8.1.1 Determination of Essentiality
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 (2) in England that secured strong evidence of the essentiality of boron for the broad bean (Vicia faba L.), and later Brenchley and Warington (3) 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 (4), Sommer (5), and other investigators who followed them.
Since its discovery as an essential trace element, the importance of boron as an agricultural chemical has grown very rapidly. Its requirement differs markedly within the plant kingdom. It is essential for the normal growth of monocots, dicots, conifers, and ferns, but not for fungi and most algae. Some members of Gramineae, for example, wheat (Triticum aestivum L.) and oats (Avena sativa L.) have a much lower requirement for boron than do dicots and other monocots, for example, corn.
Of the known micronutrient deficiencies, boron deficiency in crops is most widespread. In the last 80 years, hundreds of reports have dealt with the essentiality of boron for a variety of agricultural crops in countries from every continent of the world.
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 (6). Some of the main functions of boron are summarized below.
Boron deficiency rapidly inhibits the elongation and growth of roots. For example, Bohnsack and Albert (7) showed that root elongation of squash (Cucurbita pepo L.) seedlings declined within 3h 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 to18h. Josten and Kutschera (8) 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.
Root elongation is the result of cell elongation and cell division, and evidence suggests that boron is required for both processes (9). When boron is withheld for several days, nucleic acid content decreases. Krueger et al. (10) demonstrated that the decline and eventual cessation of root elongation in squash seedlings was correlated temporally with a decrease in DNA synthesis, but preceded changes in protein synthesis and respiration.
Lenoble et al. (11) concluded that boron additions may need to be increased under acid, high-aluminum soils, because applications of boron prevented aluminum inhibition of root growth on acid, aluminum-toxic soils.
220.127.116.11 Protein, Amino Acid, and Nitrate Metabolism
Protein and soluble nitrogenous compounds are decreased in boron-deficient plants (12). 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 (13). For example, Dave and Kannan (14) 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 (15,16).
Shelp (16) reported that the partitioning of nitrogen into soluble components (nitrate, ammonium, and amino acids) of broccoli (Brassica oleracea var. botrytis L.) was dependent on the plant organ and whether boron was supplied continuously at deficient or toxic levels. Boron deficiency did not substantially affect the relative amino acid composition (16) but did enhance the proportion of inorganic nitrogen, particularly nitrate, in plant tissues and translocation fluids (13). A number of researchers reported increases in nitrate concentration as well as corresponding decreases in nitrate reductase activity in sugar beet (Beta vulgaris L.), tomato (Lycopersicon esculentum Mill.), sunflower, and corn plants (17,18) due to boron deficiency. Boron deficiency in tobacco (Nicotiana tabacum L.) resulted in a decrease in leaf N concentration and reduced nitrate reductase activity (19). Boron-deficient soybeans (Glycine max Merr.) showed low acetylene reduction activities and damage to the root nodules (20).
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 (21). Under boron deficiency, the pentose phosphate shunt comes into operation to produce phenolic substances (22). Boron-deficient sunflower seeds showed marked decrease in nonreducing sugars and starch concentrations, whereas the reducing sugars accumulated in the leaves (23). 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) (24). Camacho and Gonzalas (19) 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 (25). In boron-deficient pea (Pisum sativum L.) leaves, the concentration of sugars and starch increased, but they decreased in the pea seeds and thus lowered the seed quality (26). Evidence on the impact of boron deficiency on starch concentration is conflicting. It is difficult to explain whether the differences are due to a variation in crop species.
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 (27). It has been established that adventitious roots develop on stem cuttings of bean only when boron is supplied (28,29). 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 (30,31), but more recently, Ali and Jarvis (28) reported that without boron, RNA synthesis decreases markedly within and outside the region from which roots ultimately develop.
There are many reports in the literature of phenol accumulation under long-term boron deficiency (32). Since boron complexes with phenolic compounds such as caffeic acid and hydrox-yferulic acid, Lewis (33) proposed a role for boron in lignification. Absence of boron would therefore cause reactive intermediates of lignin biosynthesis and other phenolic compounds to affect changes in metabolism and membrane function, resulting in cell damage. However, the available evidence indicates that lignin synthesis may actually be enhanced by boron deficiency.
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 (34). Plants subjected to boron deficiency have been observed to result in sterility or low germination of pollen in alfalfa (Medicago sativa L.) (35), barley (Hordeum vulgare L.) (36), and corn (37). Even under moderate boron deficiency, plants may grow normally and the yield of the foliage may not be affected severely, but the seed yield may be suppressed drastically (38).
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. Dugger (15) summarized early reports that illustrate changes in membrane structure and organization in response to boron deficiency. Boron may give stability to cellular membranes by reacting with hydroxyl-rich compounds. Consistent with this view is evidence suggesting that a major portion of the cellular boron is concentrated in protoplast membranes from mung bean (Phaseolus aureus Roxb.) (39).
The involvement of boron in inorganic ion flux by root tissue (40-42) and in the incorporation of phosphate into organic phosphate (43) was evident from earlier research. In general, the absorption of phosphate, rubidium, sulfate, and chloride was suppressed in boron-deficient root tissues, but it could be restored to normal or nearly normal rates by a concomitant addition of boron or pre-treatment with boron for 1 h. This effect could be explained by a rapid reorganization of the carrier system, with boron functioning as an essential component of the membrane (15). The movement of monovalent cations is associated with membrane-bound ATPases. Boron-deficient corn roots had a limited ATPase activity, which could be restored by boron addition for only 1 h before enzyme extraction (40).
Recently, Tang and Dela Fuente (44,45) demonstrated that potassium leakage (as a measure of membrane integrity) from boron- or calcium-deficient sunflower hypocotyl segments was completely reversed by the addition of boron or calcium for 3 h. It was not possible to reverse the inhibited process by replacing one deficient element with the other. Seedlings deficient in both boron and calcium showed greater effects than seedlings deficient in one element only. Basipetal auxin transport was also inhibited by boron or calcium deficiency, but the addition of boron for 2h did not restore the process reduced by boron deficiency. This reduction in auxin transport was not related to reduced growth rate, acropetal auxin transport, lack of respiratory substrates, or changes in calcium absorption, suggesting that boron had a direct effect on auxin transport.
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