Dark Fixation of CO 2 by Crassulacean Plants

Self Cite

The concentration of glucan, mucilage, soluble carbohydrates, and mal;c acid were determined In Opuatia bigelovii EngehnL during a 23-week period. The experiment began during the dry summer by irrigation to stimulate Crassulacean acid metaboUsm and was followed by 13 weeks of drought. Spoehr (8). The latter were completed before there was much understanding of CAM physiology and ecology. In particular, the early researchers were perplexed by the diurnal acidity changes they sometimes observed, and their carbohydrate investigations were overshadowed by the pentosan theory of succulence (2). Our work reexamines the question of carbohydrate metabolism by cacti growing in a changing desert environment.

Section: Methodsmentioning

confidence: 99%

Carbohydrate Metabolism of Cactus in a Desert Environment

Sutton¹,

Ting²,

Sutton³

1981

Self Cite

“…The same air stream was used during the chase period. For the labeling, approximately 100 to 200 ,uCi of 04CO2 (6 ,uCi/,umol) were used (28). During the course of the continuous feeding or pulse-chase experiments, 1-ml samples of the air in the labeling cuvette were taken and the 14CO2 adsorbed in 20% (v/v) ethanolamine in methanol for determination of specific radioactivity.…”

mentioning

confidence: 99%

C₄ Pathway Photosynthesis at Low Temperature in Cold-tolerant Atriplex Species

Caldwell

Osmond²,

Nott³

1977

Two species of Atriplex were grown under low temperature (8 C day/ 6 C night) and high temperature (28 C day/20 C night) regimes. The photosynthetic capacity of these plants was studied as a function of temperature in a leaf gas exchange cuvette. Both species showed substantial photosynthetic capacity between 4 and 10 C and this was not enhanced by growth at low temperatures but rather, was somewhat greater in plants grown at higher temperature. Photosynthetic capacity of low temperature-grown plants at high temperature was greater in Atriplex confertifolia (Torr. and Frem.) S. Watts., a native of cool deserts, than in Atriplex vesicaria (Hew. ex. Benth.) from warmer desert areas. Leaves of both species were also subjected to 4CO% pulsechase and steady-state feeding experiments under controlled temperature conditions. These experiments revealed that the kinetics of carbon assimilation through the intermediates of the C4 pathway is not substantially disrupted at low temperature in either species. There was, however, a substantial interchange of label between aspartate and malate at low temperature which was not evident at high temperature. There was also an increase in the pool sizes of the C4 acids involved in photosynthesis of A. confertifolia. Speculation as to the explanation of these changes and their possible significance in promoting low temperature C4 photosynthesis in these plants is presented.The C4 pathway of photosynthesis has been most commonly associated with plants of tropical or warm and environments, a notion supported by recent statistical evaluation of the percentage of C4 species in regional grass and dicotyledon floras in North America in relation to climatic factors (27,31). These studies indicate that the occurrence of C4 grasses is best correlated with night temperature during the growing season and that the occurrence of C4 dicotyledons is best correlated with summer pan evaporation. It would be difficult to assess the performance, as opposed to the occurrence, in such a comprehensive way, but the few published studies of C4 photosynthesis as a function of temperature further indicate that most of the grasses examined are low temperature-limited. Thus, several C4 tropical grasses show low temperature compensation points between 6.5 and 10 C (18) and many are chilling-sensitive, showing impaired photosynthesis and Chl breakdown after relatively brief exposure to temperatures to 10 C (20,24,30). A C4 dicotyledon,

“…Recently this contention gained some support from the observation of labeled 3-PGA after dark CO2 fixation of isolated cells of Sedum telephium (2). Reinvestigating the early reports of Bradbeer et al (3) and Jolchine (6), Sutton and Osmond (10) reported that malate was labeled exclusively in the C-4 position after a short time of CO2 dark fixation in leaves of four CAM species, whereas after longer fixation times label also appeared in the C-1 position. These investigators attributed the formerly observed 2:1 ratio to an erroneous method of malate degradation.…”

mentioning

confidence: 82%

Equilibration of Label in Malate during Dark Fixation of CO₂ in Kalanchoë fedtschenkoi

Dittrich¹

1976

In vitro studies of dark 14CO2 fixation with isolated cell aggregates of Kalanchocefedtschenkoi showed that malate synthesized after 20 Bradbeer et al. (3) to occur via two CO2 fixation steps: first, RuDP2 carboxylase was thought to fix CO2 generating two molecules of 3-PGA, which subsequently were converted to PEP; second, this PEP should be subject to a second carboxylation step yielding OAA, finally giving rise to malate. This hypothesis was based upon the finding that label in carbons 4 and 1 in malate was distributed at a constant ratio of 2:1 after short and long periods of dark CO2 fixation. Recently this contention gained some support from the observation of labeled 3-PGA after dark CO2 fixation of isolated cells of Sedum telephium (2). Reinvestigating the early reports of Bradbeer et al. (3) and Jolchine (6), Sutton and Osmond (10) reported that malate was labeled exclusively in the C-4 position after a short time of CO2 dark fixation in leaves of four CAM species, whereas after longer fixation times label also appeared in the C-1 position. These investigators attributed the formerly observed 2:1 ratio to an erroneous method of malate degradation. Additional insight into this matter was provided by in vitro studies of Kluge et al. (7). Upon feeding PEP to tissue slices of two CAM plants during 14CO2 fixation in the dark, the incorporation of 14C was stimulated, but the ratio of label in C-4 to C-1 remained about 7:3, similar to that of the control. This observation was a first indication that the labeling pattern in malate might not be a consequence of carboxylation reactions, but of reactions occurring after carboxylation. This paper presents additional evidence that carboxylation of RuDP is not involved during synthesis of malate in the dark and that label appearing in C-1 of malate is presumably due to the action of fumarase. MATERIALS AND METHODSPlant Material. Mature plants of Kalanchoe fedtschenkoi were kept in an environmental control chamber at a rhythm of 9 hr light (40,000 lux fluorescent tubes plus red light) at 28 C and 40% relative humidity and corresponding 15 hr dark at 17 C and 78% relative humidity. Diurnal changes of acidity in medium size leaves were about 100 ueq/g fresh weight.Selection of Incubation Medium. At the end of the light period, pieces of 2 cm2 were cut out of mature leaves and the epidermis of the lower side was carefully peeled off. These pieces of tissue were floated on water upside down and allowed to take up 14CO2 in the dark for 10 min. After rinsing with distilled H20 for 5 min, the tissue was incubated at 25 C in 5 ml of various solutions, which were renewed each hour. Solution one was distilled H20, solution two contained 50 mm HEPES, pH 7.5, 0.1 mm Ca(NO3)2, 1 mM MgCl2, and 1 mm K2HPO4; solutions three to six contained buffer as solution two plus 0.4, 0.6, 0.8, and 1 M sorbitol, respectively.After 5 hr the leaf tissues were rinsed with distilled H2O and extracted in boiling ethanol. The leakage of radioactivity from the tissues was determined and com...