Shade-avoidance is a major adaptive response of plants, and is usually considered to be controlled by phytochromes through the perception of changes in the red:far red light ratio. However, few studies on the effects of blue light (BL) and of light intensity [photo- synthetically active radiation (PAR)] on light-grown plants have been conducted, especially concerning changes in PAR at constant BL. The objective here was to quantify the photocontrol of aerial morphogenesis by BL and PAR. Experiments were conducted varying BL and PAR independently, with three BL levels (4, 38, and 83 lmol m22 s21) at constant PAR (300 lmol m22 s21) and three PAR levels (338, 705, and 163 lmol m22 s21) at constant BL (36 lmol m22 s21). Effects on morphogenetic processes were analysed as quantita- tive modulations of ontogenic trends and response curves were produced. White clover (Trifolium repens L.) was used, as it is a typical shade-avoider display- ing the whole syndrome of shade-avoidance in a purely vegetative stage. Morphological responses were strongly controlled by both BL and PAR changes, through antagonist effects on leaf appearance rate and additive effects on petiole elongation. All the other responses appeared to be the indirect consequences of changes in the leaf appearance rates. BL acted as a light signal for plant morphogenesis. However, the PAR control probably implicates two distinct mech- anisms, such as a trophic effect and a signal. Both PAR and BL actions involved organ-specific differ- ences, which are central in the control of the shade- avoidance responses.