THE USE OF UV-RADIATION TO CONTROL THE ARCHITECTURE OF SALVIA SPLENDENS PLANTS .2. RELATIONSHIPS BETWEEN PAR LEVELS AND UV-RADIATION IN THEPHOTOREGULATION OF STEM ELONGATION

The effects of UV irradiation on stem elongation of Salvia splendens p lants preadapted to two acid grown under four different irradiances of visible light, were studied using linear voltage differential transdu cers. The levels of radiant energy during the experimental phase showe d a temporary and opposite effect during the day-time and the night-ti me: increasing levels of photosynthetically active radiation (PAR)dagg er reduced stem growth during the day and enhanced elongation growth d uring the night. It appears, therefore, that similar final stem elonga tion in plants grown under very different PAR levels is the result of the algebraic sum of different and sometimes opposite effects of PAR o n stem growth. Except for the controls, the plants received one UV tre atment 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 result s show that the preadapting PAR conditions changed the sensitivity of the plants to both UV and to the following PAR conditions. The sensiti vity of S. splendens to UV radiation is inversely correlated to the PA R levels before and during the UV treatments. Furthermore the presence of active photosynthetic and photomorphogenic systems, i.e. the prese nce of visible light during the UV treatment, decreases the sensitivit y of the plants to UV radiation. Depending on PAR levels, the UV treat ments given during the night induced a temporary inhibition of growth followed by a promotion of Stem elongation.