The mechanism of stomatal movement in Allium cepa L.

In the guard cells of Allium cepa leaves, no starch was found either when the stomata were open or closed. The lack of other soluble polysaccharides that could be hydrolyzed during the opening reaction of the stomata (Schnabl, Planta 1977, in press) leads to the question, how is the osmotic effect, which is the basis of the stomatal movement, achieved in Allium? It is shown in this paper, by histochemical and microprobe analyses, that in Allium — as in other plant species—the K⁺ concentration of the guard cells increases during stomatal opening. The charges of the K⁺ ions in the guard cells seem to be fully compensated by imported Cl^- ions. This could mean that if starch is present in the guard cells, as in the majority of plant species, its major role in the mechanism of stomatal movement is to deliver the cuunteranions for the imported K⁺ ions.     

Autotrophy in maize husk leaves : evaluation using natural abundance of stable isotopes

The natural abundance of carbon and hydrogen isotopic composition, expressed as a δ¹³C value of plant dry matter and cellulose in the hypsophylls (husk leaves) of maize (Zea mays L.) was measured and compared with that of leaves and cobs. The δ¹³C values of outer hypsophylls were usually 2 to 3%‰ more negative than leaves or other tissues, and became more negative with increasing chlorophyll content, indicating significant local C₃ pathway fixation of CO₂ in the outer hypsophylls. The δD values indicated a significant part of hypsophyll cellulose was derived from heterotrophic sources (sucrose from C₄ photosynthesis in other tissues). Isotopic mass balance calculations allowed quantitative estimation of these carbon sources and, in the samples examined, about 16% of hypsophyll cellulose was derived from local C₃ photosynthesis, about 62% from local C₄ photosynthesis, and about 22% from sucrose imported from other leaves.     

Metabolism of the alpha-Pyridone Ring of Ricinine in Ricinus communis L

Chemically synthesized ricinine-3,5-¹⁴C was used to study the metabolism of this alkaloid in the plant which produces it, Ricinus communis L. In a time course study, ricinine-3,5-¹⁴C was administered to a series of castor plants (Ricinus communis L.) and the radioactivity recovered in the ricinine samples showed a decrease with increase in time. It was also observed that the alkaloid was translocated to the seed. The in vivo conversion of ricinine-3,5-¹⁴C to respiratory ¹⁴CO₂ occurred in both light and dark and indicated that the α-pyridone ring of ricinine could be degraded.     

<Emphasis Type="Italic">Virola sebifera</Emphasis> Aubl. (Myristicaceae) leaf chemical composition and implications on leaf-cutter ant foraging choice

Leaf-cutter ant plant material choice is essential for colony maintenance and growth. Plant material is used as a substrate for cultivating symbiotic fungus, and the ants’ preference for particular leaves, tends to be determined by vegetal age-related physicochemical factors. The plant species Virola sebifera Aubl. (Myristicaceae), for example, shows a large number of leaf surface trichomes. Although non-glandular, V. sebifera trichomes may gradually retain an increasing amount of chemical compounds over the lifetime of the leaf. Thus, the present study aims to investigate the role of plant chemical compounds on Atta sexdens rubropilosa preference for V. sebifera leaves of different ages. For this purpose, the chemical composition of trichomes on young and senescent leaves was analyzed, and ants’ preference tested. The chemical compositions differ between V. sebifera young and senescent leaves, with triacontane (C30) predominance in young leaves and tetratriacontane (C34) predominance in senescent leaves. Ants’ preference choice was tested by randomly offering leaves of different ages to A. sexdens rubropilosa workers, following six different treatments: (1) young leaf fragments; (2) young leaf fragments with few trichomes removed; (3) loose trichomes from young leaves; (4) senescent leaf fragments; (5) senescent leaf fragments with few trichomes removed, and (6) loose trichomes from senescent leaves. Ants’ preference was observed for young leaves fragments with a few trichomes removed and also for young leaves loose trichomes. Ants’ preference might be due to specific volatile compounds (GLV) preset in V. sebifera young leaves. Results suggest occurrence of ants’ selectivity resulting from changes on trichomes chemical composition between V. sebifera leaves different age stages.     

