ASSIMILATE TRANSPORT IN MAIZE (ZEA-MAYS L.) SEEDLINGS AT VERTICAL LOW-TEMPERATURE GRADIENTS IN THE ROOT-ZONE

Even moderate chilling temperatures may cause important modifications in assimilate movement in maize seedlings from the shoot to the roots, but there is no information on long-distance transport of assimilates in plants subjected to vertical gradients of moderately low temperatu res in the root zone. Seedlings of a chilling-tolerant (KW1074) and a chilling-sensitive inbred line (CM109) of maize were grown in a system that allowed the maintenance of temperature gradients between the top soil (0-10 cm) and the subsoil (10-50 cm). After pregrowth at 24 degre es C until the third-leaf stage, plants were subjected to chilling-str ess regimes for 6 d (17/17/17 degrees C, 17/17/12 degrees C, 12/12/12 degrees C, 12/12/17 degrees C, air/topsoil/subsoil). The time taken fo r the assimilates to enter the phloem from the second leaf increased a t low temperatures for both lines, but to a much greater extent in CM1 09. Although mainly influenced by air and topsoil temperature, low tem perature in the subsoil also affected this trait in CM109. The speed o f assimilate transport between the second leaf and the mesocotyl in KW 1074 was strongly reduced by cool temperatures in the shoot and topsoi l as well as by 12 degrees C in the subsoil in CM109, because the latt er line had a larger portion of its root system in the subsoil as comp ared to KW1074, The portion of assimilates allocated to the root decre ased at low temperatures in both lines, but to a greater extent in CM1 09, and was controlled mostly by the subsoil temperature. After rewarm ing, values of all measured parameters of assimilate transport returne d to near pregrowth levels within a few days.