Growth Rates and Carbohydrate Fluxes within the Elongation Zone of Tall Fescue Leaf Blades

Sucrose-Phosphate Synthase and Sucrose Synthase contribute to refoliation in ryegrass, a grassland fructan-accumulating species

The perennity of grassland species such as Lolium perenne greatly depends on their ability to regrow after cutting or grazing. Refoliation largely relies on the mobilization of fructans in the remaining tissues and on the associated sucrose synthesis and transport towards the basal leaf meristems. However, nothing is known yet about the sucrose synthesis pathway. Sucrose Phosphate Synthase (SPS) and Sucrose Synthase (SuS) activities, together with their transcripts, were monitored during the first hours after defoliation along the leaf axis of mature leaf sheaths and elongating leaf bases (ELB) where the leaf meristems are located. In leaf sheaths, which undergo a sink-source transition, fructan and sucrose contents declined while SPS and SuS activities increased, along with the expression of LpSPSA, LpSPSD.2, LpSuS1, LpSuS2, and LpSuS4. In ELB, which continue to act as a strong carbon sink, SPS and SuS activities increased to varying degrees while the expression of all the LpSPS and LpSuS genes decreased after defoliation. SPS and SuS both contribute to refoliation but are regulated differently depending on the source or sink status of the tissues. Together with fructan metabolism, they represent key determinants of ryegrass perennity and, more generally, of grassland sustainability.

Half of the 18O enrichment of leaf sucrose is conserved in leaf cellulose of a C3 grass across atmospheric humidity and CO2 levels

The 18O enrichment (Δ18O) of cellulose (Δ18OCel) is recognized as a unique archive of past climate and plant function. However, there is still uncertainty regarding the proportion of oxygen in cellulose (pex) that exchanges post-photosynthetically with medium water of cellulose synthesis. Particularly, recent research with C3 grasses demonstrated that the Δ18O of leaf sucrose (Δ18OSuc, the parent substrate for cellulose synthesis) can be much higher than predicted from daytime Δ18O of leaf water (Δ18OLW), which could alter conclusions on photosynthetic versus post-photosynthetic effects on Δ18OCel via pex. Here, we assessed pex in leaves of perennial ryegrass (Lolium perenne) grown at different atmospheric relative humidity (RH) and CO2 levels, by determinations of Δ18OCel in leaves, Δ18OLGDZW (the Δ18O of water in the leaf growth-and-differentiation zone) and both Δ18OSuc and Δ18OLW (adjusted for εbio, the biosynthetic fractionation between water and carbohydrates) as alternative proxies for the substrate for cellulose synthesis. Δ18OLGDZW was always close to irrigation water, and pex was similar (0.53 ± 0.02 SE) across environments when determinations were based on Δ18OSuc. Conversely, pex was erroneously and variably underestimated (range 0.02-0.44) when based on Δ18OLW. The photosynthetic signal fraction in Δ18OCel is much more constant than hitherto assumed, encouraging leaf physiological reconstructions.

A functional structural model of grass development based on metabolic regulation and coordination rules

Shoot architecture is a key component of the interactions between plants and their environment. We present a novel model of grass, which fully integrates shoot morphogenesis and the metabolism of carbon (C) and nitrogen (N) at organ scale, within a three-dimensional representation of plant architecture. Plant morphogenesis is seen as a self-regulated system driven by two main mechanisms. First, the rate of organ extension and the establishment of architectural traits are regulated by concentrations of C and N metabolites in the growth zones and the temperature. Second, the timing of extension is regulated by rules coordinating successive phytomers instead of a thermal time schedule. Local concentrations are calculated from a model of C and N metabolism at organ scale. The three-dimensional representation allows the accurate calculation of light and temperature distribution within the architecture. The model was calibrated for wheat (Triticum aestivum) and evaluated for early vegetative stages. This approach allowed the simulation of realistic patterns of leaf dimensions, extension dynamics, and organ mass and composition. The model simulated, as emergent properties, plant and agronomic traits. Metabolic activities of growing leaves were investigated in relation to whole-plant functioning and environmental conditions. The current model is an important step towards a better understanding of the plasticity of plant phenotype in different environments.

