Changes in stomatal density in rice grown under various CO 2 regimes with natural solar irradiance

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Rice ( Oryza sativa L. cv. IR-30), grown from seed under natural solar irradiance, was exposed to CO 2 concentrations ranging from 160 to 900 μl CO 2/l air from 9 days after planting until senescence. Stomatal density was determined from leaf impressions at two growth stages: on leaf number 7 (31 days after planting), and on flag leaves (104 days after planting). Increasing CO 2 concentrations resulted in a rise in stomatal density of leaves at both growth stages. The effect was greatest on the flag leaves, which exhibited a 54% increase in abaxial stomatal density (from 550 to 810 stomata/mm 2) at 500 as compared with 160 μl CO 2/l. Stomatal density increased with increasing CO 2 up to 330 μl CO 2/l; enrichment above this level resulted in no further significant increase in stomatal density. For both leaf ages, the abaxial stomatal density was more influenced by increases in CO 2 than the adaxial surface. The increase in stomatal density was largely the result of a rise in the number of stomata per row, although on the abaxial surface more rows across the leaf also contributed to the response. Flag leaf area was not significantly different among the CO 2 treatments, so the number of stomata per leaf followed similar trends to the stomatal density. This indicated the CO 2 effect was on stomatal, rather than leaf area, development. The response of stomatal density to rising CO 2 seems to be a species-dependent phenomenon, that varies with leaf surface and CO 2 range utilized.