Microscopy,Culture, and Quantitative Real-Time PCR Examination Confirm Internalization of Mycobacteria in Plants

The environment is a reservoir of nontuberculous mycobacteria and is considered a source of infection for animals and humans. Mycobacteria can persist in different types of environments for a relatively long time. We have studied their possible internalization into plant tissue through intact, as well as damaged, root systems of different types of plants grown in vitro and under field conditions. The substrate into which plants were seeded was previously contaminated with different strains of Mycobacterium avium (10⁸ to 10¹⁰ cells/g of soil) and feces from animals with paratuberculosis. We detected M. avium subsp. avium, hominissuis, and paratuberculosis in the stems and leaves of the plants by both culture and real-time quantitative PCR. The presence of mycobacteria in the plant tissues was confirmed by microscopy. The concentration of mycobacteria found inside plant tissue was several orders of magnitude lower (up to 10⁴ cells/g of tissue) than the initial concentration of mycobacteria present in the culture medium or substrate. These findings led us to the hypothesis that plants may play a role in the spread and transmission of mycobacteria to other organisms in the environment.     

The relationship of the pyridine nucleotide cycle to ricinine biosynthesis in Ricinus Communis

Quinolinic acid-6-¹⁴C feeding experiments were performed using Ricinus communis L. plants to determine the efrect of selected inhibitors on the incorporation of this precursor into the alkaloid, ricinine, and into the pyridine nucleotide cycle intermediates. Administration of azaserine and azaleucine resulted in a decrease in the incorporation into ricinine and an increase in the radioactivity remaining in quinolinic acid. Administration of excess ricinine and ethionine yielded similar results. This mutual inhibition indicated an interdependency between the conversion of quinolinic acid to ricinine and to the pyridine nucleotide cycle intermediates.     

Nitrogen Metabolism in Senescent Flag Leaves of Wheat (Triticum aestivum L.) in the Light

Nitrogen metabolism was examined in senescent flag leaves of 90- to 93-day-old wheat (Triticum aestivum L. cv Yecora 70) plants. CO₂ assimilation and the levels of protein, chlorophyll, and nitrogen in the leaves decreased with age. Glutamine synthetase activity decreased to one-eighth of the level in young flag leaves. Detached leaves were incubated (with the cut base) in ¹⁵N-labeled NH₃, glutamate, or glycine in the light (1.8 millieinstein per square meter per second) at 25°C in an open gas exchange system under normal atmospheric conditions for up to 135 minutes. The ¹⁵N-enrichment of various amino acids derived from these ¹⁵N-substrates were examined. The amido-N of glutamine was the first ¹⁵N-labeled product in leaves incubated with ¹⁵NH₄Cl whereas serine, closely followed by the amido- and amino-N of glutamine, were the most highly ¹⁵N-labeled products during incubation with [¹⁵N]glycine. In contrast, aspartate and alanine were the first ¹⁵N-labeled products when [¹⁵N] glutamate was used. These results indicate that NH₃ was assimilated via glutamine synthetase and glutamate synthase activities and the photorespiratory nitrogen cycle remained functional in these senescent wheat flag leaves. In contrast, an involvement of glutamate dehydrogenase in the assimilation of ammonia could not be detected in these tissues.     

Relationship between leaf development,carboxylase enzyme activities and photorespiration in the C4-plant Portulaca oleracea L.

Summary Ribulose diphosphate (RuDP) and (PEP) phosphoenolpyruvate carboxylase enzyme activities were studied in young, mature, and senescent Portulaca oleracea leaves. While the absolute amount of both the C₃ (RuDP) and C₄ (PEP) carboxylase is less in senescent leaves than in mature leaves, RuDP carboxylase activity is reduced to a lesser degree. In senescent leaves, PEP carboxylase activity equals 10% of that in mature tissue, but RuDP carboxylase is 27% of that in mature leaves. The same ontogenetic series was also used to determine photorespiration rates and responses to several gas treatments. Young and mature leaves were unaffected by changes in the light regime or oxygen concentrations, and exhibited typical C₄-plant light/dark ¹⁴CO₂ evolution ratios. Senescent leaves, on the other hand, have photorespiration ratios similar to C₃-plants. In addition, senescent leaves were affected by minus CO₂, 100% O₂ and N₂ in a manner expected of C₃-plants, but not C₄-plants. These results are discussed in terms of a relative increase in activity of the C₃ cycle in later developmental stages in this plant.Abbreviation RuDP ribulose diphosphate - PEP phosphoenolpyruvate - PGA phosphoglyceric acid     

