Grain weight, radiation interception and use efficiency as affected by sink-strength in Mediterranean wheats released from 1940 to 2005

Bread wheat has been frequently characterised as sink-limited during grain filling but the degree of sink-limitation could vary with the contribution of breeding in increasing the number of grains per unit land area. This hypothetical change in the level of sink-limitation due to breeding has been insufficiently documented. Two source-sink manipulation experiments under field conditions with three released cultivars and an advanced breeding line representing important steps in wheat breeding in the Mediterranean area of Spain were conducted in order to quantify whether genetic improvement of grain yield in Mediterranean wheat modified the source-sink balance during grain filling, and how it affected grain weight and post-anthesis photosynthetically active radiation intercepted by the crop (IPAR) and radiation use efficiency (RUE). Average grain weight of control and trimmed spikes during grain filling was not significantly affected by halving the number of growing grains in the two oldest cultivars, but it was significantly increased in the most modern line, and had an intermediate response in the intermediate cultivar. In those cases in which a certain degree of source-limitation during grain growth occurred the magnitude of the response reflected a co-rather than a source-limitation. Considering grains from different positions within the spikes the smaller (distal) grains responded more markedly than the larger (proximal) grains. No differences in post-anthesis IPAR were found between the trimmed and control sub-plots for any of the genotypes. However, trimming the spikes reduced post-anthesis RUE, a fact corroborated by a simultaneous reduction in leaf net photosynthetic rate at noon. It seems that bread wheat breeding has tended to reduce the strength of the sink-limitation during post-anthesis even under Mediterranean conditions, and consequently the most modern line presented a sort of co-limitation.