The use of UV radiation to control the architecture of Salvia splendens plants .2. Relationships between PAR levels and UV radiation in the photoregulation of stem elongation

The effects of UV irradiation on stem elongation of Salvia splendens plants preadapted to two acid grown under four different irradiances of visible light, were studied using linear voltage differential transducers. The levels of radiant energy during the experimental phase showed a temporary and opposite effect during the day-time and the night-time: increasing levels of photosynthetically active radiation (PAR)dagger reduced stem growth during the day and enhanced elongation growth during the night. It appears, therefore, that similar final stem elongation in plants grown under very different PAR levels is the result of the algebraic sum of different and sometimes opposite effects of PAR on stem growth. Except for the controls, the plants received one UV treatment from Philips TL 12 40 W fluorescent tubes either in the middle of the light period or at the beginning of the dark period, The results show that the preadapting PAR conditions changed the sensitivity of the plants to both UV and to the following PAR conditions. The sensitivity of S. splendens to UV radiation is inversely correlated to the PAR levels before and during the UV treatments. Furthermore the presence of active photosynthetic and photomorphogenic systems, i.e. the presence of visible light during the UV treatment, decreases the sensitivity of the plants to UV radiation. Depending on PAR levels, the UV treatments given during the night induced a temporary inhibition of growth followed by a promotion of Stem elongation.