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"Treatment consisted of 24 mol kg-1 soil either as a Ca or Mg salt or as a mixture in a 1:1 molar ratio of Ca and Mg. Control received 8 mmol each of CaCO3 and MgCO3 kg-1 soil. bValues followed by a common letter do not differ significantly at P £ 0.05 by Duncan's multiple range test.

"Treatment consisted of 24 mol kg-1 soil either as a Ca or Mg salt or as a mixture in a 1:1 molar ratio of Ca and Mg. Control received 8 mmol each of CaCO3 and MgCO3 kg-1 soil. bValues followed by a common letter do not differ significantly at P £ 0.05 by Duncan's multiple range test.

Source: Adapted from Gupta U.C., in Boron and Its Role in Crop Production. CRC Press, Boca Raton, FL, 1993, pp. 87-104.

and magnesium, not shown in the table, were not related to the applications of boron. Table 8.4 shows that after the crop was harvested, lower quantities of hot-water-soluble boron were found in the soil that received calcium or magnesium sulfates than in soil that received calcium or magnesium carbonates.

Unpublished data (83) on podzol soils with a pH range of 5.4 to 7.8 showed that liming markedly decreased the boron content of pea plant tissue from 117 to 198 mg kg-1 at pH 5.4 to 5.6, to 36 to 43 mg kg-1 at pH 7.3 to 7.5. At pH values higher than 7.3 to 7.5, even tripling the amount of lime did not affect the boron content of plant tissue.

No clear relationship was found between the Ca/B ratio in the leaf blades and the incidence of brown-heart in rutabaga (189). However, it was noted that an application of sodium increased the calcium concentration in rutabaga tissue, thereby affecting the Ca/B ratio and possibly the incidence of brown-heart. It should be pointed out that use of the Ca/B ratio in assessing the boron status of plants should be viewed in relation to the sufficiency of other nutrients in the growing medium and in the plant.

8.7.1.2 Macronutrients, Sulfur, and Zinc

Among the macronutrients, nitrogen is of utmost importance in affecting boron accumulation by plants. Chapman and Vanselow (191) were among the pioneers in establishing that liberal nitrogen applications are sometimes beneficial in controlling excess boron in citrus. Under conditions of high boron, application of nitrogen depresses the level of boron in orange (Citrus sinensis Osbeck) leaves (192). Lysimeter experiments showed that tripled fertilization (NPK) rates and irrigation increased boron accumulation by plants on tested soils (193).

Boron concentrations in boot-stage tissue of barley and wheat increased significantly with increasing rates of compost additions (59). Such increases in boron were attributed to a high concentration of 14 mg B kg 1 in the compost. The authors reported that boron concentrations decreased with increasing rates of nitrogen. Additions of nitrogen decreased the severity of boron toxicity symptoms. The form of nitrogen can affect plant boron accumulation. Wojcik (194) reported that on boron-deficient, coarse-textured soils, nitrogen as calcium and ammonium nitrates increased the availability and uptake of boron by roots. This increase was attributed to the fact that nitrate inhibited boron sorption on iron and aluminum oxides, and increased boron in soil solution.

Increasing rates of nitrogen applied to initially nitrogen-deficient soils significantly decreased the boron concentration of boot-stage tissue in barley and wheat in a greenhouse study, but field experiments did not show any significant effect of nitrogen on boron concentration (195). The ineffectiveness of nitrogen in alleviating boron toxicity in cereals under field conditions is due to the fact that nitrogen failed to decrease the boron concentration in boot-stage tissue. Furthermore, nitrogen deficiency was more severe under greenhouse conditions than under field conditions. The decreases in boron concentrations were greater with the first level of added nitrogen than with the higher rates (195). This result may indicate that application of nitrogen is helpful in alleviating boron toxicity on soils low in available nitrogen.

Little difference in boron concentration of alfalfa was detected, and symptoms of boron deficiency progressed with increasing potassium concentration in the growth media (196). The authors suggested that the accentuating effect of high potassium on boron toxicity or deficiency symptoms might be due to the influence of potassium on cell permeability, which is presumably regulated by boron. Long-term experiments on cotton indicated positive yield responses to boron fertilization when associated with potassium applications (197). Yield increases were related to increased leaf potassium and boron concentrations.

