1. Resistance to grasshoppers

A. New data on linkage with P. In a previous paper (Anales del Instituto Fitotecnico de Santa Cataline 2:25‑52. 1940), we reported that gene ag, for resistance to grasshoppers, is in chromosome 1. In those cultures, F₂ and backcrosses, AG and P were linked in the coupling phase, giving about 20% recombination. Here we are reporting data from a three‑point test, where Ag and P enter in the repulsion phase. Burnham's pa is present also. All data belong to two 1946 cultures.

Three‑point test:	ag P pa	x	ag p pa
	+ p +

(0)		(1)?		(2)		(1+2)?
ag	+	ag	+	ag	+	ag	+
P	p	p	P	P	p	p	P	Total
pa	+	+	pa	+	pa	pa	+
65	84	8	10	23	26	6	8
149		18		49		14		230
		7.8%		21.3%		6.1%

There is no question about ag being located to the left of pa. But, owing to the fact that single crossovers in region 1 and double crossovers appear with almost the same frequency, the relative position of ag and P is not settled. However, there is a slight indication favoring the sequence and approximate spacing of genes to be as follows:

ag 13.9 P 27.4 pa

B. Its linkage with P makes easier the task of transferring ag to commercial varieties as mny of them (American dent varieties) are P^wr (colorless pericarp and colored cob) and others are p (colorless pericarp and cob). The F₁ (P^wr/p Ag/ag) in coupling or repulsion -‑ as the case might be -- is repeatedly backcrossed to the commercial variety and subsequently selfed selecting for that pericarp type which is linked with resistance. So the costly tests with insects are relegated to the final steps of the work. Susceptible P^wr lines are changed into p resistant ones; and, conversely, p susceptible lines are transformed into P^wr resistant.

A new aid in selection for resistant plants is afforded by the "basket‑worm" (Oiketicus kirbyi Guilding). This insect is abundant in the Buenos Aires region. Its poliphagous larva feeds on the leaves of many horticultural plants causing great damage especially to trees and shrubs. These larvae attack the common varieties of maize, but not the one resistant to grasshoppers. When the small larvae emerge in the spring, they are spread by wind, covering all plantations. At this time, with a "at saturation" invasion over the experimental field of maize, it is easy to classify resistant plants from susceptible ones. This allows the elimination of most of the susceptible material before the final test with grasshoppers is made.

C. Regional tests were conducted at 50 geographical places in order to demonstrate the behavior of common varieties as compared to hybrids between selfed lines resistant to grasshoppers. The picture here enclosed shows the results of one of those tests, after a heavy invasion of grasshoppers. It is more demonstrative than any written description could be. (Ed. note: Dr. Horovitz's pictures are very striking and are on file at the Department of Plant Breeding, Cornell University, for anyone who might wish to see them.)

D. Relation between resistance to grasshoppgrs and resistance to other insects. The "amargo" maize (= ag) is resistant to the Acrididae: Schistocerca cancellata (= S. paranensis), Scyllina variabilis and Dichroplus arrogans, among which there are locusts and grasshoppers. It is also resistant to Oiketicus kirbyi, a lepidopterous belonging to the Psychidae. All of these insects are leaf‑biters. (It must be said, by the way, that grasshoppers eat some restricted tissues of the "amargo" plant, as anthers, silks and the auricular region at the base of the leaf which is lacking chlorophyll.) On the other hand, "amargo" maize is not resistant to sucker insects (corn aphides) nor to feeders on internal tissues as stalkworm (Diathraea sacharalis) and ear‑worm (Holliothis sp.).

As there is no corn borer (Pyrausta nubilaris) yet in Argentina, we have had no opportunity to test the ag maize with this insect. But Marston, in Michigan, found resistance to corn‑borer in a corn which came to him from Argentina under the name "amargo". Marston transferred that resistance to Michigan lines of maize. Some years ago, Marston kindly sent us samples of his new corn‑borer resistant corn as well as the original "amargo" used by him as the source for resistance. All of them -‑ Marston's original "amargo" included -‑ have been proven completely susceptible to grasshoppers in our tests.

