Sterility of basal spikelets in wheat: predetermined fate or a matter of resources?

Grains per spike in wheat (Triticum aestivum L.) are not uniformly distributed. Basal spikelets are typically sterile (or rudimentary) under common agronomic field conditions leading to the assumption that this is a constitutive trait. Yet, it is unknown whether high resource availability per plant could prevent floret mortality in basal spikelets. We present data from two field experiments assessing spike fertility in the four most basal spikelets of spikes from the main shoot and tillers. A collection of genotypes with different years of release and tillering habits were grown under contrasting sowing densities (agronomic densities versus isolated plants) to generate a condition of high assimilate supply to the spike. Under agronomic densities, the four most basal positions presented sterility in almost all cases in the main shoot spikes and without exception in the tiller spikes. When plants were isolated in each experiment, sterility in basal spikelets was absent and less frequent in spikes from the main shoot and tillers. Moreover, in the isolated condition, modern genotypes had a higher number of grains per spikelet than their older counterparts, whereas genotypes with low tillering habit showed the least response in spikelet fertility. These results indicate that while the predetermined lanceolate shape of spikes leads to the sterility of basal spikelets at high densities, this sterility can be reversed if assimilates become more available. Thus, sterility of basal spikelets in common agronomic conditions is not a constitutive condition of the spike.