Heritable differences in glucose levels among sugary corn lines and sporadic maltose accumulation in heterozygous sugary enhancer sweet corn kernels

The sugary enhancer (se) endosperm mutant was identified as a recessive modifier of sugary (su) in the sweet corn line IL677a (J. E. Ferguson et al., J. Hered. 69:377-380, 1978; and R. A. Brink, MNL 52:110-112, 1978). Homozygous sugary-sugary enhancer (su se) kernels are characterized by elevated sucrose at the eating stage (about 21 days after pollination) and by elevated maltose at the mature dry stage (J. E. Ferguson et al., Plant Physiol. 63:416-420, 1979). A breeding program was undertaken to transfer se from IL677a to a number of different su backgrounds. Selection on segregating ears was based on kernel appearance; su se kernels are thought to be slower to dry during maturation, and to be lighter colored and more finely wrinkled at the mature dry stage than su Se kernels. Immature self-pollinated ears from plants grown from the selected seed were tasted in the field to test for the expected levels of sweetness.

Sugar and sorbitol levels in mature dry seed of the lines developed in the breeding program were analyzed by gas-liquid chromatography to check the success of selection for se. Sucrose and maltose are the diagnostic sugars, but sorbitol, fructose, and glucose were included for completeness. Most lines analyzed were found to have elevated maltose levels, indicating successful selection for se, but a few lines had low maltose levels. The low maltose lines generally had higher glucose levels than their high maltose counterparts (E. E. Carey, M.S. Thesis, Univ. of Illinois, 1981). Five standard su Se inbred lines and IL677a were analyzed at the same time for comparison with the lines from the se breeding program. The glucose levels of the su Se lines varied markedly (Table 1), with IL14h having very low levels, IL442a and IL11a having intermediate levels, and IL454a and IL459a having high levels. IL442a, one of the parents of IL677a, also had low levels of maltose. That sugar levels of the inbred lines were stable over successive generations was determined by analysis of remnant seed of four successive generations (E. E. Carey, M.S. Thesis).

In an attempt to compare the behavior of the glucose trait in the lines from the se breeding program with those in the su Se lines, a number of crosses were made (some reciprocal) and sugars in the mature dry F1 seed were analyzed. Table 1 lists the sugar and sorbitol levels of the parents (selfs) and the crosses. IL11a through IL459a were the su Se lines, IL751a and ILB5765 were the high glucose-low maltose lines from the se breeding program, and IL677a and IL753a were the su se lines (IL753a derived from the se breeding program).

In crosses between the high glucose lines from the se breeding program (IL751a and ILB6765) and the high glucose su Se lines (IL454a and IL459a), glucose levels were consistently elevated. High glucose was dominant to low glucose in the cross IL454a X IL14h and its reciprocal, but not in IL14h X IL677a which accumulated almost no glucose. Heritable differences in the levels of reducing sugars in sweet corn lines might be of interest since canning quality is adversely affected by elevated levels of reducing sugars.

In crosses between su Se and su se lines, maltose levels were generally low, but in a few cases maltose accumulated when the female parent was se. It is difficult to draw conclusions about the cause of this maltose accumulation because of the limited number of crosses that were made and the inconsistency with which it occurred - for instance, IL753a X IL442a accumulated maltose but IL677a X IL442a did not.

Accumulation of maltose in heterozygous se and non-se (i.e., IL442a) kernels indicates that conclusions about the allelic state of se should not be made on the basis of maltose alone. A more thorough understanding of the expression of se and its effects on su will require more complete genetic studies. These will probably involve analysis of individual kernels from segregating ears, and development of near-isogenic lines for more controlled physiological genetic studies.

Table 1.

Edward E. Carey
 
 


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