Boron and Plants

Munir Ozturk, Serdal Sakcali, Salih Gucel, and Huseyin Tombuloglu

Abstract Boron is found naturally in the earth's crust in the oxidized form as borax and colemanite, particularly in the oceans, sedimentary rocks, coal, shale, and some soils. It is never found in the elemental form in nature possessing a complex chemistry similar to that of silicon, with properties switching between metals and non-metals. Boron has become an important and strategic element in terms of developing technologies. It is released into the environment mainly through the weathering of rocks, volatilization from oceans, geothermal steam, burning of agricultural refuse and fuel wood, power generators (coal/oil combustion), glass industry, household use of boron-containing products (including soaps and detergents), borax mining and processing, leaching from treated wood and paper, chemical plants, and sewage/sludge disposal, but a major proportion originates from the weathering of rocks. Boron is regarded as an essential element for human beings, animals and plants. Boron occurs in soils at concentrations ranging from 10 to 300 mg kg-1 depending on the type of soil, amount of organic matter, and amount of rainfall. The treatments lead to significant increases in the productivity of some plants but in certain cases a decrease is seen as the boron level increases with the boron content of irrigation water, in particular on the soils with

Botany Department, Ege University, 35100 Bornova, Izmir, Turkey e-mail: [email protected]

Biology Department, Fatih University, Istanbul, Turkey e-mail: [email protected]

Near East University, Institute of Environmental Sciences, Nicosia, Cyprus e-mail: [email protected]; [email protected]

H. Tombuloglu (B)

Biology Department, Fatih University, Istanbul, Turkey e-mail: [email protected]

Dedicated to Prof. Dr. Yusuf VARDAR (Ege University) and Prof. Dr. Hubert ZIEGLER (Munich Technical University) on their sad demise in 2009.

M. Ashraf et al. (eds.), Plant Adaptation and Phytoremediation, 275

DOI 10.1007/978-90-481-9370-7_13, © Springer Science+Business Media B.V. 2010

a heavy texture, high CaCO3 and clay content. Lack of boron in plants results in necrosis but excess amounts are said to produce poisonous effects. Turkey produces more than 60% of the world's borax, with important boron reserves located in Susurluk, Bigadic and Sindirgi regions of Balikesir, Kestelek-Bursa, Emet-Kutahya, the largest reserves occur in Kirka-Eskisehir. Therefore, there is a naturally occurring high level of boron in the ground waters in some of these areas due to the excess amounts of boron given out to the environment during washing and purification processes which result in the pollution of cultivated areas. An attempt will be made here to present an overview of the plant diversity on the boron contaminated soils in Turkey, effects of different concentrations of boron on the germination ability of some plants and possible candidates for phytomining of the soils showing boron toxicity symptoms.

Keywords Boron ■ Toxicity ■ Phytoremediation ■ Genotoxicity ■ Polygonum Contents

1 Introduction 276

2 Boron Production and Usage 277

3 Boron and Living Beings 278

4 Boron and Plants 278

4.1 Boron Tolerance, Deficiency and Toxicity in Plants 280

4.2 Boron Uptake By Plants 283

4.3 Molecular Basis of Boron Uptake and Transport 284

4.4 Boron Remobilization 286

5 Boron Pollution 287

6 Phytoremediation 291

7 Boron and Seed Germination 293

8 Boron and Genotoxicity in Plants 297

9 Conclusion 301

References 305

1 Introduction

Elemental boron (B) is a member of Group IIIA of the periodic table, along with aluminum, gallium, indium, and thallium, differing distinctly in its chemical properties from aluminum but resembles silicon (Si), arsenic (As), and germanium (Ge) possessing a very complex chemistry (Cotton and Wilkinson 1988; Marschner 1995). Tanaka and Fujiwara (2008) have recorded it as a member of metalloid group of elements belonging to group V, because its characteristics lie between metals and non-metals (Marschner 1995), being a semiconductor rather than a metallic conductor.