The effects of phosphorus, potassium, and sulfur are less clear than those of nitrogen on the availability of boron to plants. Studies conducted in China (198) showed that rape (Brassica napus L.) plant boron concentration decreased with increasing potassium, and that lower potassium levels enhanced boron accumulation. The authors concluded that the optimum K/B ratio in rape plants was 1000:1.

Tanaka (199) showed that boron accumulation in radish increased with an increase in phosphorus supply. Malewar et al. (200) found that increasing the phosphorus fertilization rate resulted in higher phosphorus in cotton and groundnut. Experiments conducted on cotton also demonstrated that boron concentration in leaves was greatest with phosphorus fertilization (201). On the other hand, the presence of phosphorus can affect boron toxicity in calcareous soils. In studies on maize genotypes, boron was more toxic in the absence, rather than in the presence of, phosphorus, and thus boron toxicity in calcareous soils of the semiarid regions could be alleviated with applications of phosphorus (202).

Sulfate may have a slight effect on accumulation of boron in plant tissues (199). Field studies in Maharashtra, India, showed that boron applied with gypsum gave increased dry pod yield of groundnuts (203). The experimental results from a number of crops indicated that sulfur applications had no effect on boron concentration of peas, cauliflower, timothy (Phleum pratense L.), red clover, and wheat, but such applications significantly decreased the boron content of alfalfa and rutabaga (83). It is possible that various crops behave differently. For example, with soybean, applications of gypsum at 1000 kg ha1 did not alleviate boron toxicity resulting from the application of 10kg B ha^1 (204).

Recent studies showed that applied zinc played a role in partially alleviating boron toxicity symptoms by decreasing the plant boron accumulation (205). Zinc treatments partially depressed the inhibitory effect of boron on tomato growth (150).

8.7.1.3 Soil Texture

The texture of soil is an important factor affecting the availability of boron (206). A study on soils from eastern Canada showed that higher quantities of hot-water-soluble boron occurred in fine-textured soils than in coarse-textured soils (207). Studies in Poland showed that boron accumulation in potatoes and several cereals was less on sandy soils than on loamy soils (193).Page and Cooper (208) reported that leaching losses from acid, sandy soils after addition of 12.5 cm of water, account for as much as 85% of the applied boron. Movement is less rapid in heavy-textured soils because of increased fixation by the clay particles (119).

In Brazil, response to boron by cotton was significantly higher on Alic Cambisol, and the reverse was true for a dystrophic dark red latosol (209). It was suggested that high sand content (87%) and low clay (10%) and low organic matter (1.3%) in the latter soil could have resulted in toxic concentrations of boron in solution. The type of clay and the soil pH can significantly influence the amount of boron adsorbed. Hingston (210) reported that increasing pH resulted in an increase in the monolayer adsorption and a decrease in bonding energy for Kent sand kaolinite and Marchagee montmorillonite, and a slight increase in bonding energy for Willalooka illite up to pH 8.5. On a mass basis, illite adsorbed most boron over the range of pH values commonly occurring in soils; montmorillonite adsorbed appreciable amounts at higher pH, and kaolinite adsorbed the least.

Fine-textured soils generally require more boron than do the coarse-textured soils to produce similar boron concentrations in plants. Boron concentrations in solutions of 3.5 mg kg1 in sandy loam and 4.5 mg kg1 in clay loam resulted in similar boron concentrations in gram (Cicer arietinum L.) (211).

8.7.1.4 Soil Organic Matter

Organic matter is one of the chief sources of boron in acid soils, as relatively little boron adsorption on the mineral fraction occurs at low pH levels (212). The hot-water-soluble boron in soil has been positively related to the organic matter content of the soil (207). Addition of materials such as compost rich in organic matter resulted in large concentrations of boron in plant tissues and in phytotox-icity (60). Boron in organic matter is released in available form largely through the action of microbes (213). The complex formation of boron with dihydroxy compounds in soil organic matter is considered to be an important mechanism for boron retention (214). The influence of organic matter on the availability of boron in soils is amplified by increases in pH and clay content of the soil.

