Accumulation Of Total Nonstructural Carbohydrates Research Articles

Adaptive responses of carbon and nitrogen metabolisms to nitrogen-deficiency in Citrus sinensis seedlings

BackgroundIn China, nitrogen (N)-deficiency often occurs in Citrus orchards, which is one of the main causes of yield loss and fruit quality decline. Little information is known about the adaptive responses of Citrus carbon (C) and N metabolisms to N-deficiency. Seedlings of ‘Xuegan’ (Citrus sinensis (L.) Osbeck) were supplied with nutrient solution at an N concentration of 0 (N-deficiency), 5, 10, 15 or 20 mM for 10 weeks. Thereafter, we examined the effects of N supply on the levels of C and N in roots, stems and leaves, and the levels of organic acids, nonstructural carbohydrates, NH4+-N, NO3−-N, total soluble proteins, free amino acids (FAAs) and derivatives (FAADs), and the activities of key enzymes related to N assimilation and organic acid metabolism in roots and leaves.ResultsN-deficiency elevated sucrose export from leaves to roots, C and N distributions in roots and C/N ratio in roots, stems and leaves, thus enhancing root dry weight/shoot dry weight ratio and N use efficiency. N-deficient leaves displayed decreased accumulation of starch and total nonstructural carbohydrates (TNC) and increased sucrose/starch ratio as well as a partitioning trend of assimilated C toward to sucrose, but N-deficient roots displayed elevated accumulation of starch and TNC and reduced sucrose/starch ratio as well as a partitioning trend of assimilated C toward to starch. N-deficiency reduced the concentrations of most FAADs and the ratios of total FAADs (TFAADs)/N in leaves and roots. N-deficiency reduced the demand for C skeleton precursors for amino acid biosynthesis, thus lowering TFAADs/C ratio in leaves and roots. N-deficiency increased (decreased) the relative amounts of C-rich (N-rich) FAADs, thus increasing the molar ratio of C/N in TFAADs in leaves and roots.ConclusionsOur findings corroborated our hypothesis that C and N metabolisms displayed adaptive responses to N-deficiency in C. sinensis seedlings, and that some differences existed between roots and leaves in N-deficiency-induced alterations of and C and N metabolisms.

Effects of perennial ryegrass competition on bermudagrass and hybrid bermudagrass cover, biomass, and total nonstructural carbohydrate accumulation

AbstractTo extend golf play in early summer or hasten athletic field preparation in late summer, turf managers often desire to allow perennial ryegrass (Lolium perenne L.) to persist and compete with overseeded bermudagrass (Cynodon dactylon L. Pers.) or hybrid bermudagrass (Cynodon dactylon × C. transvaalensis Burtt Davy) during optimal growing conditions. Understanding how competition‐free growing degree‐days (CFGDD) influence bermudagrass total nonstructural carbohydrates (TNC), biomass, and visual green cover may aid in turf‐use scheduling. Twenty‐four weekly treatments of foramsulfuron eliminated perennial ryegrass competition in replicate plots and allowed assessment of competition‐free bermudagrass coverage, biomass, and TNC as related to cumulative heat and light units. Results from this study indicate that bermudagrass coverage is a function of cumulative CFGDD and competition‐free daily light integral (CFDLI) but is dependent on locations or years where cultivars or seasonal legacy are variable. At three locations, ‘Patriot’ and ‘Midiron’ hybrid bermudagrass and ‘Riviera’ bermudagrass required 603, 709, and 959 CFGDD and 2,447, 3,271, and 4,556 mol m–2 CFDLI to reach 97% coverage, respectively. Trends of TNC per unit area, however, continued to increase after full coverage was achieved and up to the point at which no more heat or light units were accumulated in fall. Absence of a clear asymptote in the trends suggests that heat or light units that were seasonally accumulated in Blacksburg, VA, were insufficient to achieve maximum bermudagrass carbohydrate reserves before winter dormancy.

Interspecific differences in how sink-source imbalance causes photosynthetic downregulation among three legume species.

