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

Abstract

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 μmol.m-2.s-1 and returned to 50 μmol.m-2.s-1 after 60 min, the CO2 assimilation rate (A) increased and stabilized after 15 min in both species. Stomatal conductance (gs), 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 gs 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 μmol.m -2.s-1, was submitted to a step increase of 600 μmol.m-2.s-1, there was an oscillatory behavior of A and gs requiring 150 min to stabilize. The causes of this behavior are discussed with respect to improved intrinsic water use efficiency by stomatal closure.