Flowering Phenology and Synchrony between Volunteer and Cropped Spring Wheat: Implications for Pollen‐Mediated Gene Flow

Abstract

Genetically engineered (GE) wheat (Triticum aestivum L.) volunteers could present a problem in cultivated wheat because they may facilitate movement of a GE trait to other volunteers or non‐GE wheat crops. However, volunteers can emerge periodically throughout the growing season and, thus, flowering overlap with the crop may be largely asynchronous, presenting a significant barrier to gene flow. Field experiments were initiated to determine the influence of volunteer wheat emerging at various densities and emergence times relative to the crop on flowering phenology and synchrony between cropped and volunteer spring wheat populations. Using a novel analytical method that treats individual flowering curves as a series of probability density functions, we characterized flowering phenology and quantified flowering synchrony between cropped and volunteer spring wheat. The results indicated that volunteer wheat initiated and ceased flowering earlier when emergence occurred before crop emergence. Thermal time requirements for completion of 5, 50, and 75% flowering decreased as volunteer emergence was progressively delayed. Flowering was further accelerated when emergence coincided with the crop and, consequently, flowering synchrony between volunteer and crop plants was significantly higher. Levels of flowering synchrony peaked at 86% and were highly dependent on volunteer emergence time relative to the crop. Our results suggest that pollen‐mediated gene flow (PMGF) in spring wheat will be dependent on emergence timing of volunteers and, thus, if hybridization between cultivated wheat and volunteers or neighboring wheat crops is going to transpire, it will most likely occur when volunteer emergence occurs within a hybridization window of 75 growing degree‐days on either side of crop emergence. Coupled with seed‐mediated gene flow, the additional admixture caused by PMGF could be problematic for coexistence between GE and non‐GE wheat.

Low crop plant population densities promote pollen-mediated gene flow in spring wheat (Triticum aestivum L.)

“…However, the merits and limitations of pairedpollinator rows versus random, heterogeneous plant distributions remain unknown with regards to PMGF, and further investigation is certainly warranted. Paired-pollinator rows have, nevertheless, been utilized in several previous studies investigating flowering phenology and gene flow in wheat (Martin 1990;Hucl 1996;Willenborg et al 2009). …”

Identification of a hybridization window that facilitates sizeable reductions of pollen-mediated gene flow in spring wheat

“…1). The greatest flowering synchrony in all years and locations was obtained when volunteer emergence coincided with the crop, but there was always at least 1 day of flowering overlap between volunteer and cropped wheat populations when volunteers emerged within 150 GDD of the wheat crop (Willenborg et al 2009b). A total of 545,407 wheat seedlings were screened for hybridization among both locations and years.…”

“…However, spring wheat phyllochron (time between elongation of successive leaves) intervals range between 80 and 100 GDD (Bauer et al 1984;Shirtliffe et al 2000) and therefore, we can expect wheat plants possessing one more or one less leaf to have emerged (and probably flower) within the hybridization window (Willenborg et al 2009b). …”

“…Volunteer wheat is an abundant weed in western Canada and can emerge either in a single cohort early in the growing season (De Corby et al 2007) or in multiple cohorts spanning the length of the growing season (Anderson and Soper 2003;Harker et al 2005). Willenborg et al (2009b) demonstrated that emergence timing of volunteers was fundamental to flowering synchrony between cropped and volunteer wheat populations, and it is highly probable that similar effects will be evident for PMGF. Towards this end, we designed a field experiment with two main objectives: (1) to determine whether variable emergence timing of volunteer spring wheat could temporally isolate conspecific volunteer and cropped spring wheat populations, thus minimizing PMGF (2) to identify a hybridization window in spring wheat crops based on the emergence timing of volunteers and the resulting temporal isolation.…”

Crop Genotype and Plant Population Density Impact Flowering Phenology and Synchrony between Cropped and Volunteer Spring Wheat

“…Because wheat can flower in as few as 3–5 d (Hucl, 1996) and volunteer wheat plants will emerge and flower within wheat crops, flowering asynchrony with volunteer populations may be an important barrier to gene dispersal between cropped and volunteer wheat populations. Willenborg et al (2009) showed that volunteer wheat emerging after a spring wheat crop exhibited considerable flowering synchrony with the crop. However, volunteer plants emerging after a spring wheat crop cannot be selectively removed from the crop, and thus present a challenge with regard to minimizing PMGF.…”