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.
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 kg-1 under normal conditions, but only from 87 to 135 mg kg-1 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).
Method of Cultivation and Cropping
The method of ploughing has been shown to affect plant boron accumulation. For example, Lal et al. (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).
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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