Several reports in the literature indicate a temperature component in the expression of virescent traits in maize. Phinney and Kay (Hilgardia 23:185, 1954) reported that the rate of greening for a virescent mutant, pale-yellow-1, was inversely related to temperature. In the most extensive study to date, Millerd and McWilliam (Plant Physiol. 43:1967, 1968) reported the inability of the mutant M11 to green at temperatures below 17 C; at 27 C, the M11 mutant greened as well as normal seedlings.

We have examined five virescent lines in order to define their temperature characteristics. The five lines are v, v3, v16 and v18, all in the Ohio 43 background, and v12 in an unknown inbred background. Greening responses were tested at six different temperatures ranging from 19 to 30 C. Greening was assayed by the in vivo technique previously described (Hopkins, Hayden and Walden, Can. J. Bot. 53:2720, 1975). In order to compensate for differences in the rate of seedling development at different temperatures, seedlings were assayed at the midpoint of the second leaf when that leaf was 5 to 7 cm in length.

With the exception of v, all mutants exhibited a threshold temperature below which greening did not occur (Table 1). Above the threshold, greening was a linear function of temperature. Greening of the wild-type was not influenced by temperature over the range tested. The extent of greening by v, v12 and v16 at 30 C was within 10 percent of the wild-type; greening of v18 at 30 C was only 25 percent of the wild-type and increased to only 50 percent at 35 C. The one mutant which did not appear to be markedly influenced by temperature was v. These seedlings exhibited considerable variability, greening to about 25 to 50 percent of the wild-type over the temperature range tested, but with no threshold.

Table 1. Threshold temperatures for greening of virescent mutants.
 

Genotype	Threshold Temperature (C)
v3/v3	23
v12/v12	20
v16/v16	25
v18/v18	22

Several temperature-shift experiments were conducted with v16. From these experiments we determined that at least three days at the lower temperature were required to produce a significant depression in chlorophyll content. When seedlings were transferred from 30 to 20 C, only those tissues which expanded at the lower temperature were bleached. There was no loss of chlorophyll in tissues which had already greened at the higher temperature. Following transfer from 20 C to 30 C, the newly expanded tissues greened but tissue which had been bleached at the lower temperature showed at best only a moderate tendency to green and then only in the region of the mid-vein.

The temperature characteristics described here correlate closely with our qualitative observations in the field. Those lines which show lower threshold temperatures generally green more rapidly in the field than do those with higher thresholds. The v18, which in laboratory trials never greened more than 50 percent of the wild-type, grows more slowly in the field and at maturity is of smaller stature than the wild-type or other virescent lines.

From these results we conclude that temperature sensitivity is a principal component in the expression of the virescent trait and that the response is allele specific.

W. G. Hopkins and D. B. Walden

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