Uptake and metabolism of carbohydrates by epidermal tissue

Isolated epidermis of Commelina communis L. and Tulipa gesneriana L. assimilated ¹⁴CO₂ into malic acid and its metabolites but not into sugars or their phosphates; epidermis could not reduce CO₂ by photosynthesis and therefore must be heterotrophic (Raschke and Dittrich, 1977). If, however, isolated epidermis of Commelina communis was placed on prelabelled mesophyll (obtained by an exposure to ¹⁴CO₂ for 10 min), radioactive sugars appeared in the epidermis, most likely by transfer from the mesophyll. Of the radioactivity in the epidermis, 60% was in sucrose, glucose, fructose, 3-phosphoglyceric acid and sugar phosphates. During a 10-min exposure to ¹⁴CO₂, epidermis in situ incorporated 16 times more radioactivity than isolated epidermal strips. Isolated epidermis of Commelina communis and Tulipa gesneriana took up ¹⁴C-labelled glucose-1-phosphate (without dephosphorylation), glucose, sucrose and maltose. These substances were transformed into other sugars and, simultaneously, into malic acid. Carbons-1 through-3 of malic acid in guard cells can thus be derived from sugars. Radioactivity appeared also in the hydrolysate of the ethanol-insoluble residue and in compounds of the tricarboxylic-acid cycle, including their transamination products. The hydrolysate contained glucose as the only radioactive compound. Radioactivity in the hydrolysate was therefore considered an indication of starch. Starch formation in the epidermis began within 5 min of exposure to glucose-1-phosphate. Autoradiograms of epidermal sections were blackened above the guard cells. Formation of starch from radioactive sugars therefore occurred predominantly in these cells. Epidermis of tulip consistently incorporated more ¹⁴C into malic and aspartic acids than that of Commelina communis (e.g. after a 4-h exposure to [¹⁴C]glucose in the dark, epidermis, with open stomata, of tulip contained 31% of its radioactivity in malate and aspartate, that of Commelina communis only 2%). The results of our experiments allow a merger of the old observations on the involvement of starch metabolism in stomatal movement with the more recent recognition of ion transfer and acid metabolism as causes of stomatal opening and closing.Abbreviation G-1-P glucose-1-phosphate     

Abscisic acid levels and metabolism in the leaf epidermal tissue of Tulipa gesneriana L. and Commelina communis L.

We have shown the presence of abscisic acid (ABA) in abaxial epidermal strips taken from Tulipa gesneriana and Commelina communis and that the ABA level rises in the epidermis when leaves are water stressed. ABA levels had risen 50% in the abaxial epidermis of C. communis 30 min after the leaves lost 10% of their fresh weight. Epidermis from both T. gesneriana and C. communis metabolize [¹⁴C]ABA to several products probably including phaseic acid (PA) and dihydrophaseic acid (DPA).Abbreviations ABA abscisic acid - RIA radioimmunoassay - PA phaseic acid - DPA dihydrophaseic acid - TLC thin-layer chromatography - GC gas chromatography     

Carbon dioxide fixation and related properties in sections of the developing green maize leaf

Light and dark ¹⁴CO₂ assimilation, pulse-chase (¹⁴CO₂ followed by ¹²CO₂) labeling experiments both in the light and in the dark, photorespiratory activity and some enzymes (ribulose 1,5-bisphosphate (RuBP) carboxylase, phosphoenolpyruvate (PEP) carboxylase, and NADP-malic enzyme) were followed in sections of 2.5 centimeters from the base (younger tissue) to the tip (oldest tissue) of the green maize leaf. Tissue was taken from the third leaf of 12- to 16-day-old plants consisting of sections 0 to 2.5 centimeters (base), 4.5 to 7.0 centimeters (center) and 9.0 to 11.5 centimeters (top) measured from the base. Some of these properties were also determined in the intact leaves of 4-day-old maize plants.     

Carbon dioxide enhances the development of the ethylene forming enzyme in tobacco leaf discs

Since CO₂ is known to stimulate ethylene production by promoting the conversion of 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene, the effect of CO₂ on the activity and the development of the ethylene forming enzyme (EFE) was studied in tobacco (Nicotiana tabacum L. cv Havana 425 and Xanthi) leaf discs. In addition to previous observations that EFE activity is dependent on CO₂ concentration and is saturable with 2% CO₂, present data show two saturation curves at 2% and 10% CO₂. Promotion of EFE development was dependent also on CO₂ concentration (saturated at 2% CO₂) and duration (maximum at 24 in the dark), and was abolished by 20 micromolar cycloheximide. Application of exogenous ethylene (20 microliters per liter) or light treatment further increased the CO₂-enhanced development of EFE, implying that these two factors can also affect EFE development via interaction with CO₂. The results suggest that CO₂ exerts its stimulatory effect on the conversion of ACC to ethylene by enhancing not only the activity but also the synthesis of EFE in leaf discs.     