It is extensively distributed in low concentrations throughout nature in the form of various inorganic borates constituting about 10 mg kg-1 of the Earth's crust, ranging from 5 mg kg-1 in basalts to 100 mg kg-1 in shales (Woods 1994), and occurs in soils at concentrations ranging from 10 to 300 mg kg-1 (average 30 mg kg-1), depending on the type of soil, amount of organic matter, and rainfall. Economic reserves of borate minerals are rare and are usually found in arid desert regions with a geological history of volcanic and/or hydrothermal activity (Mellor 1980). The majority of the boron occurs in the ocean, at an average concentration of about 4.5 mg L-1 (Weast et al. 1985), but is also released from anthropogenic (agricultural, industrial and domestic) sources to a lesser extent (Butterwick et al. 1989). Natural weathering of clay-rich sedimentary rocks, coal and shale on land surfaces accounts for a large proportion of the boron, mobilized into the soils and the aquatic environment, in the form of borates. Boron in soil solution is present as boric acid and easily leached out of the soil due to its high solubility (Shorrocks 1997; Yan et al. 2006). It is adsorbed onto the surfaces of soil particles, with the degree of adsorption depending on the type of soil, pH, salinity, organic matter content, iron and aluminum oxide content, iron-and aluminum-hydroxy content, and clay content (Kekeg 2008; Ayvaz 2002).

The availability of B in soil is limited in many regions in the world with a high rainfall and seasonal water availability. On the contrary, in the arid and semiarid regions, ground water reaches the topsoil by capillary action and evaporates to leave solutes in soil. In regions with high-boron groundwater, boron concentration in topsoil reaches to a toxic level for plants and reduces crop yields. South Australia, Egypt, Iraq, Jordan, Libya, Morocco, Syria, Turkey, California, and Chile are regions/countries with boron toxicity problems in agricultural lands (Yau et al. 1995).

2 Boron Production and Usage

Borate minerals have been employed in a wide range of uses for many centuries, dating from at least the eighth century when they were used primarily as a flux for assaying and refining gold and silver as well as production of wall plaster and ceramics (Ayvaz 2002; Bayca et al. 2008; Batar et al. 2009). Their valuable properties and relative rarity has stimulated international trade in borates. Marco Polo claimed to have transported Chinese borate minerals from Tibet to Europe and Venice was the center for borate imports (Travis and Cocks 1984). It is wildly used in the industry. A large number of minerals contain boric oxide, but five of them are the most important from a worldwide commercial standpoint. The most widely used commercial productions and materials of boron include borax-pentahydrate, borax, sodium perborates, colemanite, ulexite as well as boric acid. These are produced in a limited number of countries, dominated by the Turkey and United States, which together furnish about 90% of the world's borate supplies (Lyday 1993; Culver et al. 1994). The principal end usage for borate include insulation and textile-grade fiberglass, laundry bleach (sodium perborate), borosilicate glass, fire retardants, chemical fertilizers and herbicides (as a trace element), and enamel coating, frit and ceramic glazes, as well as several other applications (Etiproducts 2005; WHO 1998). Other minor usage include cosmetics and pharmaceuticals (as a pH buffer), boron neutron capture therapy (for cancer treatment), and pesticides. The cancer treatment application which preferentially accumulates in tumor versus normal tissue, utilizes a boron compound made with 10B isotope, (Barth and Soloway 1994).

3 Boron and Living Beings

The lowest lethal dose for humans exposed to boric acid is reported to lie around 640 mg kg-1 body weight by oral exposure, 8600 mg kg-1 body weight by dermal exposure, and 29 mg kg-1 body weight by intravenous injection (Stokinger 1981). After establishment of essentiality, understanding a role(s) of boron became the major task in boron biology, however, its essentiality in humans has not been established, although its beneficial effect has been reported. Boric acid and borax were widely used in medicine at the beginning of the century for therapeutic purposes, both locally as well as orally. Boric acid was used to treat various diseases, such as epilepsy and infectious diseases. Several case studies reviewed by Kliegel (1980) describe mild to severe responses to boron compounds. Linden et al (1986) have published a retrospective review of 364 cases of boric acid exposure. Vomiting, diarrhea and abdominal pain were the most common symptoms given by the 276 cases exposed.

