North African knapweed (Centaurea diluta Aiton) is an annual weed that is widespread in southern Spain and is of increasing concern in dryland cropping systems. Despite its expanding range in Spain, there is limited information on the emergence timing and pattern of this species, knowledge of which is critical for developing more timely and effective management strategies. Therefore, there is a need to develop simple and reliable models to predict the timing and emergence of this annual weed under dryland conditions. A multi-location field experiment was established across Spain in 2016 to 2017 to assess the emergence of C. diluta. At each of 11 locations, seeds were sown in the fall, and emergence was recorded. Overall emergence averaged 39% in the first year across all sites and 11% in the second year. In both years, the main emergence flush occurred at the beginning of the growing season. A three-parameter Weibull function best described seedling emergence of C. diluta. Emergence models were developed based on thermal time (TT) and hydrothermal time (HTT) and showed high predictability, as evidenced by root mean-square error prediction values of 10.8 and 10.7, respectively. Three cardinal points were established for TT and HHT at 0.5, 10, and 35 C for base, optimal, and ceiling temperatures, respectively, while base water potential was estimated at −0.5 MPa.

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This study deals with hybrids between Aegilops geniculata and bread wheat, Triticum aestivum L., detected in two Ae. geniculata populations in the "Meseta Central", Spain's central plateau where wheat is a major crop. Morphological traits and pentaploid chromosome numbers were used to identify hybrids in 2004 and 2005. The frequency of hybridization under natural conditions was calculated for one of these populations. Six hybrids were detected from a sample of 3158 seeds from this population. The hybrids were mainly sterile but seed set can be found. This study extends the range of documented hybridization to include central Spain. The potential risks associated with natural hybridization in the context of transgenic wheat cultivation are discussed.

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Isolation distance is the main barrier to crop-to-crop gene-flow. A 3-year study assessed the maximum potential outcrossing under field conditions between two wheat cultivars (Triticum aestivum L.) and between wheat and durum wheat (Triticum turgidum L. var. durum). Outcrossing was measured by seed set on emasculated recipient plants placed at four sides with different distances from a 3 m · 3 m T. aestivum (cultivar Chinese Spring) pollen source. Frequencies of seed set at 0 m distance were 45% (37-56%) for T. aestivum cultivars and 18% (5-30%) with T. turgidum. These values agree with hybridization in non-limiting pollen conditions measured by manual crosses in greenhouse. The number of pollen grains and the outcrossing frequencies decreased at increasing distances influenced by the prevailing wind direction. Under semiarid conditions of this assay, viable pollen was found 14 m from the pollen source, with a maximum distance of 8 m at which crosspollination decreases below 1%. Ambient conditions affect pollen viability, hybridization and pollen dispersal. Data presented in this paper emphasize the major role played by environmental conditions in outcrossing. Data obtained in one area may therefore not coincide with the prevailing situation in different locations and climates.