Non-photosynthetic Tissues Research Articles

Genes encoding cytochrome c oxidase subunit 5c from sunflower ( Helianthus annuus L.) are regulated by nitrate and oxygen availability

Subunit 5c is the smallest subunit of plant cytochrome c oxidases and has no defined counterpart in animal and fungi. To gain insight about the function and regulation of its expression, we have isolated two cDNAs encoding this subunit from a sunflower ( Helianthus annuus L.) root library. The encoded proteins are 91% identical at the amino acid level. Northern blots using specific probes indicated that both genes have similar expression patterns, with higher transcript levels in non-photosynthetic tissues. According to in situ hybridization experiments, expression in roots is localized within developing cortex and vascular tissues, with relatively low levels in the root cap and epidermis. Treatment of plants with solutions containing different nitrate concentrations produced an increase in transcript levels in roots and leaves. Light and/or sugars produced no significant changes in expression. Transcript levels were severely reduced upon incubation under hypoxia, and recovered to initial levels when plants were returned to normoxic conditions. A similar behavior was observed for the cytochrome c gene. We conclude that nitrate and oxygen are important factors regulating the expression of this cytochrome oxidase subunit, which is only partially coordinated with the expression of other components of the cyanide-sensitive respiratory pathway.

Two Types of Ferrochelatase in Photosynthetic and Nonphotosynthetic Tissues of Cucumber: THEIR DIFFERENCE IN PHYLOGENY, GENE EXPRESSION, AND LOCALIZATION

Ferrochelatase catalyzes the insertion of Fe(2+) into protoporphyrin IX to generate protoheme. In higher plants, there is evidence for two isoforms of this enzyme that fulfill different roles. Here, we describe the isolation of a second ferrochelatase cDNA from cucumber (CsFeC2) that was less similar to a previously isolated isoform (CsFeC1) than it was to some ferrochelatases from other higher plants. In in vitro import experiments, the two cucumber isoforms showed characteristics similar to their respective ferrochelatase counterparts of Arabidopsis thaliana. The C-terminal region of CsFeC2 but not CsFeC1 contained a conserved motif found in light-harvesting chlorophyll proteins, and CsFeC2 belonged to a phylogenetic group of plant ferrochelatases containing this conserved motif. We demonstrate that CsFeC2 was localized predominantly in thylakoid membranes as an intrinsic protein, and forming complexes probably with the C-terminal conserved motif, but a minor portion was also detected in envelope membranes. CsFeC2 mRNA was detected in all tissues and was light-responsive in cotyledons, whereas CsFeC1 mRNA was detected in nonphotosynthetic tissues and was not light-responsive. Interestingly, tissue-, light-, and cycloheximide-dependent expressions of the two isoforms of ferrochelatase were similar to those of two glutamyl-tRNA reductase isoforms involved in the early step of tetrapyrrole biosynthesis, suggesting the existence of distinctly controlled tetrapyrrole biosynthetic pathways in photosynthetic and nonphotosynthetic tissues.

Adaptive significance of evergreen vs. deciduous leaves: solving the triple paradox