Sink-source imbalance could cause an accumulation of total non-structural carbohydrates (TNC; soluble sugar and starch) in source leaves. We aimed to clarify interspecific differences in how sink-source imbalance and TNC causes the downregulation of photosynthesis among three legume plants. The TNC in source leaves was altered by short-term manipulative treatments, and its effects on photosynthetic characteristics were evaluated. Soybean, French bean and azuki bean were grown under high nitrogen availability. After primary leaves were fully expanded, they were subjected to additional treatments: defoliation except for two primary leaves; transfer to low nitrogen conditions; transfer to low nitrogen conditions and defoliation; or irradiation by light-emitting diodes. Physiological and anatomical traits such as TNC content, maximum photosynthetic rate, cell wall content and δ13C values of primary leaves and whole-plant growth were examined. Among the three legume plants, the downregulation of maximum photosynthesis and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content was co-ordinated with an increase in TNC only in French bean. Rubisco did not decrease with an increase in TNC in soybean and azuki bean. The defoliation treatment caused an increase in cell wall content especially in soybean, and maximum photosynthesis decreased despite resulting in a higher Rubisco content. This indicates that a decrease in mesophyll conductance could cause photosynthetic downregulation, which was confirmed by an increase in δ13C. The present results suggest that a downregulation of photosynthesis in response to increased levels of TNC in source leaves can result not only from decreases in Rubisco content, but also from anatomical factors, such as an increase in cell wall thickness leading to reduced chloroplast CO2 concentrations.

Growth, Carbohydrate Assimilation and Leaf Gas Exchange Characteristics of Elaeis guineensis Jacq Seedlings under Nitrogen Fertilization

Aims: This study was conducted to characterise the growth, carbohydrate and leaf gas exchange patterns of oil palm seedlings performance (Deli Yangambi) under nitrogen fertilisation. Study Design: The palms were fertilised with three nitrogen rates: (1) Control (90 g/palm); (2) twice the control (180 g/palm), and (3) thrice the control (270 g/palm). Randomized complete block design (RCBD) with three replications was used during the experiment using Deli Yangambi seedlings Place and Duration of Study: Ladang 2, Universiti Putra Malaysia from March 2017 to June 2017. Methodology: The nitrogen treatments started when the oil palm seedlings reached four months old using Urea (46% N) as a source of nitrogen. Growth, carbohydrate and leaf gas exchange properties were measured at the end of the treatment period of two months. Results: As the level of nitrogen application increased, from 90 - 270 g/palm, the plant height, number of frond per plant, basal diameter, leaf area per seedling, root biomass, total dry matter, net assimilation rate (NAR) and relative growth rate (RGR) was statistically significantly increased between nitrogen treatments. The production of total non-structural carbohydrate (TNC) was reduced under high rates of nitrogen. The reduced in TNC under high nitrogen rates was supported by a decrease in sucrose and starch content under low nitrogen fertilisation. The net photosynthesis (A), stomatal conductance (gs), transpiration (E) and Intercellular CO2 also was found to be increased under high nitrogen rates indicated that higher rates of nitrogen would enhance the leaf gas exchange characteristics of the palms. Conclusion: In conclusion, the growth of oil palm seedlings growth was enhanced with high rates of nitrogen supplementation. As the nitrogen rates (90>270 g/palm), more production of TNC was produced and increased the sink strength of the palms. The enhanced growth and TNC accumulation also increased the leaf gas exchange properties that were showed by increased in A, gs, E, Ci and Water use efficiency in the oil palm seedlings.

Sink-Source Balance and Down-Regulation of Photosynthesis in Raphanus sativus: Effects of Grafting, N and CO2.

To clarify whether excessive accumulation of total non-structural carbohydrate (TNC) causes down-regulation of photosynthesis in Raphanus sativus, we manipulated sink-source balance to alter TNC levels in source leaves and examined its effects on photosynthetic characteristics, whole-plant biomass allocation and anatomical characteristics of leaves and petioles. Comet and Leafy varieties with large and small hypocotyls were reciprocally grafted to change hypocotyl sink strength. They were grown at high or low nitrogen (N) availability and at elevated or ambient CO2. Maximum photosynthetic rate, which was highly correlated with Rubisco and leaf N contents, was hardly correlated with TNC across the grafting combinations and growth conditions. Biomass allocation to petioles and hypocotyls and accumulation of TNC in each organ were significantly higher at low N. TNC and structural carbohydrates such as cellulose and hemicellulose were higher and the proportion of intercellular air space in source leaves was lower at low N and elevated CO2. We conclude that excess TNC does not cause severe down-regulation of photosynthesis, and cell walls and petioles are also major carbohydrate sinks responding to changes in sink-source and carbon-nitrogen balances, which contribute to alleviating further accumulation of TNC to avoid its negative effects in source leaves.