Changes in beta-1,3-Glucan Synthase Activity in Developing Lima Bean Plants

A plasma membrane-enriched fraction was isolated from various tissues of developing lima bean seedlings, Phaseolus lunatus var Cangreen, to study β-1,3-glucan synthase activity changes. All tissues contained an active β-glucan synthase, including the cotyledons that will be senescent in mature lima bean plants. Young primary leaves exhibited a very active β-glucan synthase; but this activity dropped markedly, about fivefold, as the leaves gained weight and became photosynthetic. Some tissues, such as the hypocotyl and young stem, exhibited an increase in β-glucan synthase activity as the tissues were growing and a decrease as the growth rate slowed. Roots exhibited a high activity early in development that only decreased slightly, about 30%, as root growth increased. Surprisingly the senescent cotyledons contained an activity equivalent to some other tissues that was maintained over our measurement time of 21 days. Perhaps this callose synthesis activity is related to translocation processes as the cotyledons transfer their reserves to the growing seedling. We concluded that β-glucan synthase was not a good indicator of sink strength in these lima bean tissues. The plasma membrane fractions also were tested for other enzymes that might be present because an electron microscope study revealed a low contamination by other types of membranes. The membrane fractions had low but detectable activities of sucrose synthase, UDPglucose pyrophosphorylase, UDPase, alkaline invertase, and a general phosphatase; but these enzymes exhibited no consistent pattern(s) of activity change with plant development.     

Oil body-mediated defense against fungi: From tissues to ecology

Oil bodies are localized in the seed cells and leaf cells of many land plants. They have a passive function as storage organelles for lipids. We recently reported that the leaf oil body has an active function as a subcellular factory that produces an antifungal oxylipin during fungal infection in Arabidopsis thaliana. Here, we propose a model for oil body-mediated plant defense. Remarkably, senescent leaves develop oil bodies and accumulate α-dioxygenase1 (α-DOX1) and caleosin3 (CLO3) on the oil-body membrane, which catalyze the conversion of α-linolenic acid to the phytoalexin 2-hydroxy-octadecatrienoic acid (2-HOT). The model proposes that senescent leaves actively produce antifungal oxylipins and phytoalexins, and abscised leaves contain a mixture of antifungal compounds. In natural settings, the abscised leaves with antifungal compounds accumulate in leaf litter and function to protect healthy tissues and young plants from fungal infection. Plants might have evolved this ecological function for dead leaves.     

Activation of the de novo pathway for pyridine nucleotide biosynthesis prior to ricinine biosynthesis in castor beans

The ricinine content of etiolated seedlings of Ricinus communis increased nearly 12-fold over a 4-day period. In plants quinolinic acid is an intermediate in the de novo pathway for the synthesis of pyridine nucleotides. The only known enzyme in the de novo pathway for pyridine nucleotide biosynthesis, quinolinic acid phosphoribosyltransferase, increased 6-fold in activity over a 4-day period which preceded the onset of ricinine biosynthesis by 1 day. The activity of the remainder of the pyridine nucleotide cycle enzymes in the seedlings, as monitored by the specific activity of nicotinic acid phosphoribosyltransferase and nicotinamide deamidase, was similar to that found in the mature green plant. In the roots of Nicotiana rustica, where the pyridine alkaloid nicotine is synthesized, the level of quinolinic acid phosphoribosyltransferase was 38-fold higher than the level of nicotinic acid phosphoribosyltransferase, whereas in most other plants examined, the specific activity of quinolinic acid phosphoribosyltransferase was similar to the level of activity of enzymes in the pyridine nucleotide cycle itself. A positive correlation therefore exists between the specific activity of a de novo pathway enzyme catalyzing pyridine nucleotide biosynthesis in Ricinus communis and Nicotiana rustica and the biosynthesis of ricinine and nicotine, respectively.     

Bionomics of <Emphasis Type="Italic">Momordica cochinchinensis</Emphasis> Fed <Emphasis Type="Italic">Aulacophora foveicollis</Emphasis> (Coleoptera: Chrysomelidae)

The effects of feeding on root by the larvae and three types of Momordica cochinchinensis Spreng (Cucubitaceae) leaves (young, mature and senescent) by the adults of Aulacophora foveicollis Lucas (Coleoptera: Chrysomelidae) were studied under laboratory conditions. Total larval developmental time was 19.7 ± 0.2 days by feeding on young roots. Adult males lived for 28.4 ± 1, 65.7 ± 1.1 and 22.8 ± 1.3 days on young, mature and senescent leaves, respectively; whilst adult females lived for 34.3 ± 1.2, 68.5 ± 0.9 and 26.4 ± 1.4 days on young, mature and senescent leaves, respectively. Fecundity was highest in mature leaves fed insects (202.2 ± 10.6). Total carbohydrate, protein, lipid, nitrogen and amino acid were much higher in root followed by mature leaves than young and senescent leaves. Moisture content was highest in mature leaves than the roots, young and senescent leaves. Phenols were greatest in young leaves followed by mature leaves and least in senescent leaves and roots of the said plant. Flavonols were higher in young leaves and least in root. These results suggest that A. foveicollis adults perform better on mature leaves than young and senescent leaves for their nutrition.     

