Yield and grain number of wheat: A correlation or causal relationship?: Authors’ response to “The importance of grain or kernel number in wheat: A reply to Sinclair and Jamieson” by R.A. Fischer

R.A. Fischer offered a spirited defense of the concept that grain number is a basic cause in determining wheat yield. While grain number is often well correlated with yield, we interpret grain number as a consequence of the ability of a crop to accumulate resources, particularly carbon and nitrogen. In this response paper, we clarify points of agreement and disagreement with Fischer. A basic point of agreement is that resource accumulation prior to anthesis is critical to determining grain number. On the other hand, an important point of disagreement is the hypothesis that after anthesis a key regulation of yield formation somehow passes to a dependence on grain number. Experimental evidence shows that yield is fundamentally driven by carbon and nitrogen resource accumulation essentially independent of grain number.