Effects of light spectral quality on morphogenesis and source-sink relations in radish plants

The accumulation of dry matter and the content of major phytohormones in the aboveground and underground plant parts, as well as light curves and the diurnal course of photosynthesis in the leaves were studied in radish (Raphanus sativus L.) plants of different ages that were grown under red (RL) or blue (BL) light. As seen from the rapid increase in plant biomass, the development of storage organs (hypocotyl or tap root) started on the 14th day after the emergence of seedling of the BL plants and on the 21st day for the RL plants. Conversely, RL stimulated biomass accumulation in the aboveground parts (petioles and stems) already in the early stages of plant development. Light spectral quality only slightly affected the activity and the diurnal course of photosynthesis. The GA content was ten times higher in the aboveground parts of the RL plants than those of the BL plants. The hypocotyl of the BL plants contained much higher amounts of cytokinins and IAA than that of the RL plants. The specific responses of the source-sink relations to the light quality were related to the distribution of various phytohormones between the aboveground and underground parts of the plants: RL increased the content of gibberellins (GA) in the aboveground parts of plants, thus increasing their sink activity, whereas BL stimulated the synthesis of cytokinins and IAA in the hypocotyl and enhanced its development. Light quality-specific morphogenetic responses were reversed when plants were treated with exogenous GA or paclobutrazol, an inhibitor of GA synthesis. The treatment of the BL plants with exogenous GA stimulated petiole and hypocotyl elongation and induced stem formation. The treatment of the BL plants with paclobutrazol led to shortened petioles, the flattening of the storage organ, and the disappearance of the stem.