N

Molecules

Fig. 13.1 Permeability of biological membranes that allow or prevent the passage of molecules/solutes according to their size, charge, chemical properties, concentration and pressure (Modified from: Alberts et al. 2004)

xylem loading. Among the ten BOR1 hypothetical transmembrane domains, Takano et al. (2002) found a difference of two amino acids in the second transmembrane domain of the putative protein expressed by Arabidopsis mutants which requires higher levels of boron. Frommer and von Wiren (2002) suggested that to maintain a boron transport to the xylem, xylem sap requires borate anions. The pH is 5.6 for xylem and 7.5 for cytosole, boric acid inside the cell is converted to borate anion in the cytoplasm because of high cytosolic pH. Therefore boron can easily pass through the membrane as a form of borate anion. Then these borate anions are reconverted in the xylem to boric acid.

Frommer and von Wiren (2002) also proposed three different ways that BOR1 could export borate into the xylem: the first mechanism is diffusion that depends on the concentration gradient for borate (uniport); second is related to borate/chloride exchange coupled to a chloride gradient established by X-QUAC anion channels; and the third one is coupled counter-transport (antiport) of borate with a proton. The proton is exported to the cell wall space by H+-ATPases inside which generates a negative membrane potential (Frommer and von Wiren 2002).

NIP5;1 is identified as a boric acid channel that resides on the plasma membrane and requires boric acid uptake under boron limitations for normal growth (Takano et al. 2006). Casparian strip has an active role during the boron transport. It blocks the passage of extracellular boric acid from endodermis to the pericyle. Under boron scarcity conditions, NIPs are translated and reside on the plasma membrane of epidermal, cortical and endodermal cells on root and import of boron into the cells is limited. Boric acid can reach the pericyle and then xylem by means of these importers. The intracellular passage of boric acid between the cells is sustained by plasmodesmata. Hence, boric acid can pass to the Casparian strip and can reach to the destination point-pericyle cells before the xylem loading (Tanaka and Fujiwara 2008). The cellular boric acid needs to efflux from the pericyle cells for xylem loading. According to Tanaka and Fujiwara (2008) BOR1 proteins are expressed somehow, being regulated by posttranscriptional modifications. BOR1 exports the cytosolic boric acid to the pericyclic region under boron limited conditions, but studies have shown that BOR1 proteins are degraded via endocytosis in vacuoles under excess boron supply (30 and 100 ^M respectively) (Takano et al. 2005).

4.4 Boron Remobilization

Common idea regarding the boron transport was that it is transported towards the upper parts of the plants as a result of transpiration strength and accumulates on its destination point especially edges of the leaves. Therefore, ideally the older leaves accumulate much more boron than younger. However, studies indicated that for some species, especially significantly sugar alcohol producing species, boron concentration of young leaves is estimated to be higher than older leaves. This stresses that boron can remobilize from the different portions of plants with the help of sugar alcohols especially species that commonly produce significant amount of sugar alcohols (mannitol and sorbitol). Brown et al. (1999) showed that this remobilization is highly related to the sorbitol synthesis. In the case of enhanced production of sor-bitol synthase, transport is significantly increased. Tanaka and Fujiwara (2008) have suggested that boron can move along the flow of boron-binding sugar alcohol.

Recent metabolite study for boron toxicity tolerance in plants has shown that glucose level is increased in leaf at high boron exposure levels (1000 ^M) compared to low (5 ^M) (Roessner et al. 2006). Reid et al (2004) showed in boron intolerant plants, photosynthesis is suppressed by 23% at a high level of boron. Recently Unver et al. (2008) showed a possible role of photosystem II Protein D2 to regulate the boron toxicity in Gypsophila perfoliata by comparing the control and high boron exposed (500-1000 ^M) leaves. DDRT-PCR results showed that one of the differentially expressed transcript had high level similarity (99% positive score) in the Triticum aestivum Photosystem II protein D2. qRT-PCR analysis showed that 500 and 1000 ^M boron treated leaf samples showed 10 and 14 fold changes respectively compared to the control groups (30 ^M). Thus boron tolerant plants probably tolerate the toxic effects of boron by remobilizing the excess boron between the leaves by forming sugar-boron complexes through phloem. By reverse reaction, deficiency-tolerant plants might tolerate the boron essentiality with the same mechanism and transportation with the same way as of sugar alcohols. However, non-sugar alcohol producing plants can transport boron preferentially to young tissues as observed in Arabidopsis (Noguchi et al. 2000), Brasica napus (Stangoulis et al. 2001), and Helianthus annuus (Matoh and Ochiai 2005) in case of the limited boron exposures (Tanaka and Fujiwara 2008). It is proposed that non-sugar alcohol producing plants have to activate different mechanism to translocate boron into the young portions of the plants. Boron transporters and channels may be involved in this translocation (Noguchi et al. 2000). Also Tanaka and Fujiwara (2008) hypothesized that plants are capable of sensing boron levels and regulate the transport under limited conditions.

