Floristic characterization and pollen morphology of plants visited by Apis mellifera L. in caatinga areas in Bahia, Brazil

ABSTRACT The aim of this study was to identify plant species visited by Apis mellifera L. in honey producing areas with typical Caatinga vegetation in the State of Bahia, as well as morphologically characterize pollen grains of the most representative species. Flowering specimens were collected from both areas, herborized, identified and deposited at the HUNEB herbarium. Analyses of floristic similarities were performed between eleven municipalities close to the study areas. Pollen was collected from all specimens, acetolyzed, measured, statistically analyzed, morphologically described, and photographed in light microscopy. Of the total of species recorded, 67.46 % were regarded as having beekeeping importance, with the richest botanical families being Fabaceae, Malvaceae and Asteraceae. Additionally, 37.5 % of the recorded species were herbs. The analyzed municipalities showed 84 % of floristic similarity. Of the total species recorded with visits by A. mellifera, 25.52 % had their pollen grains already described in the literature as monads, tetrads and polyads; isopolar, apolar and heteropolar; and mostly prolate spheroidal shape. Sizes varied from small to large, and the amb circular was predominant. The exine ornamentation was greatly diversified, varying from psilate to echinate. The obtained data corroborate the palynological knowledge of plants regarded as having beekeeping importance within the Caatinga.


Introduction
Northeast Brazil is known for its areas with great beekeeping potential due to the diversity of environments and native Caatinga flora, especially in the State of Bahia, a biome distributed through 70 % of this region (Sena 2011;Oliveira & Santos 2014;Liberato & Morais 2016).According to data obtained by Instituto Brasileiro de Geografia e Estatística (IBGE 2021), in 2021, the production of honey in Brazil had an increase of 6.4 % more than the previous year, and the Northeast was one of the three regions that boosted this growth, participating with 36.3 % of the national production, estimated in 55.8 thousand tons.In this scenario, the state of Bahia stands out occupying the fifth place in the honey production ranking, accounting for 8.2 % of the Brazilian honey produced in 2021.
Nonetheless, for beekeepers to have an ideal use of quality and production of honey in a specific region, it is necessary to know the favourable flora for bees, besides analysing the soil, local climate, and support capacity of this region (Pereira et al. 2006;Santos & Ribeiro 2009;Ribeiro et al. 2019).Plants with beekeeping potential usually show intense flowering events, being attractive food sources for different species of bees.These plant species might show floral morphological characteristics, besides specific odours, that enable pollinator attraction, being the researcher or beekeeper's job to infer their utility for bees (Freitas & Silva 2006;Santos et al. 2006a;Melo et al. 2018).
The biological knowledge of plant species is important for beekeeping as flowering times, and pollen morphology helps to estimate the beekeeping potential of these species from the bee pasture and contribute to the creation of a flowering calendar for local bee plants (Freitas & Silva 2006;Lima et al. 2006;Pereira et al. 2006;Moreti et al. 2007).Studies on the floristic diversity of interest to bees allow the creation of future strategies to rationally use the beekeeping flora and the conservation of their ecosystems, contributing to a possible production increase of honey (Mendonça et al. 2008).
Several floristic studies have revealed the rich diversity and structure of the Caatinga biome (Araújo et al. 2010;Santos et al. 2011;Lima et al. 2012;Sanquetta et al. 2014;Luna et al. 2015;Rocha et al. 2017), as well as studies focusing on bee plant species for the Caatinga and other ecosystems.The description of pollen morphology from these areas, coupled with palynological analyses of bee products, has contributed to the identification of plant species foraged by bees, including the species Apis mellifera L. (Ferguson & Skvarla 1982;Aguiar et al. 2002;Almeida et al. 2006;Lima et al. 2006;Viana et al. 2006;Almeida 2007;Chaves et al. 2007;Vidal et al. 2008;Oliveira et al. 2010;Nascimento et al. 2014;Silva et al. 2014a;Silva et al. 2016b;Costa et al. 2018;Rasoloarijao et al. 2019;Mander et al. 2020;Moraes et al. 2020;Silva et al. 2020;Lu et al. 2021;Reis et al. 2021;Dias et al. 2022).A. mellifera forages to flowers for both pollen and nectar.The species' preference for floral types and available resources can be altered by several factors, including the period of the year, available flowering in the surrounding area, and the nutritional needs of the colony.In addition, these bees may have their foraging behavior modified as a function of temperature, but this is not a limiting factor for the species (Malerbo- Souza & Silva 2011;Moura et al. 2011;Araújo et al. 2014).
Thus, the aim of this study was to conduct a floristic study in two areas of Caatinga vegetation to characterize bee plant species, especially for colonies of A. mellifera, in apiaries located in the State of Bahia, as well as morphologically characterize pollen grains of the most representative species.

