Relation of plant vigor to number of kernel rows.

For a genetic analysis, it is obviously important to choose a quantitative character that is little affected by diversities of environment. To undertake it with such characters of maize as height of stalk, weight of plant, weight of grain and the like would be to court failure. The effects of soil heterogeneity and of weather diversity might be expected to mask genetic differences. There are two quantitative characters of maize, however, that are influenced only slightly by enviromental diversity, namely, number of nodes or leaves on the stalk and number of kernel rows on the ear. The seedling leaves of maize are usually lost long before the plant has completed its growth and the first internodes are very short. While this does not make it impossible to record accurately the number of nodes, it makes accuracy more difficult. An even more important objection to the use of number of nodes in a genetic study in this latitude is the close correlation between number of nodes and lateness of maturity. One can find varieties of early and of late maize that are either relatively tall or relatively short, but maize with a large number of nodes is, in my experience, always late in maturity.

Number of kernel rows can usually be determined accurately and with little difficulty. The only difficulties encountered are crooked or zigzag rows and tapering and fasciated ears on which the number of rows varies from butt to tip of the ear. These difficulties can be overcome largely by selection for straight rows and non‑tapering and non‑fasciated ears. When the use of such ears is not easily avoided, it has been my practice to record the number of rows at about one‑quarter of the distance from the butt toward the tip of the ear. Since two ears on the same stalk occasionally differ in number of rows by two and rarely by four rows, I have used the upper -‑ usually the better developed -‑ ear.

It should be obvious that conditions of growth could affect number of nodes and numbor of kernel rows, if at all, only during the early stages of the plant's development. Usually about five leaf primordia ara found in the embryo of the mature kernel. In relatively young seedlings, all leaves or leaf primordia are visible and the tassel primordium appears, after which no more nodes can arise. Even the ear shoot is laid down and thereby the number

of kernel rows determined relatively early in the life of the young maize plant.

Differential soil fertility. Some years ago, two 12‑row inbred lines of maize, A and B, and the F₁ and F₂ of the cross, A‑B, were grown during two summers on two small plots, one of almost pure sand and the other of very fertile soil. On the sand plot the plants were from two to four feet tall whiLe on the rich soil plot they reached a height of seven to nine feet. An open‑pollinated stock of the 16‑row variety, Cornell 11, also was tested on the rich soil and sand plots. More recently three 12‑row (2,4,39), two 8‑row (51, XI) and one 16‑row (Pr) inbred stocks were grown on relatively rich garden soil and duplicated on similar soil on which oats had been seeded heavily at planting time. The oats evidently robbed the maize plants of much of the available fertilizer elements, for, at the time of tasseling, the maize plants were only one to two feet tall and were more nearly yellow than green in color. A heavy side dressing of ammonium nitrate after the oat plants had been removed resulted in the development of usable ears on only a part of the plants. Frequency distributions of all the lots subjected to these differential soil‑fertility tests are presented in table 1.

In every one of the 11 tests, the mean number of kernel rows was lower on the low‑fertility plot than on the high‑fertility one. Differences in means varied from 0.27 to 3.69. All lots combined, involving 1189 plants, showed a difference of 1.59. In general the differences are statistically significant, but of small magnitude. There is, therefore, no doubt that extreme differences in soil fertility influence mean number of kernel rows, and it is equally without doubt that the relatively small differences in fertility encountered in any small experimental field in any one season may well be disregarded.

Table 1. Effect of extreme differences in soil fertility on number of kernel rows

Maize stock	Soil fertility level	Number of kernel rows										Total	Mean	Difference
		4	6	8	10	12	14	16	18	20	22
XI	High	7	1	24								32	7.06
	Low	23	1	12								36	5.39	1.67
51	High	1	6	52	2							61	7.80
	Low	3	6	26	3							38	7.53	.27
A	High			1	11	35	6					53	11.74
	Low			2	28	35	3					68	11.15	.59
B	High				5	49	16					70	12.31
	Low			5	50	28	1					84	10.60	1.71
A‑B	High				5	33	30					68	12.74
F1	Low			4	27	45	7					83	11.83	1.41
A‑B	High			1	14	52	24	1				92	12.22
F2	Low			2	16	22	5					45	11.33	.89
2	High			5	8	29	10					52	11.69
	Low	2	2	1	2	2						9	8.00	3.69
4	High			2	3	44	25	1				75	12.53
	Low			5	2	2						9	9.33	3.20
39	High				2	49	14					65	12.37
	Low			4	11	13						28	10.64	1.73
Pr	High						2	17	24	1		44	17.09
	Low					4	13	7				24	14.25	2.84
Cornell	High					1	13	41	15	10	3	83	16.70
"	Low					15	33	18	4			70	14.41	1.29
All tests	High	8	7	85	50	292	140	60	39	11	3	695	12.47
	Low	28	9	61	139	166	62	25	4			494	10.88	1.59
												1189

Heterosis. Since extreme differences in vigor of growth, induced by differences in soil fertility affect number of kernel rows to some degree, it is important to know what effect may be induced by hybrid vigor of plants, or heterosis. A comparison of mean row number of 13 12‑row inbreds and of all the 78 F₁ crosses between them were made. Similar comparisons of mean row number of threee 10-row inbreds with the three F₁s and of six 8‑row inbreds and the 15 F₁s were made.

Of the 14 12‑row inbreds 6081 and of the 91 F₁s 7217 individuals were recorded, an average of 434 per inbred and 79 per F₁. The average of the mean number of kernel rows for the inbreds was 12.14 and for the F₁s 13.03, a difference of 0.89 rows, presumably an effect of the greater vigor of F₁ plants. Some of the F₁s showed no significantly greater number of rows than the average of the two inbred parents, and two of them exhibited a slightly lower number. The fact that 89 F₁s showed plus and only two minus differences when compared with the averages of the inbred parents indicates a significant though small effect of plant vigor on number of kernel rows.

The three 10‑row inbreds, with 1543 individuals recorded, had an average of mean row numbers of 10.33 and their three F₁s, including only 319 individuals, an average of 11.56, an average positive difference of 1.23 rows. If these records are combined with those from the 12‑row lots, the 17 inbreds averaged 11.89 and their 94 F₁s 12.98 rows, a difference of approximately 1.1 rows. This supposed effect of plant heterosis is of the same order of magnitude as that earlier ascribed to differences in vigor of plant associated with extreme differences in soil fertility.

The seven 8‑row inbreds and their 19 F₁s showed significant differences in only a few cases. There were 14 plus and five minus differences, and the average difference was only 0.18. This questionable slight effect of plant heterosis on number of kernel rows in case of 8‑row inbreds and their F₁ hybrids parallels a similar lack of effect of excessive crowding compared with wide spacing of 8‑row inbreds.

R. A. Emerson