EFFECTS OF SHADE ON REPRODUCTION AND SOME MORPHOLOGICAL-CHARACTERISTICS OF ABUTILON-THEOPHRASTI MEDICUS, DATURA-STRAMONIUM L AND SORGHUM-HALEPENSE L PERS

Relations between availability of light and reproductive activity in several of the most damaging summer weeds (Abutilon theophrasti Medicus, Datura stramonium L., Sorghum halepense L. Pers.) were studied, in order to investigate seed production capacity of these weed species in the unfavourable conditions occurring under a crop canopy simulated by shade cloths. Decrease in seed production under shading appeared to be due to a decrease in the number of fruits per plant, while the quantity of seed per fruit proved to be fairly stable. Biomass partitioning within the plant reduced the percentage of dry matter accumulated in reproductive organs, leading to a marked decrease in the harvest index. Furthermore, shading caused a species-dependent delay in onset of the flowering and ripening stages. In Datura stramonium shading led to a greater decrease in seed production and, consequently, in the harvest index than in the other species examined. Finally, laboratory tests demonstrated that the percentage of dormant seeds remained unmodified in all species examined.