Study area
The bee flora was sampled from an area of native Caatinga vegetation with surrounding crops near the Caldeirão do Mulato and Roça da Fonte apiaries in the municipalities of Antônio Gonçalves (12º26'07"S; 39º07'11"W; altitude 191 m) and Campo Formoso (10º30'27"S; 40º19'17"W; altitude 556 m), respectively.These municipalities are located in the Mid-Northern semiarid region of the State of Bahia (Fig. 1), near the plains of the Jacaré and Salitre rivers in the Jacobina mountain complex (SEI 2020).
The climate in these municipalities varies from subhumid to dry (Antônio Gonçalves) and semiarid (Campo Formoso), with mean temperatures from 14.5 ºC to 33 ºC, and rainfall varying from 500 to 1,100 mm, annually.The vegetation from this region is diverse and adapted to scarce rainfall, being dense to sparse typical arboreal Caatinga with a gramineous herbaceous layer.The municipality of Antônio Gonçalves is characterized by a mosaic of Caatinga-dry forest-cerrado, while Campo Formoso is characterized by open to dense arboreal Caatinga and remnants of Atlantic rainforest (SEI 2020).

Collection and processing of botanical specimens
Specimen collection visited by A. mellifera was performed from September 2016 to May 2017 and from October 2019 to January 2020 at Antônio Gonçalves, while at Campo Formoso, specimen collection only took place from May 2019 to January 2020.All specimens were monthly collected in both municipalities based on the direct observation of A. mellifera foraging their flowers, with a duration of five minutes.All field collections were performed from eight to nine hours per day, considering the vegetation layers of trees, shrubs, subshrubs, herbs and lianas (Rizzini & Rizzini 1983).Specimen collection took place on random pre-existent trails surrounding the study areas in a ray of 1,500 m (Marques et al. 2011) from Caldeirão do Mulato and Roça da Fonte apiaries.Flowering specimens were collected using usual botanical techniques (Bridson & Forman 1998), annotating floral attributes such as colour, attraction unit, symmetry, size, and odour presence (Percival 1965) to aid in specimen characterization, mostly for identification and labelling purposes.All identifications were based on comparisons between specimens deposited at Herbarium of the Universidade do Estado da Bahia (HUNEB) and Herbarium of the Universidade Estadual de Feira de Santana (HUEFS), using identification keys from the specialized literature, and by sending duplicates to taxonomic specialists.The classification system adopted in Acta Botanica Brasilica, 2023, 37: e20220264 this study was the Angiosperm Phylogenetic Group IV (The Angiosperm Phylogeny Group et al. 2016), and specimens were deposited at the HUNEB herbarium.
Monthly rainfall estimates were retrieved for both municipalities based on the data from the INMET (2020) website for all periods of our field studies: Antônio Gonçalves (2016-2017) and Campo Formoso (2019-2020).A floristic similarity analysis was performed between the bee flora from the studied municipalities with other areas of Caatinga vegetation in the Brazilian semiarid (Table 1).The checklist was used to elaborate a binary matrix of absence/ presence for all species compiled from our fieldwork and the selected studies from the Brazilian semiarid to observe floristic similarity patterns in these areas.Subsequently, the Unweighted Pair Groups Method Average (UPGMA) was used as the weighted average algorithm to identify the floristic relationship between the studied areas.A dendrogram generated from a cluster analysis using the Jaccard index was generated, alongside all analyses, using the PC-ORD 7.0 software (Mccune & Mefford 2011).