Impact of Carbon Dioxide and Increasing Nitrogen Levels on Growth, Photosynthetic Capacity, Carbohydrates and Secondary Metabolites in Kacip Fatimah (Labisia pumila)

Aims: This study was designed to investigate how CO2 and nitrogen availability alters growth, leaf gas exchange, primary and secondary metabolites production (total phenolics and flavonoid) in three varieties of L. pumila. namely var alata, var pumila and var lanceolata. under CO2 enrichment (1200 µmol mol-1) combined with four levels of nitrogen fertilization (0, 90, 180 and 270 kg N ha-1) for 15 weeks. Study Design: Three-month old L. pumila seedlings of var. alata, var. pumila and var. lanceolata were left for a month to acclimatize in a nursery until ready for the treatments. Carbon dioxide enrichment treatment started when the seedlings reached 4 months old where plants were exposed to 1200 µmol-1 mol-1 CO2 and fertilized with four levels of nitrogen concentrations viz. 0, 90, 180 and 270 kg N Ha-1. The fertilization with nitrogen levels were split into three applications. A control at ambient CO2 (400 µmol-1 mol-1) with standard N fertilization (180 kg N ha-1) was included to compare plant responses to high CO2 combined with different levels of N. Place and Duration of Study: Faculty of Agriculture Glasshouse Complex, Universiti Putra Malaysia from February to May 2011. Methodology: The seedlings were raised in specially constructed growth houses receiving 12-h photoperiod and average photosynthetic photon flux density of 300 µmol m-2 s-1. Carbon dioxide at 99.8% purity was supplied from a high–pressure CO2 cylinder and injected through a pressure regulator into fully sealed 2 m x 3 m growth houses at 2-h daily and applied continuous from 800 to 1000. Total plant biomass was taken by calculating the dry weight of root, stem and leaf per seedling. Total leaf area per plant was measured using a leaf area meter (LI-3100, Lincoln Inc, USA). The measurement of photosynthesis was obtained from a closed system of infra-red gas analyzer LICOR 6400 Portable Photosynthesis System. The response of net CO2 exchange (A) to changing intercellular CO2 concentration (Ci) was conducted at 27°C, 50% relative humidity and at light saturating conditions of 800 µmol m-2s-1. Anthrone method was used to determine the content of total available carbohydrate. Follin–Ciocalteau reagent was used to determine total phenolics and flavonoid using spectrophotometer. Results: In all parameters measured there were no significant was observed in the three varieties. As the levels of nitrogen increased from 0 to 270 kg N ha-1, plant growth, photosynthesis and total chlorophyll content was enhanced. However, carbohydrate, total phenolics and flavonoid was reduced with the increase in nitrogen fertilization. It was found that the high application of nitrogen can increased the photosynthetic capacity of L. pumila by having high Vcmax (RuBisCO CO2 fixation capacity), A sat (CO2 saturation point) and low dark respiration rate in the analysis of A/Ci curves. The production of secondary metabolites was related to increased total non-structural carbohydrate (TNC) under low nitrogen condition. Photosynthesis, displayed a significant negative relationship with secondary metabolites production. Total chlorophyll content exhibited very significant negative relationships with total soluble sugar, starch and total non-structural carbohydrate. Conclusion: This study demonstrated that enrichment with high levels of CO2 is able to enhance the production of plant secondary metabolites. However, increased nitrogen fertilization can reduce the production of these plant secondary metabolites under CO2 enrichment. When there were accumulation of TNC in plant leaves and reduction in photosynthesis, the production of plant secondary metabolites might be up-regulated. The increase in the production of plant secondary metabolites was indicated by reduction in chlorophyll contents.