Atmospheric CO2 and VPD alter the diel oscillation of leaf elongation in perennial ryegrass: compensation of hydraulic limitation by stored-growth

We explored the effects of atmospheric CO₂ concentration (C_a ) and vapor pressure deficit (VPD) on putative mechanisms controlling leaf elongation in perennial ryegrass. Plants were grown in stands at a C_a of 200, 400 or 800 μmol mol^-1 combined with high (1.17 kPa) or low (0.59 kPa) VPD during the 16 h-day in well-watered conditions with reduced nitrogen supply. We measured day : night-variation of leaf elongation rate (LER_day  : LER_night ), final leaf length and width, epidermal cell number and length, stomatal conductance, transpiration, leaf water potential and water-soluble carbohydrates and osmotic potential in the leaf growth-and-differentiation zone (LGDZ). Daily mean LER or morphometric parameters did not differ between treatments, but LER_night strongly exceeded LER_day , particularly at low C_a and high VPD. Across treatments LER_day was negatively related to transpiration (R²  = 0.75) and leaf water potential (R²  = 0.81), while LER_night was independent of leaf water potential or turgor. Enhancement of LER_night over LER_day was proportional to the turgor-change between day and night (R²  = 0.93). LGDZ sugar concentration was high throughout diel cycles, providing no evidence of source limitation in any treatment. Our data indicate a mechanism of diel cycling between daytime hydraulic and night-time stored-growth controls of LER, buffering C_a and daytime VPD effects on leaf elongation.

Identification of Metabolite and Lipid Profiles in a Segregating Peach Population Associated with Mealiness in Prunus persica (L.) Batsch

The peach is the third most important temperate fruit crop considering fruit production and harvested area in the world. Exporting peaches represents a challenge due to the long-distance nature of export markets. This requires fruit to be placed in cold storage for a long time, which can induce a physiological disorder known as chilling injury (CI). The main symptom of CI is mealiness, which is perceived as non-juicy fruit by consumers. The purpose of this work was to identify and compare the metabolite and lipid profiles between two siblings from contrasting populations for juice content, at harvest and after 30 days at 0 °C. A total of 119 metabolites and 189 lipids were identified, which showed significant differences in abundance, mainly in amino acids, sugars and lipids. Metabolites displaying significant changes from the E1 to E3 stages corresponded to lipids such as phosphatidylglycerol (PG), monogalactosyldiacylglycerol (MGDG) and lysophosphatidylcholines (LPC), and sugars such as fructose 1 and 1-fructose-6 phosphate. These metabolites might be used as early stage biomarkers associated with mealiness at harvest and after cold storage.

leafkin-An R package for automated kinematic data analysis of monocot leaves

Growth is one of the most studied plant responses. At the cellular level, plant growth is driven by cell division and cell expansion. A means to quantify these two cellular processes is through kinematic analysis, a methodology that has been developed and perfected over the past decades, with in-depth descriptions of the methodology available. Unfortunately, after performing the lab work, researchers are required to perform time-consuming, repetitive and error-prone calculations. To lower the barrier towards this final step in the analysis and to aid researchers currently applying this technique, we have created leafkin, an R-package to perform all the calculations involved in the kinematic analysis of monocot leaves using only four functions. These functions support leaf elongation rate calculations, fitting of cell length profiles, extraction of fitted cell lengths and execution of kinematic equations. With the leafkin package, kinematic analysis of monocot leaves becomes more accessible than before.

The δ18 O and δ2 H of water in the leaf growth-and-differentiation zone of grasses is close to source water in both humid and dry atmospheres

The oxygen and hydrogen isotope composition of water in the leaf growth-and-differentiation zone, LGDZ, (δ¹⁸ O_LGDZ , δ² H_LGDZ ) of grasses influences the isotopic composition of leaf cellulose (oxygen) and wax (hydrogen) - important for understanding (paleo)environmental and physiological information contained in these biological archives - but is presently unknown. This work determined δ¹⁸ O_LGDZ and δ² H_LGDZ , ¹⁸ O- and ² H-enrichment of LGDZ (∆¹⁸ O_LGDZ and ∆² H_LGDZ ), and the ¹⁸ O- and ² H-enrichment of leaf blade water (∆¹⁸ O_LW, ∆² H_LW ) in two C₃ and three C₄ grasses grown at high and low vapor pressure deficit (VPD). The proportion of unenriched water (p_x ) in the LGDZ ranged from 0.9 to 1.0 for ¹⁸ O and 1.0 to 1.2 for ² H. VPD had no effect on the proportion of ¹⁸ O- and ² H-enriched water in the LGDZ, and species effects were small or nonsignificant. Deuterium discrimination caused depletion of ² H in LGDZ water, increasing (apparent) p_x -values > 1.0 in some cases. The isotopic composition of water in the LGDZ was close to that of source water, independent of VPD and much less enriched than previously supposed, but similar to reported xylem water in trees. The well-constrained p_x will be useful in future investigations of oxygen and hydrogen isotopic fractionation during cellulose and wax synthesis, respectively.