8.7.1.5 Soil Adsorption

When boron is released from soil minerals, mineralized from organic matter, or added to soils by means of irrigation or fertilization, part of the boron remains in solution, and part is adsorbed (fixed) by soil particles. An equilibrium exists between the solution and adsorbed boron (215). Usually more boron is adsorbed by soils than is present in solution at any one time (216), and fixation seems to increase with time (207).

Boron retention in soil depends upon many factors such as the boron concentration of the soil, soil pH, texture, organic matter, cation exchange capacity, exchangeable ion composition, and the type of clay and mineral coatings on clays (210,215,217,218). Of the clays, illite is the most reactive with boron, and kaolinite is the least reactive on a mass basis (210,219).

8.7.1.6 Soil Salinity

An antagonistic relationship existed between soil boron application levels and sodium adsorption ratio (SAR) of irrigation waters (220). Visible effects of boron toxicity developed in sugar beet plants at 0.5 SAR at high boron levels, and the symptoms intensified with plant age. However, effects of excess boron were markedly reduced at 20 and 40 SAR. Increasing soil salinity levels decreased the boron concentration in chickpea (gram) plants; such effects were accentuated at the higher boron levels (221).

8.7.2 Other Factors

8.7.2.1 Plant Genotypes

Data on the effect of plant genotypes on boron uptake are meager. Susceptibility to boron deficiency is controlled by a single recessive gene (222), as shown by the tomato cultivars T 3238 (B-inefficient) and Rutgers (B-efficient). Studies (222,223) have shown that T 3238 lacks the ability to transport boron to the top of the plants and confirms the differential response of T 3238 and Rutgers to a given supply of boron. Gorsline et al. (106) observed that corn hybrids exhibited genetic variability related to boron uptake and leaf concentration. One study conducted by E.G. Beauchamp, L.W. Kannenberg, and R.B. Hunter at the University of Guelph, Ontario (personal communication), indicated that the corn inbred CG 10, compared with several others, was the least efficient in boron accumulation as measured by the boron content of leaves sampled at the anthe-sis stage. These researchers, in a study of 11 hybrids, also found that decreased boron accumulation was associated with higher stover yield.

Some wheat cultivars in Asia, were tolerant of boron deficiency, whereas several sensitive genotypes failed to set grain in the absence of boron (224). Experiments conducted in China showed that roots of some wheat varieties secreted more organic acids, resulting in low pH and increased availability of boron, zinc, and phosphorus (225).

8.7.2.2 Environmental Factors

One of the chief environmental factors affecting the response of plants to the availability of nutrients is the intensity of light. The faster the plant grows, for example, under high light conditions, the faster it will develop boron deficiency symptoms in a particular growth period. Observations by Broyer (226) indicated that deficiencies as well as toxicities are revealed earliest or most intensely in the summer. Experiments conducted with duckweed (Lemna paucicostata Hegelm.) showed that reducing light intensity decreased the response to boron deficiency or toxicity (227). In the absence of boron, severe deficiencies were observed in cultures under continuous illumination from a daylight fluorescent lamp at 5500 lux, but not at 1000 lux. Over the range of 0.5 to 2.5 mg B L"1 in the culture solution, plant boron accumulation was reduced with decreasing light intensity. Studies conducted on young tomato plants grown in solution culture showed that in the absence of boron deficiency, symptoms developed more rapidly at high than at low light intensity (228). Plants supplied with boron did not exhibit symptoms.

An interaction appears to occur between temperature and lighting conditions. Rawson et al. (229) reported that low light alone reduced floret fertility in wheat by around 8%; however, in combination with a marginal boron supply, low light amplified the boron deficiency effect by some 60%. Furthermore, reduced light had the most deleterious effect at high temperature. Field studies in Bangladesh (230) demonstrated that some of the factors responsible for sterility in wheat are low temperatures over many days during flowering, and saturated or waterlogged soil. These factors affect transpiration, which in turn affects boron transport in the plant during the critical preflowering or flowering period.

