ABSTRACT

High crop yields are determined by ability of plants to produce high levels of photoassimilate and/or to partition large proportions of carbohydrate efficiently into harvested organs (Daie, 1985; Faville et al., 1999). Assimilate-producing plant parts such as leaves are known as the source, and plant parts to which assimilate is trans­ located such as grains and fruits are known as the sink. Thus, sourcesink relationships may be defined as relationships between plant parts in which one part serves as the producer of materials (source) translocated to other plant parts where materials accumulate or are consumed (sink) (CSS A, 1992). Limitations to grain yields of cereals that cannot be explained by unfavorable factors such as diseases, de­ ficiencies or toxicities of nutrients and elements, or water disorders may be assessed for potential assimilate translocation to developing grains (source capacity) and potential to accumulate assimilates (sink capacity) (Uhart and Andrade, 1995). Source-sink relationships are important for determining quantity of crop yields. Several studies have shown maize grain yield to be limited by sink capacity (Golds­ worthy and Coleogrove, 1974; Allison, Wilson, and Williams, 1975; Barnett and Pearce, 1983; Jones and Simmons, 1983). In contrast, Tollenaar and Daynard (1978) noted source limitations for yield potentials at high latitudes, and Uhart and Andrade (1991) reported limitations by both source and sink capacities at cool temperatures. Changes in source/sink ratios affect C partitioning during grain fill­ ing so that carbohydrate accumulates in stems when sinks are limit­ ing and carbohydrates are remobilized when sources are limiting (Jones and Simmons, 1983; Uhart and Andrade, 1991). Nitrogen

partitioning can also be affected by source/sink ratios during grain filling. Greater leaf and stem N concentrations occur when sinks are limiting (Christensen, Below, and Hageman, 1981; Reed et al., 1988), and greater vegetative N remobilization generally occurs when sources are limiting (Reed et al., 1988).