Kinematic Analysis of Cell Division and Expansion: Quantifying the Cellular Basis of Growth and Sampling Developmental Zones in Zea mays Leaves

Growth analyses are often used in plant science to investigate contrasting genotypes and the effect of environmental conditions. The cellular aspect of these analyses is of crucial importance, because growth is driven by cell division and cell elongation. Kinematic analysis represents a methodology to quantify these two processes. Moreover, this technique is easy to use in non-specialized laboratories. Here, we present a protocol for performing a kinematic analysis in monocotyledonous maize (Zea mays) leaves. Two aspects are presented: (1) the quantification of cell division and expansion parameters, and (2) the determination of the location of the developmental zones. This could serve as a basis for sampling design and/or could be useful for data interpretation of biochemical and molecular measurements with high spatial resolution in the leaf growth zone. The growth zone of maize leaves is harvested during steady-state growth. Individual leaves are used for meristem length determination using a DAPI stain and cell-length profiles using DIC microscopy. The protocol is suited for emerged monocotyledonous leaves harvested during steady-state growth, with growth zones spanning at least several centimeters. To improve the understanding of plant growth regulation, data on growth and molecular studies must be combined. Therefore, an important advantage of kinematic analysis is the possibility to correlate changes at the molecular level to well-defined stages of cellular development. Furthermore, it allows for a more focused sampling of specified developmental stages, which is useful in case of limited budget or time.

Measures of light in studies on light-driven plant plasticity in artificial environments

Within-canopy variation in light results in profound canopy profiles in foliage structural, chemical, and physiological traits. Studies on within-canopy variations in key foliage traits are often conducted in artificial environments, including growth chambers with only artificial light, and greenhouses with and without supplemental light. Canopy patterns in these systems are considered to be representative to outdoor conditions, but in experiments with artificial and supplemental lighting, the intensity of artificial light strongly deceases with the distance from the light source, and natural light intensity in greenhouses is less than outdoors due to limited transmittance of enclosure walls. The implications of such changes in radiation conditions on canopy patterns of foliage traits have not yet been analyzed. We developed model-based methods for retrospective estimation of distance vs. light intensity relationships, for separation of the share of artificial and natural light in experiments with combined light and for estimation of average enclosure transmittance, and estimated daily integrated light at the time of sampling (Q(int,C)), at foliage formation (Q(int,G)), and during foliage lifetime (Q(int,av)). The implications of artificial light environments were analyzed for altogether 25 studies providing information on within-canopy gradients of key foliage traits for 70 species × treatment combinations. Across the studies with artificial light, Q(int,G) for leaves formed at different heights in the canopy varied from 1.8- to 6.4-fold due to changing the distance between light source and growing plants. In experiments with combined lighting, the share of natural light at the top of the plants varied threefold, and the share of natural light strongly increased with increasing depth in the canopy. Foliage nitrogen content was most strongly associated with Q(int,G), but photosynthetic capacity with Q(int,C), emphasizing the importance of explicit description of light environment during foliage lifetime. The reported and estimated transmittances of enclosures varied between 0.27 and 0.85, and lack of consideration of the reduction of light compared with outdoor conditions resulted in major underestimation of foliage plasticity to light. The study emphasizes that plant trait vs. light relationships in artificial systems are not directly comparable to natural environments unless modifications in lighting conditions in artificial environments are taken into account.

Comparison of a colorimetric and a high-performance liquid chromatography method for the determination of fructan in pasture grasses for horses

BACKGROUND

Pasture (fresh or conserved as hay/haylage) forms the basis of most equid diets and contains varying amounts (0 to ≥ 200 g kg⁻¹ dry matter (DM) or more) of fructans. Over-consumption of fructan is associated with the onset of laminitis in equids, an agonizing condition that may necessitate euthanasia. To enable appropriate dietary management of animals susceptible to laminitis, it is essential that fructans can be properly quantified in fresh and conserved pasture. For research purposes, fructans are frequently quantified by high-performance liquid chromatography (HPLC), but these methods are costly for routine screening. However, an inexpensive colorimetric method for measuring fructans in human foods is commercially available. The aim here was to determine the suitability of the commercially available colorimetric method for determining the fructan content of pasture grasses for horses.

RESULTS

Pasture grasses (Phleum pretense, Festuca rubra, Dactylis glomerata, Lolium perenne) managed for grazing (sampled from April to November) and a further set managed for conservation (sampled in July) were analysed for fructan content by HPLC and the colorimetric technique. HPLC values ranged from 83 to 299 g fructan kg⁻¹ DM (mean 154); corresponding colorimetric values were 5-238 g fructan kg⁻¹ DM (mean 82). Discrepancies in values between the two methods varied with time of sampling and plant species. Comparison of selected samples before and after incubation with the fructan hydrolases used in the colorimetric method revealed incomplete fructan hydrolysis from the pasture grasses, resulting in underestimates of their fructan content.

