Measurement of the FE-deficiency response mechanism in sorghum

Abstract

Growth chamber studies were conducted to develop a visual test for differentiating sorghum cultivar Fe-efficiency in CaCO(,3) buffered nutrient culture. Nine sorghum cultivars were germinated in vermiculite and grown for seven days before being transferred to a Fe-stress nutrient solution buffered to pH 7.5 with excess CaCO(,3). Twenty replications were grown in a growth chamber with a 26(DEGREES)C/15 hour light (300 (mu)E m('-2)s('-1)) and a 22(DEGREES)C/9 hour dark period. Leaf chlorosis was visually scored on a 0 to 4 rating scale after 10 days in nutrient solution and chlorophyll content was determined spectrophotometrically after extraction with 80% (v/v) acetone. Differences in leaf chlorophyll production by visual scores correlated significantly with measured levels of chlorophyll in both growth chamber and field grown sorghum. Sorghum cultivars were readily separated into Fe-inefficient (SC0118-14-5, BTx378, SC0033-14E), Fe-efficient (RTx2536, RTx430), and intermediate (77CS256, RTx7000, 74CS5388, BTx399) groupings. This visual rating scheme is both rapid and efficient and could be readily used in evaluation of Fe-efficiency in crop plants. Root reductants were released from Fe-stressed sorghum into CaCO(,3) buffered nutrient solution. Reductants measured in the CaCO(,3) solutions required considerably more time than did extraction of reductants from excised roots in 35 ml of pH 3 nutrient solution and 1 mM glucose. An Fe-efficient cultivar, RTx2536, and an FE-inefficient cultivar, BTx378, were differentiated both by measurement of root reductants released into CaCO(,3) buffered nutrient solution and by an excised root extraction method; however, neither method was as time effective as the visual rating method. A qualitative test for p-coumarate hydroxylase showed that all ten sorghum cultivars tested contained the enzyme and could not be differentiated for Fe-efficiency on this basis. The effect of NH(,4)('+)-N on root reductant levels developed during Fe stress was investigated in plants grown in a CaCO(,3) buffered nutrient solution. After a three day period of Fe stress, sorghum root reductant levels were not increased by the presence of NH(,4)('+)-N in the growth medium or in the extracting solution. Since reductant degration over time was pH dependent, experiments measuring root reductant release into nutrient solution would be confounded by treatments with different solution pH values. In an unbuffered nutrient solution a differential NH(,4)('+) uptake due to the cultivar would affect solution pH and interfere with a reliable interpretation of reductant measurements.