Boron (B) is one of the eight essential micronutrients, also called trace elements, required for the normal growth of most plants. It is the only nonmetal among the plant micronutrients. Boron was first recognized as an essential element for plants early in the twentieth century. The essentiality of boron as it affected the growth of maize or corn (Zea mays L.) plants was first mentioned by Maze (1) in France. However, it was the work of Warington  in England that secured strong evidence of the essentiality of boron for the broad bean (Vicia faba L.), and later Brenchley and Warington extended the study of boron to include several other plant species. The essentiality of boron to higher plants was decisively accepted after the experimental work of Sommer and Lipman , Sommer , and other investigators who followed them.

Boron  FUNCTIONS IN PLANTS

Deficiency of boron can cause reductions in crop yields, impair crop quality, or have both effects. Some of the most severe disorders caused by a lack of boron include brown-heart (also called water core or raan) in rutabaga (Brassica napobrassica Mill.) and radish (Raphanus sativus L.) roots, cracked stems of celery (Apium graveolens L.), heart rot of beets (Beta vulgaris L.) brown-heart of cauliflower (Brassica oleracea var. botrytis L.), and internal brown spots of sweet potato (Ipomoea batatas Lam.). Some boron deficiency disorders appear to be physiological in nature and occur even when boron is in ample supply. These disorders are thought to be related to peculiarities in boron transport and distribution. The initial processes that control boron uptake in plants are located in the roots

1.Root Elongation and Nucleic Acid Metabolism
Boron deficiency rapidly inhibits the elongation and growth of roots. For example, Bohnsack and Albert  showed that root elongation of squash (Cucurbita pepo L.) seedlings declined within 3 h after the boron supply was removed and stopped within 24 h. If boron was resupplied after 12 h, the rate of root elongation was restored to normal within 12 to18 h. Josten and Kutschera  reported that the presence of boron resulted in the development of numerous roots in the lower part of the hypocotyl in sunflower (Helianthus annuus L.) cuttings. Consequently, the numerous adventitious roots entirely replaced the tap root system of the intact seedlings.

2 Protein, Amino Acid, and Nitrate Metabolism
Protein and soluble nitrogenous compounds are decreased in boron-deficient plants . However, the influence of organ age, i.e., whether the organ was actively involved in the biosynthesis of amino acids and protein or remobilization of amino acids from protein reserves, has often been ignored . For example, Dave and Kannan  reported that 5 days of growth without boron increased the protein concentration of bean (Phaseolus vulgaris L.) cotyledons compared to control seedlings, suggesting that nitrogen remobilization is hindered due to boron deficiency. By contrast, protein concentrations in the actively growing regions could be reduced by lower rates of synthesis caused by boron deficiency .

3 Sugar and Starch Metabolism

Boron is thought to have a direct effect on sugar synthesis. In cowpeas (Vigna unguiculata Walp), acute boron deficiency conditions increased reducing and nonreducing sugar concentrations but decreased starch phosphorylase activity . Under boron deficiency, the pentose phosphate shunt comes into operation to produce phenolic substances . Boron-deficient sunflower seeds showed marked decrease in nonreducing sugars and starch concentrations, whereas the reducing sugars accumulated in the leaves . This finding indicates a specific role of boron in the production and deposition of reserves in sunflower seeds. High concentrations of nonreducing sugars were also found in boron-deficient mustard (Brassica nigra Koch) . Camacho and Gonzalas  also found higher starch concentration in boron-deficient tobacco plants. In low-boron sunflower leaves, starch decreased, but there was an increase in sugars and protein and nonprotein nitrogen fractions

4 Auxin and Phenol Metabolism

Boron regulates auxin supply in plants by protecting the indole acetic acid (IAA) oxidase system through complexation of o-diphenol inhibitors of IAA oxidase. Excessive auxin activity causes excessive proliferation of cambial cells, rapid and disproportionate enlargement of cells, and collapse of nearby cells . It has been established that adventitious roots develop on stem cuttings of bean only when boron is supplied . Auxin initiates the regeneration of roots, but boron must be supplied at relatively high concentrations 40 to 48 h after cuttings are taken, for primordial roots to develop and grow. It was initially proposed that boron acted by reducing auxin to concentrations that were not inhibitory to root growth , but more recently, Ali and Jarvis  reported that without boron, RNA synthesis decreases markedly within and outside the region from which roots ultimately develop.

