Genes that retard water loss in maturing sweet corn kernels

The water content of sweet corn at the time it is harvested for fresh use or preserving (17-24 days after pollination, depending on variety and temperature) is near 70%. Slow-drying is one of the important effects of the sugary-1 (su) gene as seed development advances. The soft, fully distended condition of the su kernel when it is most desirable for use as food is transitory. The seed later becomes firm and wrinkled, and eventually, in the ripe ear, hard and glassy.

Mutant genes that intensify the water retaining effect of the su factor, and thus prolong the harvesting period, are of interest as a possible means of significantly expanding the food uses of sugary corn. The writer has given special attention to three genes among the several that alter sugary-1 expression in this way, namely, floury-2 (fl2), opaque-7 (o7) and, as noted in the immediately preceding news item, sugary enhancer (se).

Quantitative data on the retarding effect of the fl2 gene on water loss in sugary and non-sugary kernels at Madison, Wisconsin, in 1977, are presented in Figure 1. In this experiment su fl2 and su Fl2 plants derived from hybrids between W64A fl2 and Seneca Chief, a commercial sweet corn hybrid, were crossed with W64A fl2 and the standard dent corn inbred line W64A (fl2), respectively. The su fl2/W64A Su fl2 hybrid was then pollinated using pollen from a closely related su fl2 stock, and the su fl2/W64A Su Fl2 hybrid was pollinated with su Fl2 pollen of similar derivation. Thus about 3/4 of the residual inheritance carried by the resulting four classes of kernels, Su fl2, Su fl2, su fl2, and su fl2 was derived from the W64A inbred strain, and the balance from Seneca Chief. Collections for moisture determination were begun at 30 days following pollination, the earliest time at which the Su and su kernels could be clearly distinguished from each other on the ear. Weights of fresh and oven-dried kernels were taken on two independently collected 100-kernel samples (each based on 4 ears) of each of the four classes of seed, at 30, 40, 45, 55 and 73 days after pollination.

Figure 1.

The retarding effect of su on water loss is shown by the positions of the Su Fl2 and su Fl2 entries in Figure 1. At 55 days after pollination, for example, the su seeds contain 17.7% more water than their non-sugary counterparts. A large difference (11%) persists even to 73 days.

The water content of Su kernels is already about 10% lower at 30 days than that of su seeds, as shown by the points of origin of the four lines in Figure 1. In contrast, the differentiating effect of fl2 in this respect is little expressed until after 30 days. The fl2 gene alone however, strongly retards water loss. At 73 days after pollination the moisture content of floury-2 (Su fl2) kernels is 33.2%, a value which is about the same as that of sugary (su Fl2) kernels (34.5%) at this stage.

Especially noteworthy is the fact that fl2 interacting with su holds the kernel water content above 60% up to 45 days after pollination. This means that, insofar as moisture content at the picking stage determines the suitability of sweet corn for food, fl2 in conjunction with su extends the harvesting period to 40 days or more, and thus well beyond the upper limit for ordinary sweet corn of about 25 days.

Numerical data have not been taken on the effects of the su o7 and su se genotypes on water loss from maturing kernels. Field observations indicate, however, that the retarding action of both o7 and se are pronounced. Furthermore, J. W. Gonzalez, A. W. Rhodes and D. B. Dickinson (Plant Physiol. 58:28-32) report that Ill.677a sugary seeds which, as shown in the preceding news item, are homozygous for the sugary enhancer (se) gene, tend similarly to retain moisture as development advances. Older data show that the waxy gene (wx) in combination with su, in contrast, has only a small effect on water retention in the maturing seed (R. H. Andrew, R. A. Brink and N. P. Neal, J. Agric. Res. 69:355-371).

