Water is allocated differently to chloroplasts in sun and shade leaves

Hydrogen-1 nuclear magnetic resonance spectroscopy was used to study water allocation in cell compartments of sun and shade leaves. NMR spectra of Acer platanoides were resolved into two peaks that were assigned to chloroplast and nonchloroplast water. Sun leaves contained 1.7 times more water per unit area of surface than shade leaves, and the water was allocated differently. Chloroplasts in sun leaves contained 17% of the total leaf water versus 47% in shade leaves. Comparing equal leaf surface areas, the chloroplasts in shade leaves contained 60% more water than those in sun leaves.     

Overwintering Periwinkle (Vinca minor L.) Exhibits Increased Photosystem I Activity

The effects of natural, overwintering conditions on photosystem I and photosystem II activity were examined in isolated thylakoids of periwinkle (Vinca minor L.), an endemic, cold-tolerant, herbaceous evergreen. DCMU-Insensitive photosystem I activity (ascorbate/dichlorophenolindophenol → methylviologen) exhibited a twofold increase in light-saturated rates upon exposure to low temperature and freezing stress with no effect on the apparent quantum yield of this reaction. DCMU-Sensitive photosystem II activity (H₂O → dichlorlophenolindophenol) exhibited only minor fluctuations in light-saturated rates but a 50% decrease in the apparent quantum yield of this reaction upon exposure to overwintering conditions. This was correlated with a decrease in the 77°K fluorescence emission at 694 nanometers. These functional changes occurred with no detectable changes in the relative chlorophyll contents of the chlorophyll-protein complexes or the chlorophyll-thylakoid protein. The chlorophyll a/b varied less than 10% during any single growth year. Analyses of total leaf extracts indicated that all lipid classes exhibited increased levels of linoleic and linolenic acid. Neither the trans-Δ³-hexadecenoic acid level nor the ratio of oligomeric:monomeric light harvesting of photosystem II was affected by exposure to winter stress. The content of the major chloroplast lipids monogalactosyldiacylglycerol, digalactosyldiacylglycerol, phosphatidyl-diacyl-glycerol, and sulfoquinovosyldiacylglycerol exhibited minor fluctuations, whereas phosphatidylcholine and phosphatidylethanolamine content doubled on a mole percent or chlorophyll basis. We conclude that the previously reported increase in photosystem I activity during controlled, low temperature growth is observed during exposure to natural overwintering conditions. This appears to occur with minimal changes in the structure and composition of the photosynthetic apparatus of periwinkle.     

Arabinogalactan-Proteins from Primary and Mature Roots of Radish (Raphanus sativus L.)

Organ-specific variations in blood group H-like activity were observed in developing radish plants. A temporary increase in serological activity was found to occur in the roots at the earlier stages of development. Arabinogalactan-proteins (AGPs) were isolated from primary and mature roots, and investigated for changes in their physicochemical properties, structure, and serological activities. These root AGPs were composed mainly of l-arabinose and d-galactose but were distinguishable from each other in their contents of l-fucose as well as of protein and hydroxyproline. The structures of the carbohydrate moieties of the root AGPs were essentially similar to those of AGPs isolated from seeds and mature leaves in that they consisted of consecutive (1→3)-linked β-d-galactosyl backbone chains having side chains of (1→6)-linked β-d-galactosyl residues, to which α-l-arabinofuranosyl residues were attached in the outer regions. One prominent feature of the primary root AGPs was that they contained appreciable amounts of l-fucose, which was presumably responsible for expression of the serological activity. In their immunological reactions with rabbit anti-radish leaf AGP antibody, the root AGPs were shown to share common antigenic determinant(s) with those of seed and leaf AGPs.     

Characterization of fructan from mature leaf blades and elongation zones of developing leaf blades of wheat, tall fescue, and timothy

Water-soluble carbohydrate composition of mature (ceased expanding) leaf blades and the elongation zone of developing leaf blades was characterized in wheat (Triticum aestivum L.), tall fescue (Festuca arundinacea Schreb.), and timothy (Phleum pratense L.). These species were chosen because they differ in mean degree of polymerization (DP) of fructan in the mature leaf blade. Our objective was to compare the nature and DP of the fructan. Vegetative plants were grown with a 14-hour photoperiod and constant 21°C at the leaf base. Gel permeation chromatography of leaf blade extracts showed that the apparent mean fructan DP increased in the order wheat < tall fescue < timothy. Apparent mean DP of elongation zone fructan was higher than that of leaf blade fructan in wheat and timothy, but the reverse occurred for tall fescue. Low DP (≤10) and high DP (>10) pools were found in both tissues of tall fescue and wheat, but concentration of low DP fructan was very low in either tissue of timothy. All three species have high DP fructan. Comigration with standards on thin-layer chromotography showed that wheat contained 1-kestose and a noninulin fructan oligomer series. Tall fescue contained neokestose, 1-kestose and higher oligosaccharides that comigrated with neokestose-based compounds and inulins. Thin-layer chromatography showed that small amounts of fructose-containing oligosaccharides were present in timothy.     