5 Boron Pollution

In recent years, there has been a great increase in the use of boron at the industrial level as well as water desalination processes for healthy irrigation. The mining processes lead to a dramatic increase in the accumulation of boron in agricultural soils (Parks and Edwards 2005). The arid and semiarid regions are potentially having risk with boron toxicity, due to capillary action and evaporation of boron rich ground waters. Under these circumstances boron concentration reaches to a toxic level for plants and reduces crop yields by polluting agriculrural areas (Tanaka and Fujiwara 2008).

Turkey is the important producer of naturally occurring borax fertilizers (Norman 1998). More than 50% of the world boron reserves are found in Turkey (Roskill 1999; Kalafatoglu and Ors 2000). It has become an important and strategic element in terms of developing technologies (Kose et al. 2003; Oren et al. 2006).The proven reserves are 375 million tons, whereas possible reserves are 483 million tons. This is equivalent to the 72.2% of the world reserves (Bayca et al. 2008). These are found in Susurluk, Bigadig, Sindirgi regions of Balikesir (Fig. 13.2), Kestelek

Fig. 13.2 Setallite images of Boron mines in Bigadig, Balikesir (White spots indicate boron mines)

District of Bursa, Emet District of Kutahya and Kirka District of Eskisehir. The largest reserves are found in Emet, Bigadig, Kirka and Mustafakemalpa§a Districts (72% of the world boron reserves). These are located in an area of 100 x 200 km2. Mines are situated alongside the drainage areas of Simav and M. Kemalpasa rivers. During the mining processes, boron containing drainage waters, cause pollution of Simav Creek, which is used for the irrigation of nearly 40,000 ha of agricultural area in Balikesir, Kepsut, Susurluk and Karacabey plains (Sener and Ozkara 1989; Uygan and Qetin 2004). The boron carried by the Simav Creek is over 2 mg L-1 and threatens the fertile agricultural soils (Sener and Ozkara 1989). Watery wastes from the mining areas in general contain 14-18% B2O3 which flows in to the collection ponds (Kose et al. 2003). A total of 60.000 tons of wastes are produced every year from the boron extraction mining areas (Batar et al. 2009). The boron concentration in the collection ponds is above the limits given by WHO (Oren et al. 2006). Some work has been done to purify these wastewaters (Kalafatoglu et al. 1997). Very few studies have been carried out on the soil-plant interactions in relation to boron in Turkey. Dundar and Qepel (1979) have reported harmful effects of boron on the leaves of some species in the forest vegetation around Emet (Kutahya) Borax Production Plant. Through the wastewaters of the river Simav the boron is spread to a wide area and causes boron pollution in agricultural soils of this area, rendering the soil infertile (Onel 1981).

Especially in the areas around the boron reserves in Turkey industrialization and urbanization have developed dramatically and this pollution can be seen intensively. The wastewater with a high boron content flowing into the rivers like Simav adversely affects the agricultural areas in the region (Sener and Ozkara 1989). The washing waters, rich in boron which are released from boron mines are collected in the Qamkoy Dam (Fig. 13.3). However, other waters rich in boron from inactive

Fig. 13.3 The wastewater from the Boron mines flown into the Qamkoy Collector Dam

Fig. 13.4 Boron mines which are not used but cause environmental pollution through rain and underground waters

Fig. 13.4 Boron mines which are not used but cause environmental pollution through rain and underground waters and closed boron mines are flown into the river Simav which reach the agricultural areas through rain as well as underground waters (Fig. 13.4).

According to Uslu and Turkmen (1987) boron levels recommended for permanent usage should be up to 0.75 mg L-1, and 2 mg L-1 for short term usage. The samples taken from Simav Creek and its environs in Bigadigshowed boron levels as 22.56 (open mine surface water); 22.85 (Qamköy Dam water); 23.07 (water taken after ore washing); 23.07 (water from collected pools); 11.35 (water from Simav River); 1.64 (water from the Simav River-500 m away from the mine); and 16.89 mg L-1 (open mine surface water). Soils associated with these reserves are high in boron and host a plant diversity with tolerance to high levels of boron.

