Early planting of corn (Zea mays L.) is a strategy to avoid excessive heat and drought that often negatively influence grain production during its reproductive phase. An experiment was conducted by imposing very low (day/night, 21/13°C), low (25/17°C), and optimum (29/21°C) temperatures during seed germination and seedling growth stages under optimum moisture and nutrient conditions. Above‐ and belowground growth parameters were assessed at 18 d after seeding. Several root morphological traits were assessed using the WinRHIZO root image analysis system. Corn hybrids varied significantly for many traits measured, particularly plant component weights and root morphological parameters. Principal component analysis (PCA) and total low‐temperature response index (TLTRI) methods were used to categorize corn hybrid tolerance to low temperature and to group corn hybrids as cold tolerant, moderately cold tolerant, moderately cold sensitive, and cold sensitive. Total leaf and root weights and cumulative root length and length per unit volume were the most important morphological traits in describing hybrid tolerance to cold temperature. Based on the TLTRI method, relative scores were provided for each hybrid, which ranged from 22.45 to 29.52 among the hybrids. The hybrids CR8410VT3P, D57VP51, and R22BHR43 were classified as cold sensitive and AR1262, DKC6697, DKC6804, and M2V707 as cold tolerant based on PCA and TLTRI techniques. Based on the relative scores assigned in this study, corn producers could select hybrids to maximize corn production in an early planting production system.

Global food demand has risen continuously because of increasing population with greater food and energy needs. Corn production, however, is constrained by current and possible increased future variability in the weather. Earlier planting is a strategy for U.S. Mid-South corn producers to avoid typical summer droughts. However, planting early will increase the likelihood of seedlings exposure to cold temperatures. The objectives of this study were to evaluate corn hybrids planted when the conditions are desirable followed by low and moderately low temperatures to assess the variability among the vegetative and physiological parameters and to classify hybrids into different cold tolerant groups. Twenty one commercially-grown hybrids were subjected to three day/night temperature treatments; 29/21°C (optimum), 25/17°C (moderately low), and 21/13°C (low) from 15 d after planting (DAP) for plants grown at optimum temperature. Shoot, root, and physiological parameters were measured, 32-34 DAP. Significant differences and interactions were observed among the temperature treatments and hybrids for most of the traits measured. Based on relative scores, developed in this study, AR1262 and P1636YHR were classified as cold tolerant and H68B and ST11504VT3 as cold sensitive. Cold tolerant hybrids and their associated morpho-physiological characteristics may be useful for breeders to develop new hybrids that could withstand low and variable temperatures during seedling growth and developmental period.Key words: cold tolerance, maize, fluorescence, growth indices, photosynthesis, stomatal conductance.Résumé : La demande mondiale d'aliments ne cesse d'augmenter en raison de la croissance démographique, qui accentue les besoins en nourriture et en énergie. Malheureusement, la production de maïs est affectée par la variabilité actuelle du climat, laquelle pourrait s'intensifier dans l'avenir. Les producteurs du centre-sud des États-Unis sèment le maïs hâtivement pour éviter les sécheresses usuelles de l'été. Cependant, les semis précoces augmentent la probabilité que les plantules soient exposées au froid. La présente étude devait évaluer des hybrides plantés à un moment où les conditions étaient favorables, avant une période de fléchissement important ou modéré des températures, cela en vue d'établir la variabilité des paramètres végétatifs et physiologiques de ces hybrides et de les classer en différents groupes en fonction de leur tolérance au froid. Vingt-et-un hybrides commerciaux ont été soumis à trois régimes de températures diurnes/ nocturnes : 29/21°C (régime optimal), 25/17°C (régime modérément bas) et 21/13°C (régime bas), 15 jours après repiquage des plants cultivés à température optimale. Les auteurs ont évalué les pousses, les racines et les paramètres physiologiques entre le 32 e et le 34 e jour suivant la plantation. D'importantes variations et interactions ont été relevées pour la plupart des caractères examinés entre les différents traitements et hybrides. Selon les notes relatives établies dans le cadr...