Light and thermal environments as modified by a wheat crop: effects on weed seed germination.
Knowledge of the regulatory effects of the crop canopy on weed seed germination is necessary to understand fully the behaviour of weed seed banks during a crop cycle. It is well known that canopy presence interferes with seed germination through modifications to the light and thermal environment, but the changing effect of a growing canopy has not been assessed, thus precluding the identification of shading-intensity thresholds for triggering the different types of canopy-detection mechanisms in different seed populations. Field experiments were performed with artificially modified thermal and light environments, using two types of seed banks (seeds buried or located at the soil surface). Conditions below a wheat canopy were modified to match light and thermal conditions prevailing on bare soil (i.e. soil without vegetation). Most weed emergence patterns during the early stages of crop establishment were not modified by the thermal regime produced by the incipient canopy compared with a bare soil control. However, the reduction of the red:far-red ratio from the bare soil value of 1.2-0.9 below the wheat canopy reduced germination of some weed species located at the soil surface, and the effect could by reversed by far-red filters. The weed Galinsoga parviflora developed two generations during the crop cycle. The modified light and thermal environments beneath an establishing wheat canopy was not sufficient to inhibit the germination of Galinsoga parviflora, even if seeds were located at the soil surface. Only for seeds of the second generation, dispersed from seeds of the first plant generation, was there sufficient modification of the photothermal environment below the wheat canopy to interfere with dormancy termination. Synthesis and applications. Understanding seed responses to modifications in the photothermal environment below a crop canopy (e.g. wheat crop) should allow us to improve weed management strategies by manipulating crop canopy attributes. This could be achieved by modification of sowing date, crop density, spatial arrangement and genotype. For example, increasing the crop plant density would diminish the number of weeds emerging during the first phase of crop establishment. This strategy would be appropriate where weed seeds are predominantly located at the soil surface, typically found in a no-till cropping system.