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EnglishInteraction effects between nitrogen and UV-B radiation were studied in sunflower (Helianthus annuus L. variety IAC-Iarama) plants grown in a greenhouse under natural photoperiod conditions. Plants were irradiated with 0.8W m-2 (control) or 8.0W m-2 (+UV-B) of UV-B radiation for 7h per day. The plants were grown in pots containing vermiculite and watered with 70% of full strength nitrogen-free Long Ashton solution, containing either low (42.3ppm) or high (282ppm) nitrogen as ammonium nitrate. High nitrogen increased dry matter of stem, leaves and shoot, photosynthetic pigments and photosynthesis (A) without any alteration in stomatal conductance (gs) nor transpiration (E) while it reduced the intercellular CO2 (Ci) concentration, and malondialdehyde (MDA) content. High UV-B radiation had negative effects on dry matter production, A, gs and E with the effects more marked under high nitrogen, whereas it increased Ci under high nitrogen. Activity of PG-POD was reduced by high UV-B radiation under low nitrogen but it was not changed under high nitrogen. The UV-B radiation increased the MDA content independently of nitrogen level. Results indicate that the effects of UV-B radiation on sunflower plants are dependent of nitrogen supply with high nitrogen making their physiological processes more sensitive to UV-B radiation. Key words: antioxidants; growth; malondialdehyde; peroxidation; photosynthesis portuguesOs efeitos da interacao entre nitrogenio e a radiacao UV-B foram estudados em plantas de girassol (Helianthus annuus L. variedade IAC-Iarama), cultivadas em casa de vegetacao sob condicoes fotoperiodicas naturais. As plantas foram irradiadas com 0.8W m-2 (controle) ou 8.0W m-2 (+UV-B) durante 7h por dia. As plantas foram cultivadas em vasos contendo vermiculita e regadas com solucao nutritiva de Long Ashton 70%, contendo baixa (42,3ppm) ou alta (282ppm) dose de nitrogenio na forma de nitrato de amonio. A alta dose de nitrogenio aumentou a materia seca das folhas, caule e parte aerea, pigmentos fotossinteticos e a fotossintese (A), sem alterar a condutância estomatica (gs) ou transpiracao (E), enquanto reduziu a concentracao intercelular de CO2 (Ci) e o conteudo de malondialdeido (MDA). A elevada radiacao UV-B reduziu a producao de materia seca, A, gs e E, sendo os efeitos mais acentuados sob alta dose de nitrogenio, enquanto aumentou Ci sob alta dose de nitrogenio. A atividade da PG-POD foi reduzida sob alta radiacao UV-B e baixo nitrogenio, mas nao foi alterada sob alta dose de nitrogenio. A radiacao UV-B aumentou o conteudo de MDA independente do nitrogenio. Os resultados indicam que os efeitos da radiacao UV-B em plantas de girassol sao dependentes da disponibilidade de nitrogenio sendo que a alta dose de nitrogenio torna seus processos fisiologicos mais sensiveis a radiacao UV-B. Palavras-chave: antioxidantes; crescimento; malondialdeido; peroxidacao; fotossintese

The sensitivity of sunflower (Helianthus annuus L.) plants to UV-B radiation is altered by nitrogen status