Impact of potato psyllid density and timing of infestation on Zebra chip disease expression in potato plants

The impact of vector density and timing of infestation on potato were investigated. Healthy potato plants at different growth stages (4, 5, and 7 weeks after germination) were exposed separately to four different B. cockerelli densities (0, 5, 20, and 40 psyllids per cage) in field cages and Zebra chip (ZC) symptoms, leaf photosynthetic rates, tuber yield, and total nonstructural carbohydrate accumulation in leaves and tubers of healthy and B. cockerelli-infested plants were monitored. Potato psyllid nymph and egg populations reached a seasonal peak at 6 weeks after the exposure to insect. Younger plants at 4-week growth stage after germination were more susceptible to B. cockerelli infestation and ZC expression than older plants. As few as five B. cockerelli adults were enough to transmit the ZC pathogen and cause ZC expression both in foliage and tuber. At the density of 20 psyllids per cage, more than 50% of plants showed ZC symptoms in tubers. Furthermore, B. cockerelli infestation reduced leaf photosynthesis rates (P<sub>n</sub>), resulting in less starch and more reducing sugars in tubers, and hence reduced tuber weight and yield, especially when psyllid infestation occurred at the early growth stages. The results indicate that early B. cockerelli infestation of younger plants was associated with more severe ZC expression in both foliage and tubers, leading to earlier dead plants. The data suggest that strategies for controlling B. cockerelli during early potato crop development could thus lessen the severity of ZC development.

Manipulation of the hypocotyl sink activity by reciprocal grafting of two Raphanus sativus varieties: its effects on morphological and physiological traits of source leaves and whole-plant growth.

To reveal whether hypocotyl sink activities are regulated by the aboveground parts, and whether physiology and morphology of source leaves are affected by the hypocotyl sink activities, we conducted grafting experiments using two Raphanus sativus varieties with different hypocotyl sink activities. Comet (C) and Leafy (L) varieties with high and low hypocotyl sink activities were reciprocally grafted and resultant plants were called by their scion and stock such as CC, LC, CL and LL. Growth, leaf mass per area (LMA), total non-structural carbohydrates (TNCs) and photosynthetic characteristics were compared among them. Comet hypocotyls in CC and LC grew well regardless of the scions, whereas Leafy hypocotyls in CL and LL did not. Relative growth rate was highest in LL and lowest in CC. Photosynthetic capacity was correlated with Rubisco (ribulose 1·5-bisphosphate carboxylase/oxygenase) content but unaffected by TNC. High C/N ratio and accumulation of TNC led to high LMA and structural LMA. These results showed that the hypocotyl sink activity was autonomously regulated by hypocotyl and that the down-regulation of photosynthesis was not induced by TNC. We conclude that the change in the sink activity alters whole-plant growth through the changes in both biomass allocation and leaf morphological characteristics in R. sativus.

Harvest index combined with impaired N availability constrains the responsiveness of durum wheat to elevated CO2 concentration and terminal water stress.

Despite its relevance, few studies to date have analysed the role of harvest index (HI) in the responsiveness of wheat (Triticum spp.) to elevated CO2 concentration ([CO2]) under limited water availability. The goal of the present work was to characterise the role of HI in the physiological responsiveness of durum wheat (Triticum durum Desf.) exposed to elevated [CO2] and terminal (i.e. during grain filling) water stress. For this purpose, the performance of wheat plants with high versus low HI (cvv. Sula and Blanqueta, respectively) was assessed under elevated [CO2] (700μmolmol-1 vs 400μmolmol-1 CO2) and terminal water stress (imposed after ear emergence) in CO2 greenhouses. Leaf carbohydrate build-up combined with limitations in CO2 diffusion (in droughted plants) limited the responsiveness to elevated [CO2] in both cultivars. Elevated [CO2] only increased wheat yield in fully watered Sula plants, where its larger HI prevented an elevated accumulation of total nonstructural carbohydrates. It is likely that the putative shortened grain filling period in plants exposed to water stress also limited the responsiveness of plants to elevated [CO2]. In summary, our study showed that even under optimal water availability conditions, only plants with a high HI responded to elevated [CO2] with increased plant growth, and that terminal drought constrained the responsiveness of wheat plants to elevated [CO2].