Patterns in the dominance of evergreen vs. deciduous plants have long interested ecologists, biogeographers, and global modellers. But previous models to account for these patterns have significant weaknesses. Bottom-up, mechanistic models – based on physiology, competition, and natural selection – have often been non-quantitative or restricted to a small range of habitats, and almost all have ignored belowground costs and whole-plant integration. Top-down, ecosystem-based models have succeeded in quantitatively reproducing several patterns, but rely partly on empirically derived constants and thresholds that lack a mechanistic explanation. It is generally recognized that seasonal drought can favor deciduous leaves, and that infertile soils can favor long-lived evergreen leaves. But no model has yet explained three great paradoxes, involving dominance by 1) evergreens in highly seasonal, boreal forests, 2) deciduous larch in many nutrient-poor peatlands, and 3) evergreen leaf-exchangers in nutrient-poor subtropical forests, even though they shed their leaves just as frequently as deciduous species. This paper outlines a generalized optimality model to account for these and other patterns in leaf longevity and phenology, based on maximizing whole-plant carbon gain or height growth, and building on recent advances in our understanding of the quantitative relationships of leaf photosynthesis, nitrogen content, and mass per unit area to leaf life-span. Only a whole-plant approach can explain evergreen dominance under realistic ecological conditions, or account for the boreal paradox, the larch paradox, the leaf-exchanger paradox, and expected shifts in shade tolerance associated with leaf phenology. Poor soils favor evergreens not merely by increasing the costs of nutrient acquisition, but also by depressing the maximum rate of photosynthesis and thus the seasonal contrast in photosynthetic return between leaves adapted to favorable vs. unfavorable conditions. The dominance of evergreens in western North America beyond the coastal zone of mild winters and winter rainfall appears related to the unusually long photosynthetic season for evergreen vs. deciduous plants there. Future models for optimal leaf phenology must incorporate differences between evergreen and deciduous plants in allocation to photosynthetic vs. non-photosynthetic tissue, rooting depth, stem allometry, xylem anatomy, and exposure to herbivores and leaching, and analyze how these differences interact with the photosynthetic rate, transpiration, and nutrient demands of leaves with different life-spans to affect rates of height growth in specific microsites.

Phospho enolpyruvate carboxylase from cherimoya fruit: properties, kinetics and effects of high CO 2

Phospho enolpyruvate carboxylase (PEPC, EC 4.1.1.31) regulatory properties were studied in non-photosynthetic (mesocarp) and photosynthetic (peel) tissues from cherimoya ( Annona cherimola Mill.) fruit stored in air, in order to gain a better understanding of in vivo enzyme regulation. Analyses were also performed with fruit treated with 20% CO 2–20% O 2 to define the role of PEPC as part of an adaptive mechanism to high external carbon dioxide levels. The results revealed that the special kinetic characteristics of the enzyme from mesocarp—high V max and low sensibility to l-malate inhibition - are related to the active acid metabolism of these fruits and point to a high rate of reassimilation of respired CO 2 into keto-acids. With respect to fruit stored in air, PEPC in crude extracts from CO 2-treated cherimoyas gave a similar V max (1.12±0.03 μkat·mg −1 protein), a lower apparent K m (68±9 μM for PEP) and a higher I 50 of l-malate (5.95±0.3 mM). These kinetic values showed the increase in the affinity of this enzyme toward one of its substrate, PEP, by elevated external CO 2 concentrations. The lower K m value and lower sensitivity to l-malate are consistent with higher in vivo carboxylation reaction efficiency in CO 2-treated cherimoyas, while pointing to an additional enzyme regulation system via CO 2.

Two types of MGDG synthase genes, found widely in both 16:3 and 18:3 plants, differentially mediate galactolipid syntheses in photosynthetic and nonphotosynthetic tissues in Arabidopsis thaliana.

In Arabidopsis, monogalactosyldiacylglycerol (MGDG) is synthesized by a multigenic family of MGDG synthases consisting of two types of enzymes differing in their N-terminal portion: type A (atMGD1) and type B (atMGD2 and atMGD3). The present paper compares type B isoforms with the enzymes of type A that are known to sit in the inner membrane of plastid envelope. The occurrence of types A and B in 16:3 and 18:3 plants shows that both types are not specialized isoforms for the prokaryotic and eukaryotic glycerolipid biosynthetic pathways. Type A atMGD1 gene is abundantly expressed in green tissues and along plant development and encodes the most active enzyme. Its mature polypeptide is immunodetected in the envelope of chloroplasts from Arabidopsis leaves after cleavage of its transit peptide. atMGD1 is therefore likely devoted to the massive production of MGDG required to expand the inner envelope membrane and build up the thylakoids network. Transient expression of green fluorescent protein fusions in Arabidopsis leaves and in vitro import experiments show that type B precursors are targeted to plastids, owing to a different mechanism. Noncanonical addressing peptides, whose processing could not be assessed, are involved in the targeting of type B precursors, possibly to the outer envelope membrane where they might contribute to membrane expansion. Expression of type B enzymes was higher in nongreen tissues, i.e., in inflorescence (atMGD2) and roots (atMGD3), where they conceivably influence the eukaryotic structure prominence in MGDG. In addition, their expression of type B enzymes is enhanced under phosphate deprivation.

