--P. F. Byrne, E. H. Coe, L. L. Darrah and K. B. Simpson

While investigating resistance of maize to the corn earworm (CEW), Heliothis zea (Boddie), we noticed an association between a low level of CEW damage, relatively high silk pH, and silks which do not turn brown when crushed (Byrne et al., Environ. Entomol. in press). Coe and Han (MNL 59:40 and 60:50) presented evidence that the silk browning trait was controlled by the P locus, and Miranda (MNL 56:30) suggested that this same locus was among the loci that were associated with resistance to CEW in the race Zapalote Chico.

We report here the results of an experiment that examined these three traits in crosses that were expected to segregate 1:1 for browning and nonbrowning silks. Plants were grown in the field in a split-plot design in which whole plots were designated either for measurement of pH and the browning reaction, or for artificial infestation with CEW eggs. Subplots were progeny of the crosses listed in the tables which follow. pH was measured on six-to-eight-day-old silks using a Corning portable pH meter equipped with a pair of microelectrodes (Lazar Research Labs, Inc., Los Angeles, CA 90046). Evaluations of resistance were obtained by measuring the length of CEW ear tunnels 14 days after infestation.

In all crosses, nonbrowning segregates had significantly higher mean pH values than segregates with browning silks (Table 1). Because we have found discrepancies between silk pH values obtained with the Corning/Lazar equipment and those obtained with a standard laboratory Beckman pH meter, the pH values reported here may not indicate solely the hydrogen ion concentration of silk tissue. Ceska et al. (MNL 54:118) reported that plants with the WW allele (nonbrowning silks) at the P locus accumulate flavonol glycosides due to a blocked flavone pathway. Perhaps the Lazar electrodes were sensitive to the increased concentration of these compounds in nonbrowning silks.

Table 1. Mean pH values of browning and nonbrowning silk classes.

Mean tunnel length of ears with nonbrowning silks was significantly lower than that of ears with browning silks in progeny of the cross (W23 x K55) x K41 (Table 2). For the other two crosses, however, significant differences among browning and nonbrowning silk classes were not detected.

Table 2. Mean CEW tunnel lengths in ears having browning and nonbrowning silks.

Our data indicate that the nonbrowning silk trait is associated with chemical changes detectable with the equipment we used. While this trait might be useful as a selection criterion for CEW resistance in some materials, it would not be a generally applicable criterion for such resistance.

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