Assimilate export from leaves of chilling-treated seedlings of maize. The path to vein

The structure of the vascular bundles of maize leaves was examined in order to compare it with the changes in 〚14C〛-assimilate export at low temperature. Physiological measurements (photosynthesis, 〚14C〛-assimilate export parameters) were made on two model inbred lines: chilling-tolerant KW 1074 (flint) and chilling-sensitive CM 109 (dent). When the temperature was decreased from 24 to 14 °C, the movement of 〚14C〛-assimilates from the source cell to the phloem loading zone was inhibited more strongly in the dent-type CM 109 line than in the flint-type KW 1074 inbred line. CM 109 seedlings also showed a reduction in radioactivity exported from the 14CO2 feeding area, while KW 1074 seedlings did not. We determined the plasmodesmata frequency on four flint and four dent lines (including KW 1074 and CM 109 lines). This structural analysis indicated that the companion cell/thin-walled sieve tube complex was isolated symplastically from adjoining cells in flint-type inbred lines leaves, but it was not isolated in dent-type inbred lines. We hypothesise that the contribution of the symplastic route of phloem loading, being particularly susceptible to low temperature, might be one of the reasons for the higher chilling-sensitivity of dent genotypes, as compare to flint genotypes.