Boron is also required by animals, including zebrafish, trout (Rowe and Eckhert 1999), and frogs (Fort et al. 1998). Its deprivation causes impaired growth, abnormal bone development, increase in urinary calcium excretion, and change of macro-mineral status in animals (Devirian and Volpe 2003), also affecting carbohydrate and mineral metabolism, energy consumption, and regulation of the activity of several enzymes; however, the molecular basis of boron function in animals is not well understood (Devirian and Volpe 2003). Excessive boron intake causes acute neurological effects, diarrhea, anorexia, weight loss, and testicular atrophy in mice, rats, and dogs. It also causes decrease in fetal body weight and increase in skeletal malformation and cardiovascular defects in pregnant female animals (Yazbeck et al. 2005; Pawa and Ali 2006). Several investigators have studied the effects of borates on bacteria, protozoa and algae. The effective concentrations for the bacterium Pseudomonas putida range widely (Schöberl and Huber 1988; Guhl 1996; Bringmann and Kuhn 1980). Nitrogen-fixing cyanobacteria require boron for proper functioning of the heterocyst cell wall (Bonilla et al. 1990). Mateo et al. (1986) concluded that boron is essential for nitrogen fixation in Anabaena.

4 Boron and Plants

Since the discovery of boron as an essential element for plants, evidence has been accumulating that boron is an essential element not only for vascular plants, but also for diatoms, cyanobacteria, and a number of species of marine algal flagellates (Marschner 1995). Initial phase of the studies was based on the symptoms of boron deprived plants. It is considered to be involved in the metabolism of nucleic acids, carbohydrates and proteins, indole acetic acid, phenol, cell wall synthesis and structure, membrane integrity and function; however, molecular basis of these roles is mostly unknown (Marschner 1995; Goldbach et al. 2001). It is an essential micronutrient for higher plants, with interspecies differences in the levels required for optimum growth and plays an important role in some plant functions such as metabolic pathways, uptake of Ca2+, sugar translocation, pollen germination, hormone action, root development, flower and fruit formation, normal growth and functioning of the apical meristem, water translocation from roots to the upper portions of the plant body and membrane structure and function (Abdulnour et al. 2000; Liu et al. 2000; Lou et al. 2001). Nobel (1981) studied the effect of several boron compounds on photosynthesis in submerged macrophytes, watermilfoil (Myriophyllum alterniflorum), buttercup (Ranunculus penicillatus) and waterweed (Elodea canadensis).

Early investigation of the effects of boric acid and borax on the field bean (Vicia faba) and other plants indicated the role of boron in plant nutrition (Ayvaz 2002). There is an overlap of the beneficial and injurious effects of boron between species; therefore, three broad categories of tolerance (sensitive, semi-tolerant, and tolerant) have been established (Ayvaz 2002). The sensitive species can tolerate 0.5 mg L-1 of boron but tolerant species can tolerate up to 4 mg L-1 (Batar et al. 2009). Plants in general use less than 5% of boron in the soils (Uygan and Qetin 2004). The tolerant plants endure a wide range of boron concentrations with little effect, and the sensitive plants exhibit a strong reaction to either too much or too little boron. Phytoremediation is the use of plants to make soil contaminants non-toxic and is one form of bioremediation. The term phytoremediation generally refers to phytostabi-lization and phytoextraction. In phytostabilization, soil amendments and plants are used to alter the chemical and physical state of the heavy metal contaminants in the soil. In phytoextraction, plants are used to remove contaminants from the soil and are then harvested for processing.

Boron is an essential element for higher plants. Many studies have shown that certain boron concentrations are necessary for biochemical, physiological and morphological development of plants. Our studies revealed that boron is an essential requirement for maize. The growth rate of radicule and genomic stability increased at 10 mg L-1 boron concentration. Similar findings have been reported by Kocacaliskan and Olcer (2006) and Konuk et al. (2007). Boron toxicity may limit crop productivity in boron rich agricultural soils. In dry seasons/conditions, boron supply to roots is reduced due to reduced mass flow from soil to the root (Shorrocks 1997).

