciae was studied at Hazeva in the 

northern Arava Valley in Israel. Flowers at anthesis had 

red anthers, a red stigma and a green corolla which 

turned red as a postfloral phenomenon. Their flowering 

period was approximately 10 months long (from mid- 

June until mid-April) during which time two main flowering 

patterns were distinguished. Some plants flowered 

twice a year, with separate summer and winter flowering 

periods; other plants flowered continuously, with two 

peaks, one in the summer and one in the winter. Several 

significant differences between summer and winter flowering 

and fruiting were found: (1) the summer flowering 

period was shorter than that of winter, (2) flowering synchrony 

between individual plants was lower in summer 

than in winter, (3) in summer the plants produced a larger 

proportion of female flowers, whereas in winter most 

of the plants produced a larger proportion of hermaphrodites, 

(4) in summer a limited number of plants produced 

smaller flowers while the majority produced normalsized 

flowers, whereas in winter the entire population 

produced only normal-sized flowers, and (5) fruit set 

percentage was lower in summer than in winter. L. acaciae 

was found to be self-compatible, but, since it was 

not spontaneously self-pollinated, it showed high dependence 

on pollinator activity. In summer the flowers were 

visited by a wide spectrum of pollinators, both birds and 

insects, while in winter flowers were visited almost exclusively 

by the orange-tufted sunbird (Nectarinia osea 

osea, Nectariniidae). These seasonal changes in flowering 

characteristics and pollinator activity could explain 

why reproductive success is higher in winter than in 

summer. 

Y. Vaknin · Y. Yom Tov 

Department of Zoology, George S. Wise Faculty of Life Sciences, 

Tel Aviv University, Tel Aviv 69978, Israel 

Y. Vaknin (✉) · D. Eisikowitch 

Department of Botany, George S. Wise Faculty of Life Sciences, 

Tel Aviv University, Tel Aviv 69978, Israel; 

Tel.: 3–6409849; Fax: 3–6409380; e-mail: vaknin@post.tau.ac.il&/ 

fn-block: 

&kwd:Key 

words Loranthus acaciae · Nectarinia osea osea · 

Gynomonoecious · Male sterility · Self-compatible&bdy: 

Introduction 

In pollination systems of angiosperms, plants often ensure 

successful reproduction through phenological and 

morphological adaptations which serve as effective attractions 

for pollinators. In several plant species, pollination 

is accomplished by a single pollinator and the flowers 

are highly specialized. However, most plant species 

are pollinated by several species of pollen vectors (Bertin 

1982). 

Plants pollinated by birds usually show the following 

features (Faegri and Pijl 1979): diurnal anthesis during 

which nectar is secreted in abundant quantities and sugar 

concentration is usually lower than optimum for most insects, 

and may also be lower than optimum for birds; 

long tubular or trumpet-shaped corollas with nectar located 

at the base; odorless flowers with vivid colours; 

absence of landing platforms or nectar guides; and ovary 

and other flower organs usually surrounded by hard 

flower walls and other protective tissue. These features 

make the flowers inconspicuous and/or non-economical 

for visitors such as short-tongued insects, and better 

adapted for nectar-feeding birds (Rebelo 1987). Phenological 

flowering patterns are also factors involved in 

bird-pollinated systems, especially in plants with extended 

flowering periods, since seasonal availability of pollinators 

may select for flowering times of animal-pollinated 

species (Rathcke and Lacey 1985). 

Loranthus acaciae (Loranthaceae) is a perennial 

green semiparasitic mistletoe with an east Sudanian distribution 

area (Zohary 1966). In Israel it occurs along the 

Arava, Dead Sea and Jordan Valleys. Predominant hosts 

are Acacia raddiana Savi and Acacia tortilis (Forssk.) 