Biosynthesis of sterols and triterpenes in Pelargonium hortorum

Although all parts of the geranium plant (Pelargonium hortorum) are capable of synthesizing sterols and triterpenes and their esters in vitro from mevalonic acid-[2-¹⁴C], the aerial portions are more active than other tissues. All plant parts were shown to incorporate mevalonic acid-[2-¹⁴C] into isoprenoids for at least 3 days. The leaves and petioles had the greatest incorporation on a wet weight basis. Chopped preparations showed comparable incorporations of mevalonate whereby rootlets incorporated about one half as much as most parts; the flower petals incorporated five times the average amount. In leaves the principal sterol synthesized was sitosterol. Metabolic studies with isolated leaves indicated a fairly rapid conversion of free tetracyclic triterpenes to 4-desmethyl-sterols, while β-amyrin was synthesized at a different rate than α-amyrin. Esterified tetracyclic triterpenes exhibited only a slight amount of conversion to 4-desmethylsterols.     

NaCl胁迫下沙枣幼苗生长和阳离子吸收、运输与分配特性

沙枣(Elaeagnus angustifolia L.)耐盐性强,是我国北方生态脆弱地区造林绿化的一个先锋树种。为探讨沙枣的盐适应机制,研究了不同浓度NaCl(0、100和200 mmol/L)胁迫30d对其水培幼苗生物量累积以及不同组织(根、茎、叶)K+、Na+、Ca2+和Mg2+吸收、运输与分配的影响。结果表明:盐胁迫不同程度地促进了沙枣苗根系生长;100 mmol/L NaCl胁迫对幼苗生物量累积无明显影响,而200 mmol/L则显著抑制了生物量累积;盐胁迫幼苗根、茎、叶中Na+含量以及K+-Na+选择性运输系数(S K,Na)和Ca2+-Na+选择性运输系数(S Ca,Na)显著或大幅度增加,而K+、Ca2+、Mg2+含量以及K+/Na+、Ca2+/Na+和Mg2+/Na+比值则显著或大幅度下降;200 mmol/L NaCl胁迫沙枣根Na+含量和根Na+净累积量分别为22.15 mg/g干重和1.87 mg/株(是对照的16.20倍和20.06倍),根成为Na+净累积量增加幅度最大的组织和Na+含量最高的组织;200 mmol/L NaCl胁迫沙枣茎、叶中的Na+含量以及冠组织Na+净累积量分别高达5.15、7.71 mg/g干重和3.29 mg/株(是对照的7.22倍、9.58倍和5.45倍),但幼苗仍能正常生长。综合分析认为,沙枣的盐适应机制是根系拒盐和冠组织耐盐,主要通过根系的补偿生长效应、根系对Na+的聚积与限制作用以及冠组织对Na+的忍耐来实现的,同时也与根、茎和叶对K+、Ca2+选择性运输能力显著增强有关。     

Variation with age in the photosynthetic carbon fixation pattern by leaves of Amaranthus paniculatus and Oryza sativa: Change in the primary carboxylation but no shift from C4 or C3 pathway

Abstract The pattern of photosynthetic carbon fixation by leaves of Amaranthus paniculatus L. (a C₄ plant) and Oryza sativa L. (a C₃ plant) varied with age. Younger leaves of A. paniculatus incorporated ¹⁴CO₂ into malate and aspartate while senescent leaves fixed predominantly into phosphoglycerate (PGA) and sugar phosphates. Only developing leaves of O. sativa formed malate/aspartate whereas mature and senescent leaves produced PGA/sugar phosphates as the initial labelled products. Correspondingly the ratio of phosphoenolpyruvate/ribulose bisphosphate (RuBP) carboxylase activities was higher in younger leaves of A. paniculatus and developing leaves of O. sativa than in older leaves. However, pulse chase experiments revealed that the main donors of carbon to end products, irrespective of leaf stage, were C₄ acids and PGA in A. paniculatus and O. sativa respectively. The results suggest that although an apparent change from initial β-carboxylation to RuBP carboxylation occurs during leaf ontogeny in both the plants, the overall leaf photosynthesis remains C₄ or C₃. The high rate of ¹⁴CO₂ incorporation into PGA/sugar phosphates by senescent leaves of A. paniculatus is suggested to be partly due to the increased intercellular spaces in their mesophyll, allowing greater access of CO₂ directly to RuBP carboxylase in the bundle sheath.     

