Selective Control o Egyptian Broomrape (Orobanche Aegyptiacapers.) by Glyphosate and its Amino Acid Status in Relation to Selected Hosts
Nandula, Vijay K. II
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Broomrapes are achlorophyllous holoparasites of many economically important dicotyledonous crops. As weeds, they cause reductions in crop yield, adversely affect crop quality, and result in loss of cultivated land due to reduced crop alternatives. Few effective control measures exist for broomrapes. One of the most promising approaches is the use of low rates of glyphosate in hosts with tolerance to the herbicide. Recently, availability of glyphosate-resistant crops has provided an alternative in broomrape infested areas. Knowledge about the nitrogen status of broomrapes is essential for developing new control strategies. Broomrapes have two potential sources of amino acids. First, the haustorium aids in the translocation of amino acids from the host plant to the parasites. Second, broomrapes may be able to synthesize some amino acids themselves and obtain the rest from the host. However, the relative importance of these two modes of acquiring amino acids by broomrapes is not clear. Osmotic stress has been implicated as a possible reason for inhibition of broomrape germination by nitrogen. To date, there has been no attempt to correlate osmotic potential with nitrogen induced inhibition of broomrape germination. Optimum temperatures for conditioning and germination are different among broomrape species. Although temperature is known to influence germination in broomrape, its effect on subsequent development of the parasitic seedling has not been studied. Studies were conducted to determine the use of glyphosate in controlling broomrape in common vetch that is tolerant to low rates of glyphosate, and to compare this response with broomrape control in oilseed rape that has been genetically engineered for glyphosate resistance. Glyphosate dose response studies using a commercial formulation and patterns of absorption, translocation, and metabolism, using ¹⁴C-glyphosate, were determined for both host crops. Glyphosate significantly reduced the growth of broomrape at 0.18 and 0.36 kg ae ha⁻¹> in common vetch and 0.25 to 0.75 kg ha⁻¹ in oilseed rape. More than 25% of translocated ¹⁴C-glyphosate in both host crops accumulated in broomrape tubercles. Broomrape parasitism caused a redistribution of translocated ¹⁴C-glyphosate in the roots of both host crops. Glyphosate was metabolized up to 25% in common vetch, but remained intact in oilseed rape. Studies were conducted to analyze amino acid composition of both nonparasitized and broomrape-parasitized hosts and associated broomrape after hydrolysis and phenylisothiocyanate derivatization of amino acids. Results indicated that amino acid concentrations of leaves of parasitized carrot plants were lower than those of the leaves of nonparasitized carrot plants. Broomrape tubercles had equal or higher amino acid concentrations compared to those of the leaves of nonparasitized carrot plants. Levels of free alanine and arginine concentrations of broomrape callus were higher than those of any other tissue of either carrot or broomrape. The effect of glyphosate on the host-broomrape interaction regarding amino acid metabolism was examined. Glyphosate generally increased the amino acid concentrations in common vetch and oilseed rape plants, and broomrape attachments. The aromatic amino acids, phenylalanine and tyrosine, did not differ from this pattern. Concentrations of certain amino acids in broomrape were similar to those of parasitized common vetch and parasitized oilseed rape, whereas levels of several others, were higher in broomrape attachments compared to the host plants. In vitro studies were conducted to determine the influence of osmotic potential and temperature on broomrape germination. Osmotic potential significantly affected germination and radicle elongation of broomrapes. No correlation was found between osmotic potential and ammonium-induced inhibition of germination of broomrapes. Temperature significantly influenced germination and radicle elongation of all broomrape species tested.
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