ABSTRACT
Rates of phloem unloading are an important component and offers a further point of potential regulation of photoassimilate movement. Regulation and mechanisms of phloem unloading vary between species, developmental stage and sink function. While there are no direct measures of phloem loading and unloading. Considerable indirect evidence that phloem unloading capacity is exceeded by photosynthesis and phloem loading particularly under optimal conditions. Thus, using the high pressure manifold model as proposed, resource partitioning between sinks is finely regulated by their relative hydraulic conductance of plasmodesmata linking sieve element /companion cell complexes with the surrounding phloem parenchyma cells.
Sink intensity (i.e. the competitive ability to accumulate photoassimilate per unit mass) was very high prior to anthesis of both setting and abscising flowers. Sink intensity then became very low for the first three days following anthesis after which it recovered in normally setting flowers, but failed to recover in normally abscising flowers. It is concluded that soybean reproductive abscission is determined at or very near the day of anthesis.
Studies indicated that when equilibrium is broken between sources and sinks in individual soybean plant through pods or leaf removal, the direction of assimilate transport is changed. A positive correlation between leaf area and seed weight across the main axis in soybean was reported, and was defined as source-sink parallelism.
Thus, findings suggest soybean plants are able to transport and use assimilates as far as 8 nodes distance. The successful translocation of assimilate from lower nodes is mainly used for pod formation and seed filling over a short distance, but can be translocated over a long distance and used for growth of remaining seed. This suggests the reproductive sink of soybean plants has an internal mechanism to off-set yield loss to ensure seed survival. Sink capacity is like a process of photoassimilate accumulation in a container; high volume for accumulation can accept a large amount of photoassimilate, and this results in high seed yield. In soybean both source and sink may be limiting and affect the seed yield. Thus, soybean is known as co-limited crop plant. But in common soybean crop shows source limited until environment is of disadvantage.