Resistance to corn aphis and to corn borer both might be due to the same causal condition; this having been suggested to us by the following words of Dr. R. A. Emerson in a letter of May 22, 1944: "The corn breeders of our central states have found inbreds that show strong resistance to the corn borer and the same inbreds are also resistant to aphis." We have tested with grasshoppers many corn lines of different origin, and among them several American lines carrying indications of resistance to some insects as chinch bug, corn root worms or grasshoppers. All of these lines were susceptible to grnsshoppers in our tests.

The entire informtion suggests the existence of a repellent substance in the leaves of "amrgo" corn, its distribution being restricted to green tissues only. Such a substance, conditioned by gene ag, would be, perhaps, a general repellent for leaf biter insects. Resistance to corn aphis and to corn borer is due to a different cause ‑- perhaps also a chemical repellent, but, anyway, different to the one causing resistance to grasshoppers and more widely distributed, especially through internal plant tissues.

S. Horovitz

A. H. Marchioni

E. Preliminary investigations on the nature of resistance of ag maize to grasshopper.

(a) Temperature action on resistance. Leaves of resistant corn (ag), severed from the plant, were maintained at different temperatures before submitted to insects. Leaves kept at O�C for increasing periods of time up to 96 hours did not change their resistant condition as proven in the subsequent test with grasshoppers. Leaves kept during five minutes at increasing temperatures up to 75�C, maintained their resistance. Treatments of leaves at 80�C during five minutes, slightly reduced their resistance. Treatments during a longer period of time at 80�C badly affected the condition of the leaves which did not withstand a 24‑hour test with grasshoppers without drying out. Leaves treated at 100�C during one minute, became completely susceptible.

(b) Juice was obtained by pressure from leaves of both resistant and normal corn. The remainder of pressed leaves of each kind, was supplied with juice from either resistant or susceptible leaves, and afterwards tested with grasshoppers. Fresh leaves from resistant and susceptible plants, after being impregnated with extracted juices from susceptible or resistant leaves, behaved like untreated leaves. The results are as follows:

Rest of pressed leaves	Juice from pressed leaves	Behavior against grasshoppers (rest + juice)
ag	ag	Resistant
+	ag	Susceptible
ag	+	Resistant
+	+	Susceptible

ag = resistant plants

+ = susceptible plants

These experiments show that resistance of leaves is apparently due to a thermolabile substance, not affected by low temperatures, but destroyed at 80�C. Such a substance seems to remain in the rest of pressed leaves rather than in the extracted juice.

The liquid obtained by rupture and maceration of leaves with a small amount of water, by shaking it into a test tube, in the case of resistant plants, gives a more abundant and persistent foam than that obtained from normal plants. The chemical search for saponins gave negative results. Likewise, the search for cyanoheterosides by Guignard's reaction also gave negative results.

H. G. Fisher

A. H. Marchioni

R. A. Nico

(c) Millon's reaction and resistance to grasshoppers. In order to investigate the nature of resistance to grasshoppers, plants of resistant maize were tested with reagents which served to identify some organic substances or groups of them. Millon's reaction which indicates the presence of phenolic‑groupings, gave a marked difference between some susceptible varieties and the original resistant one. Tests with leaves from the resistant corn gave a red coloration. These tests were extended to selfed lines, and cultures segregating for resistant and susceptible plants, showing a correlation between total phenol contents (as evaluated as phenic acid) and resistance. But other genetic stocks, namely, one c sh wx A B pl (c tester) stock, coming from Cornell in 1933, though susceptible to grasshoppers, gave a red coloration with Millon's test. These results could signify that Millon's reaction and "behavior to grasshoppers", perhaps depends on two pairs of linked genes, but not on a single pair. Or, otherwise, the above results might be due to the kind of phenols possessed by different lines of maize in which case certain allelic differences manifested by Millon's reaction still could be due to the pair of genes Agag. The study of phenols distribution throughout the plant shows that the largest concentration is found in green tissues already exposed to light. There is an intermediate concentration in leaves that have not yet been exposed to light. The lowest concentration is found in the still unfolded tassel, and in the whitish sub‑ligular region of the leaf, both of which show susceptibility to grasshoppers.

E. M. Sivori