Effects of elevated carbon dioxide on green leaf tissue and leaf litter quality in an intact Mojave Desert ecosystem

The effects of elevated carbon dioxide (CO₂) on plant litter are critical determinants of ecosystem feedback to changing atmospheric CO₂ concentrations. We measured concentrations of nitrogen (N) and carbon (C) and calculated C : N ratios of green leaves of two desert perennial shrubs, and the same quality parameters plus lignin and cellulose content of leaf litter from four shrub species exposed to elevated CO₂ (FACE technology; Hendrey & Kimball, 1994 ) for 3 years in an intact Mojave Desert ecosystem. Shrubs tested were Larrea tridentata, Lycium pallidum, Lycium andersonii and Ambrosia dumosa. We calculated resorption efficiency from green tissue and leaf litter N data and measured lignin and cellulose content in litter in the last year study. Green leaves of L. tridentata grown under elevated CO₂ had significantly lower N concentrations and higher C : N ratios than shrubs grown in ambient conditions in 1999 (P < 0.05). Lycium pallidum green leaves grown under elevated CO₂ had significantly lower N concentrations and higher C : N ratios than shrubs grown under ambient conditions in 2000 (P < 0.05). There was no CO₂ effect on C content of either species. We found no effect of CO₂ on N or C content, C : N ratios, or lignin or cellulose concentrations in leaf litter of L. tridentata, L. pallidum, L. andersonii, or A. dumosa. There was no significant effect of CO₂ on estimates of shrub resorption efficiency. There was a seasonal effect on green tissue and litter tissue quality for L. tridentata, with lower tissue N content in summer than in spring or winter months. These data suggest that any productivity increases with elevated CO₂ in desert ecosystems may not be limited by lower leaf litter quality and that resorption efficiency calculations are best performed on an individual leaf basis.     

Carbon metabolism in spinach leaves as affected by leaf age and phosphorus and sulfur nutrition

Spinach (Spinacea oleracea) plants were grown either continuously on complete nutrient solutions or for 2 weeks on media deficient in phosphate or sulfate. To characterize leaf carbohydrate metabolism, levels of phosphorylated intermediates, activities of enzymes involved in photosynthetic carbon metabolism, contents of soluble and acid hydrolyzable sugars were measured in leaves differing in age and mineral status and related to leaf rates of photosynthesis and assimilate partitioning. Concentrations of metabolites—particularly those which are preferentially compartmented in the cytosol—decreased from young to old leaves and were lowest in old phosphate starved leaves. Nutrient deficiency showed comparable effects on stromal and cytosolic intermediates. Whole leaf ATP to ADP ratios were dependent on the growth regime, but did not much change with leaf age. The assimilatory force increased in all leaves suffering from mineral deficiency; the assimilatory force was low when photosynthesis was high and vice versa. Sugars accumulated although enzyme activities were decreased under deficiency. The results show that growth of P- and S-starved plants is not limited by photosynthetic reactions.     

Carbon dioxide and nitrite photoassimilatory processes do not intercompete for reducing equivalents in spinach and soybean leaf chloroplasts

Previously, C Baysdorfer and JM Robinson (1985 Plant Physiol 77: 318-320) demonstrated that, in a reconstituted spinach chloroplast system, NADP photoreduction functioning at most maximal rate and reductant demand, was the successful competitor with NO₂⁻ photoreduction for reduced ferredoxin. This resulted in a repression of NO₂⁻ reduction until all NADP available had been almost totally reduced. Further experiments, employing isolated, intact spinach leaf plastids and soybean leaf mesophyll cells, were conducted to examine competition for reductant between CO₂ and NO₂⁻ photoassimilation, in situ. In isolated, intact plastid preparations, regardless of whether the demand for reductant by CO₂ photoassimilation was high (5 millimolar `CO<sub>2</sub>') with rates of CO<sub>2</sub> fixation in the range 40 to 90 micromoles CO<sub>2</sub> fixed per hour per milligram chlorophyll, low (0.5 millimolar `CO₂') with rates in the range 5 to 8 micromoles CO₂ per hour per milligram chlorophyll, or zero (no `CO<sub>2</sub>'), NO<sub>2</sub><sup>−</sup> photoreduction displayed equal rates in the range of 8 to 22 micromoles per hour per milligram chlorophyll. In the absence of `CO₂', but in the presence of saturating white light, 3-phosphoglycerate photoreduction at rates of 82 to 127 micromoles per hour per milligram chlorophyll did not repress, and occasionally stimulated concomitant rates of NO₂⁻ reduction which ranged from 23.4 to 38.5. Conversely, in plastid preparations, NO₂⁻ at levels of 50 to 100 micromolar, stimulated plastid CO₂ fixation when `CO<sub>2</sub>' was saturating with respect to carboxylation. Further, levels of NO<sub>2</sub><sup>−</sup> in the range 250 to 2500 micromolar, stimulated soybean leaf mesophyll cell net CO<sub>2</sub> fixation as much as 1.5-fold if `CO₂' was saturating with respect to CO₂ fixation. It appeared likely that, in high light in vivo, CO₂ and NO₂⁻ photoassimilatory processes are not forced to intercompete for reduced ferredoxin in the intact chloroplast.     

