Boron Toxicity and Tolerance in Crop Plants
Boron toxicity is a common problem for many crop plants, especially those growing on soil with high levels of boron and low rainfall. Boron is transported through the plant in the xylem and boron not absorbed by other tissues is deposited at the end of le
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Boron Toxicity and Tolerance in Crop Plants Robert J. Reid
1 Introduction Boron (B) is an essential plant nutrient required in relatively small quantities for normal plant growth. In many respects, boron is the most difficult of plant nutrients to manage, principally due to the fact that it is the only essential nutrient that normally exists as a neutral solute, boric acid. Combined with its small size, this lack of charge allows it to pass easily through membranes and as a result, its distribution within the plant can be hard to control. Both boron deficiency and boron toxicity are common in agricultural crops. The deficiency can be easily corrected by its fertilisation, but toxicity is much more difficult to manage. This review examines the role of boron in plants, possible targets for toxicity, and the mechanisms by which some plants have developed tolerance to excess boron.
2 Functions of Boron in Plants The essentiality of boron for plant growth was established by Warington in 1923, but it took many decades to understand which processes depended on boron. The first clue came from the discovery that boron was strongly bound in plant cell walls (Tanaka 1967) and that the boron content was well correlated with the amount of pectic polysaccharide in cell walls (Matoh 1997, 2000). From these latter studies, it is now well established that boron plays an important structural role as a component of the rhamnogalacturonan II (RGII) complex that links cell wall polysaccharides.
R. J. Reid () School of Earth and Environmental Sciences, University of Adelaide, Adelaide 5005, Australia e-mail: [email protected] N. Tuteja, S. S. Gill (eds.), Crop Improvement Under Adverse Conditions, DOI 10.1007/978-1-4614-4633-0_15, © Springer Science+Business Media New York 2013
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Other potential roles for this plant nutrient have been suggested based on the binding to cellular components of boronic acids which compete with boric acid for binding. Bassil et al. (2004) found that boronic acids interfered with cytoskeletal elements and disrupted cell to cell wall adhesion in cultured cells. Wimmer et al. (2009) identified various boron-binding proteins from microsomes by boronate affinity chromatography and proposed that boron interacts with the sugar moiety of membrane glycolipids, thereby increasing membrane stability (Wimmer et al. 2009).
3 Boron in Soils Naturally, boron-toxic soils appear in pockets around the world and are much less common than boron-deficient soils. Low boron soils are loosely considered as those with less than about 10 mg kg−1 and high boron soils are those with more than this value up to very high levels of 100 mg kg−1 (Power and Woods 1997). However, prediction of toxicity to plants based on soil boron levels can be problematic. Mertens et al. (2011) found that the boron concentration required to inhibit root growth of barley by 10 % varied from 5–52 mg B kg−1 depending on the soil type. A large proportion of this variability could be explained by differences in soil
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