ECOLOGICAL FEATURES OF FLOWERS AND THE AMOUNT OF POLLEN RELEASED IN CORYLUS AVELLANA ( L . ) AND ALNUS GLUTINOSA ( L . ) GAERTN

Hazel and alder bloom as one of the fi rst plants in climatic conditions of Poland. In the present study, detailed observations were made of the structure of fl owers of Corylus avellana L. and Alnus glutinosa (L.) Gaertn. Pollen yield of the abovementioned species was determined. The number of pollen grains in the theca, anther, fl ower and infl orescence was calculated. A comparison was made of the number of pollen grains of these taxa recorded during the atmospheric pollen seasons analysed over a period of 8 years in the conditions of Lublin. The study demonstrates that pollen yield of the common hazel infl orescence was 66 mg, whereas for black alder it was 120 mg. It was found that the number of pollen grains produced by the alder infl orescence was 2.2 times higher than by the hazel infl orescence. The atmospheric pollen season of the studied plant taxa was recorded at similar dates, but alder pollen was in much larger concentrations.


INTRODUCTION
Hazel and alder are an important source of fresh pollen for bees in early spring, since pollen fl ows from other plants are still unavailable.The pollen shedding in these plants usually starts in the second half of January during sunny, not very cool days.The several-year-long studies show that the maximum pollen release by hazel and alder in Lublin falls on different days of March or at the beginning of April (W e r y s z k o -C h m i e l e w sk a and P i o t r o w s k a , 2006).The most effi cient pollen shedding takes place at a temperature of 15-18 o C (S u s z k a , 1980).Hazel pollen is collected by bees in the form of medium-sized pollen pellets.These pellets can have different shades, as shown by data reported by different authors: bright yellow (M a u r i z i o and G r a f l , 1969), olive-yellow (L i p i ń s k i , 1982) or grey-yellow (J a b ł o ń s k i , 1998).But pellets formed from black alder pollen are often greenish-yellow (M a u r i z i o and G r a f l , 1969) or grey-green coloured (L i p i ń s k i , 1982; J a b ł o ń s k i , 1998).
Corylus and Alnus pollen grains contain one of the strongest allergens in terms of their effects on humans.In the early-spring period, they cause allergy in sensitive persons.Hazel pollen allergens exhibit cross reactions with allergens contained in alder and birch pollen (M a t t i e s e n et al. 1991; S p i e k s m a and F r e ng u e l l i , 1991; K n o x and S u p h i o g l u , 1996).
An analysis of black alder (Alnus glutinosa) and common hazel (Corylus avellana) distribution in Poland demonstrates that they are evenly distributed across the country (Z a j ą c and Z a j ą c , 2001).
The aim of the study was to determine pollen yield and the amount of pollen released in the abovementioned plant species in the conditions of Lublin and to compare these results with annual sums of pollen grains obtained from detailed analyses of atmospheric pollen seasons over a period of 8 years.

MATERIALS AND METHODS
The research material consisted of common hazel (Corylus avellana L.) shrubs growing in a line at a sunny site and black alder (Alnus glutinosa (L.) Gaertn.)trees occurring individually on the bank of the Czechówka River.Observations were made in Lublin, in the western part of the city (51 o 14' N and 22 o 32' E).
In the present study, pollen yield of the studied species was calculated based on the weight difference of 25 infl orescences before pollen shedding and after pollen release.The method of W a r a k o m s k a (1972) was used to determine the pollen weight.The average number of fl owers per infl orescence was determined based on investigations of 30 infl orescences sampled from different plants.The number of infl orescences per plant was estimated in accordance with the recommendations of D e m i a n o w i c z and H ł y ń (1960).
The number of pollen grains produced per anther was also determined.Mature stamens were sampled for the investigations before anther dehiscence.Pollen grains were washed out of the thecae with 70% alcohol onto a microscope slide using a stereoscopic microscope, and then they were counted exactly.The obtained results were recalculated per fl ower and per infl orescence.Six replications were made for each species.
The occurrence of airborne pollen was recorded using a Lanzoni VPPS 2000 pollen trap, located on a building roof at a height of 18 m above ground.Pollen monitoring was conducted in the years 2001-2008.