CONCLUSION

The colorimetric technique was not a reliable substitute for HPLC to quantify the fructan content of pasture grasses.

Diel patterns of leaf and root growth: endogenous rhythmicity or environmental response?

Plants are sessile organisms forced to adjust to their surrounding environment. In a single plant the photoautotrophic shoot is exposed to pronounced environmental variations recurring in a day-night 24 h (diel) cycle, whereas the heterotrophic root grows in a temporally less fluctuating environment. The contrasting habitats of shoots and roots are reflected in different diel growth patterns and their responsiveness to environmental stimuli. Differences between diel leaf growth patterns of mono- and dicotyledonous plants correspond to their different organization and placement of growth zones. In monocots, heterotrophic growth zones are organized linearly and protected from the environment by sheaths of older leaves. In contrast, photosynthetically active growth zones of dicot leaves are exposed directly to the environment and show characteristic, species-specific diel growth patterns. It is hypothesized that the different exposure to environmental constraints and simultaneously the sink/source status of the growing organs may have induced distinct endogenous control of diel growth patterns in roots and leaves of monocot and dicot plants. Confronted by strong temporal fluctuations in environment, the circadian clock may facilitate robust intrinsic control of leaf growth in dicot plants.

Fluxes in central carbohydrate metabolism of source leaves in a fructan-storing C3 grass: rapid turnover and futile cycling of sucrose in continuous light under contrasted nitrogen nutrition status

This work assessed the central carbohydrate metabolism of actively photosynthesizing leaf blades of a C3 grass (Lolium perenne L.). The study used dynamic (13)C labelling of plants growing in continuous light with contrasting supplies of nitrogen ('low N' and 'high N') and mathematical analysis of the tracer data with a four-pool compartmental model to estimate rates of: (i) sucrose synthesis from current assimilation; (ii) sucrose export/use; (iii) sucrose hydrolysis (to glucose and fructose) and resynthesis; and (iv) fructan synthesis and sucrose resynthesis from fructan metabolism. The contents of sucrose, fructan, glucose, and fructose were almost constant in both treatments. Labelling demonstrated that all carbohydrate pools were turned over. This indicated a system in metabolic steady state with equal rates of synthesis and degradation/consumption of the individual pools. Fructan content was enhanced by nitrogen deficiency (55 and 26% of dry mass at low and high N, respectively). Sucrose content was lower in nitrogen-deficient leaves (2.7 versus 6.7%). Glucose and fructose contents were always low (<1.5%). Interconversions between sucrose, glucose, and fructose were rapid (with half-lives of individual pools ranging between 0.3 and 0.8 h). Futile cycling of sucrose through sucrose hydrolysis (67 and 56% of sucrose at low and high N, respectively) and fructan metabolism (19 and 20%, respectively) was substantial but seemed to have no detrimental effect on the relative growth rate and carbon-use efficiency of these plants. The main effect of nitrogen deficiency on carbohydrate metabolism was to increase the half-life of the fructan pool from 27 to 62 h and to effectively double its size.

A comparative analysis of leaf shape of wheat, barley and maize using an empirical shape model

BACKGROUND AND AIMS

The phenotypes of grasses show differences depending on growth conditions and ontogenetic stage. Understanding these responses and finding suitable mathematical formalizations are an essential part of the development of plant and crop models. Usually, a marked change in architecture between juvenile and adult plants is observed, where dimension and shape of leaves are likely to change. In this paper, the plasticity of leaf shape is analysed according to growth conditions and ontogeny.

METHODS

Leaf shape of Triticum aestivum, Hordeum vulgare and Zea mays cultivars grown under varying conditions was measured using digital image processing. An empirical leaf shape model was fitted to measured shape data of single leaves. Obtained values of model parameters were used to analyse the patterns in leaf shape.

KEY RESULTS

The model was able to delineate leaf shape of all studied species. The model error was small. Differences in leaf shape between juvenile and adult leaves in T. aestivum and H. vulgare were observed. Varying growth conditions impacted leaf dimensions but did not impact leaf shape of the respective species.

CONCLUSIONS

Leaf shape of the studied T. aestivum and H. vulgare cultivars was remarkably stable for a comparable ontogenetic stage (leaf rank), but differed between stages. Along with other aspects of grass architecture, leaf shape changed during the transition from juvenile to adult growth phase. Model-based analysis of leaf shape is a method to investigate these differences. Presented results can be integrated into architectural models of plant development to delineate leaf shape for different species, cultivars and environmental conditions.