Comparative Growth Behaviour and Leaf Nutrient Status of Native Trees Planted on Mine Spoil With and Without Nutrient Amendment

The effect of nutrient amendment on growth of nine indigenous tree species planted on coal mine spoil was studied. Greater growth in fertilized plots was accompanied by greater foliar N and P concentrations in all species. The response to fertilization varied among species and was greater in non-leguminous than in leguminous species. Furthermore, leguminous species exhibited higher growth rates compared to non-leguminous species. The log-transformed height-diameter relationships were significant for all tree species and treatments. Acacia catechu, Dalbergia sissoo, Gmelina arborea and Azadirachta indica fitted the elastic similarity model of tree growth; whereas Pongamia pinnata and Phyllanthus emblica followed the constant stress model. Tectona grandis was the only species which fitted the geometric similarity model. In Albizia lebbeck and Terminalia bellirica, the b -values (the gradients of the log-transformed height to diameter relationships) were considerably lower (<0.5), and these two species did not fit any model of tree growth. In several cases, the b -values were considerably influenced by nutrient amendment. The log-transformed crown mass and trunk mass relationships were significant for all treatments and species. The slope of the crown mass:trunk mass relationship was near unity in A. indica, D. sissoo, G. arborea, P. emblica, P. pinnata, T. grandis, and T. bellirica. However, in A. catechu and A. lebbeck, this slope was well below unity suggesting a greater allocation to non-photosynthetic tissue. Fertilizer amendment resulted in a heavier crown relative to trunk in A. indica, T. grandis and T. bellirica.

Molecular biology of the plastidic phosphorylated serine biosynthetic pathway in Arabidopsis thaliana

Serine biosynthesis in plants proceeds by two pathways; the glycolate pathway which is associated with photorespiration and the pathway from 3-phosphoglycerate which is presumed to take place in the plastids. The 3-phosphoglycerate pathway (phosphorylated pathway) involves three enzymes catalyzing three sequential reactions: 3-phosphoglycerate dehydrogenase (PGDH), 3-phosphoserine aminotransferase (PSAT) and 3-phosphoserine phosphatase (PSP). cDNA and genomic clones encoding these three enzymes from spinach and Arabidopsis thaliana were isolated by means of heterologous probe screening, homologous EST clones and genetic complementation in an Escherichia coli mutant. The identity of the isolated cDNAs was confirmed by functional complementation of serine auxotrophy in E. coli mutants and/or the detection of catalytic activity in the recombinant enzymes produced in E. coli. Northern blot analyses indicated the most preferential expression of these three genes in light-grown roots. In contrast, the mRNAs of two proteins involved in the glycolate pathway (H-protein of glycine decarboxylase multienzyme complex and serine hydroxymethyltransferase) accumulated to high levels in light-grown shoots. Environmental stresses, such as high salinity, flooding and low temperature, induced changes in mRNA levels of enzymes in the plastidic phosphorylated serine biosynthetic pathway but not in that of the glycolate pathway. These results indicate that the plastidic 3-phosphoglycerate pathway plays an important role in supplying serine in non-photosynthetic tissues in plants and under environmental stresses.

Transcriptional expression characteristics and subcellular localization of ADP-glucose pyrophosphorylase in the oil plant Perilla frutescens.