RESULTS
Hazel and alder belong to plants with unisexual fl owers, arranged monoeciously.They form male Amentifl orae-type infl orescences with a long, slender axis, the so-called base.
Catkin male infl orescences of Corylus avellana grow on shoots in clusters, most frequently grouping 3-4 of them.Flowers are arranged spirally on the axis and initially they form a rigid, relatively compact arrangement (Figs 1, 2).They are formed in the summer of the previous year.The fi rst-formed fl ower primordia were observed in the second half of June.At the beginning of September, when their average length was 1.4 cm, pollen grains were found to be already fully developed.In the period which immediately preceded pollen shedding, the infl orescences reached the length of 3.6 cm, on the average.During fl owering, the infl orescence extended by over 100%, it became slender, susceptible to wind blasts.The bracts were characteristically bent back, forming in its upper part a bowl-shaped hollow (Fig. 4) in which a part of pollen settled from the higher located anthers; it is favourable for the pollen dispersal by wind and, at the same time, facilitates pollen collection by bees.
Hazel male fl owers are composed of four stamens subtended by bracts (Fig. 5).The stamen fi lament is divided as far as the base, and two thecae are not joined by the connective (Fig. 3), which is a special adaptation to anemophily.The bracts which enclose the stamens exhibit a roof-tile-like arrangement and they are covered with numerous hairs (Fig. 2).One outer bract and two inner bracts -with total dimensions of 3.22 mm x 2.45 mm -are fused together.In the examined material, the thecae reached the average length and width dimensions of 1.16 mm x 0.61 mm.Initially, the fi lament was short -0.2 mm, but during pollen shedding it extended     its length up to about 1 mm.In the apical portion of the thecae, tufts of hairs were observed with the length of 0.2-0.3mm, forming a characteristic little beard (Fig. 3).
Average pollen yield of the common hazel infl orescence obtained based on the weight difference of pollen-bearing aments and aments which have shed pollen was 66 mg.The hazel shrubs developed, on the average, 7 basal stems on which 2430 infl orescences were found.Pollen yield of the common hazel shrub was estimated at 168 g.It was also found that 100 running meters of the shrub line may produce 8400 g of pollen.
The calculations show that the male hazel infl orescence is composed of an average of 240 fl owers, producing a total of 1920 thecae.In a single theca, there was an average of 4 550 pollen grains, hence the number of grains produced by one infl orescence was 8 736 000 (Tab. 1).
Alnus glutinosa male infl orescences grow at the top of twigs in clusters of 3-5.Their dark brown-claret colour is given by the fused bracts: the outer bract, two inner bracts and two bracteoles, from underneath of which bright yellow stamens come out (Fig. 8).On the surface of the infl orescence, yellowish resin substances occur which are also collected by bees.In the angles of the partially lignifying bracts, cymes grow which are arranged alternately in relation to the main infl orescence axis.The cymes are composed of three male fl owers (Fig. 9), differently than in hazel where single fl owers occur enclosed with bracts.The black alder fl ower with a small 4-segmented perianth, with 4 stamens, reaches a diameter of 2.5 mm (Fig. 12).The anthers with the length and width dimensions of 0.92 mm x 1.06 mm are partially divided and they have strongly developed connective (Figs 10,12).
It was calculated that a single black alder tree produced an average of 7370 infl orescences.The average pollen weight per infl orescence was determined at the level of 120 mg, whereas pollen yield of one black alder tree was 884 g.

Corylus Alnus
geous effect, since thereby hazel pollen is more easily accessible (L i p i ń s k i , 1982).
The number of pollen grains produced in the thecae of common hazel and black alder was similar and it was 4550 and 4210, respectively.However, it was found that a single alder infl orescence, characterised by a larger number of fl owers, produced over two times more pollen than a hazel catkin.The number of pollen grains in the Corylus avellana infl orescence, calculated by the present paper's author, is 8 736 000, and for Alnus glutinosa 19 534 000, whereas according to literature data it is estimated at 3 933 000 for hazel and 4 445 000 for alder (P o h l , 1937 after E r d t m a n , 1954; M a u r i z i o and G r a f l , 1969).The above differences may be attributable to many reasons, among others, habitat conditions, meteorological factors before and during fl owering which may disturb the microsporogenesis and also affect the infl orescence length.M o l i n a et al. (1996) report that the number of pollen grains determined in stamens from different trees of the same species may be various, for example, for Quercus rotundifolia it was within the range of 1530 -9800.The present paper's author did not observe large deviations in the number of pollen grains in the stamens of the studied plant species., 1993).Signifi cant differences were found between the extreme dates of the pollen season start in the years 2001-2005 in Lublin, and for hazel this difference was 47 days, whereas in the case of alder it reached 51 days ( W e r y s z k o -C h m i e l e w s k a and P i o t r o w s k a , 2006).Fluctuations in the start dates of pollen seasons occur not only between particular years, but also between regions.We r y s z k o -C h m i e l e w s k a and R a p i e j k o (2007) observed a difference amounting to 6-16 days between the start dates of the alder pollen seasons in Lublin and Warsaw in the same years of study.