Hayne trees along the Arava and the Dead Sea Valleys 

and Ziziphus spina christi (L.) Desp. along the Jordan 

Valley (Zohary 1980, 1982). According to Zohary 

(1966), the flowers are hermaphroditic, with five petals

280 

connate in the lower part forming a tubular and curved 

corolla about 40 mm long, and with a short calyx 

2–3 mm long. The ovary is inferior. Nahari (1980) found 

that the flowers were pollinated only when they were 

green, and she described their later red phase as a postpollination 

phenomenon. Preliminary observations revealed 

the presence of female (male-sterile) flowers, 

with white to light brown anthers, next to hermaphrodite 

flowers on the same plants, indicating that the plant is 

“gynomonoecious”. It was also found that L. acaciae is 

self-compatible, but its inability to spontaneously selfpollinate 

suggested a high dependency on pollinator activity 

(Vaknin 1994). 

L. acaciae is considered to be pollinated almost exclusively 

by the orange-tufted sunbird (Nectarinia osea 

osea Bonaparte), a small passerine bird which feeds on 

flower nectar and supplements its diet with insects and 

other small invertebrates, especially during the breeding 

season (Bodenheimer 1935). The birds probe their long, 

curved bills through the flower entrance, and may thus 

effect pollination. In this article we present data on seasonality 

in flowering phenology and flower characteristics 

of L. acaciae, with reference to its pollination biology 

and reproductive success. 

Materials and methods 

Study area 

This study took place betwen July 1992 and April 1994 at Hazeva 

in the northern Arava Valley, 30 km south of the Dead Sea 

(30°49′N, 35°15′E), 130 m below sea level. The climate in Hazeva 

is very arid with large annual variations in temperature. The mean 

maximal temperature fluctuates between 34°C in August and 14°C 

in January. Average annual relative humidity is 40–45% and average 

annual rainfall is 50 mm (Kadmon 1956). All L. acaciae 

plants within a perimeter of 2 km around Hazeva were located and 

numbered, and their hosts noted. 

Phenology of the flower 

Mature flower buds were tagged and bagged and a full protocol of 

flower development was maintained throughout the flowering period, 

including flower morphology, colour, nectar secretion and 

pollen release (in hermaphrodite flowers). A similar procedure 

was applied on flowers that were hand pollinated as soon as they 

opened. 

Phenology of the plant and the population 

A plant was considered to have flowered if it produced at least one 

open green flower. The flowering period length of each plant was 

documented through weekly observations. The number of flowers 

on each plant was determined every 1–3 weeks throughout the 

flowering period by counting the flowers in a large section of the 

plant. The total number of flowers present was estimated by extrapolation 

(Yan 1993). 

Flowering synchrony 

The estimated average number of days that the flowering of an individual 

overlapped with the flowering of every other plant in the 

sample was calculated according to Augspurger (1981) and Gomez 

(1993). The limits for complete asynchrony and full synchrony 

within the population were defined as 0 and 1, respectively. We 

arbitrarily defined the summer flowering period to be from 16 

June 1993 to 25 November 1993 and the winter flowering period 

from 26 November 1993 to 18 April 1994. 

Flower dimensions 

Dimensions of flowers from six plants (ten flowers from each 

plant) were measured using calipers to an accuracy of ±0.1 mm. 

Effects of the flowering season on flower dimensions 

Seasonal changes in flower dimensions were determined by summer 

and winter measurements of corolla tube length and width in 

three plants (ten flowers from each plant) which had smaller flowers 

in the summer. 

Frequencies of hermaphrodite and female flowers 

The frequencies of hermaphrodite and female flowers were determined 

for each plant by sampling 50 flowers on randomly selected 

branches. Each plant was sampled once in summer 1993 and once 

in winter 1993–1994. 

Effects of the flowering period on reproductive success 

Flower buds were tagged and remained undisturbed for the rest of 

the flowering period. Fruit set percentage was calculated for summer 

and winter flowering periods. 

Pollinator activity 

Flower visitors were observed at different hours of the day 

throughout the entire flowering period and their behaviour as potential 

pollen vectors and pollinators was documented. During the 

summer of 1993, flower buds in six plants were bagged in perforated 

nets (20×25 mm hole size) large enough to allow free passage 

of insects but prevent birds from reaching the flowers. Adjacent 

unbagged branches were tagged and left available to all flower 

visitors. Fruit set percentage was calculated for bagged and unbagged 

flowers. 

