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The objective of this study was to evaluate the effects of changes in the photosynthetic photon flux density (PPFD) on net gas exchange of Citrus limon, a woody species, and Nicotiana tabacum, an herbaceous species. When PPFD was increased from 50 to 350 mumol.m-2.s-1 and returned to 50 mumol.m-2.s-1 after 60 min, the CO2 assimilation rate (A) increased and stabilized after 15 min in both species. Stomatal conductance (g s), however, continued to increase. After returning to low PPFD, A immediately diminished to a low value for both lemon and tobacco. Stomatal conductance of lemon diminished slowly over 60 min, whereas g s for tobacco took only 15 min to decrease. This difference in behavior is probably due to over-sensitivity of stomata of woody species, such as Citrus, when exposed to high light, retarding stomatal closure on return to low PPFD. Furthermore, when lemon, growing at a PPFD of 300 mumol.m-2.s-1, was submitted to a step increase of 600 mumol.m-2.s-1, there was an oscillatory behavior of A and g s requiring 150 min to stabilize. The causes of this behavior are discussed with respect to improved intrinsic water use efficiency by stomatal closure. O objetivo desse estudo foi avaliar o efeito da variação da densidade de fluxo de fótons fotossintéticos (DFFF) nas trocas gasosas de Citrus limon, uma espécie lenhosa, e Nicotiana tabacum, uma espécie herbácea. Quando a DFFF foi aumentada de 50 para 350 mimol.m-2.s-1 durante 60 min, retornando em seguida a 50 mimol.m-2.s-1, a taxa de assimilação de CO2 (A) aumentou e estabilizou após 15 min, mas a condutância estomática (g s) continuou crescendo, em ambas as espécies. Quando se retornou à baixa DFFF, A diminuiu instantaneamente nas duas espécies, mas a g s do limoeiro diminuiu lentamente durante 60 min, enquanto em tabaco, a g s levou apenas 15 min para decrescer. Essa diferença nas respostas é provavelmente devida à hipersensibilidade dos estômatos de espécies lenhosas, como Citrus, quando submetidas à alta DFFF, retardando o seu fechamento, quando de volta à baixa DFFF. Além disso, quando o limoeiro, crescendo sob DFFF de 300 mimol.m-2.s-1, foi submetido subitamente a 600 mimol.m-2.s-1, iniciou-se uma flutuação de A e g s, levando 150 min para se estabilizar. As causas dessa oscilação são discutidas em função do ajuste da eficiência intrínseca no uso de água, devido ao controle estomático

Effects of changes in the photosynthetic photon flux density on net gas exchange of Citrus limon and Nicotiana tabacum