Bibliothek

Notizen: Abstract Evidence is drawn from previous studies to argue that C3—C4 intermediate plants are evolutionary intermediates, evolving from fully-expressed C3 plants towards fully-expressed C4 plants. On the basis of this conclusion, C3—C4 intermediates are examined to elucidate possible patterns that have been followed during the evolution of C4 photosynthesis. An hypothesis is proposed that the initial step in C4-evolution was the development of bundle-sheath metabolism that reduced apparent photorespiration by an efficient recycling of CO2 using RuBP carboxylase. The CO2-recycling mechanism appears to involve the differential compartmentation of glycine decarboxylase between mesophyll and bundle-sheath cells, such that most of the activity is in the bundlesheath cells. Subsequently, elevated phosphoenolpyruvate (PEP) carboxylase activities are proposed to have evolved as a means of enhancing the recycling of photorespired CO2. As the activity of PEP carboxylase increased to higher values, other enzymes in the C4-pathway are proposed to have increased in activity to facilitate the processing of the products of C4-assimilation and provide PEP substrate to PEP carboxylase with greater efficiency. Initially, such a ‘C4-cycle’ would not have been differentially compartmentalized between mesophyll and bundlesheath cells as is typical of fully-expressed C4 plants. Such metabolism would have limited benefit in terms of concentrating CO2 at RuBP carboxylase and, therefore, also be of little benefit for improving water- and nitrogen-use efficiencies. However, the development of such a limited C4-cycle would have represented a preadaptation capable of evolving into the leaf biochemistry typical of fully-expressed C4 plants. Thus, during the initial stages of C4-evolution it is proposed that improvements in photorespiratory CO2-loss and their influence on increasing the rate of net CO2 assimilation per unit leaf area represented the evolutionary ‘driving-force’. Improved resourceuse efficiency resulting from an efficient CO2-concentrating mechanism is proposed as the driving force during the later stages.

Notizen: Abstract Photosynthetic metabolism was investigated in leaves of five species of Flaveria (Asteraceac), all previously considered to be C4 plants. Leaves were exposed to 14CO2 for different intervals up to 16s. Extrapolation of 14C-product curves to zero time indicated that only F. trinervia and F.bidentis assimilated atmospheric CO2 exclusively through phosphoenolpyruvate carboxylase. The proportion of direct fixation of 14CO2 by ribulose-I, 5-bisphosphate carboxylase/oxygenase (Rubisco) ranged from 5 to 10% in leaves of F. australasica. F. palmeri and F. vaginata.Protoplasts of leaf mesophyll and bundle sheath cells were utilized to examine the intercellular compartmentation of principal photosynthetic enzymes. Leaves of F. australasica, F. palmeri and F. vaginata contained 5 to 7% of the leaf's Rubisco activity in the mesophyll cells, while leaves of F. trinervia and F. bidentis contained at most 0.2 to 0.8% of such activity in their mesophyll cells. Thus, F. trinervia and F. bidentis have the complete C4 syndrome, while F. australasica, F. palmeri and F. vaginata are less advanced, C4-like species.

Notizen: [Auszug] Nasutins have been detected in Nasut^termes exitiosus Hill, N. fumigatus Brauer, N. magnus Froggatt and N. walkeri Hill; but they appear to be absent from N. dixoni Hill and from all species of other genera (Porotermes, Heterotermes, Hamitermes and Drepanotermes) so far examined. With N. exitiosus, ...

Notizen: [Auszug] WE have recently made a few experiments bearing on the phenomena exhibited by Mr. C. E. Benham's artificial spectrum top (see NATURE, November 29, 1894, p. 113), and the explanation of them suggested by Prof. Liveing (NATURE, December 13, 1894, p. 167), and have obtained results which we ...

Notizen: [Auszug] The FAO/UNEFs8'9 estimate of deforestation in the tropics is compared in Table 1 with estimates made by Myers10'12 and the FAO's Production Yearbook13. Comparison of the rates given by FAO/UNEP and the Production Yearbook assumes that deforestation of the open and closed forests of the former is ...