Root and shoot respiration of perennial ryegrass are supplied by the same substrate pools: assessment by dynamic 13C labeling and compartmental analysis of tracer kinetics

The substrate supply system for respiration of the shoot and root of perennial ryegrass (Lolium perenne) was characterized in terms of component pools and the pools' functional properties: size, half-life, and contribution to respiration of the root and shoot. These investigations were performed with perennial ryegrass growing in constant conditions with continuous light. Plants were labeled with (13)CO(2)/(12)CO(2) for periods ranging from 1 to 600 h, followed by measurements of the rates and (13)C/(12)C ratios of CO(2) respired by shoots and roots in the dark. Label appearance in roots was delayed by approximately 1 h relative to shoots; otherwise, the tracer time course was very similar in both organs. Compartmental analysis of respiratory tracer kinetics indicated that, in both organs, three pools supplied 95% of all respired carbon (a very slow pool whose kinetics could not be characterized provided the remaining 5%). The pools' half-lives and relative sizes were also nearly identical in shoot and root (half-life < 15 min, approximately 3 h, and 33 h). An important role of short-term storage in supplying respiration was apparent in both organs: only 43% of respiration was supplied by current photosynthate (fixed carbon transferred directly to centers of respiration via the two fastest pools). The residence time of carbon in the respiratory supply system was practically the same in shoot and root. From this and other evidence, we argue that both organs were supplied by the same pools and that the residence time was controlled by the shoot via current photosynthate and storage deposition/mobilization fluxes.

Partitioning of water soluble carbohydrates in vegetative tissues of Lolium multiflorum Lam. ssp. italicum cv. Lema

In temperate grasses, fructans are the major storage polysaccharides, being accumulated mainly in mature leaf sheaths, and also in the roots. The partitioning of carbohydrates within different organs regulates plant growth and development. The aim of the present work was to analyze the partitioning of water soluble carbohydrates in five different parts (elongating leaf blades, expanded leaf blades, upper and lower segments of the stubble, and roots) of plants of L. multiflorum cv. Lema, in order to contribute to an understanding of soluble carbohydrates distribution in these plants. Soluble carbohydrates and total fructose were analyzed in plants cultivated during 4 months in a glasshouse, by colorimetric, TLC and HPAEC-PAD techniques. Results showed that the greatest portion of total soluble carbohydrates was constituted of free and combined fructose, in all parts of the plants. The stubble contained the highest level of carbohydrates, followed by the elongating leaf blades, expanded leaf blades and roots. The leaf sheaths were not analyzed separately from the stubble, which explains the high levels of carbohydrates found in this part of the plant. The high metabolism of the elongating leaf blades, when compared to that of the expanded leaf blades, could explain the increased amounts of fructans stored in those tissues. Analysis by HPAEC-PAD showed that the elongating leaf blades and the roots had the highest proportions of low molecular weight fructans that could be readily mobilized, supplying the demand of growing tissues in other organs.

Differential expression of XET-related genes in the leaf elongation zone of F. pratensis

Festuca pratensis Huds. is a forage grass with the ability to withstand harsh climatic conditions. However, its potential agronomic use is limited by its poor competitive ability, which can be traced to limitations in leaf growth. In order to characterize this process and to identify genes which might function as markers for leaf growth, three XET-related genes in the leaf elongation zone (LEZ) of F. pratensis are reported. A detailed expression analysis is presented of the three genes in two F. pratensis genotypes with contrasting leaf growth characteristics grown under two nitrogen levels. By means of a detailed spatial analysis of growth and XET encoding transcript pattern along the LEZ, a specific correlation is shown between FpXET1 expression and tissue elongation that is maintained under the different growth conditions, while the two other XETs expressed in the LEZ show different transcript dynamics. Tissue localization of FpXET1 and FpXET2 transcripts indicate an accumulation throughout young tissue, which is consistent with the encoded proteins playing roles in cell wall modification processes during growth. It is proposed that FpXET1 is a potential marker for tissue elongation and leaf growth in F. pratensis.

Cloning and functional analysis of sucrose:sucrose 1-fructosyltransferase from tall fescue

Enzymes of grasses involved in fructan synthesis are of interest since they play a major role in assimilate partitioning and allocation, for instance in the leaf growth zone. Several fructosyltransferases from tall fescue (Festuca arundinacea) have previously been purified (Lüscher and Nelson, 1995). It is surprising that all of these enzyme preparations appeared to act both as sucrose (Suc):Suc 1-fructosyl transferases (1-SST) and as fructan:fructan 6(G)-fructosyl transferases. Here we report the cloning of a cDNA corresponding to the predominant protein in one of the fructosyl transferase preparations, its transient expression in tobacco protoplasts, and its functional analysis in the methylotrophic yeast, Pichia pastoris. When the cDNA was transiently expressed in tobacco protoplasts, the corresponding enzyme preparations produced 1-kestose from Suc, showing that the cDNA encodes a 1-SST. When the cDNA was expressed in P. pastoris, the recombinant protein had all the properties of known 1-SSTs, namely 1-kestose production, moderate nystose production, lack of 6-kestose production, and fructan exohydrolase activity with 1-kestose as the substrate. The physical properties were similar to those of the previously purified enzyme, except for its apparent lack of fructan:fructan 6(G)-fructosyl transferase activity. The expression pattern of the corresponding mRNA was studied in different zones of the growing leaves, and it was shown that transcript levels matched the 1-SST activity and fructan content.

