Most of the chlorine in plants is present in the form of the anion, chloride. However, more than 130 natural chlorine-containing compounds have been isolated from plants (11). They may include polyacetylenes, thiophenes, iridoids, sesquiterpene lactones, pterosinoids, diterperenoids, steroids and gibberellins, maytansinoids, alkaloids, chlorinated chlorophyll, chloroindoles and amino acids, phenolics, and fatty acids. Although the functions of naturally occurring chlorine-containing compounds in plants have not received much attention in plant nutrition, the fact that these compounds often exhibit a strong biological activity suggests a need to investigate their potential importance. Some chlorine-containing compounds may behave as hormones in the plant, or they may have a function in protection against attack from other organisms.
The amount of chlorine accumulation required to prevent deficiency symptoms in most crops however, is much less than that which is typically accumulated (Table 9.2). A laboratory study (7) that determined the chlorine requirements of 11 different crop species estimated that plants require 1 lb of chlorine for each 10,000 lb of dry matter produced, or a concentration of about 0.1 g kg-1. On a land area basis, large crops may need about 2.24 kg ha-1 or more of chlorine. This estimate for plant chlorine requirement is presumed to be for biochemical functions (2). The benefits that are sometimes observed from higher concentrations of chlorine are likely due to its osmoregulatory role in plants (36).
Distribution in Plants
Most of the chlorine in plants is not incorporated into organic molecules or dry matter, but remains in solution as chloride and is loosely bound to organic molecules. Chloride concentrations expressed on a tissue-water basis may typically range from 50 to 150 mmol L-1 (4). A study (25) that determined chloride in the tissue water and the dry matter of whole corn plants at 35 days after emergence found a concentration of 66 mmol Cl L-1 (1.83 g kg-1 dry matter basis) for corn grown on soil fertilized with 200 kg Cl ha-1 applied as KCl and only 10 mmol Cl L-1 (2.5 g kg-1 dry matter basis) for corn plants grown on unamended soil. In general, chloride concentrations are higher in tissues that have high water content. Chloride concentrations are presumably highest in the rapidly expanding zones of root and shoot tissue. Pulvini and guard cells also have higher concentrations of chloride than the bulk tissue (4).
Vegetative plant tissues usually accumulate increasing concentrations of chloride with increasing supply of chloride, but plants parts can also exclude chloride (4,25,63). Corn seed may have only 0.44 to 0.64 g Cl kg-1 on a dry weight basis, and chloride accumulation in the grain is not influenced by chloride supply (45). In many crops, chloride transport from roots to shoots is restricted by a mechanism that resides in the roots (4,64,65). Soybean cultivars that exclude chloride from the shoots are more salt-tolerant than cultivars that accumulate chloride (57).
Reports on critical tissue concentrations of chloride for crops grown in the field are few in number (Table 9.2). Studies conducted in the Great Plains of the United States have examined the relationship between tissue chloride concentration and relative yield of wheat. In wheat plants at head emergence, a critical chloride concentration of 1.5 g kg-1 was given in a 1986 report (66). In a more recent and larger study (67) that was based on an assessment of 219 wheat cultivars, three zones of chloride status were identified: (i) a deficiency zone with a plant chloride concentration <1.0 g kg-1, (ii) an adequate chloride status zone with concentrations 4.0 g kg-1, (iii) and a transition, or critical range, between these two zones. A study (45) of corn grown in high-yield environments in New Jersey suggested a critical ear-leaf chloride concentration of 3.2 g kg-1, derived from a comparatively small database.
Chlorine Concentrations in Crops
A review (4) of chlorine nutrition tabulated the concentrations of chloride in a wide variety of crops. The compilation of data in Table 9.2 shows that concentrations of chloride classified as deficient, normal, or toxic vary widely among plant species.
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