Nitrogen fertilization levels and timing affect the plasticity of yield components in bread wheat (Triticum aestivum L.)

The plasticity of grain yield components typically follows a hierarchy, with early-forming components having greater plasticity than late components (tiller number > spike number > grain number > grain weight). The first aim of this study was to assess the plasticity of two previously unexplored yield sub-components in wheat, i.e. spikelets per spike and grains per spikelet, and of other yield-related variables, i.e. straw mass, plant height, ovary mass and harvest index. The second aim was to test the potential limits of the phenotypic plasticity of yield components, and the relationships between them, by applying sudden strong changes in resource availability (via nitrogen fertilization)at different times during the crop cycle. Nitrogen (N) fertilization was applied only early (60 kg ha-1 of N, early tillering) or only late (120 kg ha-1, early stem elongation), very late (120 kg ha-1, late stem elongation) or extremely late (120 kg ha-1, pollination), never (N0) or multiple times (total of 300 kg ha-1) during the growth cycle of both a small-fruited (Bologna) and a large-fruited (Bora) bread wheat (Triticum aestivum L.) cultivar. Yield and grain number increased with fertilization levels in both cultivars, with the 300 kg N ha-1 treatment having the highest yield. Grain weight also increased with fertilization levels, but only when considering different N levels applied early.Delaying fertilization reduced yield, but increased grain weight, creating a tradeoff between yield (and grain number) and grain weight. Similarly, the relationships between other yield components and variables varied when considering N timing or levels. The plasticity of yield com-ponents, expressed as total variation of the trait over the trait value for the N0 (i.e. control) treatment, was 26.5%, 30.5%, 61%, 75% and 22%, respectively for tiller number, spike number, spikelets/spike, grains/spikelet and grain weight. The plasticity of grains/spike, grain number, yield, straw biomass, plant height, ovary mass and harvest index, was 174%, 236% and 268%, 417%, 54%, 54% and 36.5%, respectively. The results show that the plasticity of yield components (and other variables) may differ with different sources of environmental variation, explaining why, in the literature, two traits may be found to correlate both positively, negatively (tradeoff) or not at all. These results further our knowledge on the plasticity of, and the relationships among, yield components, and may be useful for further improvements of yield with agronomic intervention or breeding.