Elevated carbon dioxide affects the patterning of subsidiary cells in Tradescantia stomatal complexes

The influence of elevated CO2 concentration (670 ppm) on the structure, distribution, and patterning of stomata in Tradescantia leaves was studied by making comparisons with plants grown at ambient CO2. Extra subsidiary cells, beyond the normal complement of four per stoma, were associated with nearly half the stomatal complexes on leaves grown in elevated CO2. The extra cells shared characteristics, such as pigmentation and expansion, with the typical subsidiary cells. The position and shape of the extra subsidiary cells in face view differed in the green and purple varieties of Tradescantia. Substomatal cavities of complexes with extra subsidiary cells appeared larger than those found in control leaves. Stomatal frequency expressed on the basis of leaf area did not differ from the control. Stomatal frequency based on cell counts (stomatal index) was greater in leaves grown in CO2-enriched air when all subsidiary cells were counted as part of the stomatal complex. This difference was eliminated when subsidiary cells were included in the count of epidermal cells, thereby evaluating the frequency of guard cell pairs. The extra subsidiary cells were, therefore, recruited from the epidermal cell population during development. Stomatal frequency in plants grown at elevated temperature (29 degrees C) was not significantly different from that of the control (24 degrees C). The linear aggregations of stomata were similar in plants grown in ambient and elevated CO2. Since enriched CO2 had no effect on the structure or patterning of guard cells, but resulted in the formation of additional subsidiary cells, it is likely that separate and independent events pattern the two cell types. Plants grown at enriched CO2 levels had significantly greater internode lengths, but leaf area and the time interval between the appearance of successive leaves were similar to that of control plants. Porometric measurements revealed that stomatal conductance of plants grown under elevated CO2 was lower than that of control leaves and those grown at elevated temperature. Tradescantia was capable of regulating stomatal conductance in response to elevated CO2 without changing the relative number of stomata present on the leaf.