The marked increase in toughness and resistance to mastication of the pericarp that occurs from about 20 days post-pollination onward need not be a deterrent to using more mature sweet corn for food. The kernels on older husked ears can be scarified at the crown with a sharp instrument and the endosperms and embryos can then be squeezed out with a suitably shaped hand tool, leaving much of the pericarp on the cob. This operation can be carried out efficiently, however, only on kernels that are soft and fully distended. The writer has processed ears from su fl2 and su se plants in this way up to 40 days after pollination without difficulty. The kernels on ordinary sweet corn, on the other hand, are too firm at 40 days to permit this operation. The procedure whereby endosperm and embryo were separated from pericarp is similar in certain respects to the factory process whereby cream-style corn is prepared for canning, and so lends itself to mechanization. A desirable departure from the usual commercial procedure, however, is scarification of the crown instead of cutting of the upper part of the kernel with its tough outer covering, before squeezing out the embryo and remaining endosperm.

Freeing sweet corn from the limitations as a food imposed by a pericarp that becomes coarsely fibrous and tough as kernel development advances is a useful gain in itself. Greater flexibility in time of harvesting is thus made possible. The amount and character of the food products obtainable also are expanded. Mention is made in the immediately preceding news item that certain sugary-sugary enhancer (se se) homozygotes, for example, were of excellent table quality when harvested at about 20 days after pollination. A different product is forthcoming at later developmental stages. Preliminary trials show that canned or frozen corn from su fl2 or su se plants picked at 40 days and processed so as to leave much of the pericarp on the cob was less sweet than corresponding 20-day corns but had a pleasing texture and mellow taste. It is evident from the results of these tests that incorporation in the sugary-1 genotype of one or another gene, like sugary enhancer, opaque-7, or floury-2, might significantly expand the range of nutritious and palatable food products that can be obtained from corn.

Attempts to broaden the culinary use of sweet corn by incorporating in the su genotype a second mutant that significantly delays water loss from the endosperm during kernel maturation encounters important mechanical and biological production problems. These include injury during machine harvesting and processing of the seed and fitness of the double mutants to meet the various challenges of the external environment. Perhaps the seed injury problem can be made manageable by the development of inbred lines with husks loose enough during ripening to facilitate more rapid field curing before machine picking.

Major components of the fitness system in sweet corn relate to seed germination and seedling establishment and reaction of the plant to ear and stalk rotting organisms.

The three mutants mentioned above, sugary enhancer, opaque-7, and floury-2, in homozygous condition reduce mature seed weight and, when in combination with the sugary gene, impair germination and seedling development. The su gene alone lowers mature seed weight about 20%, as compared with Su kernels on segregating ears. The results of a 1977 experiment on the weight of mature seeds from selfed su/+ and su fl2/+ plants in which the normal (+) parents in each set were nine inbred dent corn strains are instructive in showing the adverse effect of the fl2 gene on sugary kernel development. If the mature "weight index" of the +/+ (i.e., nonsugary, non-floury-2) kernels on segregating ears is taken as 100, then the mean weight index for the su/su (non-floury) seeds was 80.6 and that for su fl2/su fl2 kernels was 60.9. (The su and fl2 genes are linked with about 8% crossing over between them. No adjustment in calculating the weight index was made for the recombinants, so the 60.9 value is a maximum).

It was found that the fl2 gene alone on the background of W64A, a dent inbred line, reduced mature seed weight about 20%, as was the case also with the su gene alone, as noted earlier. Thus the adverse effects of the su and fl2 genes on mature seed weight are additive. Reduction of the seed weight index from 80 for the su or the fl2 gene alone to about 60 for the su fl2 double mutant results in a mature, dry kernel that borders on chaffiness in some genetic backgrounds. The yield of edible products would be expected to decline accordingly, and seed germination and plant establishment would be less reliable. These results make it doubtful that homozygous su fl2 strains would be economically viable.

Tests comparable in scale to those of fl2 on seed weight, in combination with su, have not been made with opaque-7 and sugary enhancer. Preliminary observations indicate, however, that whereas both these genes reduce the dry weight of mature sugary seeds, the net effects in this respect are significantly less than that of the fl2 factor.

R. A. Brink

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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