The appearance of chlorophyll derivatives in senescing tissue

Chlorophylls a-1 and b′, which are breakdown products of chlorophylls a and b respectively, were found in senescing leaves of Phaseolus vulgaris and Hordeum vulgare following excision from the plant. Chlorophyll a-1 was not detected in healthy plants, in senescing attached leaves or in chlorophyll-proteins isolated from senescent tissue. Chlorophyll a-1 formation in excised leaves increased with time for up to 10 days as chlorophyll a levels fell.     

Arabidopsis cysteine proteinase inhibitor AtCYSb interacts with a Ca-dependent nuclease,AtCaN2

Plant cysteine proteinase inhibitors (cystatins) play important roles in plant defense mechanisms. Some proteins that interact with cystatins may defend against abiotic stresses. Here, we showed that AtCaN2, a Ca²⁺-dependent nuclease in Arabidopsis, is transcribed in senescent leaves and stems and interacts with an Arabidopsis cystatin (AtCYSb) in a yeast two-hybrid screen. The interaction between AtCYSb and AtCaN2 was confirmed by in vitro pull-down assay and bimolecular fluorescence complementation. Agarose gel electrophoresis showed that the nuclease activity of AtCaN2 against λDNA was inhibited by AtCYSb, which suggests that AtCYSb regulates nucleic acid degradation in cells.     

Photoinhibition and photoprotection in senescent leaves of field-grown wheat plants

Photosynthesis, photosystem II (PSII) photochemistry, photoinhibition and the xanthophyll cycle in the senescent flag leaves of wheat (Triticum aestivum L.) plants grown in the field were investigated. Compared to the non-senescent leaves, photosynthetic capacity was significantly reduced in senescent flag leaves. The light intensity at which photosynthesis was saturated also declined significantly. The light response curves of PSII photochemistry indicate that a down-regulation of PSII photochemistry occurred in senescent leaves in particular at high light. The maximal efficiency of PSII photochemistry in senescent flag leaves decreased slightly when measured at predawn but substantially at midday, suggesting that PSII function was largely maintained and photoinhibition occurred in senescent leaves when exposed to high light. At midday, PSII efficiency, photochemical quenching and the efficiency of excitation capture by open PSII centers decreased considerably, while non-photochemical quenching increased significantly. Moreover, compared with the values at early morning, a greater decrease in CO₂ assimilation rate was observed at midday in senescent leaves than in control leaves. The levels of antheraxanthin and zeaxanthin via the de-epoxidation of violaxanthin increased in senescent flag leaves from predawn to midday. An increase in the xanthophyll cycle pigments relative to chlorophyll was observed in senescent flag leaves. The results suggest that the xanthophyll cycle was activated in senescent leaves due to the decrease in CO₂ assimilation capacity and the light intensity for saturation of photosynthesis and that the enhanced formation of antheraxanthin and zeaxanthin at high light may play an important role in the dissipation of excess light energy and help to protect photosynthetic apparatus from photodamage. Our results suggest that the well-known function of the xanthophyll cycle to safely dissipate excess excitation energy is also important for maintaining photosynthetic function during leaf senescence.     

Metabolomic investigations of Ricinus communis for cultivar and provenance determination

Eight specimens of six known cultivars of Ricinus communis were investigated for differences in their metabolome that could be used to determine both provenance and cultivar. Seven replicates of three seeds per specimen were subjected to ¹H NMR analysis, with the collected data further investigated using multivariate statistical analysis (OPLS-DA), resulting in class separations according to provenance. Analysis of loadings plots in addition to further chemical analysis of the extracts allowed for the identification of phenylalanine, ricinine, the N-demethyl and O-demethyl analogues of ricinine, and sucrose as important molecular markers for particular cultivars. To test the strength of the model, extracts generated from blinded specimens were used as a prediction set and were correctly classified according to provenance and cultivar.