In many countries, the absence of B in the soil causes deficiency problems in plants (Shorrocks 1997). However, in Turkey high levels more commonly end up in the toxicity (Ataslar et al. 1995). According to Ayvaz (2002) and Kekeg (2008) the symptoms of boron deficiency in plants include cessation of root and leaf growth, necrosis of leaf primodia and primary root tips, necrosis of stem and leaf phloem, bark splitting, retardation of enzyme reactions, reduced pollen germination, and even death. Normal growth will usually resume if boron is added to the growth medium. A boron-deficient nutrient solution also inhibits mitosis in the root tip of the field bean. A 10 mg L-1 boron solution produces optimum cell division and elongation of the root tip; however, 50 mg L-1 boron causes a reduction in mitosis. The studied on the effects of boron deficiency and toxicity in Pinus radiata seedlings grown in water culture have revealed that profound changes occur in cell wall morphology, suggesting that boron is critical to cell wall expansion (Cakmak and Romheld 1997). It has been proposed that this structural, cross-linking function of boron is involved with the pectin fraction, which contains apiose and other hydroxylated fragments amenable to complexation by borate (Loomis and Durst 1992). Hu et al. (1996), studied the fourteen species of crop plants, and it was concluded that high pectin content requires more boron for forming cell walls or that pectin forms a tightly held boron complex that depletes boron availability for other critical functions, thereby increasing the overall demand for boron. Kobayashi et al. (1996) have isolated and characterized a rhamnogalacturonan Il/borate complex from enzyme-digested cell wall pectin.

Recently, one of the primary functions of boron in higher plant has been reported at the molecular level. It cross-links pectins in cell walls, and this cross-linking is essential for normal expansion of leaves. Pectins, important components of plant cell wall, are complex polysaccharides, including homogalacturonans and rhamno-galacturonans I and II (RG-I and RG-II). It was demonstrated that the RG-II is cross-linked by a 1:2 borate-diol diester and forms the dimeric RG-II (Kobayashi et al. 1996). O'Neill et al. (2001, 2004) have demonstrated that the cross-link between RG-IIs formed by borate cis-diol ester bonds is essential for normal leaf expansion through analysis of the mur1 mutant in Arabidopsis thaliana, which has abnormal sugar composition of RG-II. It is clear that this role of boron in cross-linking of pectin is among the number of roles of boron in plants.

4.1 Boron Tolerance, Deficiency and Toxicity in Plants

Boron is of great importance to plants. However, the amount needed is very little. The amount of boron useful for the growth of plants varies between 0.5 and 2.0 mg L-1. Generally the soils containing less than 0.5 mg L-1 of boron are poor in terms of boron and boron deficiency symptoms can be observed in the plants. In the soil where the rate of boron is over 2.0 mg L-1 there is boron pollution and consequent decrease in production and defects in the products can be seen (Taiz and Zeiger 1991).

Many studies have shown that certain concentrations of boron are necessary for biochemical, physiological and morphological developments (Hale and Orcutt 1987). There is a very narrow range between boron deficiency and toxicity as more than 5.00 mg L-1 available boron can be toxic to many agronomic crops. Lack of boron often limits production of forage legumes (alfalfa, clover, trefoil) and some vegetable crops. The tolerant species are Alfalfa, Beet, Cotton, Grain, sorghum, Oat, Sugar beet and Tomato; moderately tolerant species being Barley, Cabbage, Celery, Corn, Squash, Sweet clover and Turnip, and moderately sensitive species are

Broccoli, Carrot, Cucumber, Pea, Pepper, Potato and Radish. The sensitive species are Avocado, Bean, Grape, Grapefruit, Lemon, Orange and Wheat. The growth of Viciafaba grown under a medium without boron supplementation is reduced, but a recovery occurs by supplying boron. It is toxic when present at higher concentrations. Thus, it is essential to maintain concentration of boron in media/soil within an appropriate range for maximum yields. In plant, symptom of boron deficiency occurs mainly in growing or expanding organs in the plant body.