The role of fructan in flowering of Campanula rapunculoides

Inulin type fructan was detected in all vegetative organs of Campanula rapunculoides L. plants. All flower parts contained fructan at some developmental stage. A steady decrease was found in sepals during development. Petals, however, stored fructan in the bud stage. A rapid breakdown during opening of the flower resulted in high concentrations of glucose and especially fructose that may contribute to the osmotic driving force involved in petal expansion. Before complete shrivelling, the hexoses were apparently exported from flower parts. Fructans were hydrolysed and exported from the stamen and style tissue upon flower opening. Similarly, the major fructan reserves in the ovary were broken down almost simultaneously with those in other flower parts. Hexoses did not reach high levels in the ovary, probably because they were rapidly metabolized and/or incorporated by developing seeds.

The effects of elevated CO(2) concentrations on cell division rates, growth patterns, and blade anatomy in young wheat plants are modulated by factors related to leaf position, vernalization, and genotype

This study demonstrates that elevated [CO(2)] has profound effects on cell division and expansion in developing wheat (Triticum aestivum L.) leaves and on the quantitative integration of these processes in whole-leaf growth kinetics, anatomy, and carbon content. The expression of these effects, however, is modified by intrinsic factors related to genetic makeup and leaf position, and also by exposure to low vernalizing temperatures at germination. Beyond these interactions, leaf developmental responses to elevated [CO(2)] in wheat share several remarkable features that were conserved across all leaves examined. Most significantly: (a) the contribution of [CO(2)] effects on meristem size and activity in driving differences in whole-blade growth kinetics and final dimensions; (b) an anisotropy in cellular growth responses to elevated [CO(2)], with final cell length and expansion in the paradermal plane being highly conserved, even when the rates and duration of cell elongation were modified, while cell cross-sectional areas were increased; (c) tissue-specific effects of elevated [CO(2)], with significant modifications of mesophyll anatomy, including an increased extension of intercellular air spaces and the formation of, on average, one extra cell layer, while epidermal anatomy was mostly unaltered. Our results indicate complex developmental regulations of sugar effects in expanding leaves that are subjected to genetic variation and influenced by environmental cues important in the promotion of floral initiation. They also provide insights into apparently contradictory and inconsistent conclusions of published CO(2) enrichment studies in wheat.

Fructosyltransferase Activities in the Leaf Growth Zone of Tall Fescue

High concentrations of water-soluble carbohydrates, mainly fructan, accumulate in the growth zone of tall fescue (Festuca arundinacea Schreb.) leaf blades. We studied sucrose-hydrolyzing activities in the leaf growth zone because of their importance in carbohydrate partitioning. Sucrose hydrolysis in the basal 1.5 cm was largely due to fructosyltransferases, which had activities up to 10 times higher than in fully developed leaf tissue. Three fructosyltransferases (F1, F2, and F3) were purified from the leaf growth zone. Each synthesized, from either sucrose or 1-kestose, a mixture of trisaccharides and higher-order oligofructans identical with the low-degree of polymerization fructan extracted from similar plant tissue. The highly purified fructosyltransferases retained ability (13%) to transfer fructose from sucrose to water. Time-dependent and substrate-dependent studies, using sucrose as the substrate, showed proportional production of fructose and glucose, indicating that both products are from the same enzyme. Fructosyltransferase was calculated to contribute about half the total transfer of fructose to water in the basal 1.5 cm. Invertase activity increased to near 2.0 cm when fructosyl transfer to sucrose and other oligofructans decreased. Invertase was the major activity for sucrose hydrolysis at positions distal to 3.0 cm.

