A Direct Confirmation of the Standard Method of Estimating Intercellular Partial Pressure of CO₂

Abstract

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:

Stomatal Behavior and CO2 Exchange Characteristics in Amphistomatous Leaves

“…For (2). However, using a technique similar to ours, Sharkey et al (11) estimate ri in Xan:hium to be closer to 1.0 m2 s mol-', and state that they believe the estimate of 3.2 m2 s mol-' to be high. Based on these data and our measurements on sunflower and cocklebur, it seems that estimated ri values are quite variable, possibly due to differ-ences in technique, plant material, and because of the difficulty in accurately measuring the low resistances which apparently exist.…”

Salinity Effects on Photosynthesis and Growth in Alternanthera philoxeroides (Mart.) Griseb.

“…NaCl was added to culture solutions in daily, 50 mM increments to produce five different salinity treatments (0, 100, 200, 300, and 400 mM). Salt CCas= -CC2 -(1.6 x Pn/g.-v) (1) where c"' is the CO2 concentration outside the leaf, and Pn is the net rate of CO2 uptake (20).…”

The Sites of Evaporation within Leaves

“…There are two corollaries of this hypothesis. First, if evaporation occurs close to the epidermis, the value of c i inferred from g s will overestimate the value of c i deeper within the mesophyll (hypothesis 9; Sharkey et al, 1982). Second, if the sites of evaporation are deeper within the leaf, the intercellular airspaces adjacent to stomata will be farther below saturation because vapor concentration must decline from those sites to the epidermis in order to drive vapor flux (hypothesis 10).…”