Three ADP-glucose pyrophosphorylase clones were isolated from the cotyledon cDNA library of the oil plant, Perilla frutescens, and their intracellular localization investigated. Two of three cDNAs (PfagpS1 and PfagpS2) were homologous to the catalytic small subunit of AGPases found in other plants, while the third clone (PfagpL) was highly similar to the large subunit type. Transcripts for PfagpS1 and PfagpS2 were observed in both photosynthetic and non-photosynthetic tissue, showing the highest expression in the stem, while PfagpL transcripts were abundantly expressed in stem and cotyledon. To evaluate the subcellular localization of PfagpS2 and PfagpL as well as the maize BT2, N-terminus-GFP DNA fusion were constructed and transformed into tobacco plants. Immunoblot analysis showed that the expressed PfagpS2- and PfagpL-GFP fusions were targeted to the plastid in the heterologous tobacco system whereas the BT2-GFP remained intact, suggesting a cytoplasmic location. These intracellular assignments were confirmed by direct confocal microscopic examination. GFP signals were localized to the cytoplasm as well as in the nucleus in BT2-GFP plants, and to the plastids in PfagpS2- and PfagpL-GFP plants. Our results indicate that Perilla cotyledons contain multiple AGPase subunits, of which at least two isoforms and very likely the third, are plastidial in nature.

Effects of light and phytohormones on the expression of cucumber ferrochelatases

Ferrochelatase catalyzes the co-ordination of Fe2+ as a central metal into protoporphyrin IX to form protoheme. A ferrochelatase of cucumber (CsFeC1) was localized only in non-photosynthetic tissues [Suzuki et al. (2000) Plant Cell Physiol. 41: 192-199]. In the present study, we isolated a second ferrochelatase cDNA from cucumber (CsFeC2), the amino acid sequence of which showed a relatively low homology with that of CsFeC1. The C-terminal region of CsFeC2 (but not of CsFeC1) showed homology with the conserved LHC motif of light-harvesting chlorophyll-binding protein, and CsFeC2 belonged to a phylogenetic group composed of several plant and cyanobacterial ferrochelatases containing the conserved sequence. It was localized mainly in thylakoid membrane, whereas minor was also detected in the envelope membrane. It was an intrinsic protein bound to the thylakoid membrane and formed complexes, probably with the C-terminal LHC motif. Its mRNA was detected in all tissues and was light responsive in cotyledons, whereas CsFeC1 mRNA was detected in non-photosynthetic tissues and was not light responsive. Cycloheximide treatment in cotyledons had no effect on the level of CsFeC2 mRNA in contrast with the elevation of CsFeC1 mRNA level. Treatment with benzyl adenine increased the level of CsFeC2 mRNA but not of CsFeC1 mRNA. These results suggested that CsFeC2 might mainly involved in the heme biosynthesis in chloroplast. The results of import experiment and measurement of ferrochelatase activity will be reported and discussed in terms of their physiological roles.

Spatial and temporal expression of two cytosolic glutamine synthetase genes in Scots pine: functional implications on nitrogen metabolism during early stages of conifer development.

Ammonium assimilation during the initial stages of Scots pine growth involves two cytosolic glutamine synthetase (GS, EC: 6.3.1.2) isoenzymes encoded by separate genes, GS1a and GS1b. GS1a was most exclusively expressed in photosynthetic tissues of the seedling whereas GS1b was expressed ubiquitously showing higher levels in non-photosynthetic tissues such as root and hypocotyl. Temporal expression analysis has shown that when germination starts GS1b is the predominant form in the embryo, however, its relative abundance in the tissue decreased in the postgerminative stages when green cotyledons are developed. In contrast GS1a was present at a low level in the embryo but its abundance increased markedly during germination and seedling growth. These data suggest that GS1a and GS1b genes display different and non-redundant roles in the nitrogen metabolism of conifers. The precise localization of individual transcripts by in situ hybridization strongly supports this possibility. GS1 gene products are mainly expressed in different cellular types: GS1a in chlorophylic parenchyma and GS1b in the vascular bundles of all tissues examined in the seedling. Our data support that glutamine biosynthesis in pine seedlings follows a different pattern related to angiosperms involving two cytosolic GS proteins: one of them a typical cytosolic GS which may be involved in the generation of glutamine for N transport and a second cytosolic GS generating amino donors for the biosynthesis of major N compounds in photosynthetic tissues, a closer role to angiosperm chloroplastic GS. The results are discussed with regard to recent studies on N mobilization and metabolism during the initial stages of conifer development.

