PhotosynthesisResearch 44: 253-260, 1995. 0 1995KluwerAcademicPublishers. Printedin the Netherlands. Regular paper
Leaf cavity CO2 concentrations and CO2 exchange in onion, Allium cepa L. George T. Byrd 1'*, T. Loboda2, Clanton C. Black Jr.2 & R. Harold Brown 1 1Crop and Soil Sciences Department; 2Biochemistry and Molecular Biology Department, University of Georgia, Athens, GA, 30602-7272, USA; *Address for correspondence~reprints: George T. Byrd, Department of Ecology and Evolutionary Biology, Rice University, P.O. Box 1892, Houston, TX, 77251, USA Received 10 February1995;acceptedin revisedform3 May 1995
Key words: CO2 conductance, CO2 recycling, membrane, photosynthesis
Abstract Onion (Allium cepa L.) plants were examined to determine the photosynthetic role of CO2 that accumulates within their leaf cavities. Leaf cavity CO2 concentrations ranged from 2250 #L L - 1 near the leaf base to below atmospheric (< 350 #L L - 1 ) near the leaf tip at midday. There was a daily fluctuation in the leaf cavity CO2 concentrations with minimum values near midday and maximum values at night. Conductance to CO2 from the leaf cavity ranged from 24 to 202 #tool m - 2 s- 1 and was even lower for membranes of bulb scales. The capacity for onion leaves to recycle leaf cavity CO2 was poor, only 0.2 to 2.2% of leaf photosynthesis based either on measured CO2 concentrations and conductance values or as measured directly by 14CO2labeling experiments. The photosynthetic responses to CO2 and 02 were measured to determine whether onion leaves exhibited a typical C3-type response. A linear increase in CO2 uptake was observed in intact leaves up to 315 #L L-1 of external CO2 and, at this external CO2concentration, uptake was inhibited 35.4 4- 0.9% by 210 mL L - 1 Qcompared to 20 mL L - 1 O2. Scanning electron micrographs of the leaf cavity wall revealed degenerated tissue covered by a membrane. Onion leaf cavity membranes apparently are highly impermeable to CO2 and greatly restrict the refixation of leaf cavity CO2 by photosynthetic tissue.
Abbreviations: Ca - external
CO 2 concentration; Ci - intercellular CO 2 concentration; I" - CO2 compensation concentration; PPFR - photosynthetic photon fluence rate
Introduction Leaves of onion (Allium cepa L.) form cylindrical tubes with large central cavities in which rather high CO2 concentrations develop, especially in the dark (Darbyshire et al. 1979). Other plants which accumulate CO2 in leaves generally have aerenchyma tissue with extensive gas spaces. Many of these plants are wetland species that passively move O2 downward to flooded roots and CO2 and other gases in the opposite direction (Teal and Kanwisher 1966; Dacey 1980; Constable et al. 1992). Teal and Kanwisher (1966) directly measured gaseous 02 and CO2 diffusion through the salt water marsh grass, Spartina alterniflora Loisal, with high internal CO2 concentrations resulting from CO2 transport out of the root. Constable et al. (1992)
found for leaves of cattail (Typha Iatifolia L.) that CO2 concentrations varied diurnally from near atmospheric during midday to over 18 times atmospheric levels just before dawn. They suggested that photosynthesis in cattail as well as other wetland species possessing aerenchyma is supplemented by elevated leaf CO2 levels, probably arising from microbial activity and plant respiration. However, any significant contributions of aerenchyma CO2 to overall photosynthesis may be optimistic. Singer et al. (1994) found that aerenchyma CO2 contributes only a small fraction (
Leaf cavity CO2 concentrations and CO2 exchange in onion, Allium cepa L. George T. Byrd 1'*, T. Loboda2, Clanton C. Black Jr.2 & R. Harold Brown 1 1Crop and Soil Sciences Department; 2Biochemistry and Molecular Biology Department, University of Georgia, Athens, GA, 30602-7272, USA; *Address for correspondence~reprints: George T. Byrd, Department of Ecology and Evolutionary Biology, Rice University, P.O. Box 1892, Houston, TX, 77251, USA Received 10 February1995;acceptedin revisedform3 May 1995
Key words: CO2 conductance, CO2 recycling, membrane, photosynthesis
Abstract Onion (Allium cepa L.) plants were examined to determine the photosynthetic role of CO2 that accumulates within their leaf cavities. Leaf cavity CO2 concentrations ranged from 2250 #L L - 1 near the leaf base to below atmospheric (< 350 #L L - 1 ) near the leaf tip at midday. There was a daily fluctuation in the leaf cavity CO2 concentrations with minimum values near midday and maximum values at night. Conductance to CO2 from the leaf cavity ranged from 24 to 202 #tool m - 2 s- 1 and was even lower for membranes of bulb scales. The capacity for onion leaves to recycle leaf cavity CO2 was poor, only 0.2 to 2.2% of leaf photosynthesis based either on measured CO2 concentrations and conductance values or as measured directly by 14CO2labeling experiments. The photosynthetic responses to CO2 and 02 were measured to determine whether onion leaves exhibited a typical C3-type response. A linear increase in CO2 uptake was observed in intact leaves up to 315 #L L-1 of external CO2 and, at this external CO2concentration, uptake was inhibited 35.4 4- 0.9% by 210 mL L - 1 Qcompared to 20 mL L - 1 O2. Scanning electron micrographs of the leaf cavity wall revealed degenerated tissue covered by a membrane. Onion leaf cavity membranes apparently are highly impermeable to CO2 and greatly restrict the refixation of leaf cavity CO2 by photosynthetic tissue.
Abbreviations: Ca - external
CO 2 concentration; Ci - intercellular CO 2 concentration; I" - CO2 compensation concentration; PPFR - photosynthetic photon fluence rate
Introduction Leaves of onion (Allium cepa L.) form cylindrical tubes with large central cavities in which rather high CO2 concentrations develop, especially in the dark (Darbyshire et al. 1979). Other plants which accumulate CO2 in leaves generally have aerenchyma tissue with extensive gas spaces. Many of these plants are wetland species that passively move O2 downward to flooded roots and CO2 and other gases in the opposite direction (Teal and Kanwisher 1966; Dacey 1980; Constable et al. 1992). Teal and Kanwisher (1966) directly measured gaseous 02 and CO2 diffusion through the salt water marsh grass, Spartina alterniflora Loisal, with high internal CO2 concentrations resulting from CO2 transport out of the root. Constable et al. (1992)
found for leaves of cattail (Typha Iatifolia L.) that CO2 concentrations varied diurnally from near atmospheric during midday to over 18 times atmospheric levels just before dawn. They suggested that photosynthesis in cattail as well as other wetland species possessing aerenchyma is supplemented by elevated leaf CO2 levels, probably arising from microbial activity and plant respiration. However, any significant contributions of aerenchyma CO2 to overall photosynthesis may be optimistic. Singer et al. (1994) found that aerenchyma CO2 contributes only a small fraction (
