Final grain weight in wheat is not strongly influenced by sugar levels or activities of key starch synthesising enzymes during grain filling

We investigated the extent to which final grain weight in wheat is limited by starch synthesis capacity during grain filling under field conditions. A total of 54 wheat genotypes including elite varieties and landraces was grown in two successive years in fields in the east of England. The weight, water content, sugars, starch and maximum catalytic activities of two enzymes of starch biosynthesis, ADPglucose pyrophosphorylase and soluble starch synthase, were measured during grain filling. The relationships between these variables and the weights and starch contents of mature grains were analysed. Final grain weight showed few or no significant correlations with enzyme activities, sugar levels or starch content during grain filling, or starch content at maturity. We conclude that neither sugar availability nor enzymatic capacity for starch synthesis during grain filling significantly influences final grain weight in our field conditions. We suggest that final grain weight may be largely determined by developmental processes prior to grain filling. Starch accumulation then fills the grain to a physical limit set by developmental processes. This conclusion is in accord with those from previous studies in which source or sink strength has been artificially manipulated.