Internal CO₂ Measured Directly in Leaves

Abstract

Observations of nonuniform photosynthesis across leaves cast doubt on intemal CO2 partial pressures (p) calculated on the assumption of uniformity and can lead to incorrect conclusions about the stomatal control of photosynthesis. The problem can be avoided by measuring pi directly because the assumptions of uniformity are not necessary. We therefore developed a method that allowed pi to be measured confinuously in situ for days at a time under growth conditions and used it to investigate intact leaves of sunflower (Helianthus annuus L.), soybean (Glycine max L. Merr.), and bush bean (Phaseolus vulgaris L.) subjected to high or low leaf water potentials (I,) or high concentrations of abscisic acid (ABA). The leaves maintained a relatively constant differential (Ap) between ambient CO2 and measured pi throughout the light period when water was supplied. When water was withheld, ', decreased and the stomata began to close, but measured p increased until the leaf reached a *w of -1.76 (bush bean), -2.12 (sunflower) or -3.10 (soybean) megapascals, at which point Ap = 0. The increasing pi indicated that stomata did not inhibit CO2 uptake and a Ap of zero indicated that CO2 uptake became zero despite the high availability of CO2 inside the leaf. In contrast, when sunflower leaves at high Iw were treated with ABA, pi did not increase and instead decreased rapidly and steadily for up to 8 hours even as I, increased, as expected if ABA treatment primarily affected stomatal conductance. The accumulating CO2 at low Iw and contrasting response to ABA indicates that photosynthetic biochemistry limited photosynthesis at low Iw but not at high ABA.Water deficits have a large impact on plant growth and productivity by reducing leaf turgor, cell expansion, stomatal conductance, and photosynthesis and increasing ABA and solute concentrations in the tissues (2, 4). Among these responses, the inhibition of photosynthesis is central and often thought to be caused by reduced stomatal conductance, perhaps because of high ABA, that limits the availability of CO2 inside the leaf (8-10, 29, 32, 33). However, the '

Stress-Induced Cytokinin Synthesis Increases Drought Tolerance through the Coordinated Regulation of Carbon and Nitrogen Assimilation in Rice

“…Wild-type rice plants under water stress displayed a reduction in stomatal conductance and an inhibition of photosynthetic activity and C assimilation. Lauer and Boyer (1992) demonstrated that, during drought stress, the limitation in C assimilation was due to the reduction of the biochemical capacity of photosynthesis. The authors showed that although a simultaneous reduction of stomatal opening occurred during water deficit, calculated substomatal CO 2 concentration (Ci) was maintained or even increased.…”

Tracing Photosynthetic Response Curves with Internal CO₂ Measured Directly

“…1). In the cup, the CO2 equilibrated with that in the stomatal pores adjacent to the airspace Lauer and Boyer, 1992;Boyer and Kawamitsu, 2011). The cup was connected in a closed loop with an IRGA (LI-840A; LI-COR) and a micro blower (109P0412H309; Sanyo Denki Co., Ltd., Osaka, Japan) which allows the air to gently circulate around the loop (ca.…”

“…These are considered problems for the A Ci analysis, which usually depends on calculated Ci and thus the diffusive behavior of water vapor. Lauer and Boyer (1992) and Boyer and Kawamitsu (2011) considered the direct method to be a robust approach for tracing A Ci curves. In this study, we incorporated a direct Ci measurement system into the LI-6400 open gas exchange system (LI-COR, Lincoln, NE, USA).…”

“…The applicability of the system for A Ci curve measurement was tested in the sunflower (Helianthus annuus L.) leaf. We took special care to minimize bulk air flow through the leaf (Lauer and Boyer, 1992;Boyer and Kawamitsu, 2011), diffusion leaks (Flexas et al, 2007;Rodeghiero et al, 2007), and changes in the CO2 gradient across the leaf mesophyll (Mott and O'Leary, 1984;Parkhurst et al, 1988;Parkhurst and Mott, 1990) that can be important for the measurement.…”

Response to water deficit of Nothofagus dombeyi plants inoculated with a specific (Descolea antárctica Sing) and non-specific (Pisolithus tinctorious (Pers.) Coker & Couch) ectomycorrhizal fungi

“…Many studies have shown that the decrease of photosynthesis under water stress can be associated with perturbations of the photochemical processes (Lauer & Boyer 1992, Yordanov et al 2003. In particular, photosystem II (PSII) is very sensitive to water stress, because it results in damage of the electron transport system linked to PSII (He et al 1995).…”