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The partal pressure of CO2 inside leaves of several species was measured directly. Small gas exchange chambers were clamped above and below the same section of an amphistomatous leaf. A flowing gas stream through one chamber aflowed normal CO2 and water vapor exchange. The other chamber was in a closed circuit consisting of the chamber, an infrared gas analyzer, and a peristaltic pump. The CO2 in the closed system rapil reached a steady pressure which it is believed was identical to the CO2 pressure inside the leaf, because there was no flux of CO2 across the epidermis. This measured partial pressure was In close agreement with that estimated from a consideration of the fluxes of CO2 and vapor at the other surface.The partial pressure of CO2 inside leaves, cj,2 is affected by and affects both rate of CO2 assimilation and stomatal conductance. An analysis of the physics of gas exchange by Penman and Schofield (11) first indicated how ci could be estimated, by assuming that the stomatal conductance found from measurements of transpiration rate under a known vapor pressure difference also defined the relationship between rate of assimilation and the difference in partial pressure of CO2 across the leaf epidermis, when that conductance is corrected for the differing diffusion coefficients for CO2 and water vapor in air. Gaastra (5) used the concept of ci in several of his equations, and, in 1963, Moss and Rawlins (10) published the first estimates of ci calculated from the gas exchange ofsingle leaves. Since then, the method ofestimating ci has become standard practice in gas exchange studies, although it has not been experimentally verified. The experiments we report here show that it is essentially correct. After these experiments were begun, it came to our attention that Laisk (7) has done similar experiments in attempting to answer a different question. His measurements agree with those reported here.Throughout this paper we will refer to the partial pressure of CO2 inside the leaf, as determined by a consideration of gas exchange measurements as the estimated ci to distinguish that value from the ci measured directly.MATERIALS AND METHODS Plant Material. Xanthium strumarium L. was grown in 4-L plastic pots in potting soil. The plants were automatically watered 'Permanent address:

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Tropospheric ozone (O 3 ) decreases photosynthesis, growth, and yield of crop plants, while elevated carbon dioxide (CO 2 ) has the opposite effect. The net photosynthetic rate (P N ), dark respiration rate (R D ), and ascorbic acid content of rice leaves were examined under combinations of O 3 (0, 0.1, or 0.3 cm 3 m -3 , expressed as O 0 , O 0.1 , O 0.3 , respectively) and CO 2 (400 or 800 cm 3 m -3 , expressed as C 400 or C 800 , respectively). The P N declined immediately after O 3 fumigation, and was larger under O 0.3 than under O 0.1 . When C 800 was combined with the O 3 , P N was unaffected by O 0.1 and there was an approximately 20 % decrease when the rice leaves were exposed to O 0.3 for 3 h. The depression of stomatal conductance (g s ) observed under O 0.1 was accelerated by C 800 , and that under O 0.3 did not change because the decline under O 0.3 was too large. . Excluding the stomatal effect, the mesophyll P N was suppressed only by O 0.3 , but was substantially ameliorated when C 800 was combined. Ozone fumigation boosted the R D value, whereas C 800 suppressed it. An appreciable reduction of ascorbic acid occurred when the leaves were fumigated with O 0.3 , but the reduction was partially ameliorated by C 800 . The degree of visible leaf symptoms coincided with the effect of the interaction between O 3 and CO 2 on P N . The amelioration of O 3 injury by elevated CO 2 was largely attributed to the restriction of O 3 intake by the leaves with stomatal closure, and partly to the maintenance of the scavenge system for reactive oxygen species that entered the leaf mesophyll, as well as the promotion of the P N .