Effects on the Stem of Winter Barley of Manipulating the Source and Sink during Grain-filling

Source and sink activities of field-grown barley plants were manipulated by shading the whole plant or by de-earing at various times after anthesis. The changes in the masses of the internodes, ears, grains and leaves were followed until the grain matured. Shading reduced the mass of ears and grains and, in addition, shading to 45% of incident radiation from anthesis prevented mature internodes from increasing in mass. Severe shading (to 9% of incident radiation) from 11 d after anthesis caused the normal decrease in mass of internodes to occur earlier than in unshaded plants. Thus, only assimilate in excess of that required for tructural growth was stored in internodes. Lower internodes of plants shaded from anthesis lost more of the mass that had been accumulated prior to anthesis than control plants, giving a greater potential pre-anthesis contribution to grain yield. Internodes from plants that were de-eared accumulated more mass than internodes of control plants in one year but not in another, i.e. the potential for internodes to store assimilate appears to be exploited in some years but not others. The mass of internodes from de-eared plants either decreased after that of control plants or their mass stayed constant. De-eared plants had an increased number of tillers compared with control plants; these tillers could have been an alternative sink for assimilate stored in the internodes of de-eared plants. Mobilization of assimilate from internodes is in response to demand for assimilate from grains being in excess of current photosynthesis.