Capacity for NADPH/NADP turnover in the cytosol of barley seed endosperm: The role of NADPH-dependent hydroxypyruvate reductase

Barley ( Hordeum vulgare L.) endosperm from developing seeds was found to contain relatively high activities of cytosolic NAD(P)H-dependent hydroxypyruvate reductase (HPR-2) and isocitrate dehydrogenase (ICDH). In contrast, activities of peroxisomal NADH-dependent hydroxypyruvate reductase (HPR-1) and glycolate oxidase as well as cytosolic NAD(P)H-dependent glyoxylate reductase were very low or absent in the endosperm both during maturation and seed germination, indicating the lack of a complete glycolate cycle in this tissue. In addition, activities of cytosolic glucose-6-phosphate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase were low or absent in the endosperm. The endosperm HPR-2 exhibited similar properties to those of an earlier described HPR-2 from green leaves, e.g. activities with both hydroxypyruvate and glyoxylate, utilization of both NADPH and NADH as cofactors, and a strong uncompetitive inhibition by oxalate ( K i in the order of micromolar). In etiolated leaves, both HPR-1 and HPR-2 were present with the same activity as in green leaves, indicating that the lack of HPR-1 in the endosperm is not a general feature of non-photosynthetic tissues. We conclude that the endosperm has considerable capacity for cytosolic NADP/NADPH cycling via HPR-2 and ICDH, the former being possibly involved in the utilization of a serine-derived carbon.

Analysis of the Expression of CLA1, a Gene That Encodes the 1-Deoxyxylulose 5-Phosphate Synthase of the 2-C-Methyl-d-Erythritol-4-Phosphate Pathway in Arabidopsis

The discovery of the 2-C-methyl-D-erythritol-4-phosphate pathway for the biosynthesis of isoprenoids raises the important question of the nature and regulation of the enzymes involved in this pathway. CLA1, a gene previously isolated from Arabidopsis, encodes the first enzyme of the 2-C-methyl-D-erythritol-4-phosphate pathway, 1-deoxy-D-xylulose-5-phosphate synthase. We demonstrate this enzyme activity by complementation of the cla1-1 mutant phenotype and by direct enzymatic assays. Based on mRNA and protein expression patterns this enzyme is expressed mainly in developing photosynthetic and non-photosynthetic tissues. The beta-glucuronidase expression pattern driven from the CLA1 gene regulatory region supports the northern and protein data while also showing that this gene has some level of expression in most tissues of the plant. A mutation in the CLA1 gene interferes with the normal development of chloroplasts and etioplasts, but does not seem to affect amyloplast structure. Microscopic analysis also shows a pleiotropic effect of the CLA1 gene mutation in mesophyll tissue formation.

Expression of thioredoxins f and m, and of their targets fructose‐1,6‐bisphosphatase and NADP‐malate dehydrogenase, in pea plants grown under normal and light/temperature stress conditions

Thioredoxins (Trxs) f and m, as well as their targets chloroplast fructose-1,6-bisphosphatase (FBPase) and NADP+-malate dehydrogenase (NADP-MDH), displayed transcriptional expression in both photosynthetic and non-photosynthetic organs of pea plants (Pisum sativum L. cv. Lincoln) grown for 50 d under normal irradiance. However, whereas Trx m and both target enzymes were poorly expressed in non-photosynthetic tissues, the content of the precursor form of the Trx f-specific mRNA was high in pea roots. In contrast, the translational expression of Trx f was low in this organ. The high FBPase activity in immature seeds, and the low activity of leaves, must be related to high starch synthesis in the first, and with high sucrose formation in the second. The transcriptional expression of FBPase and NADP+-MDH, and to a lesser extent that of Trxs f and m, was inhibited under low irradiance in plants grown under both normal and high temperatures. Pea plants grown at low temperature displayed a high level of mRNAs for Trxs and their targets, especially when the growth was carried out at low light. To a lesser extent, similar behaviour was observed at the protein level. Chloroplasts of mesophyll leaf cells of pea plants grown under saturating light, or under sub-saturating continuous irradiance, showed broken envelopes, distorted structural elements and disorganized starch grains, as a consequence of a photobleaching process and high starch accumulation.

