Effect of Boron Deficiency and Toxicity in Plants

Charu Lata, Ashwani Kumar, Parvender Sheoran, Anita Mann and Pooja*

ICAR-Central Soil Salinity Research Institute, Karnal – 132001, Haryana

*ICAR-Sugarcane Breeding Institute, Regional Centre, Karnal – 132001, Haryana

In nature, boron (B) toxicity is not as widespread as B deficiency. Boron toxicity is an

important disorder that can limit plant growth on soils of arid and semi arid environments throughout

the world. Boron is one of the eight essential micronutrients for healthy crop growth and its deficiency

is a widespread problem in relatively humid areas of the world. Though the biochemical role of B in

plant is still not well understood, the evidence generally showed that B is important in cell division

and is apparently a necessary component of the cell wall (Cohen and Lepper, 1977). Boron also

played an important role in the synthesis of proteins and the translocation of sugars. Tanada (1978)

suggested that B is required by plants to stabilize a positive electrostatic charge in the plasma

membrane that is generated by the actions of phytochrome and gravity. There is a certain minimum

requirement of B for a plant, below which a deficiency symptom will develop. As well, there is a

certain maximum level of tolerance, above which toxicity symptoms appear. Boron requires special

attention among the essential mineral nutrients because the need of B by plants is relatively small; the

range between deficiency and excess is narrow. Since it is not practical to leach out excess B because

management of B is difficult due to its high permeability, presence as an uncharged molecule at

physiological pH, high solubility and can be easily leached under high rainfall conditions, leading to

deficiencies in plants. Toxicity can occur under three main conditions:

(l) In soils inherently high in B or in which B has naturally accumulated.

(2) As a result of over fertilization with minerals high in B and

(3) Through the use of irrigation waters high in B leading to B accumulation and concentration

in soil.

Under low rainfall conditions, the opposite is often true that it is not sufficiently leached and

therefore may accumulate to a level in the leaves that become toxic to plant growth and metabolism.

There are few reports that showed underground water for irrigation contains toxic amounts of B in

arid or semiarid states of India, such as Uttar Pradesh, Rajasthan, Haryana, Punjab, and Gujrat.

Ground waters of Agra (Uttar Pradesh), Gurgaon and Hisar (Haryana), Bhatinda, Sangrur and

Amritsar (Punjab) have high B content (up to 7.3 ppm). Ground waters of the coastal regions may also

have high B content. Kanzaria and Patel (1985) reported that 11% of waters in north Gujarat, Kutch,

Kaira, Vadodara and Bharauch had B concentration beyond the tolerance limit of most crop plants.

Toxicity symptoms are slow to develop, or are only observed with extreme B treatments. The

physiological effects of boron toxicity include reduced root cell division, decreased shoot and root

growth, decrease in leaf chlorophyll, inhibition of photosynthesis, lower stomatal conductance,

deposition of lignin and suberin, reduced proton extrusion from roots, increased membrane leakiness,

lipid peroxidation and altered activities of antioxidation pathways.

Information on the symptoms of B deficiency and B toxicity in a number of vegetable and

field crops has been well documented (Gupta, 1979). In the plant, B translocates readily through the

xylem in the transpiration stream. Boron is not readily translocated from old to young plant parts, the

first symptom of B deficiency will be in the growing points - the stem tips, root tips, new leaves and

flower buds. Both B deficiency and excess will result in reduction of crop yield and/or in impairment

of crop quality. For most plant species, deficiencies in the field occur when the B level reaches less

than 15 µg/g (dry matter basis) while B concentration of 20-100 µg/g are adequate for growth. Boron

toxicity under field conditions generally occurs when plant tissue concentration exceeds 200 µg/g.

The uneven distribution of B between and within plant parts presents some difficulties in defining the

critical level for B. As above mentioned, agricultural regions that contain insufficient or toxic levels

of B in soil have problems with yield and quality of many crops. Hence, understanding the

mechanisms that are involved in B uptake and distribution in plants can be critical to improve

agricultural production. Physiological studies also have shown the occurrence of an active B uptake

by roots under low B conditions (Dannel et al., 2000). This active absorption of B is supported by the

fact that B uptake was inhibited by both metabolic inhibitors and cold treatment in roots. Once B has

been absorbed by root cells this micronutrient must be loaded into xylem. In well B supplied plants

this process is mediated by a passive mechanism that involves both B diffusion across lipid bilayer

and facilitated permeation of boric acid via MIPs channel (MIPs). After being loaded into xylem, B is

transported through this vascular system to shoot in a process mediated by transpiration stream.

However, B can be also transported via phloem to both reproductive and vegetative tissues (Shelp et

al., 1995), although this capacity varies among species (Brown and Shelp, 1997). One mechanism that

has been suggested to mediate phloem transport of B involves the formation of boron-diol complexes

as transport molecules (Brown and Hu, 1996). In fact, B can readily bind to cis hydroxyl groups of

sugar alcohols (mannitol and sorbitol), which allow B to be transported through phloem. However, B

transport via phloem, especially to young tissues, also occurs in plants that are not able to produce

these types of carbohydrates (Stangoulis et al., 2001a). Very recently it has been demonstrated that B

is transported from mature leaves into actively growing reproductive organs via phloem in white lupin

(Huang et al., 2008). Nevertheless, the molecular mechanism involved in this B-phloem transport is

still unknown.

Reference

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Vol.31. Academic Press,Inc., New York.