TRANSPIRATION AND STOMATAL CONDUCTANCE OF ROSES CV SONIA GROWN WITH SUPPLEMENTAL LIGHTING.

Roses cv Sonia were grown in the winter in a greenhouse with or without supplemental lighting (20 h SON/T; 5 W/m2). The transpiration rate of plants grown with supplemental lighting (SL-plants) was higher than of plants grown without supplemental lighting (NL-plants), both in the light and in the dark period. Stomates of NL-plants closed during the night, while stomates of SL-plants remained open during the 4 hours of darkness. The number of stomates per area was the same for both plants. During the first two days of vase life at 12 h light/day, the transpiration rate on leaf area base of SL-flowers was almost twice that of NL-flowers and stomatal resistance of SL-flowers was lower than that of NL-flowers. Stomates of SL-flowers closed only partial during the dark period in contrast with NL-flowers. When cut flowers were placed in constant darkness transpiration rate of SL-flowers was higher than of NL-flowers and the stomates of SL-flowers remained open during more than 40 hours, while the stomates of NL-flowers closed almost immediately.