Photosynthate partitioning in Basal zones of tall fescue leaf blades

Elongating grass leaves have successive zones of cell division, cell elongation, and cell maturation in the basal portion of the blade and are a strong sink for photosynthate. Our objective was to determine dry matter (DM) deposition and partitioning in basal zones of elongating tall fescue (Festuca arundinacea Schreb.) leaf blades. Vegetative tall fescue plants were grown in continuous light (350 micromoles per square meter per second photosynthetic photon flux density) to obtain a constant spatial distribution of elongation growth with time. Content and net deposition rates of water-soluble carbohydrates (WSC) and DM along elongating leaf blades were determined. These data were compared with accumulation of (14)C in the basal zones following leaf-labeling with (14)CO(2). Net deposition of DM was highest in the active cell elongation zone, due mainly to deposition of WSC. The maturation zone, just distal to the elongation zone, accounted for 22% of total net deposition of DM in elongating leaves. However, the spatial profile of (14)C accumulation suggested that the elongation zone and the maturation zone were sinks of equal strength. WSC-free DM accounted for 55% of the total net DM deposition in elongating leaf blades, but only 10% of incoming (14)C-photosynthate accumulated in the water-insoluble fraction (WIF approximately WSC-free DM) after 2 hours. In the maturation zone, more WSC was used for synthesis of WSC-free DM than was imported as recent photosynthate.

Fructan and cryoprotection in ryegrass (Lolium perenne L.)

Changes in carbohydrate content and composition of ryegrass (Lolium perenne L.) cv. Reveille field-grown in Normandy were investigated from May 1987 to May 1988. During the winter period, the concentrations of high DP (degree of polymerization) fructan were the lowest whilst those of ethanol-soluble carbohydrates were the highest. Ethanol-soluble carbohydrates also accounted for most of the non-structural carbohydrates of four cultivars of Lolium perenne field-grown in central France in January 1989. The accumulation of osmotically active carbohydrates during winter leads only to a small depression of freezing point and consequently, these sugars cannot be considered as efficient cryoprotectants in ryegrass.

Spatial distribution of growth rates and of epidermal cell lengths in the elongation zone during leaf development in Lolium perenne L

Relative elemental growth rates (REGR) and lengths of epidermal cells along the elongation zone of Lolium perenne L. leaves were determined at four developmental stages ranging from shortly after emergence of the leaf tip to shortly before cessation of leaf growth. Plants were grown at constant light and temperature. At all developmental stages the length of epidermal cells in the elongation zone of both the blade and sheath increased from 12 μm at the leaf base to about 550 μm at the distal end of the elongation zone, whereas the length of epidermal cells within the joint region only increased from 12 to 40 μm. Throughout the developmental stages elongation was confined to the basal 20 to 30 mm of the leaf with maximum REGR occurring near the center of the elongation zone. Leaf elongation rate (LER) and the spatial distributions of REGR and epidermal cell lengths were steady to a first approximation between emergence of the leaf tip and transition from blade to sheath growth. Elongation of epidermal cells in the sheath started immediately after the onset of elongation of the most proximal blade epidermal cells. During transition from blade to sheath growth the length of the blade and sheath portion of the elongation zone decreased and increased, respectively, with the total length of the elongation zone and the spatial distribution of REGR staying near constant, with exception of the joint region which elongated little during displacement through the elongation zone. Leaf elongation rate decreased rapidly during the phase when only the sheath was growing. This was associated with decreasing REGR and only a small decrease in the length of the elongation zone. Data on the spatial distributions of growth rates and of epidermal cell lengths during blade elongation were used to derive the temporal pattern of epidermal cell elongation. These data demonstrate that the elongation rate of an epidermal cell increased for days and that cessation of epidermal cell elongation was an abrupt event with cell elongation rate declining from maximum to zero within less than 10 h.

Rate of accumulation of fructan oligomers in wheat seedlings (Triticum aestivum L.) during the early stages of chilling treatment

The accumulation of soluble carbohydrates was studied during chilling of 2-week-old wheat (Triticum aestivum L.) seedlings. The extracts were analysed using a C18 HPLC method separating hexoses, sucrose and a wide range of oligofructan. Isomers were recognized and their content determined. The amounts of hexoses, sucrose and the trisaccharides 1-kestose and 6-kestose increased at the beginning of the treatment. The accumulation rate of 1-kestose, the trisaccharide generally considered to be the precursor of fructan, increased ten fold with some delay. Heavier fructan accumulated even later. In leaf blades, 1-3 molecular species with a degree of polymerization from 4 to 8 were detected. Five of these accounted for 90% of this pool at day 5. In leaf bases the amount of carbohydrates was lower but more species of fructan were detected. The sequence of accumulation was the same as in the blades. These results are discussed in relation to current models ot fructan synthesis.

