SALINAS, CALIFORNIA
Western Maize Genetics

Is the “Country Gentlemen” phenotype a better bet than prolificacy in maize yield enhancement?

— Shaver, DL

In the maize spikelet, the lower floret normally aborts, leaving only the upper floret to produce a fertile caryopsis. Simple arithmetic indicates that kernel-per-ear nunber might be doubled by failure of abortion on the part of the lower floret. A duplicate-factor genotype is established in maize for such failure, and is commonly found in “Country Gentlemen,” and “Shoepeg”, controlled by the doubly recessive condition at the duplicate factor loci pi1 and pi2.

A greater kernel number per plant is interesting because it has been shown that kernel number per se is of pararount importance in the determination of net “sink strength” demand upon the plant’s CH2O economy. Hence, the stronger the sink strength, the greater the tendency for success in grain yield. Breeders have long been concerned with two earedness to achieve the same end of higher seed number per plant. Unfortunately, the literature on prolificacy agrees with the results reported here in Table 1, below, that 2-earedness is fatal to good stalk rot resistance, and precludes having satisfactory levels of standability in practical applications. This is because of the fact that at the point of ear-branch attachment to the main culm, the phloem anastomoses serving the ear-branch connect assymetrically with the ad-axial descending phloem traces descending from the source leaves above the ear placement node. In single-eared plants, the phloem tubules distal to the ear anastomoses allow the uninterupted downward flow of photosynthate to continue on to the lower stalk and root system, where it is necessary to allow the plant to continue its defence against invasion by root and stalk rotting pathogens. In sum, then, a single-eared plant allows a proportion of photosynthate to pass on down to the root area, while in two-eared plants there is a double-dip subtraction of photosynthate which starves the lower stalk and root beyond an acceptable threshold level.

This defines a reason for interest in doubling kernel number on a single ear, rather than on two separate ears. As shown in Table One, such a phenotype does not entail a large frequency of stalk rot failure, whereas two-earedness clearly does.

Table 1. Comparison of rate of stalk rot stalk failure in plants in a pi1-pi2 synthetic, 2004 season at Greenfield, CA. comparing different grain-bearing phenotypes.

The present work involves two separate mass-selection populations, now after 15 generations of work. In both, Country Gentleman was used as the source of pi alleles, because the source of Shoepeg that was availble proved not to have the pi phenotype. To begin, then, C.G. was crossed to agronomic Lancaster lines on the one hand, and with Stiff Stalk lines on the other. After 3 generations of selection to recover the pi phenotype, a further backcross was made to the same recurrent agronomic parental lines. After another 3 gnerations of mass selection to again recover the pi phenotype, another cross was made to agronomic Lfy inbreds, representative of the Lanc. and SS heterotic groupings. Since this latter infusion of agronomic germplasm, a further 6 generations of mass selection in isolated populations has been practiced to solidify the pi phenotype.

It should be noted that although the pi1-pi2 genotype is undoubtedly necessary for the expression of the CC phenotye, penetrance is poor, and as far as breeding art is concerned, it is better to operate on the idea that this is a quasi-multigenic trait, because it seems that penetrance is dependent upon unspecified background genes. While progress is deliberate with each generation, there appears to be quite a long ways to go, to achieve economically significant agronomic types.

Figure 1 shows a sample ear from the two pi synthetics Syn. I is based upon SS-derived recurrent parent lines. In Syn. I, ear faciation is often found as a means of accomodating all the extra kernel positions, and is a major criterion to select against. Also, in this Syn. there is a strong tendency for psuedo kernel rowing, making for a semblance of normal appearance for the pi phenotype. It has an estimated 2,896 kernels.

Figure one also depicts an ear from the pi Syn II, based upon Lancaster-derived recurrent parent lines. Its kernel placement is typical of that originally found in the CC non-recurrent parent, with no apparent regular kernel rowing. It has an estimated 2,258 kernels.

The author considers the pi materials here to be part of a long-range program to produce an economic alternative to conventional maize. A start has been made, but he expects that a long ways still remains. All things considered, he believes that this material should be offered as an addition to the public domain, and seeds will be available without condition to interested, responsible parties.

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