English

The pattern of total nonstructural carbohydrate (TNC) accumulation in strawberry ( Fragaria ananassa Duch.) nursery runner plants, cv. eCamarosaf, was determined for three growing seasons. Plant growth and fruit production patterns were also evaluated. The experiments were carried out on plants propagated in high latitude (41‹50' N) and high elevation (1292 m) nurseries in Siskiyou County, California. Plants were sampled beginning in late summer through early autumn and analyzed for dry mass (DM) and TNC. Plants from different digging dates were established in growth chambers (GC) at UC Davis or fruit evaluation plots in Irvine, California. In the nursery, TNC concentration in storage tissues increased steadily from the second week of September to the third week of October, and crown and root TNC concentration was positively correlated with the accumulation of chilling units (hours .7.2‹C). The root TNC concentration consistently increased from 6 to 10% DM from mid-September to the first week of October. Transplant growth and fruiting pattern were affected by digging date. Overall, the roots were more sensitive to chilling in terms of TNC accumulation, than the crowns. Therefore, roots would be the appropriate organ for assessing TNC status and potential digging dates of strawberry nursery runner plants early in the fall. Key words : Transplant, carbohydrate, chilling, growth analysis.

Tree size affects accumulation of carbon and nitrogen metabolites in white spruce seedlings during short day-induced bud formation

The present study documents the changes in carbon and nitrogen metabolites occurring in apical buds and previous year stems of white spruce seedlings ( Picea glauca [Moench] Voss) with contrasting growth phenotypes (tall vs. small) after transfer to short day (SD; 8 h) photoperiod to induce bud formation. Concentrations of total nonstructural carbohydrates markedly increased in the developing buds within the days after transfer to SD, mainly as a result of increased concentrations of monosaccharides such as glucose, fructose, and pinitol. At the same time, starch levels declined, with the resulting carbohydrates presumably used to meet early carbon requirements of the SD-induced apical bud. Concentrations of glutamine, glutamic acid and proline also decreased immediately after transfer to SD in both organs. Later stages of SD-induced bud formation were characterized by an increase in starch, sucrose, and glutamine concentrations in previous year stems, concomitant with an increase in the steady-state levels of UDP-glucose pyrophosphorylase and glutamine synthetase protein. In contrast, arginine levels increased after 2 weeks of SD exposure, indicating a transition in arginine metabolism at the time of initiation of shoot stem primordia and bud elongation. Higher accumulation of total nonstructural carbohydrates and total amino acids in previous year stems of small trees could indicate lower sink strength of apical bud during its development, which could in turn impact subsequent tree growth.

Optimal partitioning theory revisited: Nonstructural carbohydrates dominate root mass responses to nitrogen

Under optimal partitioning theory (OPT), plants preferentially allocate biomass to acquire the resource that most limits growth. Within this framework, higher root mass under low nutrients is often assumed to reflect an allocation response to build more absorptive surface. However, higher root mass also could result from increased storage of total nonstructural carbohydrates (TNC) without an increase in non-storage mass or root surface area. To test the relative contributions of TNC and non-storage mass as components of root mass responses to resources, we grew seedlings of seven northern hardwood tree species (black, red, and white oak, sugar and red maple, American beech, and black cherry) in a factorial light x nitrogen (N) greenhouse experiment. Because root mass is a coarse metric of absorptive surface, we also examined treatment effects on fine-root surface area (FRSA). Consistent with OPT, total root mass as a proportion of whole-plant mass generally was greater in low vs. high N. However, changes in root mass were influenced by TNC mass in all seven species and were especially strong in the three oak species. In contrast, non-storage mass contributed to increased total root mass under low N in three of the seven species. Root morphology also responded, with higher fine-root surface area (normalized to root mass) under low vs. high N in four species. Although biomass partitioning responses to resources were consistent with OPT, our results challenge the implicit assumption that increases in root mass under low nutrient levels primarily reflect allocation shifts to build more root surface area. Rather, root responses to low N included increases in: TNC, non-storage mass and fine-root surface area, with increases in TNC being the largest and most consistent of these responses. The greatest TNC accumulation occurred when C was abundant relative to N. Total nonstructural carbohydrates storage could provide seedlings a carbon buffer when respiratory or growth demands are not synchronized with photosynthesis, flexibility in responding to uncertain and fluctuating abiotic and biotic conditions, and increased access to soil resources by providing an energy source for mycorrhizae, decomposers in the rhizosphere, or root uptake of nutrients.