The natural plant cover of the boron mining areas around Kirka-Eski§ehir is represented by the taxa like (Türe and Bell 2004); Gypsophila perfoliata L. var. Perfoliata. Catapodium rigidum (L.) C.E. Hubbard ex Dony subsp. rigidum var. rigidum; Juniperus oxycedrus L. subsp. oxycedrus; Adonis flammea Jacq.; Glaucium leiocarpum Boiss.; Papaver rhoeas L.; Hypecoum imberbe Sibth. & Sm.; Alyssum pateri Nyar. subsp. pateri; Reseda lutea L. var. lutea; Chenopodium album L. subsp. album var. album; Melilotus officinalis (L.) Desr.; Medicago sativa L. subsp. sativa; Potentilla recta L.; Carduus nutans L. subsp. nutans; Centaurea solstitialis L. subsp. solstitialis; Centaurea depressa Bieb.; Centaurea virgata Lam.; Tragopogon latifolius Boiss. var. angustifolius Boiss.; Convolvulus linea-tus L.; Quercus trojana P. B. Webb. T; Galium verum L. subsp. verum; Allium atroviolaceum Boiss.; Aegilops cylindrica Host.; Aegilops triuncialis L. subsp. tri-uncialis; Hordeum distichon L.; Hordeum murinum L. subsp. leporinum (Link) Arc. var. leporinum: Chrysopogon gryllus (L.) Trin; Stipa lessingiana Trin. & Rupr.; Pinus nigra Arn. subsp. pallasiana (Lamb.) Holmboe; Neslia apiculata Fisch.; Matthiola longipetala (Vent.) DC. subsp. longipetala; Helianthemum canum (L.)

Baumg.; Polygala pruinosa Boiss. subsp. pruinosa; Dianthus crinitus Sm. var. crinitus; Paronychia carica Chaudhri; Hypericum avicularifolium Jaib. & Spach. subsp. depilatum; Linum hirsutum L. subsp. anatolicum (Boiss.) Hayek var. ana-tolicum; Haplophyllum thesioides (Fisch. ex DC.) G. Don; Genista aucheri Boiss.; Astragalus vulneraria DC.; Coronilla varia L. subsp. varia; Onobrychis gracilis Besser; Sanguisorba minor Scop. subsp. muricata (Spach.) Briq.; Sedum sartori-anum Boiss. subsp. sartorianum; Eryngium campestre L. var. virens Link.; Morina persica L.; Scabiosa argentea L.; Anthemis tinctoria L. var. pallida DC.; Achillea wilhelmsii C. Koch.; Onopordum tauricum Willd.; Jurinea consanguinea DC.; Centaurea urvillei DC. subsp. stepposa Wagenitz; Leontodon asperrimus (Willd.) J. Ball.; Asyneuma limonifolium (L.) Janchen subsp. limonifolium; Asyneuma virga-tum (Labill.) Bornm. subsp. virgatum; Onosma bracteosum Hausskn. & Bornm.; Anchusa officinalis L.; Anchusa stylosa Bieb.; Convolvulus compactus Boiss.; Convolvulus holosericeus Bieb. subsp. holosericeus; Lappula barbata (Bieb.) Gürke; Linaria corifolia Desf.; Orobanche alba Stephan; Acanthus hirsutus Boiss.; Globularia orientalis L.; Teucrium chamaedrys L. subsp. chamaedrys; Teucrium polium L.; Scutellaria orientalis L. subsp. pinnatifida Edmonson; Phlomis arme-niaca Willd.; Marrubium parviflorum Fisch. & Mey. subsp. parviflorum; Sideritis montana L. subsp. montana; Stachys byzantina C: Koch; Thymus leucostomus Hausskn. & Velen var. argillaceus Jalas; Salvia sclarea L.; Salvia cryptantha Montbret & Aucher ex Bentham; Acantholimon acerosum (Willd.) Boiss. var. acero-sum; Plantago lanceolata L.; Euphorbia macroclada Boiss.; Quercus pubescens Willd.; Cruciata taurica (Pallas ex Willd.) Ehrend.; Asphodelina damascena (Boiss.) Baker subsp. damascena; Muscari neglectum Guss.; Koeleria cristata (L.) Pers. and Puccinella convoluta (Homem.) P. Fourr.