Collection and processing of pollen material for the morpho-pollinic study
All pollinic studies were developed at the Laboratório de Estudos Palinológicos (LAEP) from the Universidade do Estado da Bahia (UNEB), Campus VII, Senhor do Bonfim, Bahia.Pollen samples (flowers/floral buds) were obtained from exsiccates deposited at the HUNEB herbarium from our field studies and selected based on occurring at least in three municipalities from the floristic similarity analysis.Most pollen grains were acetolyzed, according to Erdtman (1960), but for fragile pollen grains, we used the ACLAC method  (Raynal & Raynal 1971).Pollen grains were mounted using slides and coverslips with glycerine gelatine, sealed with paraffin, and analyzed and measured within seven days after mounting using a light microscope.
A total of 25 pollen grains were randomly analyzed for each specimen following technical recommendations by Salgado-Labouriau (1973) and Melhem et al. (1974).Morphometrical parameters such as polar axis (PA) and equatorial diameter (ED) in equatorial view were also analyzed.For other measurements (apocolpium side, aperture, spine distance, height and diameter of the base of spines, subspinal sexine elevation, sexine and nexine thickness), only ten pollen grains were randomly analyzed.PA and ED values of echinate pollen grains were measured from the subspinal sexine elevation.In other words, the diameter and height of spines were disregarded.For apolar pollen grains, a single measurement was done for the diameter.Alternatively, the longest (LD) and smallest (SD) diameters were measured for tetrads and polyads.
Quantitative data for 25 samples were statistically analyzed using the arithmetic average (x), sample deviation (σ), mean standard deviation (S), coefficient of variability (CV), the 95 % confidence interval (CI), and the range (R).In contrast, for measurements from 10 to smaller than 25 samples, only the arithmetic average was calculated.
Pollen grains from all studies species were illustrated based on photomicrographs obtained from a digital camera coupled with a Zeiss ® Axioskop plus microscope.Morphological descriptions included characters related to size, shape, polarity, aperture type, and exine structure and ornamentation.The terminology used in this study follows Punt et al. (2007) and Barth & Melhem (1988).

Diversity of the bee flora
The flora from the municipalities of Antônio Gonçalves and Campo Formoso in the State of Bahia was represented by 126 flowering species within the studied period.Among the identified species, 85 (67.46 %) were regarded as having beekeeping importance, distributed in 66 genera and 31 botanical families (Table 2, Fig. 2).For the municipality of Antônio Gonçalves, all plant species with beekeeping importance were collected from September 2016 to May 2017, with no records observed for the period from October 2019 to January 2020.
The species Alternanthera brasiliana (L.) Kuntze (Amaranthaceae), A. colubrina (Fabaceae), Melochia tomentosa L. (Malvaceae), M. tenuiflora and Senegalia bahiensis (Benth.)Seigler & Ebinger (Fabaceae) flowered during a period without any rainfall from December (2016) to February (2017) in the municipality of Antônio Gonçalves (Fig. 3).Even though this period is well known for the lack of rainfall, it is characterized by the relatively high diversity of bee plants flowering being visited by bees.
For example, in the municipality of Campo Formoso, the species A. leptopus (Polygonaceae) and Centratherum punctatum Cass.(Asteraceae) flowered both in the low and heavy rainfall periods from May (2019) to January (2020)      Acta Botanica Brasilica, 2023, 37: e20220264 (Fig. 3).In this municipality, the flowering peak of bee plant species was between September and November (2019) (Fig. 3), characterized by reduced rainfall.
Most herbs recorded were highlighted by offering nectar (37.5 %) as the primary flora resource offered to bees, being also the floral resource most foraged by bees in the study area (Fig. 4).Nectar-offering species represented 40 % of the total bee flora, followed by nectar-pollen-offering (32.94 %) and pollen-offering (25.88 %) species.Stigmaphyllon paralias A.Juss.(Malpighiaceae) was the single species to offer oil as the primary floral resource.Representatives from Fabaceae and Rubiaceae were among the most foraged bee plant species offering nectar and pollen.In species from Melastomataceae and Solanaceae, pollen was the sole floral resource offered, as well as in fours species of Senna (Fabaceae; S. aversiflora, S. occidentalis (L.) Link, S. rugosa (G.Don) H.S.Irwin & Barneby, and S. spectabilis).
The analysis of floristic similarity showed the formation of two distinct groups in the dendrogram, groups A and B (Fig. 5), which shared a considerable number of beekeeping plant species.In group A, the municipalities of Antônio Acta Botanica Brasilica, 2023, 37: e20220264 Gonçalves and Campo Formoso showed high floristic similarity values among them (~84 %).In comparison, the municipality of Cruz das Almas (Bahia) showed only 39.5 % of floristic similarity with the studied areas in municipalities located within the same ecosystem and with high levels of endemism, sharing only seven bee plant species: Alternanthera brasiliana, B. verticillata, Centratherum punctatum, Syagrus coronata (Mart.)Becc., Momordica charantia L., Pavonia cancellata, and Peltophorum dubium.In group B, the municipalities of Aparecida and Prata, both in the State of Paraíba, showed high floristic similarity (~90.5 %).However, when compared to the municipalities of Antônio Gonçalves and Campo Formoso, similarity levels were low.

