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.
CO2 assimilation rate; gas exchange; stomata; intrinsic water use efficiency