Zebra chip disease incidence on potato is influenced by timing of potato psyllid infestation, but not by the host plants on which they were reared

Abstract The Zebra chip (ZC) syndrome is an emerging disease of potato and a major threat to the potato industry. The potato psyllid, Bactericerca cockerelli (Sulc) is believed to be a vector of the ZC pathogen, which is now thought to be Candidatus Liberibacter, a bacterium. To further understand the relationship between potato psyllid infestation and ZC disease expression, healthy potato plants at different growth stages (4, 6 and 10 weeks after germination) were exposed separately to potato psyllids that were separately reared on four solanaceous hosts plants (potato, tomato, eggplant or bell pepper) for more than 1 year. ZC symptoms, leaf rates and total nonstructural carbohydrate accumulation in leaves and tubers of healthy and psyllid‐infested plants were monitored and recorded. Typical ZC symptoms were observed in leaves and tubers of all plants exposed to potato psyllids regardless of the host plant on which they were reared. This was also accompanied by significant reductions in net photosynthetic rate. Caged potato plants without exposure to potato psyllids (uninfested controls) did not show any ZC symptom in both foliage and in harvested tubers. Foliage damage and ZC expression were most severe in the potato plants that were exposed to potato psyllids 4 weeks after germination compared to plants infested at later growth stages. Tubers from potato psyllid‐infested plants had significantly higher levels of reducing sugars (glucose) and lower levels of starch than those in healthy plants, indicating that potato psyllid infestation interfered with carbohydrate metabolism in either leaves or tubers, resulting in ZC expression.

Effect of growth temperature and total non‐structural carbohydrate accumulation on growth coefficient in Petunia × hybrida petals

It is widely believed that growth coefficient [g, respiratory cost consumed for synthesis of a unit dry weight (DW)] is independent of growth temperature. However, some studies using regression analysis indicated an increase in g at high temperatures. These two conflicting conclusions may be affected by the differences in total non-structural carbohydrate (TNC) accumulation between growth temperatures. Here, we evaluated whether g is independent of growth temperature in Petunia x hybrida petal after excluding TNC from DW. We also examined changes in g during petal development. We measured amounts of carbon, nitrogen, minerals and TNC and theoretically estimated construction cost and g. The results showed that the estimated g greatly changed with age and differed between growth temperatures. The estimated g was markedly increased by excluding TNC from DW. The effect of accumulation of TNC on the estimated g was observed at later stage of petal development. At earlier stage, the estimated g depended on the N amount. These results indicate that g changes with growth temperature and suggest that TNC accumulation must be taken into account in the estimation.

Influence of transient shade periods on the effects of drought on photosynthesis, carbohydrate accumulation and lipid peroxidation in sunflower leaves

The effects of a slow-imposing two-week soil drying period, and subsequent re-watering, on leaf water potential ( Ψ), gas exchange rates, chlorophyll fluorescence and on the concentrations of malondialdehyde (MDA) and non-structural carbohydrates (starch, hexose and sucrose) were determined in mature leaves of sunflower plants growing under controlled-environmental conditions. To assess how transient shade periods, associated with increased cloud cover, may influence drought-induced effects on carbon assimilation, measurements were carried out both in plants kept under the photosynthetic photon flux density (PPFD) prevailing during the growth period and stress imposition (750 μmol m −2 s −1), and in plants subjected to a 5-h long period under a lower PPFD (200 μmol m −2 s −1). In plants kept under high PPFD, Ψ, stomatal conductance ( g), net CO 2 uptake rate ( A), the quantum yield of photosystem II electron transport ( Φ e), the photochemical efficiency of open PSII reaction centres ( F ′ v / F ′ m ) and the diurnal accumulation of total non-structural carbohydrates (TNC) were significantly depressed at the end of the soil drying period, whereas non-photochemical quenching (NPQ), the concentrations of MDA and the predawn pools of soluble sugars were found to increase. Under high-light level, drought-induced effects on lipid peroxidation, chlorophyll fluorescence parameters and gas exchange rates were fully reversed upon re-watering. However, the inhibition of diurnal accumulation of TNC still persisted two days following re-watering, suggesting that carbohydrate export rates were enhanced following stress relief. An overall positive effect was found upon transferring water-stressed plants to low light level, as indicated by the increases in Ψ, intrinsic water use efficiency ( A/ g), Φ e and F ′ v / F ′ m , as well as the reversal of drought-induced enhancement of both NPQ and MDA concentration. Despite g being similar in shaded well-watered and re-watered plants, the latter exhibited net CO 2 uptake rates below those found in well-watered leaves under the same light conditions, together with a diurnal decrease in the concentration of TNC that was mainly attributable to the depletion of starch and sucrose. These results indicate that, contrasting with the positive effects of shading on water-stressed plants, low PPFD may negatively affect the recovery of net photosynthesis following stress relief.