Under boron deficient conditions, leaf expansion and root elongation are inhibited. Apical dominance, flower development, and fruit and seed sets are also inhibited under boron limitation. Thus, boron deficiency causes not only the reduction in crop yield, but also the decrease in the quality. According to Stavrianakou et al. (2006), besides inhibition of growth, boron deficiency causes a notable increase in the relative concentration of 'internal' leaf and root phenolic compounds of Dittrichia viscosa (Asteraceae). It does not have any negative effect on parameters related to photosynthesis (such as stomatal density, chlorophyll concentration, photosynthetic capacity and intrinsic photochemical efficiency of PS II). As boron is not efficiently remobilized, i.e., boron tends to stay in organs where it is first distributed, it is important to maintain continuous supply of boric acid for efficient agricultural production (Marschner 1995; Shorrocks 1997; Dell and Huang 1997).

In contrast to the deficiency symptoms, typical boron toxicity symptoms occur in the marginal region of mature leaves, and these portions become chlorotic or necrotic. Boron tends to accumulate in old leaves, especially at the margin of leaves. This is because boron is transported along the transpiration streams and accumulates at the end of transpiration stream. Excess boron also reduces crop yield reduction (Yau et al. 1995). Boron toxicity is an important disorder that can limit plant growth on soils of arid and semi arid environments throughout the world. Soil is generally the primary source of trace elements for plants. However, there are exceptions in which toxic concentrations of trace elements in plants, e.g., B, can be traced directly to water from certain wells, or indirectly to land application of drainage water and soil with high B availability (Kubata 1980). However, the adsorbed and solution phases of B in the soil influence potential B toxicity effects observed in the field (Cartwright et al. 1984; Shani and Hanks 1993); and sometimes lead to decreases in crop yields grown in different regions of the world (Cartwright et al. 1986). There is also a very narrow range between boron deficiency and toxicity as more than 5.00 mg L-1 available boron can be toxic to many agronomic crops (Nable et al. 1997). The initial symptom of boron toxicity in plants is chlorosis (yellowing) of the leaf tip, progressing along the leaf margin and into the blade. Necrosis of the chlorotic tissue occurs, followed by leaf abscission. Necrosis of the leaf tissue results in a loss of photosynthetic capacity, which reduces plant productivity (Lovatt and Dugger 1984). Pollen germination and pollen tube growth may also be inhibited (Versar Inc. 1975).

Several investigators have shown a direct relationship between the boron content in leaves (foliar) and the severity of the symptoms of toxicity. Gilliam and Watson (1981) conducted an experiment in which Anderson yews (Taxus media) were grown in soil at four boron concentrations (0.5, 5.0, 25.0, or 50 mg kg-1).

