ABSTRACT The objective of the present study was to evaluate the tolerance to cadmium (Cd) of sunflower genotypes grown in greenhouse conditions, and the effectiveness of using photosynthetic parameters as physiological indicators of this tolerance. Seeds of two sunflower genotypes previously identified as tolerant (H358) and Cd-sensitive (AG960) to Cd were used. The seeds were germinated in plastic cups containing plant substrate; after 9 days, the seedlings were transplanted to plastic basins containing a nutrient solution with 0 or 10 µM of Cd, where they remained for 16 days. Samples of the plants were harvested every 5 days. The experiment was carried out in a randomized complete design, using a 4×2×2 factorial arrangement (4 days of grown in a nutrient solution with Cd, 2 sunflower genotypes, and 2 Cd levels) with four replications. Cd stress decreased CO2 net assimilation, stomatal conductance, carboxylation efficiency, photosynthetic pigment contents, potential quantum yield (Fv/Fm), and effective quantum yield of plants of the two evaluated genotypes. The decrease in photosynthetic rates of these plants was caused by both stomatal and non-stomatal limitations. Plants of the AG960 genotype showed more pronounced deleterious effects due to Cd stress than those of the H358 genotype. Thus, CO2 net assimilation rate, stomatal conductance, and chloroplast pigment content are good physiological indicators of sunflower tolerance to Cd and can at least in part, explain the greater tolerance of the H358 genotype to Cd stress when compared to the AG960 genotype. | RESUMO O presente estudo foi conduzido em casa de vegetação objetivando-se demonstrar a tolerância ao cádmio (Cd) em genótipos de girassol e avaliar a eficácia dos parâmetros fotossintéticos como indicadores fisiológicos dessa tolerância. Para isto, foram utilizadas sementes de dois genótipos de girassol previamente identificados como tolerante (H358) e sensível (AG960) ao Cd. As sementes foram germinadas em copos plásticos contendo substrato vegetal e, após 9 dias, as plântulas foram transferidas para bacias plásticas com solução nutritiva contendo 0 ou 10 µM de Cd , onde permaneceram por 16 dias. As plantas foram coletadas a cada 5 dias. O delineamento experimental foi o inteiramente casualizado, em um arranjo fatorial 4 (dias de exposição) × 2 (genótipos) × 2 (níveis de Cd), com quatro repetições. A exposição ao Cd reduziu a taxa de assimilação líquida do CO2, a condutância estomática e a eficiência de carboxilação, bem como os teores de todos os pigmentos fotossintéticos, a razão Fy/Fm e o YII das plantas de ambos os genótipos estudados. Demonstrando que a redução na taxa fotossintética dessas plantas foi ocasionada tanto por limitações estomáticas como não-estomáticas. Os efeitos deletérios do Cd foram mais pronunciados no genótipo AG960 em comparação com o H358. Assim, as variáveis A, gs e os teores de pigmentos mostraram-se bons indicadores fisiológicos da tolerância do girassol ao estresse por Cd e podem, ao menos em parte, explicar a maior tolerância do genótipo H358 em relação ao AG960.

The objective of the present study was to evaluate the tolerance to cadmium (Cd) of sunflower genotypes grown in greenhouse conditions, and the effectiveness of using photosynthetic parameters as physiological indicators of this tolerance. Seeds of two sunflower genotypes previously identified as tolerant (H358) and Cd-sensitive (AG960) to Cd were used. The seeds were germinated in plastic cups containing plant substrate; after 9 days, the seedlings were transplanted to plastic basins containing a nutrient solution with 0 or 10 µM of Cd, where they remained for 16 days. Samples of the plants were harvested every 5 days. The experiment was carried out in a randomized complete design, using a 4×2×2 factorial arrangement (4 days of grown in a nutrient solution with Cd, 2 sunflower genotypes, and 2 Cd levels) with four replications. Cd stress decreased CO2 net assimilation, stomatal conductance, carboxylation efficiency, photosynthetic pigment contents, potential quantum yield (Fv/Fm), and effective quantum yield of plants of the two evaluated genotypes. The decrease in photosynthetic rates of these plants was caused by both stomatal and non-stomatal limitations. Plants of the AG960 genotype showed more pronounced deleterious effects due to Cd stress than those of the H358 genotype. Thus, CO2 net assimilation rate, stomatal conductance, and chloroplast pigment content are good physiological indicators of sunflower tolerance to Cd and can at least in part, explain the greater tolerance of the H358 genotype to Cd stress when compared to the AG960 genotype.