Carbon dioxide supersaturation in Florida lakes

We examined data on CO₂ and related limnological and geographic information from a sample of 948 Florida freshwater lakes. The objectives for this study were (1) to determine the partial pressures of carbon dioxide (ρCO₂) in the surface waters of a large sample of Florida lakes, (2) to determine if several limnological or geographic factors are related to levels of ρCO₂ in Florida lakes, and (3) to estimate the net annual rate of loss of CO₂ to the atmosphere from the freshwater lakes of Florida. The calculated ρCO₂ for the lakes in our sample range from 0 to 81,000 μatm, with a mean of 3,550 μatm, a median of 1,030 μatm, and a geometric mean of 1,270 μatm. About 87% of the Florida lakes were supersaturated with CO₂. There were statistically significant correlations between values for ρCO₂ and several water chemistry variables; however, the R ² values were small and accounted for only a small portion of the variance. In general the ρCO₂ values were higher in the lakes with low alkalinities and low contents of dissolved salts. The best predictor of ρCO₂ is pH, with an R ² of 0.82 for a polynomial relationship. The ρCO₂ values tend to decrease from northwest to southeast across the state of Florida, which corresponds to the gradients we found for pH, alkalinity, and specific conductance. The average areal rate of carbon emission from the Florida lakes was 328 g C m⁻² y⁻¹, and the total carbon loss for the lakes and ponds of Florida was 2.0 Tg y⁻¹. This amounts to about 2% of the total carbon emissions from all the lakes of the world as estimated by previous studies. Handling editors: Darren Bade     

Carbon dioxide fixation in trypanosomatids.

Fixation of carbon dioxide has been demonstrated for extracts from Crithidia fasciculata, Trypanosoma mega and Trypanosoma brucei brucei bloodstream and culture forms. The enzymes involved in this fixation were found to be ADP-stimulated phosphoenolpyruvate carboxykinase (E.C. 4.1.1.32), 'malic' enzyme (E.C. 1.1.138-40) and pyruvate carboxylase (E.C. 6.4.1.1). The subcellular localization of these enzymes has been investigated in all three organisms. Products of short and long term fixation experiments were separated and identified. The importance of carboxylation reactions is discussed in relation to the maintenance of oxidized and reduced coenzyme levels.     

Water balance in leaf tissue

Samples of the leaf tissue (14cm²) were placed in a plexiglass chamber which consisted of three parts. Water absorbed by the leaf tissue on one side of the sample was transported through the middle part of the sample to the opposite side and was transpirated there. The intensity of transpiration the intensity of water absorption and water saturation deficit (w.s.d.) were determined simultaneously in this tissue by gravimetry. Water balance was studied either in saturated samples of leaf tissue or in tissue where w.s.d. (10%, 20%, 30%, 40%) was established in advance. Although conditions for water absorption in leaf segments were optimal, w.s.d. originated in the saturated leaf tissue under all given external conditions (evaporation from 41.7 to 17.8 mg cm^?2 h^?1). W.s.d. which was established in advance for the most part increased during the experiment and reached even high values (more than 60%). the equilibration was reached only under conditions of low evaporation and initial w.s.d. higher than 20% in young leaves and higher than 30% in adult leaves. A positive correlation between the ratio of the intensity of water absorption to the intensity of transpiration and w.s.d. was found only under conditions of lower evaporation (17.8 and 23.2mg cm^?2h^?1). The maximal values of w.s.d. were limited in this way. Water balance was studied: 1. in leaf tissue of upper, middle and lower leaves of fodder cabbage, 2. in leaf tissue of middle leaves of young and adult plants of fodder cabbage, 3. in leaf tissue of dicots (fodder cabbage) with different vessel orientation in respect to water transport, 4. in leaf tissue of monocots (banana-tree) with water transport upright to the vessel orientation. Considerable change of water balance was observed when the water transport was prolonged by two incisions in the middle part of the sample. Results of all these experiments revealed the possibility of water stress origin even in leaf tissue sufficiently supplied with water.