Symptoms of toxicity were observed when foliar boron accumulation reached concentrations ranging from 85 to 100 ^g g-1 of dry tissue. The observed symptoms included leaf tip yellowing, followed by necrosis and premature defoliation. Suppression of shoot and root growth was observed at 50 mg boron kg-1 soil. Shopova et al. (1981) found that concentrations of 16, 24, and 32 mg boron kg-1 soil resulted in a decline in plant development, yellowing of leaves, late flowering, reduction of mitotic frequency in root tip cells, and abnormalities during meiosis in the poppy (Papaver somniferum). Kluge and Podlesak (1985) found that symptoms due to boron excess begin to develop on the leaves (leaf tip necroses) of pot-grown spring barley (Hordeum vulgare) as soon as the boron content of the leaf tissue reaches 60-80 mg kg-1 dry weight. Gestring and Soltanpour (1987) grew alfalfa (Medicago sativa) in three soil types amended with sodium borate at rates of 0, 10, 20, and 40 mg boron kg-1. Alfalfa yield was significantly reduced by boron application in both the sandy loam and loam soils; however, no yield reduction was observed in the silt loam soil. Soil extractable boron did not adequately assess boron toxicity, whereas plant boron levels were a more reliable index of toxicity. Sage et al. (1989) exposed the rare serpentine plant (Streptanthus morrisonii) to boron (0, 20, 60, 240, 650, 1200, or 2400 ^mol L-1) via watering. Plants showed mild to moderate toxicity symptoms (older leaves exhibiting chlorosis and necrosis) at boron concentrations of 240 and 650 ^mol L-1. Glaubig and Bingham (1985) reported significant linear relationships between both soil and leaf tissue boron concentrations and foliar damage in four tree species endemic to California (digger pine, Pinus sabiniana; California laurel, Umbellularia californica; madrone, Arbutus menziesii; bigleaf maple, Acer macrophyllum). Under experimental conditions, Shann and Adriano (1988) demonstrated that chronic foliar aerosol exposures of boron produced phytotoxicity in relation to boron accumulation in the leaves. The authors concluded that the visual damage (leaf tip necrosis) resulting from aerosol exposure was identical to that observed from root boron toxicity for all crops tested. Boron deficiencies in terrestrial plants have been reported in many countries. Boron deficiency is more likely to occur in light-textured, acid soil in humid regions, because of boron's susceptibility to leaching.

In general, there is a small range between deficiency and toxicity. However, considerable variation exists between species in their resistance to boron. Species sensitive to boron are known to include citrus, stone fruits, and nut trees; semitolerant species include tubers and cereals; and tolerant species include most vegetables. Toxicity due to excess boron is much less common in the environment than boron deficiency. Amongst a wide variety of plant species, the typical visible symptom of B toxicity is leaf burn-chlorotic and/or necrotic patches, often at the margins and tips of older leaves (Bennett 1993; Bergmann 1992). These symptoms reflect the distribution of B in most species, with B accumulating at the end of the transpiration stream. The chlorotic/necrotic patches have greatly elevated B concentrations compared with the surrounding leaf tissues and some species (e.g., barley) show characteristic patterns for different genotypes. In species in which B is phloem mobile (e.g., Prunus, Malus, Pyrus), in which B accumulates in developing sinks rather than at the end of the transpiration stream, the symptoms of toxicity are fruit disorders (gummy nuts, internal necrosis), bark necrosis which appears to be due to death of the cambial tissues and stem die back (Brown and Hu 1996).

Although the lack of boron in the soil causes some problems in the plants, excess of boron also causes various physical and biochemical problems. These effects cause defects in the fruits and leaves of the plants (Hartmann 1981). According to researches done on the harmful effects of boron in the sunflower and bean fields the yield of sunflower is high at 0.5 mg L-1 (418 kg per 1000 m2) but the yield decreases as the density increases. The yield decreases down to 306 kg per 1000 m2 at 16 mg L-1. As for the beans the yield is 180 kg per 1000 m2 at 0.5 mg L-1 but goes down to 73 kg per 1000 m2 at 16 mg L-1 (Sener and Ozkara 1989).

Genetic variation in response to high concentrations of boron occurs at both the inter-and intra-specific levels. Boron tolerance of bread wheat (Paull et al. 1992), durum wheat (Jamjod 1996), barley (Jenkin 1993) and field pea (Pisum sativum) (Bagheri et al. 1996) is controlled by partially dominant nuclear genes. There have been many investigations on inter-specific variation, with each species or genus represented by a single variety (Maas 1987). All of these have identified a wide range in response to boron, either on the basis of plant growth, or the development of tox-icity symptoms, or both. The tolerance to boron toxicity not only operates at the level of whole plants, it also operates at the organ and cellular level (Huang and Graham 1990). In recent studies, it has been reported that high pH can limit boron uptake (Baykut et al. 1987; Hu et al. 1996). The tolerance mechanism appears to be under the control of several major additive genes and specific chromosomal locations have been identified for the genes in some species (Nable and Paull 1991; Nable et al. 1997).