Long-term effects of elevated atmospheric CO2 on species composition and productivity of a southern African C4 dominated grassland in the vicinity of a CO2 exhalation

We describe the long-term effects of a CO2 exhalation, created more than 70 years ago, on a natural C4 dominated sub-tropical grassland in terms of ecosystem structure and functioning. We tested whether long-term CO2 enrichment changes the competitive balance between plants with C3 and C4 photosynthetic pathways and how CO2 enrichment has affected species composition, plant growth responses, leaf properties and soil nutrient, carbon and water dynamics. Long-term effects of elevated CO2 on plant community composition and system processes in this sub-tropical grassland indicate very subtle changes in ecosystem functioning and no changes in species composition and dominance which could be ascribed to elevated CO2 alone. Species compositional data and soil δ13C isotopic evidence suggest no detectable effect of CO2 enrichment on C3:C4 plant mixtures and individual species dominance. Contrary to many general predictions C3 grasses did not become more abundant and C3 shrubs and trees did not invade the site. No season length stimulation of plant growth was found even after 5 years of exposure to CO2 concentrations averaging 610 μmol mol−1. Leaf properties such as total N decreased in the C3 but not C4 grass under elevated CO2 while total non-structural carbohydrate accumulation was not affected. Elevated CO2 possibly lead to increased end-of-season soil water contents and this result agrees with earlier studies despite the topographic water gradient being a confounding problem at our research site. Long-term CO2 enrichment also had little effect on soil carbon storage with no detectable changes in soil organic matter found. There were indications that potential soil respiration and N mineralization rates could be higher in soils close to the CO2 source. The conservative response of this grassland suggests that many of the reported effects of elevated CO2 on similar ecosystems could be short duration experimental artefacts that disappear under long-term elevated CO2 conditions.

Total Nonstructural Carbohydrate Assessment in Creeping Bentgrass at Different Mowing Heights

Any future precision turfgrass management system will benefit from the evaluation of turfgrass stress through indirect measurements of plant components. One such component might be the accumulation dynamics of total nonstructural carbohydrates (TNC). The effects of mowing height (MH) on TNC concentrations were measured in a field experiment. Clippings were collected from eight creeping bentgrass cultivars {Agrostis palustris Huds. [= A stolonifera var. palustris (Huds.) Farw.]} mowed at three different heights (0.64, 1.27, 1.90 cm) from 1998 through 2001. Clippings were evaluated 29 times by sampling early in the photoperiod before 1200 h. Collected clippings were instantly frozen in liquid nitrogen and freeze‐dried before TNC analyses. The TNC levels were predicted with near infrared reflectance spectroscopy (NIRS) after building a predictive equation for each carbohydrate fraction with conventional laboratory values. Seasonal trends in TNC concentrations and differences in TNC accumulation among creeping bentgrass cultivars were also evaluated. Mowing heights showed significant differences in TNC content, with 0.64‐cm mowed plots having higher TNC levels than 1.27‐ and 1.90‐cm mowed plots on 13 of the 29 dates of evaluation. No consistent differences were observed in TNC concentrations among different cultivars across different growing seasons. However, there were significant seasonal fluctuations in average TNC content at all MHs. The average TNC content of the clippings during 1999 and 2000 started decreasing in mid‐July, showing the lowest TNC content in August. The average TNC content showed an increasing trend in September, exhibiting the highest amount of TNC accumulation in October and November.

