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 th e aboveground and underground plant parts, as well as light curves and the diurnal course of photosynthesis in the leaves were studied in radish (Raph anus 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 dev elopment 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 th e RL plants. Conversely, RL stimulated biomass accumulation in the abovegro und parts (petioles and stems) already in the early stages of plant develop ment. Light spectral quality only slightly affected the activity and the di urnal 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 hypocot yl of the BL plants contained much higher amounts of cytokinins and IAA tha n that of the RL plants. The specific responses of the source-sink relation s to the light quality were related to the distribution of various phytohor mones between the aboveground and underground parts of the plants: RL incre ased the content of gibberellins (GA) in the aboveground parts of plants, t hus increasing their sink activity, whereas BL stimulated the synthesis of cytokinins and IAA in the hypocotyl and enhanced its development. Light qua lity-specific morphogenetic responses were reversed when plants were treate d with exogenous GA or paclobutrazol, an inhibitor of GA synthesis. The tre atment 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 orga n, and the disappearance of the stem.