Understanding the patterns of source-sink relationships in wheat under field conditions is crucial to determine the physiological factors limiting grain filling. The aim of this investigation was to assess the time-course of source-sink interactions in wheat after anthesis under Cuban field conditions. For this purpose, plants of wheat, cultivar Cuba C-204, were sown in the field at six dates with contrasting temperature conditions. The duration of grain filling, patterns of partitioning of C-14-photoassimilates between organs and incorporation of the label into starch in the kernels 24 h after feeding the flag leaf blade with (CO2)-C-14, were investigated to evaluate source-sink balance. The proportion of C-14 converted into starch in the kernels increased during the first 10 days of grain filling. Throughout the grain filling phase the proportion of total C-14 fixed partitioned to the grains increased, whereas that allocated to the internodes decreased. At the beginning of grain filling, the patterns of C-14-photoassimilate partitioning differed among sowing dates. Additionally, the proportion of C-14 incorporated into starch in the kernels correlated positively with the proportion of the label partitioned to the grains, but negatively with C-14 accumulation in the internodes. This supports that the accumulation of C-14 in the stem was closely related to the uptake capacity of the grains. The lack of such a relationship at later stages in grain development indicates a change from sink-limited to source-limited situation early after anthesis. In conclusion, the supply of photoassimilates under field conditions in Cuba limits grain filling, except the first days after anthesis, when it was sink-limited. (C) 2001 Elsevier Science B.V. All rights reserved.
