Fig. 2.

Changes in internal CO₂ concentration (C_i) of the leaf in Lepidium sativum plants grown under sub-ambient or super-ambient CO₂ concentrations, as related to changes in stomatal density (SD; A), pavement cell density (PCD; B) and stomatal index (SI; C). The negative values of the C_i difference show by how much C_i was lowered when C_a was reduced from 400 to 180 μmol mol^–1; the positive values indicate the increase of C_i in plants grown at 800 compared with controls kept at 400 μmol mol^–1. Stomatal and pavement cell density in the first leaves (light green triangles and light green regression line) was much more sensitive to CO₂ concentration than in cotyledons (dark green circles and dark green regression line). Each point represents a difference between the C_a treatment (180 or 800) and C_a control (400) for one of three sub-treatments (two air humidities and hypobaric plants), each grown either in air or in helox gas mixture. Data for 14-day-old leaves and 7- and 14-day-old cotyledons are shown. The SD and C_i values used in calculation of the differences were means from three independent measurements (three plants per treatment). The C_i values were calculated from δ¹³C in plant dry matter (see Appendix). Bars show the standard error of the mean, for SI calculated from the primary SD and PCD data [see eqn (6) in ‘Statistical evaluation, meta-analysis’].