Statistics 

All data are represented as means±standard error. 

Results 


A mature flower bud of L. acaciae is a narrow closed 

green tube with a bulbous tip (Fig. 1A). About a day before 

the flower opens, it begins to secrete nectar and the 

petals partially separate, producing five longitudinal slits 

(Fig. 1B); this is also known as the “Chinese lantern 

phase” (Bernhardt and Calder 1980). When the flower 

opens, the tips of the petals coil but the rest forms a curved 

and closed tube (Fig. 1C). At this stage the corolla is 

green, the stigma is red, and the five red linear anthers are 

pressed against the style, 1–2 mm below the stigma, and

are already shedding pollen. Some 2–3 days later the corolla 

turns red, nectar secretion terminates and the flowers 

are no longer receptive. If pollinated, the flowers turn red 

within 1–2 days. The red flowers remain on the branches 

for 6–12 weeks until they either produce fruit or drop off. 

A B C 

10 mm 

Fig. 1A–C Flower morphology and dimensions of L. acaciae (a 

corolla tube length, b corolla tube width, c stamen length, c–a distance 

from stigma to flower entrance) and three floral phases of L. 

acaciae: A mature bud phase; B “Chinese lantern” phase; C 

opened flower phase& ig. 

c: 

/ f 

Plant number 

33 

25 

24 

21 

18 

13 

10 

9 

8 

3 

1 

32 

31 

30 

29 

28 

27 

26 

23 

22 

17 

15 

14 

12 

11 

4 

2 

Jun. 

Jul. Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May 

Month 

Fig. 2 Flowering times of L. acaciae June 1993–April 1994& ig. 

c: 

/ f 

Table 1 Mean number of flowering days and flowering synchrony 

of L. acaciae (total (a) total flowering period in plants that flowered 

twice a year, total (b) total flowering period in plants that 

b 

a 

c 


The flowering period of Loranthus acaciae at Hazeva 

lasted 10 months, from mid-June 1993 until mid-April 

1994 (Fig. 2). During this time two main flowering patterns 

were distinguished: 59% of the plants (n=16) flowered 

twice a year, with separate summer and winter flowering 

periods, and 41% of the plants (n=11) flowered 

continuously with one or two peaks, one in the summer 

and one in the winter (Fig. 2). No significant difference 

was found between plants on two different hosts, Acacia 

raddiana and A. tortilis (n=13 and 14, respectively), in 

total flowering days (Mann-Whitney U-test, P=0.77), 

flowering synchrony in the summer (Mann-Whitney Utest, 

P=0.79) or flowering synchrony in the winter 

(Mann-Whitney U-test, P=0.70). Moreover, on both 

hosts the ratio of plants which flowered twice a year was 

almost the same (0.62 and 0.57, repectively). Therefore, 

all plants were grouped together for further analysis. No 

significant difference was found between total flowering 

days of plants that flowered once a year (n=11) and 

plants that flowered twice a year (n=16) (Mann-Whitney 

U-test, P=0.62). In plants that flowered twice a year, the 

flowering period was significantly shorter in the summer 

than in the winter (Wilcoxon signed rank test, P=0.002; 

n=16) (Table 1). Flowering synchrony was significantly 

higher in the winter than in the summer (Wilcoxon 

signed rank test, P=0.003; n=23) (Table 1). 


The following flower dimensions (given in mm) in the 

summer varied significantly among plants: corolla tube 

length, 14.1±0.40–17.4±0.31, ANOVA F 5.54=14.725, 

P

282 

Table 2 Mean (SE) length and 

width of corolla tubes of ten 

flowers from each plant with 

smaller flowers in the summer. 