Total Nonstructural Carbohydrate Assessment in Creeping Bentgrass at Different Mowing Heights

Any future precision turfgrass management system will benefit from the evaluation of turfgrass stress through indirect measurements of plant components. One such component might be the accumulation dynamics of total nonstructural carbohydrates (TNC). The effects of mowing height (MH) on TNC concentrations were measured in a field experiment. Clippings were collected from eight creeping bentgrass cultivars {Agrostis palustris Huds. [= A stolonifera var. palustris (Huds.) Farw.]} mowed at three different heights (0.64, 1.27, 1.90 cm) from 1998 through 2001. Clippings were evaluated 29 times by sampling early in the photoperiod before 1200 h. Collected clippings were instantly frozen in liquid nitrogen and freeze-dried before TNC analyses. The TNC levels were predicted with near infrared reflectance spectroscopy (NIRS) after building a predictive equation for each carbohydrate fraction with conventional laboratory values. Seasonal trends in TNC concentrations and differences in TNC accumulation among creeping bentgrass cultivars were also evaluated. Mowing heights showed significant differences in TNC content, with 0.64-cm mowed plots having higher TNC levels than 1.27- and 1.90-cm mowed plots on 13 of the 29 dates of evaluation. No consistent differences were observed in TNC concentrations among different cultivars across different growing seasons. However, there were significant seasonal fluctuations in average TNC content at all MHs. The average TNC content of the clippings during 1999 and 2000 started decreasing in mid-July, showing the lowest TNC content in August. The average TNC content showed an increasing trend in September, exhibiting the highest amount of TNC accumulation in October and November.

Resource remobilization in Quercus ilex L. resprouts

We studied the source of the nitrogen used for the growth and resprouting of holm-oak (Quercus ilexL.), and the contribution of nitrogen and carbohydrate root reserves to these processes. Three-year-old plants were grown in a greenhouse with either a sufficient or restricted nitrogen supply for one year. Half the individuals were subjected to shoot excision to provoke resprouting, and a 15N solution was given to these plants and to controls for two months. Nitrogen, Total Non-structural Carbohydrate (TNC), Total Soluble Protein content, and 15N and 13C composition were determined, and histological analyses of woody tissue were performed. Our results show that N-deprived plants used nitrogen from root reserves to support a growth rate similar to that of non-deprived plants. However, deprived plants lost their resprouting capacity in spite of the high TNC accumulation and nitrogen resupply to the soil. After the supply of nitrogen was restored to N-deprived plants, this nutrient mainly accumulated in under-ground organs, which limited the above-ground growth. Resprouting plants first remobilized the nitrogen stored in roots, and thereafter took it up from the solution. The root-crown region did not behave as a specialised reserve organ in three-year-old Quercus ilex L. plants.

Interaction Between Flooding or Drought Stress and Potato Leafhopper Injury in Alfalfa

Potato leafhopper is a predominant biotic stress in alfalfa production, whereas drought and flooding are frequent abiotic stresses. Controlled environment studies were conducted to examine the interaction between flooding or drought stress and potato leafhopper (PLH, Empoasca fabae [Harris]) feeding damage on seedlings of two cultivars of alfalfa (Medicago sativa L): one susceptible to PLH, one resistant to PLH. Flooding treatments reduced shoot dry matter accumulation approximately 50% in the absence of PLH in both cultivars and significantly reduced root dry matter in one cultivar. Drought stress significantly reduced shoot dry matter and increased root dry matter. PLH feeding combined with either flooding or drought stress reduced root dry matter accumulation but not shoot dry matter. Flooding injury and PLH feeding injury showed an additive negative response for root dry matter accumulation. Flooded and non-stressed plants accumulated less root total non-structural carbohydrates (TNC) when subjected to PLH feeding. Responses of the two cultivars were similar across all biotic and abiotic treatments. The results clearly show that the most damaging physiological response of seedling alfalfa to PLH feeding is a sharp decline in root growth and TNC accumulation. This suggests that seedlings that have experienced even minor root waterlogging damage may be especially susceptible to damage by PLH feeding. Accepted for publication 23 April 2002. Published 2 May 2002.