Diversity of the bee flora
The diversity of bee plant species in this study shows the importance of these species for beekeeping and honey production of Apis mellifera hives in the region, being important for bee foraging for pollen, nectar, resin or oils.Santos et al. (2006a) also showed the great diversity of beekeeping plant species occurring in this region of the Bahian semiarid, which are frequently foraged by A. mellifera on a daily basis.
Species from Fabaceae, Malvaceae, Asteraceae, Euphorbiaceae, and Rubiaceae comprise 55.29 % of the total species visited by Apis mellifera in the study areas.Several floristic surveys in the Caatinga domain have shown that these families represent most of the plant diversity from Northeast Brazil (Araújo et al. 2010;Bessa & Medeiros 2011;Lima et al. 2012;Sanquetta et al. 2014;Bulhões et al. 2015;Luna et al. 2015;Sabino et al. 2016).Fabaceae and Malvaceae are highlighted due to the great number of species of beekeeping importance due to offering nectar/pollen to bees in the studies of Santos et al. (2006a) and Moraes et al. (2020).
When comparing the diversity of species recorded by botanical families in this study with other floristic surveys in the State of Bahia (Nascimento et al. 2014;Carvalho & Marchini 1999;Viana et al. 2006) or with other studies regarding the interaction between plants and bees (Aguiar 2003;Milet-Pinheiro & Schlindwein 2008), one can note that Fabaceae, Asteraceae, Malvaceae, Rubiaceae and Euphorbiaceae are among the most representative families with beekeeping importance.In areas of Caatinga, species of these families are regarded as excellent suppliers of trophic resources to bees (Nascimento et al. 2014;Costa et al. 2015;Machado & Lopes 2006;Santos et al. 2006b).According to Salis et al. (2015;2017), some plant species from Fabaceae are regarded as having beekeeping potential due to offering nectar and pollen for extensive flowering periods.
The analyses of honey samples from the municipalities of Antônio Gonçalves and Campo Formoso (Reis et al. 2021), recorded a variety of pollen types corroborating the great diversity of species, genera and families of the bee flora from this region.When compared to our results, Fabaceae and Malvaceae are highlighted due to the diversity of pollen grains recorded, mostly in Mimosa (Fabaceae) and Herissantia (Malvaceae).For Oliveira & Santos (2014), these types of pollen grains recorded in honey, coupled with their morphological characters, might be related to the several species from their genera.Thus, pollen types related to Mimosa recorded by Reis et al. (2021) might represent the species Mimosa candollei, M. tenuiflora, and M. pudica recorded in the study areas.In contrast, the Herissantia pollen type might be related to the species H. crispa and H. tiubae.
Some species that flowered during the dry season in this study possibly have alternative strategies for pollinator attraction, as well as morphological and physiological adaptations, making them more resilient during the dry season (Mantovani & Martins 1988;Barbosa et al. 2003).According to Silva et al. (2008), during the dry season, there are several plant species flowering in the Caatinga domain, such as M. tenuiflora, which blooms during the first dry months of the year, followed by Anadenanthera colubrina.
The results presented here for blooming plant species important for beekeeping during the dry season corroborate data from the specialized literature from the Caatinga domain, such as Carvalho & Marchini (1999), Aguiar (2003), Costa et al. (2018), Silva-Filho et al. (2010), among others.For Silva et al. (2015), M. tenuiflora is regarded as a species of important beekeeping activities in the Caatinga, recommending its conservation and propagation in places with apiaries.Carvalho & Marchini (1999) and Vidal et al. (2008) highlighted that Melochia tomentosa and Antigonon leptopus bloom all year long, possibly being important sources of nectar and/or pollen for bees when the Caatinga vegetation shows little resources to them.The diversity of floral attributes (color, symmetry, attraction unit and scent) presented by the different species that provide resources for the bees, as described in the present study, aid determine their beekeeping importance, contributing to attracting floral visitors, especially potential pollinators (Covre & Guerra 2016;Brito et al. 2017;Melo et al. 2018;Silva et al. 2020).According to Araújo et al. (2009) and Covre & Guerra (2016), flowers arranged in inflorescences, aside from concentrating resources, also increase flower visibility and attract more pollinators.
The large representation of mainly nectariferous species reported in the present study corroborates the results obtained by Santos et al. (2006b) in the Caatinga, as well as other studies in other vegetation types, such as Viana et al. (2006) for restinga formations and Lopes et al. (2016) for the Cerrado.For the latter studies, there might not be Regarding the species of Senna reported by us, Moura et al. (2018) highlight that due to the genus' poricidal anthers, pollen is their only floral resource.The sharing of bee species between certain areas seems to indicate the existence of a foraging pattern between bee species, which includes Apis mellifera, that despite foraging a wide range of plants species, shows a clear preference for certain species in each area (Aguiar et al. 2002;Pott & Pott 1986).This can be attributed to the occurrence of distinct and predominant vegetation types in each area.

