Contribution of groundkeepers vs. weed beet to gene escape from sugar beet (Beta vulgaris spp.). Consequences for growing genetically-modified sugar beet - A modelling approach

Weed beet cannot be controlled by herbicides in sugar beet (except via height-selective applicators) as it is a crop relative, descending from accidentally flowering sugar beet (Beta vulgaris) crop plants either because of vernalization during cold springs, or presence of a dominant bolting allele in sugar beet seed lots due to cross-pollination by annual wild beet (B. vulgaris ssp. maritima) in seed production areas. A second, minor source of weed beet are crop roots lost during harvest. These roots ("groundkeepers") can reproduce in the year after sugar beet and potentially contribute to weed beet dynamics and gene flow. Bolting, flowering and seed production timing and potential of groundkeepers were measured in field experiments. Bolting and flowering were faster in groundkeepers vs. weed beet; flower and seed production was lower in groundkeepers but the latter were less sensitive to competitive crops. The measured parameters were used to introduce a ground-keeper life-cycle into the GeneSys-Beet model which quantifies the effects of cropping systems on weed beet in landscapes. Simulations over several years showed weed beet dynamics to be more sensitive to groundkeeper parameter values than to root loss at sugar beet harvest. Groundkeepers were identified as a key source of weed beet populations and of gene escape from novel sugar beet varieties (e.g. genetically-modified herbicide-tolerant varieties) in the absence of crop bolters. The control of the latter, either by manual weeding or by genetic improvement of sugar beet varieties, was shown to be essential for controlling weed beet populations and avoid the advent of herbicide-tolerant weed beet. (Résumé d'auteur