Iron uptake and phytosiderophore release by roots of sorghum genotypes1

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Abstract
Graminaceous species (grasses) have different mechanisms for mobilization and uptake of iron (Fe) than non‐graminaceous plant species. Roots of Fe deficient grasses release phytosiderophore compounds which greatly enhance mobilization and uptake of Fe. Studies were conducted to determine Fe uptake by various sorghum (Sorghum bicolor (L.) Moench) genotypes differing in age (10–22 days) and degrees of Fe deficiency chlorosis. For comparison, barley (Hordeum vulgare L. cv Europa) was used as a grass species with high resistance to Fe deficiency chlorosis. For the short‐term Fe uptake studies (4 h) Fe was supplied as ferrated phytosiderophore (1 uM labelled with 59Fe) from phytosiderophore released by roots of Fe deficient barley. Uptake rates of ferrated phytosiderophore by Fe deficient sorghum genotypes at 10, 14, and 18 days of age were relatively high and about 40–80% that of barley. Under the experimental conditions (preculture with and without 0.1 mM Fe HEDTA), the Fe nutritional status of the sorghum and barley plants did not affect the uptake rates of ferrated phytosiderophores. Mobilization of Fe (indirect determination of phytosiderophore compound release) by roots of sorghum was determined by the uptake of Fe mobilized by root exudates from 59Fe(OH)3 in dialysis tubes inserted in the Fe free nutrient solution. Sorghum genotypes mobilized considerable amounts of Fe at 11 days of age when plants were slightly chlorotic. The rates of Fe mobilization were 25 to 130% that of Fe deficient 11‐day‐old barley. The highest value of mobilization in sorghum was obtained with sudangrass [Sorghum bicolor (L.) Moench]. However, at 15 and 19 days of age when chlorosis became more severe, the sorghum genotypes mobilized essentially no Fe whereas barley still mobilized relatively high levels. Thus, the high susceptibility of sorghum to Fe deficiency chlorosis was not caused by limited capacity for uptake of ferrated phytosiderophore, but drastic decreases in mobilization of Fe with age and/or severity of Fe deficiency chlorosis.