CONCLUSIONS
1. Black alder is characterised by higher pollen yield than common hazel.The pollen weight per one hazel infl orescence is 66 mg, whereas for alder it is 120 mg.Pollen yield of a common hazel shrub was estimated at 168 g, whereas for a black alder tree it was 884 g.
According to my calculations, one male infl orescence contains an average of 580 fl owers.One stamen produced an average of 8 420 pollen grains, whereas one infl orescence 19 534 000 (Tab. 1).Alder pollen grains are classifi ed as small; they mostly have 5, sometimes 4 or 6 pores with the characteristic thickened exine (arci) between the pores.The dimensions of the examined grains reached 24.2 x 18.8 μm (Figs 13,14).
Annual sums of hazel and alder pollen grains recorded during pollen monitoring in the years 2001-2008 were markedly different.In the case of hazel, the mean annual sum of pollen grains from the 8-year study was 1052; the lowest value (796) was recorded in 2008 and the highest (1650) in 2006.The mean sum of alder pollen grains (5313) in the years 2001-2008 was fi ve times higher than in the case of hazel.In the particular years, these values ranged between 2545 (2002) and 7979 (2006) (Fig. 15).
A comparison of the hazel and alder atmospheric pollen seasons demonstrates that pollen of the abovementioned plant taxa occurs in the air at similar dates.The signifi cantly higher content of pollen of alder than hazel is also noticeable.Maximum pollen concentrations of both taxa are also recorded at similar dates.High concentrations are noted between 7 March and 8 April (Fig. 16).

DISCUSSION
Pollen yield of the common hazel infl orescence determined by the present paper's author was 66 mg.The average pollen weight per hazel infl orescence, determined earlier by W a r a k o m s k a (1970), was approximate, as it amounted to 69 mg.No data on alder pollen yield has been found in literature.
It is more diffi cult for bees to form pellets from pollen of anemophilous than entomophilous plants, since it is loose, devoid of pollenkitt, and it is also characterised by a smooth surface of the exine.However, in early spring, due to small diversity of fl owering plants, they also use this pollen fl ow.Pollen grains are the only source of protein and they are necessary for the proper development and normal functioning of the bee family (B i e ń k o w s k a , 1997; J a b ł o ń s k i , 1998).It is estimated that bees are able to collect an average of 30% of pollen from fl owers of anemophilous plants, and about 50% from fl owers of entomophilous plants (J a b ł o ń s k i , 1998).Under adverse weather conditions, it is recommended that bees should be additionally fed with pollen from hazel or alder infl orescences (L i p i ń s k i , 1982).
Bees are encountered more often in hazel infl orescences than in fl owers of other anemophilous plants due to the adaptation of bracts to pollen retention (L i p i ń s k i , 1982; B i e ń k o w s k a , 1997).The frequent occurrence of this species at sheltered sites (for example, as the forest understorey) also has an advanta-2.The study shows that the number of pollen grains in the theca of common hazel and black alder is approximate, but the alder infl orescence produces 2.2 times more pollen than the hazel infl orescence.3. The pollen weight produced by infl orescences of the studied species is correlated with the number of grains: about a twice larger number of alder pollen grains is accompanied by about twice larger weight of pollen.4.More abundant pollen shedding of alder than in hazel is also refl ected in the patterns of atmospheric pollen seasons of these taxa and annual sums of airborne pollen grains recorded.

Fig. 15 .
Fig. 15.Annual sums of Corylus and Alnus pollen grains in the air of Lublin in the years 2001-2008.
Recently, publications have appeared which show that plants have produced more pollen in the last decades as a result of warming (E m b e r l i n , 1994; F r e i , 1998; E m b e r l i n et al. 2007).Based on results of pollen monitoring conducted in Lublin in the years 2001-2005, it can be stated that hazel and alder pollen is accessible for bees in the greatest amounts, depending on the weather, in March or at the beginning of April (W e r y s z k o -C h m i e l e w sk a and P i o t r o w s k a , 2006).Multi-year observations (41 years) relating to fl owering dates of the most important melliferous plants in Poland indicate a high deviation in the start date of hazel fl owering, which is ±32.4 days (G r o m i s z

Table 1
Characteristics of male infl orescences and fl owers of Corylus avellana and Alnus glutinosa.