XVI International Botanical Congess

We have studied sunflower leaves from plants grown under controlled conditions, exposed to low doses of UV-A (320-700 nm) or UV-A+B (280-700 nm) radiation. Plants were grown under a photoperiod of 18+6 (L/D, PPFD 250 µmol.m-2s-1). Exposure to UV-radiation (80 µmol.m-2.s-1) started at 5 DFE, ending at 50 DFE. Transverse sections of leaves were used to study its anatomy. Total leaf thickness, cuticle thickness and the number of palisade layer cells did not differ between treatments. Leaf bronzing and a reduction of number and size of chloroplasts was observed in UV-A+B treated plants. This results, associated with a previous finding of a substantial diminution in the photosynthetic activity, suggest that the photosynthetic apparatus in sunflower is sensitive to UV-B when exposed at early stages of vegetative growth.