5 Flower Formation and Seed Production

The role of boron in seed production is so important that under moderate to severe boron deficiency, plants fail to produce functional flowers and may produce no seeds

6 Membrane Function
Impairment of membrane function could affect the transport of all metabolites required for normal growth and development, as well as the activities of membrane-bound enzymes

DEFICIENCY SYMPTOMS

Field and Horticultural Crops

Alfalfa (Medicago sativa L.). Symptoms are more severe at the leaf tips, although the lower leaves remain a healthy green color. Flowers fail to form, and buds appear as white or light-brown tissue . Internodes are short; blossoms drop or do not form, and stems are short . Younger leaves turn red or yellow , and topyellowing of alfalfa occurs
Barley (Hordeum vulgare L.). No ears are formed . Flowers were opened by the swelling of ovaries caused by partial sterility due to B deficiency . Boron deficiency was also associated with the appearance of ergot.
 Beet (Beta vulgaris L.). Boron deficiency results in a characteristic corky upper surface of the leaf petiole . Beet roots are rough, scabby (similar to potato scab) and off-color .
 Broccoli (Brassica oleracea var. botrytis L.). Water-soaked areas occur inside the heads, and callus formation is slower on the cut end of the stems after the heads have been harvested . Symptoms of boron deficiency included leaf midrib cracking, stem corkiness, necrotic lesions, and hollowing in the stem pith .
 Brussels sprouts (Brassica oleracea var. gemmifera Zenker). The first signs of boron deficiency are swellings on the stem and petioles, which later become suberised. The leaves are curled and rolled, and premature leaf fall of the older leaves may take place . The sprouts themselves are very loose instead of being hard and compact, and there is vertical cracking of the stem .
 Carrot (Daucus carota L.). Boron deficiency results in longitudinal splitting of roots . Boron-deficient carrot roots are rough, small with a distinct white core in the center and plants show a browning of the tops .
 Cauliflower (Brassica oleracea var. botrytis L.). The chief symptoms are the tardy production of small heads, which display brown, waterlogged patches, the vertical cracking of the stems, and rotting of the core . When browning is severe, the outer and the inner portions of the head have a bitter flavor . Stems are stiff, with hollow cores, and curd formation is delayed . The roots are rough and dwarfed; lesions appear in the pith, and a loose curd is produced .
 Clover (Trifolium spp.). Plants are weak, with thick stems that are swollen close to the growing point, and leaf margins often look burnt . Symptoms of boron deficiency in red and alsike clover may occur as a red coloration on the margins and tips of younger leaves; the coloration gradually spreads over the leaves, and the leaf tips may die .
Corn (Zea mays L.). Boron deficiency is seen on the youngest leaves as white, irregularly shaped spots scattered between the veins. With severe deficiency these spots may coalesce, forming white stripes 2.5 to 5.0 cm long. These stripes appear to be waxy and raised from the leaf surface . Interruption in the boron supply, from 1 week prior to tasselling until maturity, curtailed the normal development of the corn ear .
Oat (Avena sativa L.). Pollen grains are empty .
 Peanuts or groundnut (Arachis hypogaea L.). Boron deficiency resulted in hollow-heart in peanut kernels at a few locations in Thailand .
 Pea (Pisum sativum L.). Leaves develop yellow or white veins followed by some changes in interveinal areas; growing points die and blossoms shed . Unpublished data of Gupta and MacLeod  showed that boron deficiency in peas resulted in short internodes and small, shrivelled new leaves.
 Potato (Solanum tuberosum L.). Deficiency results in the death of growing points, with short internodes giving the plant a bushy appearance. Leaves thicken and margins roll upward, a symptom similar to that of potato leaf roll virus . Boron deficiency resulted in rosetting of terminal buds and shoots, and the new leaves were malformed and chlorotic .
 Radish (Raphanus sativus L.). Deficiency of boron in radish is also known as brown-heart, manifested first by dark spots on the roots, usually on the thickest parts . Roots upon cutting show brown coloration and have thick periderm 
 Rutabaga (Brassica napobrassica Mill.). The boron deficiency disorder in rutabaga is generally referred to as brown-heart. Upon cutting, the roots show a soft, watery area . Under severe boron deficiency the surface of the roots is rough and netted, and often the roots are elongated . The roots are tough, fibrous, and bitter, and have a corky and somewhat leathery skin .
 Snapbean (Phaseolus vulgaris L.). There is a yellowing of tops, slow flowering and pod formation .
 Soybean (Glycine max Merr.). Boron deficiency results in necrosis of the apical growing point and young growth; the lamina is thick and brittle; and floral buds wither before opening . Boron
deficiency induced a localized depression on the internal surface of one or both cotyledons of some seeds and resembled the symptoms of hollow-heart in groundnut seeds .
 Sunflower (Helianthus annuus L.). There is basal fading and distortion of young leaves with soaked areas and tissue necrosis .
 Tomato (Lycopersicon esculentum L.). The growing point is injured; flower injury occurs during the early stages of blossoming, and fruits are imperfectly filled . Failure to set fruit is common, and the fruit may be ridged, show corky patches, and ripen unevenly.
 Wheat (Triticum aestivum L.). A normal ear forms but fails to flower . In the case of severe boron deficiency, the development of the inflorescence and setting of grains are restricted .