The plant taxa recorded from Bigadig, Balikesir are (present study);

Pinus nigra Arn.; Juniperus oxycedrus L. ssp. oxycedrus; Delfinum peregy-nium; Amaranthus retroflexus L.; Chenopodium album L. ssp. album var. album; Polygonum lapathifolium L.; Polygonum aviculare L.; Polygonum equisetiforme Sibth. & Sm; Rumex Pulcher L.; Quercus ilex L.; Quercus pubescens Willd.; Silene otites; Lavatera punctata; Tamarix sp.; Sinapis arvensisL.; Neslia Apiculata Fisch.; Reseda lutea L.; Anagallis aquatica; Rosa canina L.; Malus sylvestris miller ssp. orientalis (A. Uglitzkichj Browicz var. orientalis; Crateagus monog-yna Jacq. ssp. monogyna; Spartium junceum L.; Trifolium angustifolium L. var. angustifolium; Trifolium hybridum L. var. hybridum; Ononis spinosa; Lythrum salicoria L.; Pistacia terebinthus L. ssp. terebinthus; Pistacia vera; Ruta montana (L.) L.; Tribulus terrestris L.; Linum bienne Miller; Eryngium campestre L. var. visens; Eryngium creticum; Bupleurum odontites; Ammi visagna; Bupleurum tenuissimum; Papaver rhoeas L.; Olea Europea L. var. europea; Phillyrea latifo-lia L.; Solanum nigrum. L. ssp. nigrum; Convolvulus arvensis L.; Ballota nigra ssp. anatolica; Mentha spicata ssp. spicata; Stachys byzantina; Teucrium polii; Thymbra spicata; Plantago major L.; Plantago lanceolata L.; Rubia tinctorum L.; Paliurus spina-christi; Viscum album; Osyris alba; Scabiosa columbaria L. ssp columbaria var. Columbaria; Dipsacus laciniata; Xanthium spinosum L.; Pallenis spinosa (L.) Cass.; Picnomon acarna (L.) Cass.; Carduus nutans L.; Centaurea solstitialis L. ssp. solstitialis; Centaurea ibericaTrev. ex Sprengel; Centaurea virgata; Cardopatium corymbosum (L.) Pers.; Echinops ritro L.; Scolymus hispanicus L.; Cichorium inty-bus L.; Picris altissima Delile; Helminthotheca echinoides (L.) Holub; Carthamus Lanatus; Xeranthemum annuum; Hordeum murium L.; Hordeum bulbosum L.; Loliumperenne L.; Dactylis glomerata L.; Cynosurus echinatus L.; Phragmites aus-tralis (Cav.) Trin. ex Steudel; Cynodon dactylon (L.) Pers.; Elymus elongatus ssp. eloggatus; Juncus conglomeratus; Cyperus longus L.; Draculus vulgaris; Ruscus aculeatus L. var. angustifolius Boiss.; Asparagus acutifolius L.; Asphodelus aestivus Brot.; Allium neapolitanum Cyr. and Tamus communis L. ssp. communis.

The plant diversity of the areas shows variation depending upon the boron content of the soils. The soils with lower boron concentrations (0.1-2 mg kg-1) show a rich species diversity (84 species), whereas those with higher levels (10 mg kg-1) are poor in the plant cover (28 species). According to Babaoglu et al. (2004) only five species Catapodium rigidum ssp. rigidum var. rigidum and Gypsophila per-foliata var. perfoliata show resistance to boron levels in excess of the accepted toxic levels (35 mg kg-1); these species are reported to flourish in the zone with highest boron concentration. Our investigations revealed that in Bigadiç, Balikesir boron mining area Polygonum equisetiforme was tolerating high levels of boron.

