PHYTOSIDEROPHORE RELEASE IN BREAD AND DURUM-WHEAT GENOTYPES DIFFERINGIN ZINC EFFICIENCY

The effect of the zinc (Zn) nutritional status on the rate of phytosid erophore release was studied in nutrient solution over 20 days in four bread wheat (Triticum aestivum cvs. Kirac-66, Gerek-79, Aroona and Ki rkpinar) and four durum wheat (Triticum durum cvs. BDMM-19, Kunduru-11 49, Kiziltan-91 and Durati) genotypes differing in Zn efficiency. Visu al Zn deficiency symptoms, such as whitish-brown necrosis on leaves an d reduction in plant height appeared first and more severe in Zn-ineff icient durum wheat genotypes Kiziltan-91, Durati and Kunduru-1149. Com pared to the bread wheat genotypes, all durum wheat genotypes were mor e sensitive to Zn deficiency. BDMM-19 was the least affected durum whe at genotype. Among the bread wheat genotypes, Kirkpinar was the most s ensitive genotype. In all genotypes well supplied with Zn, the rate of phytosiderophore release was very low and did not exceed 1 mu mol 32 plants(-1) 3h(-1), or 0.5 mu mol g(-1) root dry wt 3h(-1). However, un der Zn deficiency, with the onset of visual Zn deficiency symptoms, th e release of phytosiderophores was enhanced in bread wheat genotypes u p to 7.5 mu mol 32 plants(-1) 3h(-1), or 9 mu mol g(-1) root dry wt 3h (-1), particularly in Zn-efficient Kirac-66, Gerek-79 and Aroona. In c ontrast to bread wheat genotypes, phytosiderophore release in Zn-defic ient durum wheat genotypes remained at a very low rate. Among the duru m wheat genotypes BDMM-19 had highest rate of phytosiderophore release . HPLC analysis of root exudates showed that 2'-deoxymugineic acid (DM A) is the dominating phytosiderophore released from roots of Zn-effici ent genotypes. In root extracts concentration of DMA was also much hig her in Zn-efficient than in inefficient genotypes. The results demonst rate that enhanced synthesis and release of phytosiderophores at defic ient Zn supply is involved in Zn efficiency in wheat genotypes. It is suggested that the expression of Zn efficiency mechanism is causally r elated to phytosiderophore-mediated enhanced mobilization of Zn from s paringly soluble Zn pools and from adsorption sites, both in the rhizo sphere and in plants.