The molybdenum status of crops is often overlooked by the farming community, probably because of the relatively low crop requirement for molybdenum and because of a lack of education on the necessity of molybdenum in fertility programs. In addition, many commercial soil and plant analysis laboratories fail to report this nutrient in routine tissue and soil analyses. This omission may be partially due to the difficulties in accurately determining the small quantities of molybdenum that are normally present in plant tissues. It is possible that many molybdenum deficiencies in crop plants are misdiagnosed as nitrogen deficiency because of the similarity in their deficiency symptoms.
The dithiol method developed by Piper and Beckworth (75) and modified by Gupta and MacKay (76) is more sensitive and precise than other colorimetric methods used for the determination of molybdenum in plant tissues. This method is based on precipitation and extraction of a green-colored molybdenum dithiol complex after removal of interfering ions from the test solution (77). The molybdenum concentration is determined by comparing the absorbance of the sample with known standards on a light spectrophotometer. The detection limit of the dithiol method is about 20 ng Mo mL-1, and the recovery of molybdenum added to the plant material has been greater than 90% (12). Although this method is relatively inexpensive, the procedure may be too tedious and time-consuming for use in many commercial analytical laboratories. For procedures of the dithiol method, readers are referred to Gupta (73).
Trace quantities of molybdenum in plant material have been determined by flame (78) or flameless AAS (79). These procedures provide adequate sensitivity for molybdenum and are relatively rapid, but are subject to matrix interferences (77). The GF-AAS method (80) improves the accuracy and precision of determining low concentrations of molybdenum, and the procedure is applicable to a range of different plant matrices (73). The detection limits for the determination of molybdenum by AAS using flame and graphite furnace are reported to be 10 and 2 ng mL-1, respectively (78), and the recovery of molybdenum by these two methods is similar to that of the dithiol colorimetric method, ranging from 92 to 95% (12). For details of the flame and graphite furnace AAS methods, the reader is referred to Khan et al. (78) and Gupta (73).
© 2018 Biocyclopedia | All rights reserved.