Growth rates and assimilate partitioning in the elongation zone of tall fescue leaf blades at high and low irradiance

Tall fescue (Festuca arundinacea Schreb.) leaf blades elongated 33% faster at continuous low than at continuous high irradiance (60 versus 300 micromoles per second per square meter photosynthetic photon flux density) when temperature of the leaf elongation zone was held constant at 21 degrees C. Increased rate of elongation was associated with a near proportional increase in length of the elongation zone (+38%). In contrast, growth in width and thickness was decreased at low irradiance, resulting in only a 12% increase in leaf area production and 5% less total growth-associated water deposition than at high irradiance. At low irradiance dry matter (DM) import into the elongation zone was 28% less, and 55% less DM was used per unit leaf area produced. DM use in synthesis of structural components (i.e. DM less water-soluble carbohydrates) was only 13% less at low irradiance, whereas water-soluble carbohydrates (WSC) deposition was 43% less. The lower rate of WSC deposition at low irradiance was associated with a higher net rate of monosaccharide deposition (+39%), whereas net deposition rates for sucrose (-27%) and fructan (-56%) were less than at high irradiance. Still, at low irradiance, net fructan accumulation accounted for 64% of WSC deposition, i.e. 25% of DM import, demonstrating the high sink strength of the leaf elongation zone.

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 degrees 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.

Diurnal growth of tall fescue leaf blades : I. Spatial distribution of growth, deposition of water, and assimilate import in the elongation zone

Tall fescue leaf blades elongate at near constant rates during most of the light and dark periods of the diurnal cycle, with the dark rate being higher by 60 to 65%. Our objective was to determine relationships among diurnal rates of leaf elongation, deposition of water and deposition of dry matter (DM) into the elongation zone. Two separate experiments were conducted, both with a 15-hour photoperiod and constant 21 degrees C at the growth zone. Increased rates of leaf elongation in darkness were due to proportionally increased rates of elongation of 4-millimeter segments of the elongation zone. Length of the total elongation zone was 30 millimeters in both light and darkness. The spatial distribution of water contents in the elongation zone varied little during the diurnal cycle. Thus, dark stimulation of leaf elongation rate (+65%) and of water deposition (+77%) into elongation zones were similar. Water content per unit leaf length increased by 50% between the basal and distal limits of the elongation zone, indicating that tissue also grew in the lateral and vertical dimensions. Longitudinal growth of tissue, however, allowed 5 to 7 times more water deposition into the elongation zone than growth in cross-sectional area. This relationship was similar in light and darkness. In both light and darkness net rates of DM deposition (microgram per millimeter leaf length per hour) increased from the zone of cell division towards the region of most active elongation, 10 to 15 millimeters from the ligule, then decreased towards the distal end of the elongation zone. Net DM deposition rates (microgram per hour) integrated over the 30-millimeter elongation zone were similar during light and darkness. Thus, DM in the elongation zone was diluted during darkness as a result of increased water deposition. Net DM deposition rates at and above the distal end of the elongation zone were clearly positive during the light, but were close to zero or negative in darkness. Thus, DM deposition into the elongation zone and the adjacent recently expanded tissue was differentially affected in the diurnal cycle, DM deposition occurred in both tissues in light, but was restricted to the elongation zone in darkness.

Diurnal Growth of Tall Fescue Leaf Blades : II. Dry Matter Partitioning and Carbohydrate Metabolism in the Elongation Zone and Adjacent Expanded Tissue

The spatial distributions of net deposition rates of water soluble carbohydrate-free dry matter (WSC-free DM) and WSC were evaluated within and above the elongation zone of tall fescue (Festuca arundinacea Schreb.) leaf blades during light and darkness. Imported DM used for WSC-free DM synthesis during darkness (67% of the total in experiment I and 59% in experiment II) was greater than during light (47% in both experiments), suggesting that the 65% higher leaf elongation rate during darkness was accompanied by higher rates of synthesis of cellular structural components. Deposition rates of WSC in the basal and central part of the elongation zone (0-20 mm from the ligule) were similar during light and darkness, but above 20 millimeters WSC deposition occurred during light and WSC loss occurred during darkness. WSC deposition and loss throughout the elongation zone and the recently expanded tissue were mostly due to net synthesis and degradation of fructan. Fructan was predominantly low molecular weight and contributed about 50% of the total osmotic partial pressure of WSC. In the most actively growing region, where fructan synthesis was most rapid, no diurnal change occurred in molecular weight distribution of fructan. WSC solute concentrations were diluted in the most actively growing tissue during darkness because net monosaccharide and fructan deposition were unaltered and sucrose deposition was decreased, but growth-associated water deposition was increased by 77%. Net rates of fructan synthesis and degradation were not related to tissue sucrose concentration, but appeared to respond to the balance between assimilate import and assimilate use in synthesis of cellular structural components (i.e. WSC-free DM) and deposition of monosaccharides. Fructan synthesized in tissue during most active elongation was degraded when the respective tissue reached the distal limit of the elongation zone where assimilate import in darkness was insufficient to maintain synthetic processes associated with further differentiation of cells.