Understanding Source-Sink Dynamics in Wheat
Wheat is generally known as sink-limitation during the grain-filling period. First, let’s explore the definition of source-sink limitation. The source-sink relationship and the regulation of carbon allocation are determining factors in crop yield. The growth of crops can be restricted by the assimilate availability in specific crop organs or by the ability of those organs to utilize assimilates. In the former case, we refer to it as source-limitation, and in the latter case, it is known as sink-limitation.
The causes of source-limitation are as follows:
- Supply limitation: Occurs due to insufficient assimilate availability (low assimilate production).
- Transport limitation: Involves reduced flow (translocation) of assimilates towards the sink (e.g., grains`) that require them, often due to long-distance transport constraints or assimilate flow resistance.
- Competition limitation: Arises from competition among sinks (e.g., grains).
In fact, yield is subject to both source-sink limitations simultaneously. This is because not all sinks can pull assimilates equally, and the source cannot distribute assimilates evenly to all sinks. Somewhere along the line, there will be an imbalance. Therefore, it is correct to understand source-sink limitation as an inevitable factor in crop growth.
Although wheat has been known as sink-limitation during the grain-filling period, recent investigations have revealed relatively lower sink-limitation in newly bred varieties. The source-sink ratio during the grain-filling period has been consistently decreasing in wheat cultivars developed by breeders. As a result, although recent wheat varieties still experience more constraints from sinks than sources during the grain-filling period, they exhibit a relatively lower source-sink ratio when compared to past varieties.
Source-sink ratio = the ability of the leaves to produce photosynthate VS the capacity of the grain to accommodate the assimilates
This means that in recent wheat varieties, the ability of grains to assimilate resources is greater than the ability of leaves to accumulate those resources. This research conclusion underscores that grain number is a key determinant of yield and a crucial factor in crop growth after flowering. Adjusting the source-sink ratio is often achieved through the manipulation of source and sink strength. For instance, removing leaves reduces source strength, while removing seeds reduces sink strength.
- source strength = determined by assimilate supply of green leaves
- sink strength = the capacity of organs to store assimilates
Introducing past research findings, in the case of modern cultivars, when sink manipulation was applied (e.g., removing half of the grains from wheat spikes), the average grain weight increased by 50%. Furthermore, one of the research results from Argentina showed that when source manipulation was performed after flowering (e.g., removing leaves), the weight of grains located at specific positions within the spikelet decreased slightly but noticeably.
These results contradict the characteristics of sink-limitation in wheat during the grain-filling period described earlier. If wheat were indeed sink-limited, then changes in seed weight should have been minimal during source-sink regulation, regardless of whether assimilates increased (sink manipulation) or decreased (source manipulation). In other words, grain weight should respond minimally to changes in assimilate quantity, whether it increased (sink manipulation) or decreased (source manipulation).
According to the research by Acreche & Slafer (2009), historical wheat varieties mainly exhibited a low response to source-sink ratio adjustments. However, in the case of modern cultivars, when sink manipulation (i.e., de-graining) was carried out, there was an increase of approximately 33% in grain weight. This cannot be explained solely by sink-limitation.
In the case of historical varieties, wheat breeding aimed to increase the degree of source-limitation during the post-anthesis period. On the other hand, for recent varieties, it appears that breeding aimed to increase co-limitation. According to the research results of Slafer and Savin (1994b) and Borras et al. (2004), wheat seed growth during the grain-filling period is subject to sink-limitation or co-limitation, and it is not solely source-limited. In other words, sink-limitation is much stronger than source limitation.
The graph above illustrates the relationship between average seed weight and the potential grain weight of those seeds. Potential grain weight represents the maximum weight a seed can achieve. If the data points are located above the 2:1 line, it is an example of source limitation. In other words, it means that the seed’s current weight could increase by up to twice its current weight when assimilate availability increases. In this case, seed weight is influenced by the source. On the other hand, when the data points align with the 1:1 line, it is an example of sink limitation. This means that the seeds already have the maximum weight they can attain, and providing more assimilates during the grain-filling period will not increase their weight. When the data points are above the 1.5:1 line, we can interpret it as co-limitation, where both source and sink factors contribute to seed weight.
[Note 1] This theory can be applied practically as follows: Suppose a fertilizer company is visiting farms during the grain-filling period and promoting their products by saying, "Using our product can increase grain weight and yield." From a wheat physiological perspective, this statement is incorrect. Wheat experiences sink limitation during the grain-filling period, meaning that even with more nutrients supplied, grain weight changes are minimal. Moreover, the key to wheat yield is the number of grains rather than grain weight. So, to increase the number of grains, the product should be applied earlier, specifically before the flowering period when spikelet development occurs. The fertilizer company, in this case, has misunderstanding of how assimilates affect grain weight during the grain-filling period and an incorrect understanding of the critical components of wheat yield.
Therefore, based on the comparison of the increase in resource availability for growing grain and the change in seed weight due to de-graining, the research results of Acreche and Slafer (2009) suggest that recent wheat varieties can be considered both sink- and source-limited simultaneously during the grain-filling period.
These findings align with the results of Kruk et al. (1997) and Shearman et al. (2005), indicating that historical varieties are generally more sink-limited, while recent varieties experience limitations in both source and sink strength during the grain-filling period.
Reference
Acreche, M.M. and Slafer, G.A., 2009. Grain weight, radiation interception and use efficiency as affected by sink-strength in Mediterranean wheats released from 1940 to 2005. Field Crops Research, 110(2), pp.98-105.
Dejong, T.M. and Grossman, Y.L., 1995. Quantifying sink and source limitations on dry matter partitioning to fruit growth in peach trees. Physiologia Plantarum, 95(3), pp.437-443.