Soil water appears to affect the availability of boron more than that of some other elements. Studies by Kluge (231) indicated that boron deficiency in plants during drought may be only partially associated with the level of hot-water-soluble boron in soil. Reduced soil solution in connection with reduced mass flow and reduced diffusion rate, as well as limited transpiration flow in the plants during drought periods, may be causative factors of boron deficiency in spite of an adequate supply of available boron in the soil. Boron deficiencies are generally found in dry soils where summer or winter drought is severe; when adequate moisture is maintained throughout the summer, deficiency symptoms may not be common (232). In an experiment on barley, soil water had a significant effect on plant boron accumulation after boron was applied to the soil (195). The boron concentration of barley, with added boron, ranged from 162 to 312 mg kg1 under normal conditions, but only from 87 to 135 mg kg1 when the area near the boron fertilizer band was kept dry. Mortvedt and Osborn (233) likewise reported that movement of boron from the fertilizer granules increased with concentration gradient and soil moisture content.

Boron concentration of some plants has been found to be a direct function of air temperature over the 8 to 37°C range. For example, Forno et al. (234) found that Cassava (Manihot esculentum Crantz) roots grew well when the solution temperature was maintained at 28 or 33°C, but developed severe boron deficiency symptoms at 18°C. Mild symptoms of boron deficiency were also obtained at a solution temperature of 23°C.

Relative humidity also affects boron accumulation, for example, an increase in percent relative humidity from 30 to 95 resulted in a decrease from 16.5 to 9.9 mg B per plant (235). Boron deficiency symptoms observed in birdsfoot trefoil (Lotus corniculatus L.) were caused by a temporary deficiency of available boron, induced by local drought conditions (236).

Generally, soils that have developed in humid regions have low amounts of plant-available boron because of leaching. Further, plant-available boron that is present in such soils is located in the top 15 cm and in the organic matter fraction (237,238). Thus, plants growing in regosols, sandy podzols, alluvial soils, organic soils, and low humic gleys tend to develop boron deficiencies because of low soil boron reserves.

At low temperatures in spring and fall in temperate regions, availability of boron is low, as evident in crops such as alfalfa and red clover. It has been suggested that during the cool season, plants may have an increased demand for B at a time when microbial activity in the soil is depressed (David Pilbeam, Personal communication, University of Leeds, England). The lower rate of root growth during the cool season may cause the rhizosphere to become depleted of boron, and falling temperatures may make cell membranes less fluid.

Sterility has become one of the most important wheat production constraints in Nepal (239). Among environmental factors, cold temperatures during the reproductive stages at higher altitudes coupled with low availability of boron are major factors causing sterility in wheat (239). Pot experiments conducted on spring wheat also showed that cold temperatures significantly reduced the response of plants to boron, and if a cold-susceptible cultivar was cold-stressed, it accumulated less boron (240).

8.7.2.3 Method of Cultivation and Cropping

The method of ploughing has been shown to affect plant boron accumulation. For example, Lal etal. (241) reported that boron concentration in corn leaf tissue was significantly higher with mouldboard plough and ridge till than with no-till and beds. Cropping systems influence the availability of boron in soil. In a continuous cropping study in China, available boron in soil was higher after three crops of soybeans than after three crops of wheat (242).

8.7.2.4 Irrigation Water

Gupta et al. (243) reported that only a few irrigation waters have enough boron to injure plants directly. The continued use of irrigation and concentration of boron in the soil due to evapotranspiration are the reasons for the eventual toxicity problems. In arid and semiarid regions, boron concentrations of irrigation waters, especially underground waters, are often elevated and in some cases may be as high as 5 mg L"1 (244). The majority of surface waters have boron concentrations of 0.1 to 0.3 mg L"1, but well waters are more variable in boron content and often have excessive amounts (215). Some river waters used for irrigation may show high levels of boron at certain times of the year due to the contribution of spring drainage areas high in boron. Generally, ground waters emanating from light-textured soils are higher in boron than those from heavy-textured soils (245).

Boron movement in plants has been associated with transpiration. Therefore, any component of the environment that changes transpiration flux can affect boron availability. It has been proposed that decreased boron availability leading to sterility in wheat is due to water deficit as well as waterlogging in the root zone (246).

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