The double-edge sword of heterogeneous ripening pattern in winter wheat cultivar mixtures: A case study under post-anthesis water stress

Wheat cultivar mixtures with heterogeneous phenology has a less-explored potential to improve crop diversity, yield stability, and agronomic features particularly in response to the currently increased environmental stresses and uncertainties. To investigate the option of using wheat cultivar mixtures with different ripening patterns for mitigating the adverse effects of post-anthesis water stress, a two-year field experiment was conducted during 2014-15 and 2015-16 growing seasons at the research field of School of Agriculture, Shiraz University, Iran. The factorial experiment was a Randomized Complete Block Design with 3 replicates, in which 15 mixture treatments including monocultures and every 11 possible mixtures of four early- to middle-ripening wheat cultivars were grown under two normal and post-anthesis deficit-irrigation conditions. Measured traits and estimated indices included grain yield and its components, canopy temperature, soil water content, water productivity, susceptibility index, and water use efficiency. The results indicated that under the stressful condition of post-anthesis deficit-irrigation, heterogeneity in the ripening pattern of mixtures was declined. Consequently, dissimilarities in grain yields as well as various agronomic characters of mixture treatments were also lessened. This may be an evidence for the negative effect of water shortage stress on heterogeneity within agroecosystems. Although cultivar mixtures showed some casual advantages in some traits, such beneficial effects were not consistent across all conditions. Moreover, no cultivar mixture produced higher grain yield than the maximum monoculture. Despite the general expectation for beneficial ecological services from cultivar mixtures, in many cases disadvantageous blends were found which led to a considerable reduction in grain yield and water productivity. Therefore, it is suggested that unless the performance, and preferably the involved mechanisms, of cultivar mixtures are not fully understood, use of blends as an alternative for conventional high-input wheat cropping systems may lead to adverse results.