6 Phytoremediation

Plants which uptake high levels of an element from the soil are called hyperaccu-mulators; these are now being closely investigated, both by molecular techniques and by soil/plant analyses, at the sites where they occur (Karenlampi et al. 2000). The term hyperaccumulator was first used in relation to plants containing more than 1000 ^g g-1 (0.1%) Ni in dry tissue (Jaffre et al. 1976; Brooks et al. 1977). A later publication (Baker and Brooks 1989) extended the use of the term to include plants containing more than 1% Zn or Mn, or more than 0.1% Cu, Co, Cr and Pb. The ability of Thlaspi caerulescens to accumulate Zn to more than 10,000 ^g g-1 (1%) in dry tissue has been known since the 1860s, but it has become apparent from more recent work that several species of this genus can also hyperaccumulate (Reeves and Brooks 1983; Reeves 1988) from metal-rich soils and can hyperaccumulate a wider variety of metals (including Cd, Mn and Co) from amended nutrient solutions (Baker et al. 1994). There has also been recent interest in high-Cd populations of T. caerulescens from mine soils (Robinson et al. 1998; Reeves et al. 2001). A recent study of hyperaccumulators for some metals (Zn, Cd, Pb, Ni, Cu, Se and Mn) has been published (Reeves and Baker 2000). This list did not include several other elements, such as B, As and Al. As accumulation by ferns has been studied by Ma et al. (2001), and also Kochian et al. (2002) reported a plant which accumulates 3000 mg kg-1 Al, nevertheless there is not much information about boron accumulation in plants.

Recently, Gezgin et al. (2002) surveyed the boron content of 898 soil samples from 7 States in Turkey. These States include 3.5 million ha of cultivated land in Central Southern Anatolia. However, nearly 50% of soils in these areas contained low levels of available boron which can be corrected by external boron applications in the form of borax or boric acid. However, another 18% of soils contain boron at more than the critical upper level for available soil born, which is considered to be 3 mg kg-1 (Keren and Bingham 1985) for most crops. These areas can be released from this abiotic stress by phytoremediation using boron accumulating species. Soil amendments by conventional techniques such as leaching or increasing pH by liming (Nable et al. 1997) for increased boron adsorption on soil seem not to suit Central Anatolian conditions due to its low annual rainfall and water shortages, and the high lime content of the soils. For this reason, boron accumulating species appear as a solution to this problem.

First hyperaccumulation studies of boron in Turkey were undertaken by Babaoglu et al. (2004) on different taxa of Gypsophila sp. commonly growing on the boron rich areas around Kirka, Eskisehir-Turkiye. Gypsophila sphaerocephala var. sphaerocephala, G. perfoliata, Puccinellia ssp. distans and Elymus elongatus ssp. turcicus species were found in the highest boron containing sections of the mine. Out of these species, G. sphaerocephala was able to accumulate extraordinarily high concentrations of boron (Babaoglu et al. 2004). The species were found growing successfully under high total (8900 mg kg-1) and available (277 mg kg-1) soil boron concentrations. G. sphaerocephala contained considerably higher boron concentrations in its above-ground parts (2093 ± 199 SD mg kg-1, seeds; 3345 ± 341 SD mg kg-1, leaves), compared to the roots (51 ± 11 SD mg kg-1) and organs of the other species.

We also determined a boron tolerant species during our studies undertaken during 2000-2003 namely; Polygonum equisetiforme, which showed luxuriant growth over boron mining areas in the Balikesir region. It appears to us as one of the candidates as for phytoremediation of boron contaminated soils. It is a perennial deciduous taxon, with procumbent to erect stems, up to 100 cm tall, and few flowering shoots bearing pink or white flowers and distributed in Canakkale, Istanbul, Izmir, Antalya, Igel and Gaziantep. Water samples were taken from waste water of the collecting dam as well as Simav creek near the mining area.

The samples were collected around the Etibor mining area of Bigadic, Balikesir, one of the richest boron mines in the world. Plant samples along with their representative soils (0-50 cm deep) were collected from the area. Samples of surface soils were collected from pits measuring 20 x 20 x 20 cm.

All samples were put into plastic bags and directly brought to the laboratory for analyses. The plant samples were carefully washed with water to remove any traces of soil, then oven-dried at 70oC for 48 h before measuring dry weights. Samples (0.5 g) of finely ground plant material were digested with concentrated HNO3 in a microwave system (CEM). Boron in the extracts was analyzed by ICP-AES (Varian-Vista model) (Nyomora et al. 1997) in at least 4 plant samples with 3 replicates. The boron standard used was from Merck, Germany. The extractable boron concentrations in soil were determined according to the method of Cartwright et al. (1984) by extraction with 0.01 M mannitol plus 0.01 M CaCl2 using a soil solution ratio of 1:5 and a shaking time of 16 h. Boron extracted was determined by ICP-AES (Bingham 1982). The results of boron content of the soils and plants from the sampling sites is presented in Table 13.1.

Table 13.1 Boron content of the soils and plants from the sampling sites

Sampling

Boron content

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