Prunus persica plants were grown under prolonged exposure to different light treatments to determine the interaction between the blue light (BL) receptor and phytochrome and/or an independent BL response in the photoregulation of shoot and leaf development. Different light conditions were established in growth chambers by changing both the state of phytochrome and the BL photon flux density (PFD) at constant photosynthetically active radiation (PAR). Furthermore, to evaluate the independent action of the BL photoreceptor, increasing amounts of BL photons were added to the light emitted by low-pressure sodium (LPS) lamps without altering irradiance and phytochrome photoequilibrium. Applying the principle of equivalent light action, the observed blue inhibition of shoot elongation, leaf expansion and thickness were clearly related to a specific BL receptor because the state of phytochrome for each treatment was nearly identical. Increasing amounts of blue photons to light emitted from LPS lamps decreased shoot elongation, whereas leaf expansion was negatively affected only at the highest blue level, suggesting a specific fluence dependence response to BL for each organ and tissue. The BL effect was evident in reducing the thickness of all the leaf tissues except for the upper epidermis, which became thicker. This could be the result of an adaptation to protect the underlying photosynthetic apparatus. Other morphological and anatomical responses to the action of the BL receptor were greatly altered when the state of phytochrome changed in the plant tissues.; Prunus persica plants were grown under prolonged exposure to different light treatments to determine the interaction between the blue light (BL) receptor and phytochrome and/or an independent BL response in the photoregulation of shoot and leaf development. Different light conditions were established in growth chambers by changing both the state of phytochrome and the BL photon flux density (PFD) at constant photosynthetically active radiation (PAR). Furthermore, to evaluate the independent action of the BL photoreceptor, increasing amounts of BL photons were added to the light emitted by low-pressure sodium (LPS) lamps without altering irradiance and phytochrome photoequilibrium. Applying the principle of equivalent light action, the observed blue inhibition of shoot elongation, leaf expansion and thickness were clearly related to a specific BL receptor because the state of phytochrome for each treatment was nearly identical. Increasing amounts of blue photons to light emitted from LPS lamps decreased shoot elongation, whereas leaf expansion was negatively affected only at the highest blue level, suggesting a specific fluence dependence response to BL for each organ and tissue. The BL effect was evident in reducing the thickness of all the leaf tissues except for the upper epidermis, which became thicker. This could be the result of an adaptation to protect the underlying photosynthetic apparatus. Other morphological and anatomical responses to the action of the BL receptor were greatly altered when the state of phytochrome changed in the plant tissues.
