The Roles of Sink Size and Location in the Partitioning of Assimilates in Wheat Ears

Abstract

Geometric factors influencing the partitioning of 14C-labelled photosynthate between two sinks were examined in wheat ears. An awn on one spikelet was the only source of current assimilate while grains in two other spikelets were the competing sinks. The number of grains in these and their distance and vascular connection to the source were varied, while access to stem reserves could be cut off by heat ringing. Even this apparently simple experimental system presented a number of complications, such as a bias in favour of 14C movement to the upper spikelets within an ear and to the upper grains within a spikelet. These are considered before results on the effect of relative size, distance and vascular connection are described. The larger sink obtained more than its pro-rata share of 14C, usually more than the square of its size relative to the smaller sink. Partitioning between sinks of differing distance was in proportion to 1/d�, where d is the relative distance of the two sinks from the source. A sink on the opposite side of the ear received only one-tenth to one-thirtieth as much as a comparable sink on the same side as the source and the competing sink.