2Other Crops
Cotton (Gossypium hirsutum L.). Boron deficiency causes retarded internodal growth . The terminal bud often dies, checking linear growth, and short internodes and enlarged nodes give a bushy appearance that is referred to as a rosette condition  Bolls are deformed and reduced in size. Root growth is severely inhibited, and secondary roots have a stunted appearance .
 Sugar Beet (Beta vulgaris L.). Deficiency results in retarded growth, and young leaves curl and turn black . The old leaves show surface cracking, along with cupping and curling. When the growing point fails completely, it forms a heart rot .
 Tobacco (Nicotiana tabacum L.). Boron deficiency results in interveinal chlorosis, dark and brittle newly emerging leaves, water-soaked areas in leaves, and delayed flowering, and formation of seedless pods . Tissues at the base of the leaf show signs of breakdown, and the stalk toward the top of the plant may show a distorted or twisted type of growth. The death of the terminal bud follows these stages .

TOXICITY SYMPTOMS

Field and Horticultural Crops
Alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.). Boron toxicity is marked by burnt edges on the older leaves
Barley  (Hordeum vulgare L.). Boron toxicity is characterized by elongated, dark-brown blotches at the tips of older leaves . Severe browning, spotting, and burning of older leaf tips occur, gradually extending to the middle portion of the leaf . There is a reduced shoot growth and increased leaf senescence .
 Corn (Zea mays L.). Leaves show tip burn and marginal burning and yellowing between the veins . Burning of older leaf edges is more prominent .
 Cowpea (Vigna sinensis Savi). Moderate boron toxicity results in marginal chlorosis and spotted necrosis, but under severe boron toxicity, trifoliate leaves show a slight marginal chlorosis .
 Oat (Avena sativa L.). Boron toxicity in oats results in light-yellow bleached leaf tips
 Onion (Allium cepa L.). Boron toxicity results in burning of the tips of leaves, gradually
increasing up to the base, and no development of bulb occurs .
 Pea (Pisum sativum L.). Boron toxicity results in suppression of plant height and in the number of nodes. Unpublished data of Gupta and MacLeod  showed that boron toxicity results in burning of the edges of old leaves.
 Potato (Solanum tuberosum L.). Boron toxicity symptoms include arching mid-rib and downward cupping of leaves and necrosis at leaf margins .
 Rutabaga (Brassica napobrassica Mill.). The leaf margins are yellow in color and tend to curl and wrinkle. The symptoms on roots are similar to moderate boron deficiency symptoms—a watersoaked appearance of the tissues in the center of the root . Boron toxicity in turnip seedlings also results in marginal bleaching of the cotyledons and first leaves .
 Bean (Phaseolus vulgaris L.). Boron toxicity results in marginal chlorosis of the older trifoliate leaves of snapbeans; unifoliate leaves are also chlorotic with intermittent marginal necrosis . Growth is suppressed, and old leaves have marginal burning . With faba beans (Vicia faba L.), stem growth was restricted, and the young leaves were wrinkled, thick, with a dark-blue color .
 Strawberry (Fragaria x ananassa Duchesne). Slight boron toxicity was associated with marginal curling and interveinal bronzing and necrotic lesions. Under severe boron toxicity interveinal necrosis was severe, leaf margins became severely distorted and cracked, and overall plant growth was reduced .
 Wheat (Triticum aestivum L.). Boron toxicity in wheat appears as light browning of older leaf tips converging into light greenish-blue spots . In durum wheat (Triticum durum Desf.), toxicity results in retarded growth, delayed heading, increase in aborted tillers, and suppressed grain yield per tiller .