Pollen morphology of bee plant species
In the present study, we have provided the pollen morphology of 20 species with beekeeping importance, arranged into 11 plant families.According to Lima et al. (2006) and Moreti et al. (2007), understanding the pollen morphology of bee plant species is essential to enable a reliable palynological analysis, characterization and identification of the palynological content of hive products since they allow the identification of their floral and geographic origin.
According to Marques-Souza et al. (2002), Koch et al. (2017), Rasoloarijao et al. (2019) and Konzmann et al. (2020), some morphological characteristics of the pollen grains can highlight the plant-pollinator correlation, favouring the pollination process by helping adhered grains to the bee body and its posterior transfer to the stigmatic surface.Lunau et al. (2015) and Konzmann et al. (2019) highlight that some exine ornamentation elements can affect their collection and storage by bees.
The descriptions provided by us in the present study, associated with the pollen dispersal of certain taxa, are in agreement with several previous studies (Melhem et al. 2003;Lima et al. 2006;Moreti et al. 2007;Corrêa et al. 2010;Silva et al. 2016a;Lorente et al. 2017).When describing the pollen morphology of melliferous used by Apis mellifera unicolor, Rasoloarijao et al. (2019) verified that 97 % of the studied species had pollen grains dispersed as monads.This data is also supported by the present study.Only two genera, Anadenanthera and Mimosa, had pollen not dispersed as monads, being dispersed as poliads and tetrads, respectively.Mimosa-type pollen is commonly observed in apiculture products as the dominant or codominant pollen type (Oliveira et al. 2010;Nascimento et al. 2015;Santos et al. 2020).
Medium-size pollen grains were the predominant size among the studied species (42.85 %), followed by large-size grains (28.57%), with the smallest grains being observed for the tetrads of Mimosa pudica and the largest ones observed for Pavonia cancellata (Malvaceae).These results corroborate data found in other studies on pollen morphology of the taxa studied here (Lima et al. 2008;Buril et al. 2010;Corrêa et al. 2012;Matos et al. 2014;Pereira et al. 2014;Nascimento et al. 2021).
According to Culley et al. (2002), relatively small and light-weight pollen grains are common in wind-dispersed species.Large-sized pollen grains have a higher nutritional value for bees due to their increased content.Furthermore, their size does not interfere with their collection by bees but does make them more visible to these animals (Konzmann et al. 2019;Rasoloarijao et al. 2019).Marques-Souza et al. (2002), while studying biochemical and morphological aspects of the pollen grains collected by five species of meliponid bees, observed that these bees collect grains of different sizes.They were able to confirm that pollen grain size did not play an important role for the bees when selecting which flowers to visit and pollen to collect.
Regarding pollen grain polarity, most species analyzed by us presented isopolar pollen grains, with a single record of apolar and heteropolar grains in Syagurus coronata.Grains with amb (sub)circular and prolate shape, including these categories' subtypes, were predominant amongst the studied taxa.The descriptions presented by us are in agreement with Gasparino et al. (2014)  Pollen grain shape does not seem to influence pollination, as well as bee attraction and foraging (Marques-Souza et al. 2002;Rasoloarijao et al. 2019).Saba et al. (2004) described the pollen morphology for species of Melochia (Malvaceae), including M. tomentosa.They observed similar characteristics regarding the medium (short-and longstyle morphs) to the large size of the grains (long-style morph) and the oblate-spheroidal (short-and long-style morphs) to spheroidal shape (long-style morph).Saba et al. (2004) andSilveira-Júnior et al. (2017) describe the pollen grains for the studied species of Waltheria (Malvaceae).However, their descriptions for homostylic W. indica differ from the one presented by us, especially regarding pollen grain shape and size.Silveira-Júnior et al. (2012) observed that pollen grains of Richardia grandiflora were large-sized and suboblate, differing from our present observations.For Myrtaceae, our observations for Psidium guajava agree with the descriptions provided by Corrêa et al. (2018) in their small size and oblate shape.