All summer measurements 

were significantly smaller than 

winter measurements 

(Paired t-test, P

Table 4 Percentage of fruit set 

in L. acaciae as a result of pollination 

by bees (bagged flowers) 

and combined pollination 

by birds and bees (unbagged 

flowers) in Summer 1993. The 

percentage of fruit set in unbagged 

flowers was significantly 

higher than in bagged flowers 

(Wilcoxon signed rank test, 

P=0.043)&/ 

tbl. 

c: 

&tbl. 

b: 

Loranthus acaciae 

Acacia raddiana 

Acacia tortilis 

Nectarinia osea osea 

Turdoides squamiseps 

Pycnonotus xanthopygos 

Migrating birds 

Apis mellifera 

Anthophora sp. 

Xylocopy pubescens 

Discussion 


The development of the flower is accompanied by 

changes in morphology and colour from totally green to 

totally red, which may directly affect the attractiveness 

of the flowers to flower visitors. A green mature flower 

bud is unrewarding and, therefore, unattractive to flower 

visitors. At the opened flower phase, the green rewarding 

flowers with red anthers and stigma are easily located by 

flower visitors. The red non-rewarding but most attractive 

flowers remain on the plant for several weeks, indicating 

that they may serve as long distance “flags” adding 

to the attractiveness of the whole plant (Schemske 

1980) and, therefore, contribute to its success in competing 

for pollinators. The red corolla may also signal a 

non-rewarding flower, hence directing flower visitors to 

unpollinated flowers (Casper and Pine 1984; Eisikowitch 

and Rotem 1987). 


Plants with an extended flowering period may serve as a 

long-term resource (Bertin 1982; Dobkin 1984), which 

allows the presence of a constant population of pollinators 

(Stiles 1977; Waser and Real 1979). The flowering 

Plant number Bagged flowers Unbagged flowers 

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 of flowers 1091 652 

Mean 1.5 4.3 

&/ 

tbl. 

b: 

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. 

Month 

Fig. 4 Flowering periods of L. acaciae and its main hosts (bold 

lines) and activity periods of its flower visitors (thin lines)& ig. 

c: 

/ f 

283 

period of L. acaciae is long (approximately 10 months) 

(Fig. 2). However, only a small number of flowers are 

produced each day, so flower visitors probably call on 

several plants before they are satiated. This flowering 

strategy promotes cross-pollination (Augspurger 1979; 

Sazima 1977) and offers plants the following possible 

advantages (Bawa 1983; de Jong et al 1992): (1) reduced 

risk of reproductive failure; (2) the possibility of mating 

with more individuals in the population and (3) better 

control over relative investment in flowers and fruit. 

Most L. acaciae plants had two flowering peaks, one in 

the summer and one in the winter, with a strong decrease 

in flower number during the fall. Halevy and Orshan 

(1973) found similar flowering patterns in Acaciae raddiana, 

a predominant host of L. acaciae, which had two 

flowering peaks, one in June and one in November. In 

some A. raddiana plants the two flowering peaks were 

separated by a short non-flowering period. Halevy and 

Orshan (1973) pointed out that A. raddiana originated in 

Africa, and assumed that its phenological cycle in Israel 

may be the result of an endogenic rhythm which is a relict 

of its original phenology. This explanation may also 

apply to the the flowering phenology of L. acaciae. 

Since both hosts show a relatively similar ratio of parasites 

with two flowering periods a year, we can assume 

that the identity of the host is not the main factor influencing 

the polymorphism within the population, that other 

factors such as the physiological condition of the host 

may be involved. The flowering periods of both Acaciae 

species overlapped a major part of the flowering period 

of L. acaciae (Fig. 4). During this period, both host and 

parasite provide a rich source of nectar and pollen, attracting 

insects and birds, especially orange-tufted sunbirds 

which also feed on the nectar and insects provided 

by the hosts, thus contributing to their pollination as 

well. Plitman (1991) stated that, in the case of the parasite 

Cuscuta (Cuscutaceae), “the plants show phenological 

plasticity that corresponds with that of the main perennial 

hosts, usually a phase behind, thus reducing the 

deleterious effect on the host’s reproductivity”. The 

flowering peaks of L. acaciae are also a phase behind the 

flowering peaks of A. raddiana and the flowering peak 

of A. tortilis (June), but further research is required to 

clarify the nature of this relationship. 

