| Abstract: | Yield studies show that increases in grain yield are always accompanied by an increase in grain number and, hence, further increases in yield potential may require additional improvements in grain number. The improvement of modern durum wheat was mainly based on the introduction of semidwarf genes. A 2‐year field drought stress experiment, concerning two different genotype groups (landraces vs modern cultivars), was carried out under a rainout shelter in order (a) to assess the effect of water deficit on floret dynamics and grain number determination, (b) to explore the relationship between plant water status with grain number per spike and its components (i.e., spikelets per spike, fertile florets per spikelet and grain set) and (c) to quantify the importance of several plant traits in determining the final number of grains per spike and fertile florets per spike when the main source of variation is water availability. Compared to control (well irrigated), grain number per spike was reduced, depending on year, genotype and water availability level, by 12.4–58.7% and this reduction was evident almost in all spikelet positions along the spike. Although there were some doubts in the past about the increased sensitivity of semidwarf cultivars to drought stress, they were not confirmed from our results. In most of the cases, the variation in plant water status (by means of water potential index [WPI]) during floret primordia phase (FPP) explained most of the variance in grain number per spike, fertile florets per spikelet, grain set and fertile spikelets per spike. In general, increasing water stress intensity decreased grain number per spike by an average rate of 13.5 and 9.4 grains per 0.2 MPa decrease in WPI, in modern cultivars and landraces, respectively. However, seasonal and genotypic effects were evident by modifying the slopes of the linear regressions between WPI and the studied plant traits. Commonality analysis revealed that the number of fertile florets per spikelet was the best predictor of grain number per spike, indicating that there is still much space for further improvement for this trait in landraces. However, this trait has been clearly improved in modern cultivars, especially at the basal and central spikelets. Although the number of spikelets per spike was the best unique predictor of the number of grains per spike in modern cultivars, grain set presented the highest total effect. |
