Interaction of phloem-translocated amino compounds with nitrate net uptake by the roots of beech (Fagus sylvatica) seedlings

In the present study two experimental approaches were used to investigate the influence of changes in the allocation of amino compounds in the phloem of beech (Fagus sylvatica L.) seedlings on nitrate net uptake by the roots. In a first set of experiments Gin or Asp were directly fed into the phloem of the epicotyl via bark flaps. These compounds were previously found to be allocated in the phloem of adult beech trees and were shown to inhibit nitrate net uptake when supplied to beech roots. Feeding of solutions containing 100 mM of Gin or Asp plus 10 mM EDTA into the phloem resulted in a significant enrichment of the fine root tissue with the amino compound fed as compared to the roots of control plants supplied with amino acid-free EDTA solutions. Nitrate net uptake by the roots decreased by 61% (Gin) and 79% (Asp) as compared to the controls. In a second approach, shoots of young beech seedlings were exposed to 40g NH3 m^-3. NH3 uptake by shoots, nitrate net uptake by roots, and the contents and composition of total soluble non-protein nitrogen (TSNN) in leaves, phloem, and fine roots were determined and were compared to results gained with control plants exposed to charcoal-filtered air. NH3 fumigation of the shoots of beech seedlings resulted in a 35% reduction of nitrate net uptake by the roots as compared to controls. TSNN contents in leaves and phloem exudate of NH3-fumigated plants increased by 56% and 37%, respectively. This enrichment was mainly due to Arg and Glu in the leaves and Asp, Asn, Glu, and Gin, but not to Arg, in phloem exudate. The TSNN content of the fine roots was not changed by NH3 fumigation, but a significant increase in the Gin content was observed. From these results it is concluded that phloem transport of amino compounds, especially of Gin and Asp, from the shoot to the roots mediates regulation of nitrate net uptake by the roots of beech trees in order to adapt this process to the nitrogen demand of the whole plant.     

Consequences of elevated CO2, augmented nitrogen-deposition and soil type on the soluble nitrogen and sulphur in the phloem of beech (Fagus sylvatica) and spruce (Picea abies) in a competitive situation

Mixed spruce-beech plantations grown in large open-top chambers (OTC) were used to study consequences of elevated CO2, nitrogen-deposition and soil type on plant internal nitrogen and sulphur cycling of juvenile beech (Fagus sylvatica L.) and spruce (Picea abies Karst.) in a competitive situation. Processes of re-cycling as a consequence of protein turnover during leaf senescence in autumn were of further interest. For this purpose, phloem sap was collected in September 1998 and analysed for the composition and concentrations of organic and inorganic nitrogen and sulphur compounds. The phloem exudate of spruce showed higher total soluble non-protein nitrogen (TSNN) concentration on calcareous soil than on acidic soil, independent of the treatment. N-fertilization increased the N-concentration of phloem exudate significantly on both soil types, mainly by an increase of Arg and Gln concentrations. Elevated CO2 slightly increased TSNN on calcareous, but not on acidic soil. The combination of elevated CO2 and augmented N-deposition induced a further increase of TSNN on calcareous soil, but caused a lower N-effect on TSNN on acidic soil. Arg, the main TSNN component in phloem exudate, mediated this effect. Since Arg is considered to be a major nitrogen storage compound, it is concluded that in autumn elevated CO2 and augmented N-deposition, influence storage of N rather than N-supply of spruce. An effect of elevated CO2 and augmented N-deposition on GSH and sulphate concentrations in phloem exudate of spruce was not observed on acidic soil. On calcareous soil augmented N-deposition enhanced, elevated CO2 decreased phloem exudate GSH contents. In combination, elevated CO2 compensated the positive effect of N-deposition. The effects of elevated CO2 and augmented N-deposition on phloem sap N- and S-contents described above were not observed for beech trees. Apparently, elevated CO2 and augmented N-deposition did not affect plants internal S and N cycling of beech grown in spruce-beech plantations.     

Nitrogen distribution in young Norway spruce (Picea abies) trees as affected by pedospheric nitrogen supply

In numerous locations in Europe spruce trees are exposed to high loads of nitrogen. The present study was performed to characterize the distribution of nitrogen compounds under these conditions. For this purpose Norway spruce ( Picea abies [L.] Karst.) trees were cultivated under close-to-natural conditions of a forest understory in soil from an apparently nitrogen-limited field site in the Black Forest either with, or without supplementation of nitrogen as ammonium nitrate. After 11 and 20 months, growth, total nitrogen contents of the biomass, and total soluble non-proteinogenic nitrogen compounds (TSNN, i.e. nitrate, ammonium, soluble proteinogenic and non-proteinogenic amino compounds) in needles, xylem sap and phloem exudate were analysed. After 20 months of growth, N-fertilization had slightly enhanced the biomass of current-, but not of 1-year-old shoots. At both harvests, total N-content of 1-year-old needles was increased by N-fertilization, whereas current-year needles were not significantly affected. By contrast, TSNN was elevated by N-fertilization in both current-year and 1-year-old needles. The increase in TSNN was mainly attributed to an accumulation of arginine. Xylem sap analysis showed that the increase in TSNN of the needles was a consequence of enhanced nitrogen assimilation of the roots rather than the shoot. Since also TSNN in phloem exudates was enhanced, it appears that N-fertilization elevates the cycling pool of amino compounds in young Norway spruce trees. However, this pool seems to be subject to metabolic interconversion, since mainly glutamine and aspartate are transported in the xylem from the roots to the shoot, but arginine accumulated in the needles and the phloem.     