Mechanisms of starvation tolerance in pearl millet

The response of pearl millet (Pennisetum glaucum [L.]) seedlings to prolonged starvation was investigated at the biochemical and ultrastructural level. After 2 days of darkness the bulk of the seedling carbohydrate reserves were depleted. After 8 days in the dark the respiratory rate had declined to less than 50% of its initial value and the plants had lost half of their total protein content. Unlike the situation with carbohydrate depletion, protein loss was restricted to specific organs. The secondary leaf and stem (including the apical meristem) showed little or no protein loss during this period. In the primary leaf, seed, and roots, protein loss was substantial. In spite of the high rate of protein degradation in the primary leaf and roots, these organs showed no ultrastructural changes suggestive of tissue, cellular, or subcellular degradation. In addition, ribulose bisphosphate carboxylase was not preferentially degraded during starvation and only a small decline in chlorophyll content was observed after 8 days in the dark. During the period from 8 to 14 days, cell death started at the tip of the primary leaf and gradually spread downward. Both shoot and root meristems remained alive up to 14 days. Consequently, the eventual death of the plant was due to the loss of the carbohydrate-producing regions rather than the meristems. We suggest that these results provide an explanation for the high degree of starvation tolerance exhibited by pearl millet.     

Effect of Restricted Root Growth on Carbohydrate Metabolism and Whole Plant Growth of Cucumis sativus L

The effects of varied rooting volumes on root growth and source leaf carbohydrate metabolism were studied in greenhouse-grown cucumber (Cucumis sativus L cv Calypso) plants. Plants were grown for 7 weeks in container volumes that ranged from 0.4 to 5.9 liters. Plants grown in the smaller containers exhibited less leaf expansion, lower root and shoot weight, and fewer lateral stems than plants grown in the 5.9 liter containers. Shoot/root ratio was not altered by the container volume, suggesting coordination of root and shoot growth due to rooting volume. Source leaf carbon exchange rates, assimilate export rates, and starch accumulation rates for plants grown in 0.4 liter containers were approximately one-half or less in comparison to those for plants grown in 5.9 liter containers. Starch concentrations per unit leaf area were maintained at high levels in source leaves of plants grown in 0.4 liter containers over the entire day/night cycle. Lower extractable galactinol synthase activities and higher galactinol concentrations occurred in leaves of plants grown in 0.4 liter container volumes. The reduced sink demand, induced by restricted root growth, may have led to increased starch concentrations and to a reduction in stachyose biosynthesis in cucumber source leaves.     

Role of Magnesium in the Binding of Substrate and Effectors to Phosphoenolpyruvate Carboxylase from a CAM Plant

The binding of phosphoenolpyruvate, malate, and glucose 6-phosphate to phosphoenolpyruvate carboxylase purified from Crassula argentea Thunb. was measured using both the intrinsic tryptophan fluorescence of the enzyme and the extrinsic fluorescence of the complex of 8-anilino-1-napthalenesulfonate with the enzyme. It was found that the substrate phosphoenolpyruvate can bind in the absence of magnesium but is bound in greater quantities and more tightly when magnesium is present. Malate reduces the binding of phosphoenolpyruvate, while glucose 6-phosphate increases the binding of the substrate. Glucose 6-phosphate requires magnesium to bind to the enzyme, while malate does not. The general trends from the binding experiments using fluorescence methods were confirmed by activity determinations using assays performed in the absence of magnesium.     

Sink Metabolism in Tomato Fruit : II. Phloem Unloading and Sugar Uptake

Analysis of [³H]-(fructosyl)-sucrose translocation in tomato (Lycopersicon esculentum Mill.) indicates that phloem unloading in the fruit occurs, at least in part, to the apoplast followed by extracellular hydrolysis. Apoplastic sucrose, glucose, and fructose concentrations were estimated as 1 to 7, 12 to 49, and 8 to 63 millimolar, respectively in the tomato fruit pericarp tissue. Hexose concentrations were at least four-fold greater than sucrose at all developmental stages. Short-term uptake of [¹⁴C]sucrose, -glucose, and -fructose in tomato pericarp disks showed first order kinetics over the physiologically relevant concentration range. The uptake rate of [¹⁴C]-(glucosyl)-1′-fluorosucrose was identical to the rate of [¹⁴C]sucrose uptake, suggesting sucrose may be taken up directly without prior extracellular hydrolysis. Short-term uptake of all three sugars was insensitive to 10 micromolar carbonyl cyanide m-chlorophenylhydrazone and to 10 micromolar p-chloromercuribenzene sulfonic acid. However, long-term accumulation of glucose was sensitive to carbonyl cyanide m-chlorophenylhydrazone. Together these results suggest that although sucrose is at least partially hydrolyzed in the apoplast, sucrose may enter the metabolic carbohydrate pool directly. In addition, sugar uptake across the plasma membrane does not appear to be energy dependent, suggesting that sugar accumulation in the tomato fruit is driven by subsequent intracellular metabolism and/or active uptake at the tonoplast.     

First evidence for polyamine conjugation mediated by an enzymic activity in plants

An enzyme activity, found for the first time in plants, mainly located in the 22,000g supernatant of the crude extract of sprout apices of Helianthus tuberosus L. cv OB1 tubers, is able in vitro to covalently bind polyamines to endogenous substrates of different molecular weights. The major assay parameters, such as pH, dithiothreitol, and extract concentrations were optimized. The time course of the reaction, the dependence on putrescine concentration, its competition with histamine, the capacity to bind spermidine and spermine better than putrescine, the stability of the reaction product and analysis of the latter by sodium dodecyl sulfate polyacrylamide gel electrophoresis and thin-layer chromatography suggest that putrescine is linked to endogenous substrates by means of an enzyme reaction that shows some similarities with transglutaminase activities detected in animals. However, the activities of the crude extract and of a fraction eluted from a Sephadex G25 column were not affected by CaCl₂ concentrations lower or equal to 5 millimolar; 4 or 10 millimolar EGTA caused a very small reduction; higher concentrations of CaCl₂ and 15 millimolar or more of EDTA were inhibitory. N,N′-dimethylcasein was not recognized as a substrate. These data indicate that this activity also displays some characteristics which are different from those of animal transglutaminases.