Genetic Variation for Chilling Stress and Spectral Reflectance in Short-Season Maize

Increasing early season cold tolerance of maize (Zea mays L.) has the potential to lengthen its growing season and enhance yields. We evaluated cold tolerance across 44 inbred and 102 hybrid genotypes in field and controlled environment experiments. Cold tolerance was quantified as the proportion of dry weight (PDW) of plants subjected to cold temperatures or planted early, relative to dry weight of plants grown in control temperatures or planted late. Also, leaf reflectance spectroscopy was used as an alternate indicator of cold stress response. In field experiments, significant effects of genotype, planting date, environment and their interactions were detected for PDW; the mean dry weight per plant for the early planting was considerably lower than for the late planting in five field environments. Cold stress exposure resulted in higher leaf reflectance in the 500-680 nm region and lower reflectance between 758-1000 nm. We identified novel reflectance predictors, both normalized difference indices and specific wavelengths of first derivative reflectance spectra, that correlated with PDW. In some cases the strongest correlations were found when reflectance data came from unstressed plants. Multiple regression models combining first derivative reflectance at multiple wavelengths, measured on plants not exposed to cold stress, provided the best correlation with PDW; however in general these models were unique to an experiment. Genetic control and heritability of plant dry weight and leaf spectral reflectance under control and early planting conditions was investigated in the field using a North Carolina Design II mating experiment. Consistent general combining ability estimates from early and late planting indicated that additive and dominant gene effects explained hybrid biomass variation across both planting dates. A number of spectral traits were heritable, but parental contributions to hybrid values were moderate. In conclusion, we find that parents explain at most moderate levels of hybrid spectral reflectance variation, across all spectral analysis tools. Thus, selection of traits such as hybrid biomass using spectral reflectance will be inefficient. Nonetheless, we find spectral reflectances stably distinguish maize genotypes within a population irrespective of temperature treatments, so selecting inbred lines using spectral reflectance may advance breeding efforts.