Shoot-to-root relations in field-grown maize seedlings

In northern latitudes, maize (Zea mays L.) seedling growth is generally limited by low temperatures. A better knowledge of mechanisms limiting maize seedling growth at suboptimal temperatures is needed for improving growth models and, ultimately, for breeding of genotypes with improved vegetative growth at low temperatures. Shoot-to-root ratios are known to be sensitive to suboptimal temperatures, but information for held conditions is scarce. In three environments with mean soil temperatures of 14.6, 16.9, and 18.0 degrees C at the 5-cm depth, we investigated shoot-to-root relations at the 2-, 3- and 4-leaf stages in two maize inbred lines differing in chilling tolerance. Environment characteristically influenced temporal patterns of the shoot-to-root dry matter ratio (SDM/RDM), leaf area to root length ratio (LA/RL), and leaf area to root surface area ratio (LA/RSA). The chilling-sensitive Penjalinan showed close LA/RL and LA/RSA ratios only in the warmest environment, whereas the same traits in chilling-tolerant Z 7 were almost unaffected by environment, mainly due to Z 7's temperature-stable root development. Since root dry matter of Penjalinan was not affected by environments, the inefficient conversion of dry matter into root length and root surface area was responsible for the unbalanced relationship between shoot and root of this genotype at suboptimal temperatures. Our results indicate that already relatively small reductions of 2 to 3 degrees C in soil temperatures near 15 degrees C may distinctly reduce root development in chilling-sensitive maize seedlings under held conditions.