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Abstract
In 1972, a full-sib recurrent selection program for improved cold tolerance was initiated in the maize (Zea mays L.) synthetic population NS, a Nebraska version of Iowa Stiff-Stalk Synthetic. Selection was based primarily on percent emergence and visual vigor-rating under controlled (11.1/4.4$\sp\circ$C day/night) conditions. Visual selection was also practiced for agronomic traits. After an early freeze in the 1976 nursery, selection was initiated to form a freeze-tolerant subpopulation.
The first objective of this study was to evaluate progress from selection for cold tolerance, and to examine changes in agronomic traits. Materials evaluated included cycles 0, 4, 8, and 12 of the cold-tolerant population (NS-CT-C0, C4, C8, and C12) and cycles 8 and 12 of the freeze-tolerant subpopulation (NS-CT-FT-C8 and C12). In general, much more improvement in cold tolerance was noted in the growth chamber than in the field. Grain yield of NS-CT-C12 and NS-CT-FT-C12 was increased by 1.10 and 0.85 Mg ha$\sp{-1}$, respectively, relative to NS-CT-CO. The NS-CT and NS-CT-FT selections also flowered earlier, were lower in plant and ear height, and had lower grain moisture at harvest than NS-CT-C0.
The second objective was to measure genotypic variation for cold tolerance in the aforementioned entries, under cool conditions in the growth chamber and in an early planting in the field. In general, NS-CT-C0 displayed more genotypic variation for cold tolerance than did the selected populations. Selection appeared to have acted in eliminating highly susceptible genotypes in the early cycles. Genetic associations among cold tolerance traits were generally low, indicating that they are controlled by different genetic systems. Correspondence between performance in the growth chamber and in the field was low.
The third objective was to evaluate freeze tolerance of the above entries in the field and in the growth chamber. No appreciable level of freeze tolerance was noted in either environment. Nor was any differential regrowth after freezing noted. Much more damage was caused to plants by temperatures of $-$3.0 versus $-$2.5$\sp\circ$C.