4.2 Boron Uptake By Plants

Boron exists in nature (at neutral pH) primarily as undissociated boric acid-B(OH)3 which is soluble in water and exists a small amount of borate anion, B(OH)4-(Bolanos et al. 2004). Plant takes up boron from soil in the form of boric acid (Brown and Shelp 1997). As a result of being a non charged molecule, boric acid is highly permeable to the lipid bilayers and hence, passage is proportionally dependent on the concentration gradient (Brown and Shelp 1997, Tanaka and Fujiwara 2008). In order to reach the aerial parts of the plant, B needs to load xylem and transported towards the upwards proportional with the transpiration rate. Finally, B accumulates into the destination point, mostly tips and margins of the mature leaves (Brown and Shelp 1997). Uptake is reduced when soil pH increases from 4 to 9 and increases by an increase in the light intensity; the rate of boron absorption rapidly increases at temperatures ranging from 10 to 30°C and is sharply reduced above 35°C (Ayvaz 2002).

Membranes are key players during the transport of the elements, solutes and water and possess ion transporters. Common traits of some elements are their low membrane permeability co-efficiencies that make their membrane transport more difficult. But some molecules such as boric acid which are moderately permeable need a transporter. Recent studies showed that cells do not just need transporters for low permeability coefficient molecule, they also need transporters for solute, uncharged molecules and water even if, these molecules are permeable and require any energy to transport through the membrane (Alberts et al. 2002). Recent studies with artificial membrane and membranes isolated from different species have shown that the membrane permeability coefficient of boric acid is approximately 10-7. According to this data, permeability of boric acid is much higher than tryptophan, glucose and Cl- but much lower than glucose and urea. However, this value is changeable according to the type of the membrane, like lipid composition, intracellular pH.

4.3 Molecular Basis of Boron Uptake and Transport

Three mechanisms are known for across-membrane transport of boric acid: (1) passive diffusion across lipid bilayer (Dannel et al. 2000; Nuttall 2000; Dordas and Brown 2000; Frommer and von Wiren 2002; Kuchel et al. 2006 and Takano et al. 2002), (2) active transport by BOR transporter (Tanaka and Fujiwara 2008; Takano et al. 2008; Peres et al. 2002; Takano et al. 2002 and Frommer and von Wiren 2002), (3) facilitated transport by nodulin-like intrinsic protein (NIP) channel. All of these are involved in regulation of boron transport in plants.

The theory for boron uptake was that boric acid only entered in root apoplast (extracellular space) by passive transport. However, Nuttall (2000), Dordas et al. (2000) and Dordas and Brown (2000) showed that boron absorption can also occur by facilitated diffusion, through transmembrane channels- the aquaporins (Chrispeels et al. 1999). It was believed that boric acid does not require assistance of transporter called aquaporins (Benga et al. 1986; Frommer and von Wiren 2002; Kuchel et al. 2006). The findings of Agre and Kozono (2003) concluded that high permeable molecules/solutes (water, urea, glycerol etc.) can pass through the membrane with both passive diffusion and also channel-mediated transport as the membrane includes several transporters to make a rapid flux of molecules/solutes on two sides of the membrane by transporter proteins such as aquaporins (Fig. 13.1). The discovery of BOR1 (Takano et al. 2002), a boron transporter revealed that it is required for xylem loading. Takano et al. (2006) emphasized that the lower permeability of plant membranes imply the need of membrane proteins to satisfy a plant's demand of boron, especially under boron limitation.

Active transport mechanism of boric acid to the xylem and then towards the aerial parts of the plants has been reviewed at length by Tanaka and Fujiwara (2008) and Takano et al. (2008). According to these investigators the xylem loading of boron is achieved by transporter proteins. The boron absorbed by apoplast first needs to enter the cell (symplast) to reach the xylem due to the Casparian band, an apoplast barrier in the endoderm. When these solutes enter the xylem, they return to the apoplast, since vase elements are made of dead cells. The process in which a nutrient leaves symplast and enters the xylem through an ion-efflux channel is called xylem loading (Peres et al. 2002). BOR1, characterized by Takano et al. (2002), is the first protein linked to boron transport in biological systems and is related to boron

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