Growth, photosynthesis and water-use efficiency of two C4Sahelian grasses subjected to water deficits

Growth, photosynthesis and water use efficiency of two Sahel dominant C4grasses:Schoenefeldia gracilis Kunth and Dactyloctenium aegyptium (L) P. Beauv were studied under well-watered and drought conditions. D. aegyptium had a faster rate of development than S. gracilis and responded to water-deficits by decreasing leaf area in the earlier stages of development. However, under water-stress the overall reduction of leaf area suffered by S. gracilis was greater than in D. aegyptium. The decrease in net photosynthesis observed under water-deficits resulted mainly from stomatal closure. Analysis of correlation between long-term water-use efficiency and several physiological, structural and growth characteristics shows that long-term water-use efficiency in D. aegyptium was positively correlated with relative growth rates. The reduction in growth resulting from water-deficits was compensated by an increase in the short-term water-use efficiency associated with stomatal closure. In S. gracilis, the long-term water-use efficiency under moderate water-deficits increased but was not positively correlated with relative growth rate; it was associated with a high allocation to non-photosynthetic (support and vascular) tissues. The possible implications of this behaviour for the different ‘strategies’ for plant survival in the arid climate of the Sahel are discussed.

Rice sucrose-phosphate synthase: Identification of an isoform specific for heterotrophic tissues with distinct metabolite regulation from the mature leaf enzyme

Immunohistological analyses for rice (Oryza sativa) sucrose-phosphate synthase (SPS, UDP-glucose d-fructose-6-phosphate-2-glucosyltransferase, EC 2.4.1.14) show that the protein is differently localized in photosynthetic and etiolated leaves. Very little is known about SPS regulation in heterotrophic tissues; therefore, we studied the biochemical properties of the enzyme from etiolated seedlings and embryo. Two SPS forms (SPS-1 and SPS-2) were partially purified from etiolated seedlings. The effects of Glc-6-P (activator) and Pi (inhibitor) on SPS activities allowed us to differentiate the two forms. SPS-1 showed high sensitivity to Pi which also strongly decreased enzyme activation by Glc-6-P. SPS-2 was highly activated by Glc-6-P and showed low sensitivity to Pi. In vitro alkaline phosphatase treatment suggested that SPS-1 could be regulated as leaf SPS in darkness and that SPS-2 is present in a dephosphorylated state or is not regulated by protein phosphorylation. The relative MM value (116 kDa) estimated for both SPS forms in SDS-PAGE is identical to the rice leaf SPS polypeptide. Taken together, these data led us to conclude that SPS-2 is an enzyme form only present in non-photosynthetic tissues.

Overexpression, enzymatic properties and tissue localization of a ferrochelatase of cucumber.

Ferrochelatase catalyzes the insertion of Fe2+ into protoporphyrin IX to generate protoheme. A putative mature region of a cucumber ferrochelatase cDNA (hemH) was overexpressed in Escherichia coli and purified to homogeneity (40 kDa). The optimum pH was 7.7, and the apparent K(m) values for deuteroporphyrin IX and Fe2+ were 14.4 microM and 4.7 microM, respectively. The activity of the ferrochelatase was inhibited by N-methylprotoporphyrin IX (I50 = 4 nM). Western blot analysis with a polyclonal antibody raised against the recombinant ferrochelatase showed that the antibody crossreacted with protein extracts from hypocotyls and roots of cucumber but not with that from cotyledons. The antibody did not crossreact with proteins of thylakoid membranes of chloroplasts in cucumber cotyledons, although the ferrochelatase activity was mainly associated with the thylakoid membranes. Northern blot analysis also indicated that the hemH gene was expressed mainly in hypocotyls and roots, but little in cotyledons, and the level of the hemH transcripts was not light-responsive. These results demonstrated that the cucumber hemH gene encodes a ferrochelatase which presumably functions for heme biosynthesis in non-photosynthetic tissues, such as hypocotyls and roots, and suggested the presence of other types of ferrochelatase in cucumber, one of which is located in thylakoid membranes of chloroplasts.

