The importance of reduced light intensity on the growth and development of six weed species.
Crops limit light for photosynthesis and growth of weeds. We studied the effect of reduced light on performance of six weed species [one invasive species (Amsinckia micrantha), three common species (Veronica persica, Capsella bursa-pastoris, Viola arvensis) and two less common weeds (Anagallis arvensis and Scleranthus annuus)]. In two glasshouse experiments, six light levels were achieved aiming at 0%, 20%, 50%, 80%, 90% and 95% reduction of light and corresponding with daily light integrals (DLI) of 12.4, 9.63, 7.13, 2.74, 0.95 and 0.69 mol m-2 day-1 in experiment 1 and 21.2, 18.0, 10.7, 3.71, 1.64 and 1.20 mol m-2 day-1 in experiment 2. The number of leaves was strictly controlled by DLI. Chlorophyll content index, maximum photochemical efficiency of photosystem II (Fv/Fm), stomatal conductance, flowering and dry matter were strongly reduced when DLI was reduced to 0.69-3.71 mol m-2 day-1 for all species. Threshold DLI for flowering was ca. 3.71 mol m-2 day-1 for S. annuus, V. arvensis, A. arvensis and V. persica, while C. bursa-pastoris deviated by flowering at DLI of 0.95 mol m-2 day-1. This may explain why C. bursa-pastoris is common in the seedbank of Danish arable soils in spite of intensive farming with well-fertilised and dense crops.