A survey of kernel texture in Zea Mays.

The fundamental genetics of texture in Maize, though of great economic and theoretical importance has scarcely advanced beyond the pioneer efforts of East and Hayes. Our understanding of the chemical conposition of the maize kernel is advancing rapidly at the present time. For the most effective use of this information we need broad surveys of the gross morphology of the kernel in all kinds of maize.

The classification of field varieties of maize into flints, flours, dents, and pops is almost wholly artificial. Though once useful as a cataloguing device in the United States, it is now inadequate even within that restricted area. Recent work has revealed the peculiar features in the history of the U. S. corn which led Sturtevant and his predecessors into the belief that flints, dents, and pops were such fundamentally different kinds of maize. The general maize picture of commercial U. S. corns by the second half of the 19th century was a simple one, which could have been matched at no other place in the world. Across the northern part of the country and extending well down into what is now the corn belt was a widespread and uniform race of maize, the Northern flints spreading up from Mexico and already being actively mixed with the flints was a very differert group of varieties, the old white southern dents. Though there were hundreds of varieties of maize in the eastern U.S. nearly all of them came from these two very different and quite well‑marked races. The popcorn picture too, was simpler then than now. Most of the varieties were either different strains of White Rice, or some of its various hybrids. Sweet corns which we now know to be due to a single gene had originally been introduced into commercial cultivation on a northern flint background and were still prevailingly of that general growth type. Along the Gulf Coast a few Caribbean Flints complicated the picture as did occasional collections made among the Indians of the southwest and the Great Plains. Therefore, in the 80's when Sturtevant grew and studied all the types of maize he could find in the trade, or obtained from collectors, his collection did fall quite naturally into sweets, flints, dents, flours, and pops. It was not until the work progressed and he began to receive more collections from Mexico and elsewhere, that he began to see the more complicated picture presented by Zea Mays in its entirety. In the N.Y. Agricultural Eaperiment Station report for 1885 after he had grown his first large Mexican collection, we find him writing "With less material for study, these races would show a well marked separation, as illustrated in my classificatory attempt in 1884. While it is deplorable to have to change one's views as radically as this present offering shows, yet it is more deplorable to think that enlarged coUeetions may in time overset the present classification."

The special collection of maize types assembled in the herbarium of the Missouri Botanical Gaiden was utilized for the survey. From the examination of endosperm composition in this colledtion it is clear that there is one general endosperm pattem in Zea Mays, a central column of soft starch surrounded by a collar or cap of hard starch. Even in those kinds which are usually thought of as being entirely flinty, or entirely starchy, there are many of the former which have a small area of soft starch near the germ and of the latter which have a thin layer of hard starch over the entire endosperm or a small and sometimes interrupted collar of hard starch on the side away from the germ.

In making the survey, kernel sections were examined mounted on modeling clay under a dissecting microscope. Longitudinal sections were made at right angles to each other (bisecting the embryo and parallel with the embryo) as well as a series of cross sections. After a preliminary survey it was found that one cross section in the middle of the kernel was sufficient if combined with the two longitudinal sections. Well‑developed kernels were selected from the mid‑ear region, a precaution particularly necessary in varieties from Mexico and Central America where there is some variation in kernel texture on different parts of the ear. Several kernels were examined and an average condition selected for record. Most of the kernels were sectioned without any previous treatment. A few of the flintier ones were soaked for about an hour until they were easier to cut. Drawings were made on cross‑section paper at six times natural size. The chief dimensions of each section were measured and the entire drawing was carefully made to scale. The reliability of the method was checked by resampling various ears at intervals of about a fortnight.

It is not possible to make dividing lines of any kind between flints, flours, pops, and dents. There is rather a continuous series from varieties with a heavy cap of hard starch tho those with a partial collar to thase with a completely floury endosperm. Working directly with the entire collection of measured drawings it was possible to classify them roughly in ten grades.

It should be specifically pointed out that when the entire collection of kernels is examined there are no discontinuities anywhere in the ertire range. Among the flour corns one passes from extremes in which entire varieties are without any hard starch, to varieties in which some of the ears bear either a small collar or an entire cap of hard starch. At the other extreme there are apparently no varieties of maize in which all the kernels are without any soft starch. There are some pointed popcorns in which there is very little soft starch and we have found a few ears in which there are none.

As soon as the relative amounts of hard and soft starch had been graded in our collection. It was apparent that denting of the kernel was dependent upon some other factor in addition to the proportion of hard starch. While all of the kernels which were clearly dented were in the intemediate classes for percentage of hard starch, the reverse was by no means true. There were many varieties with intermiediate percentages of hard starch and no pronounced denting.

When the narrowness of the kernels was considered along with the percentage of hard starch in the endosperm, the relationship of these two variables to denting was clear. To be dented a kernel must have a complete or partial collar of hard starch (it may have a thin cap of hard starch as well) and it must be narrow. For kernels with an intermediate degree of hard starch, denting is proportional to the comparative length of the kernel. All the narrow kernels in these classes are dented, none of the broad ones are dented, and in general the narrower the kernel the more extreme the dent.

A comparison of 17 inbred lines and 17 of the single crosses derived from them was of particular interest. While the subject has apparently never been discussed in print, it is well known to most corn breeders that the inbred lines derived from U.S. corn belt dents are less dented than the open pollinated varieties from which they came. It is furthermore well known that crosses between some of these inbreds (but by no means all of them) are much more dented than either parent. The comparison suggests some of the factors which are at work in this complex situation. It is clear that the single crosses, on the average, are much narrower than the inbreds from which they were derived. This might well be a direct effect of hybrid vigor. A more vigorous plant would produce a much larger seed and unless the entire plant were proportionately larger the seed would of necessity have to be longer, and hence would be narrower in relation to its size. Single crosses tend to be more deeply dented then their parental inbreds because they tend to have longer seeds.

The genetics of texture in maize is complex. There are evidently many genes which affect it and preliminary results indicate some of these have complimentary effects. Understanding the relationship between length of kernel and denting should make it somewhat easier to work out the genetics of kernel texture.

Comparison of kernel grades for 17 inbred lines
and 17 of their derived crosses

Grade of hard starch:		3	4	5	6	7	8
	crosses	3	4	4	2	2	2
	inbreds	0	1	11	4	1	0
Length of kernels:		0.5	0.9	1.3	1.7	2.1
		0.8	1.2	1.6	2.0	2.4
	crosses	0	1	7	8	1
	Inbreds	2	3	12	0	0
Grade of denting:		1	2	3	4
	crosses	2	5	5	4
	inbreds	5	6	6	0

Edgar Anderson and Felix Agramont