Strategies for metabolic flux analysis in plants using isotope labelling

Flux measurements through metabolic pathways generate insights into the integration of metabolism, and there is increasing interest in using such measurements to quantify the metabolic effects of mutation and genetic manipulation. Isotope labelling provides a powerful approach for measuring metabolic fluxes, and it gives rise to several distinct methods based on either dynamic or steady-state experiments. We discuss the application of these methods to photosynthetic and non-photosynthetic plant tissues, and we illustrate the different approaches with an analysis of the pathways interconverting hexose phosphates and triose phosphates. The complicating effects of the pentose phosphate pathway and the problems arising from the extensive compartmentation of plant cell metabolism are considered. The non-trivial nature of the analysis is emphasised by reference to invalid deductions in earlier work. It is concluded that steady-state isotopic labelling experiments can provide important information on the fluxes through primary metabolism in plants, and that the combination of stable isotope labelling with detection by nuclear magnetic resonance is particularly informative.

Effects of light availability and sapling size on the growth, biomass allocation, and crown morphology of understory sugar maple, yellow birch, and beech

The patterns of above-ground growth, biomass allocation, crown morphology, and light attenuation were compared between small (50 to 250 cm tall) and tall (250 to 600 cm tall) yellow birch, sugar maple, and beech individuals in low (< 10% of above-canopy PPFD (Photosynthetic Photon Flux Density)) and high (10 to 40% PPFD) light environments in a mature sugar maple-birch-beech stand near Quebec city, Canada. Significant differences in above-ground growth, crown morphology, and allocation patterns were found among (i) the three co-dominating tree species, (ii) short and tall individuals, and (iii) low and high light environments. The direction of the differences in most traits investigated between low and high light environments were strikingly similar among the three species, but the magnitude of the differences often varied. Overall, yellow birch differed more in several traits in terms of its responses to light and size compared to beech and sugar maple. In general, differences found between light environments were smaller for the taller saplings, indicating that plasticity tends to decrease with increasing size in all three species. None of these crown structural differences found among species translated into differences in light attenuation within the sapling crowns. The maximum height observed in individual trees of all three species tended to decrease sharply below approximately 4% PPFD. We suggest that maximum tree height is restricted in such low light environments since the photosynthetic to non-photosynthetic tissue ratio, as measured by the leaf area ratio (LAR), declines rapidly with seedling size up to 150 cm. We suggest that these three species co-dominate in this forest due to a combination of small but effective differences in physiological, morphological, and allocational traits and responses to increases in the understory light environment.

Water Deficit Effects on Raffinose Family Oligosaccharide Metabolism in Coleus.

Variegated coleus (Coleus blumei Benth.) plants were exposed to a restricted water supply for 21 d. The relative water content in leaf tissues was reduced from 80% (control) to 60% (drought-stressed). Under drought conditions, the stomatal conductance and leaf photosynthetic rate were reduced. In green leaf tissues, drought stress also greatly decreased the diurnal light-period levels of the raffinose family oligosaccharides (RFOs) stachyose and raffinose, as well as those of other non-structural carbohydrates (galactinol, sucrose, hexoses, and starch). However, drought had little effect on soluble carbohydrate content of white, non-photosynthetic leaf tissues. In green tissues, galactinol synthase activity was depressed by drought stress. An accumulation of O-methyl-inositol was also observed, which is consistent with the induction of myoinositol-6-O-methyltransferase activity seen in the stressed green tissues. In source tissues, RFO metabolism is apparently reduced by drought stress through a combined effect of decreased photosynthesis and reduced galactinol synthase activity. Moreover, a further reduction in RFO biosynthesis may have been due to a switch in carbon partitioning to O-methyl-inositol biosynthesis, creating competition for myoinositol, a metabolite shared by both biochemical pathways.

Invertases. Primary structures, functions, and roles in plant development and sucrose partitioning.

One of the key features of plants is their ability to reduce carbon dioxide in the presence of sunlight and water to sugars, and the subsequent transport of assimilated carbon to the nonphotosynthetic tissues (sink tissues). In most plants, the transported sugar is Suc, a nonreducing disaccharide,