The colporate aperture type was the most common amongst the species analyzed in the present study.We also less frequently observed pollen grains with simple apertures (i.e., pores, colpi and sulci), as well as inaperturate grains.Aperture diversity in pollen grains is widely observed in studies that focus on the pollen morphology of different plant families, especially for those with foraging and beekeeping potential (Lima et al. 2006;Moreti et al. 2007;Moreti et al. (2007) describe the pollen grains of the main species of Fabaceae with beekeeping potential.Most of the studied grains (95 %) were colporate, while only 2.5 % were colpate.Dec & Mouga (2014) describe colporate pollen grains for some bee species of Euphorbiaceae.Nonetheless, some species did not have their aperture type described, possibly due to being inaperturate, a character commonly observed in some members of this family (Souza et al. 2016).For members of Myrtaceae, Nascimento & Carvalho (2019) report that species in this family are visited by bees and have colporate pollen grains.In the present study, most species of Myrtaceae (52.38 %) presented colporate grains.
Our present observations on exine ornamentation are in agreement with previous studies for the taxa studied by us (Melhem et al. 2003;Saba & Santos 2003;Saba et al. 2004;Melo et al. 2006;Lima et al. 2008;Buril et al. 2010;Cassino et al. 2016;Silva et al. 2016a;Souza et al. 2016;Lorente et al. 2017).Some inconsistencies between previous studies are worth mentioning, such as the exine ornamentation of Mimosa tenuiflora, described as scabrate by Buril et al. (2010) and as psilate by Lima et al. (2006).Mora et al. (2013) describe the pollen grains of Mangifera indica but fail to describe its exine ornamentation.However, the characters described by us regarding size (medium) and dispersal unit (monad) are in agreement with Mora et al. (2013).Pereira et al. (2014) and Silva et al. (2016a) characterize the pollen grains of Anacardium occidentale as having striate-reticulate exine, which disagrees with the striate-microreticulate ornamentation reported by us.Under SEM, Assis et al. (2021) described the pollen grains of A. occidentale with suprastriato-inframicroreticulate exine and M. indica with striato-perforate ornamentation, such perforations were not observed under LM.Matos et al. (2014) describe the pollen grains of Pavonia cancellata and Sida linearifolia A.St.-Hil.as having spinulate-microbaculate and spinulate-pilate exine, respectively.Nonetheless, P. cancellata, S. cordifolia, and S. galheirensis are described in the present study as spinulate-granulate.Lunau et al. (2015), when studying the mechanical protection against pollen grain collection by corbiculate bees, demonstrated that the echinate pollen of Alcea rosea L. (Malvaceae) makes it harder for them to be collected and stored in the bees' corbiculae.Nonetheless, this ornamentation type greatly facilitates the grains' adherence to animals, guaranteeing their transport for pollination.Regarding psilate grains, Culley et al. (2002) consider them to be typically associated with wind-pollinated species, together with reduced size and reduced number of apertures.Alternatively, heavily ornamented pollen grains with large size and a large number of apertures, especially the elongated ones (i.e., colpi and colpori), are common in insect-pollinated species.Nonetheless, Rasoloarijao et al. (2019) highlight that some species with psilate pollen grains offer nectar as a reward, thus being frequently visited by bees.

Conclusion
Our results provided much-needed information on the bee flora of the Caatinga visited by Apis mellifera colonies in the State of Bahia.This data aids in the recognition, preservation and multiplication of these organisms, representing a reliable tool for sustainable apicultural techniques, consequently improving the region's apicultural productivity.
The most species-rich plant families were Fabaceae, Malvaceae, Asteraceae, Euphorbiaceae and Anacardiaceae.These were also the families that had their pollen morphology described by us.These species showed a considerable morpho-pollinic variation, especially regarding their aperture type (inaperturate, porate, colpate, sulcate, or mainly colporate), size (predominantly medium-sized) and exine ornamentation (ranging from psilate to echinate).These results contribute greatly to the palynology of bee species, as well as providing data for environmental preservation studies in the Caatinga biome.

Figure 4 .
Figure 4. Distribution relation of floral resources by number of species according to habit type.

Table 1 .
Location of all areas used in the floristic similarity analysis.

Table 4 .
Morphometric characters from isopolar pollen grains from the most representative bee plant species recorded at least in three municipalities from the floristic similarity analysis.Pollen grain diameters (μm): polar axis (PA); equatorial diameter (ED); variation range (R); sexine (S); nexine (N).