Lack of neighbor perception in soft wheat increases weed-induced yield losses.

Weeds are one of the major constraints for wheat productivity, causing significant yield losses worldwide. While chemical control is the most used practice to overcome weed damage, its efficacy is challenged by increasing weed resistance to most used herbicides, which is an expanding phenomenon caused by herbicide overuse/misuse. Modern wheat varieties are less able to perceive the presence of weeds than old varieties and are therefore less competitive against them and require chemical control to ensure adequate yields. The low competitiveness of modern wheat varieties toward weeds becomes even more critical under organic farming, where chemical weeding is not allowed. The goal of this study was to evaluate the competitiveness of two wheat genetic resources, namely Rebelde, that is a modern-day cultivar and Frassineto, an accession from a Seed Bank (Banca Regionale del Germoplasma of the Regione Campania - Southern Italy). Frassineto is a landrace deriving from older varieties with higher plant height. Overall, our goal was to assess the different competitiveness of two contrasting wheat varieties differing in breeding periods (old vs modern), constitutive plant height (tall vs short), and neighbor perception (sensitive vs insensitive). Here we demonstrate that the landrace Frassineto responds to weed presence in terms of stem elongation (+46 %), increased tiller angle (+27 %), and by altering leaf total chlorophyll, chlorophyll a/b ratio (-29 %) and carotenoids (-71 %). These responses, typically linked to plant perception of altered red:far red light, were absent in Rebelde. Moreover, Frassineto showed faster growth at early phenological stages compared to Rebelde (+103 % at the tillering stage), which represents an important constitutive trait of competition. Applications of gibberellic acid, which promotes longitudinal growth in response to light, caused significant stem elongation in Frassineto (+14.3 % at 30 μM and 19.6 % at 100 μM), whereas it did not in Rebelde. Similarly, the gene expression of the Phytochrome Interactive Factor (PIF), involved in plant perception of red:far red ratio, was significantly upregulated in Frassineto (+46 %) but not in Rebelde. Altogether these responses were correlated with higher suppressive ability against weeds in Frassinato vs. Rebelde in the field and consequent higher yield stability (+198 %). These results provide important insights into those traits that should be strengthened for the development of competitive wheat varieties for a weed-resilient agro-ecosystem.