SINK STRENGTH AS A DETERMINANT OF DRY-MATTER PARTITIONING IN THE WHOLE-PLANT

Dry matter partitioning is the end result of the flow of assimilates f rom source organs via a transport path to the sink organs. The dry mat ter partitioning among the sinks of a plant is primarily regulated by the sinks themselves. The effect of source strength on dry matter part itioning is often not a direct one, but indirect via the formation of sink organs. Although the translocation rate of assimilates may depend on the transport path, the transport path is only of minor importance for the regulation of dry matter partitioning at the whole plant leve l. To understand the regulation of dry matter partitioning by the sink s, a parameter like sink strength is needed that describes a sink's ab ility to influence assimilate import and is independent of the rest of the plant. The term sink strength can be defined as the competitive a bility of an organ to attract assimilates. However, there is much deba te and confusion about the term sink strength because this term is oft en not clearly defined. Sink strength has been proposed to be the prod uct of sink size and sink activity. Although cell number is often cons idered as a suitable measure of sink size, it appears not always to be an important determinant of sink size. Moreover, sink strength may de pend on sink age rather than sink size. A model for dry matter partiti oning into generative plant parts, which is based on sink strengths of the organs, is described. The potential growth rate (potential capaci ty to accumulate assimilates) has been shown to be an important parame ter that quantitatively reflects the sink strength of an organ. The po tential growth rates of the plant's organs are not static but change d ynamically. the potential growth rate of a fruit is a function of both its age and temperature. For several crops it has been shown that the dry matter partitioning into an organ can be quantitatively described as a function of its potential growth rate relative to that of the ot her plant organs.