Waxy Corn - Genotype and Characterisation With Iodine

Genotype and Characterisation With Iodine

The wx locus is expressed in the endosperm, in the male gametophyte (pollen) as well as in the female gametophyte (embryo sac). Amylose and amylopectin have different iodine binding-properties, with maize amylose and amylopectin giving iodine affinity (IA) values of about 19 to 20 and 1%, respectively, depending upon the source. Weatherwax discovered this process in 1922.

The amount of apparent amylose can be determined either by measuring the absorbency of the starch-iodine complex (blue-value) and relating this value to that of pure amylose and amylopectin standards or by measuring the amount of iodine (mg) bound per 100 mg of starch in a potentiometric titration and relating the value to the amount bound by an amylose standard.

Values used on the iodine binding, however, are only estimates of amylose content because of differences in the binding abilities (and structure) of amylose and amylopectin among starch types. For example, amylopectin molecules with long external branches bind more iodine than those with short branches do, resulting in a small measure of apparent amylose.

Chromatographic profiles of wx-containing starches, however, reveal no amylose peak. The wavelength at which a starch-iodine complex has maximum absorbency is referred to as the lambda max.

Plants which are heterozygous on the waxy gene (Wx:wx) can be characterised by staining the pollen with iodine. Half of the pollen will be blue and half brown whereas the kernels will stay blue (very helpful in backcrossing program). If the plant is homozygous recessive (wx:wx) the whole pollen will be brown and the kernel too. Being homozygously dominant (Wx:Wx) the iodine will appear only blue.