Notizen: Summary Mushrooms were cut into vertical and horizontal sections. These sections were blotted onto nitrocellulose sheets and the sheets were then stained for tyrosinase using l-dopa. Tyrosinase was localized throughout the mushroom tissues but more enzyme was located in the epidermis of the cap, the gill region, and the stipe. Preincubation of the nitrocellulose sheets in specific inhibitors of tyrosinase completely blocked enzyme staining, suggesting that the enzyme stained areas on the nitrocellulose blots were regions of tyrosinase activity. Immunochemical localization of tyrosinase was similar to that observed by histochemical staining. Nitrocellulose blotting of mushrooms allows localizations of enzyme at the whole tissue level and may be useful for other enzymes in mushrooms as well.

Notizen: Summary An extracellular polysaccharide composed of glucose, fucose, arabinose and glucuronic acid in a molar proportion of 11:6:3:1 is a major end-product of photosynthesis by Palmella mucosa Kütz. The liberation of polysaccharide is related to the age of the culture. Glucose can substitute efficiently for CO2 as the source of carbon for polysaccharide synthesis. Nitrate-nitrogen from sodium, potassium and calcium salts can be used in the mineral salts medium with little differences in carbon metabolism of the alga. Ammonium nitrate produces an acidic medium which limits polysaccharide production. The incorporation of C14 into the polysaccharide from NaHC14O3 shows initially a trend toward intracellular synthesis. The C14 appears in the extracellular polysaccharide after prolonged exposure. Glucose-C14 is actively transformed to polysaccharide material which is an indication that glucose may play an important role in the synthesis of polysaccharide by Palmella mucosa Kütz.

Notizen: Abstract The potential for C4 photosynthesis was investigated in five C3-C4 intermediate species, one C3 species, and one C4 species in the genus Flaveria, using 14CO2 pulse-12CO2 chase techniques and quantum-yield measurements. All five intermediate species were capable of incorporating 14CO2 into the C4 acids malate and aspartate, following an 8-s pulse. The proportion of 14C label in these C4 products ranged from 50–55% to 20–26% in the C3-C4 intermediates F. floridana Johnston and F. linearis Lag. respectively. All of the intermediate species incorporated as much, or more, 14CO2 into aspartate as into malate. Generally, about 5–15% of the initial label in these species appeared as other organic acids. There was variation in the capacity for C4 photosynthesis among the intermediate species based on the apparent rate of conversion of 14C label from the C4 cycle to the C3 cycle. In intermediate species such as F. pubescens Rydb., F. ramosissima Klatt., and F. floridana we observed a substantial decrease in label of C4-cycle products and an increase in percentage label in C3-cycle products during chase periods with 12CO2, although the rate of change was slower than in the C4 species, F. palmeri. In these C3-C4 intermediates both sucrose and fumarate were predominant products after a 20-min chase period. In the C3-C4 intermediates, F. anomala Robinson and f. linearis we observed no significant decrease in the label of C4-cycle products during a 3-min chase period and a slow turnover during a 20-min chase, indicating a lower level of functional integration between the C4 and C3 cycles in these species, relative to the other intermediates. Although F. cronquistii Powell was previously identified as a C3 species, 7–18% of the initial label was in malate+aspartate. However, only 40–50% of this label was in the C-4 position, indicating C4-acid formation as secondary products of photosynthesis in F. cronquistii. In 21% O2, the absorbed quantum yields for CO2 uptake (in mol CO2·[mol quanta]-1) averaged 0.053 in F. cronquistii (C3), 0.051 in F. trinervia (Spreng.) Mohr (C4), 0.052 in F. ramosissima (C3-C4), 0.051 in F. anomala (C3-C4), 0.050 in F. linearis (C3-C4), 0.046 in F. floridana (C3-C4), and 0.044 in F. pubescens (C3-C4). In 2% O2 an enhancement of the quantum yield was observed in all of the C3-C4 intermediate species, ranging from 21% in F. ramosissima to 43% in F. pubescens. In all intermediates the quantum yields in 2% O2 were intermediate in value to the C3 and C4 species, indicating a co-function of the C3 and C4 cycles in CO2 assimilation. The low quantum-yield values for F. pubescens and F. floridana in 21% O2 presumably reflect an ineffcient transfer of carbon from the C4 to the C3 cycle. The response of the quantum yield to four increasing O2 concentrations (2–35%) showed lower levels of O2 inhibition in the C3-C4 intermediate F. ramosissima, relative to the C3 species. This indicates that the co-function of the C3 and C4 cycles in this intermediate species leads to an increased CO2 concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase and a concomitant decrease in the competitive inhibition by O2.