Flowering synchrony during the entire flowering period 

was relatively low (Table 1). According to Bawa

284 

(1983), selection for a low level of synchrony may occur 

when there is intense competition for pollinators. Flowering 

synchrony in the winter was higher than in the 

summer (Table 1) and might be the result of a longer 

flowering period in the winter than in the summer. 


The morphology and size of L. acaciae flowers could 

be considered adaptations promoting visits by birds 

with long and decurved bills, such as the orange-tufted 

sunbird. The corolla is long, tubular (Faegri and Pijl 

1979; Grant 1966; Rebelo 1987) and curved (Gill and 

Wolf 1978; Mcdade and Kinsman 1980), and the distance 

from the stigma to the flower entrance is relatively 

long. 

Seasonal changes in flower size could be a result of 

climate and environmental conditions (including temperature, 

humidity and rain) and may be considered adaptations 

for arid conditions. Plants with smaller flowers are 

probably better adapted to the greater diversity of pollinators, 

including birds and short-tongued insects, available 

in summer. In the winter, plants have more uniform 

flower size and are probably better adapted to long-billed 

birds such as the orange-tufted sunbird. 

Frequency of hermaphrodite and female flowers 

Sexual expression in plants is probably the combined result 

of endogenic (genetic) and environmental factors 

(Frankel and Galun 1977; Ilan 1977). Male sterility is 

commonly explained as an intermediate phase in the evolution 

of dioecy (a mechanism for cross pollination) (Ilan 

1977; Jordano 1993; Wolff et al 1988) and as a strategy 

to conserve pollen in the population of plants with complex 

pollination mechanisms (Ilan 1977). 

Frequency of hermaphrodite flowers in the summer 

was highly variable (CV=56.65%), whereas in the winter, 

excluding one exceptional plant, the plants had very 

high proportions of hermaphrodites (>80%) with low 

variability (CV=27.32%) (Fig. 3). This seasonal variation 

could be considered a strategy to compete for pollinators. 

Pollinator availability in winter is low and in order 

to ensure pollination plants produce a larger proportion 

of hermaphrodite flowers. However, pollinator availability 

in summer is much higher and the pollinators can 

find and visit the nectar in most of the flowers, including 

female flowers. The large proportion of female flowers 

in the summer may also promote cross-pollination. 

Pollinator activity 

The optimal pollen vector is the agent that most effectively 

transfers pollen, producing maximal seed set 

throughout the entire flowering period (Stiles 1978). 

Flower visitors of L. acaciae were mainly birds and 

bees. Flower visits by birds were legitimate, but bees, 

because of their smaller size and the large distance between 

the stigma and the flower entrance, were unable to 

make physical contact with the stigma while extracting 

nectar. Therefore, visits by bees were legitimate only 

when they were collecting pollen. Female flowers were 

pollinated almost exclusively by birds since female flowers 

lack pollen and visits to female flowers by bees were 

not legitimate. 

The slightly higher reproductive success from combined 

pollination by birds and bees over that from pollination 

by bees alone (Table 4) may indicate that L. acaciae 

is better adapted to pollination by birds, but further 

research is required. Under flowering conditions of low 

temperature, rain and overcast skies the probability of 

pollination by birds is much higher than the probability 

of pollination by poikilotherms such as insects (Cruden 

1972). Therefore, it is not surprising that in the winter 

flowering period L. acaciae is pollinated almost exclusively 

by birds (Fig. 4), the most prominent of which is 

the orange-tufted sunbird. 

In summary, this study reveals that the higher reproductive 

success of L. acaciae in the winter as compared 

to that in the summer is explained by seasonal changes in 

flowering characteristics and pollinator activity. 

&p. 

2:Acknowledgements 

We thank Amotz Zahavi for providing the 

facilities and living quarters at Hazeva and Naomi Paz for reading 

the manuscript and improving the English. This work was partially 

supported by the Ministry of Science. 

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