Competition for nitrogen sources between European beech (Fagus sylvatica) and sycamore maple (Acer pseudoplatanus) seedlings

To investigate the short‐term consequences of direct competition between beech and sycamore maple on root N uptake and N composition, mycorrhizal seedlings of both tree species were incubated for 4 days (i.e. beech only, sycamore maple only or both together) in an artificial nutrient solution with low N availability. On the fourth day, N uptake experiments were conducted to study the effects of competition on inorganic and organic N uptake. For this purpose, multiple N sources were applied with a single label. Furthermore, fine roots were sampled and analysed for total amino acids, soluble protein, total nitrogen, nitrate and ammonium content. Our results clearly show that both tree species were able to use inorganic and organic N sources. Uptake of inorganic and organic N by beech roots was negatively affected in the presence of the competing tree species. In contrast, the presence of beech stimulated inorganic N uptake by sycamore maple roots. Both the negative effect of sycamore maple on N uptake of beech and the positive effect of beech on N uptake of sycamore maple led to an increase in root soluble protein in beech, despite an overall decrease in total N concentration. Thus, beech compensated for the negative effects of the tree competitor on N uptake by incorporating less N into structural N components, but otherwise exhibited the same strategy as the competitor, namely, enhancing soluble protein levels in roots when grown under competition. It is speculated that enhanced enzyme activities of so far unknown nature are required in beech as a defence response to inter‐specific competition.     

In situ measurement of fine root water absorption in three temperate tree species—Temporal variability and control by soil and atmospheric factors

Miniature heat balance-sap flow gauges were used to measure water flows in small-diameter roots (3–4 mm) in the undisturbed soil of a mature beech–oak–spruce mixed stand. By relating sap flow to the surface area of all branch fine roots distal to the gauge, we were able to calculate real time water uptake rates per root surface area (J_s) for individual fine root systems of 0.5–1.0 m in length. Study aims were (i) to quantify root water uptake of mature trees under field conditions with respect to average rates, and diurnal and seasonal changes of J_s, and (ii) to investigate the relationship between uptake and soil moisture θ, atmospheric saturation deficit D, and radiation I. On most days, water uptake followed the diurnal course of D with a mid-day peak and low night flow. Neighbouring roots of the same species differed up to 10-fold in their daily totals of J_s (<100–2000 g m⁻² d⁻¹) indicating a large spatial heterogeneity in uptake. Beech, oak and spruce roots revealed different seasonal patterns of water uptake although they were extracting water from the same soil volume. Multiple regression analyses on the influence of D, I and θ on root water uptake showed that D was the single most influential environmental factor in beech and oak (variable selection in 77% and 79% of the investigated roots), whereas D was less important in spruce roots (50% variable selection). A comparison of root water uptake with synchronous leaf transpiration (porometer data) indicated that average water fluxes per surface area in the beech and oak trees were about 2.5 and 5.5 times smaller on the uptake side (roots) than on the loss side (leaves) given that all branch roots <2 mm were equally participating in uptake. Beech fine roots showed maximal uptake rates on mid-summer days in the range of 48–205 g m⁻² h⁻¹ (i.e. 0.7–3.2 mmol m⁻² s⁻¹), oak of 12–160 g m⁻² h⁻¹ (0.2–2.5 mmol m⁻² s⁻¹). Maximal transpiration rates ranged from 3 to 5 and from 5 to 6 mmol m⁻² s⁻¹ for sun canopy leaves of beech and oak, respectively. We conclude that instantaneous rates of root water uptake in beech, oak and spruce trees are above all controlled by atmospheric factors. The effects of different root conductivities, soil moisture, and soil hydraulic properties become increasingly important if time spans longer than a week are considered.     

Physiological performance of beech (Fagus sylvatica L.) at its southeastern distribution limit in Europe: seasonal changes in nitrogen, carbon and water balance

To assess the physiological performance of drought-sensitive European beech ( Fagus sylvatica L.) under the dry Mediterranean climate prevailing at its southeastern distribution limit in Europe, we analyzed seasonal changes in carbon, nitrogen and water balance of naturally grown adult trees. We determined the foliar C and N contents, delta13C and delta18O signatures, total soluble non-protein nitrogen compounds (TSNN) in xylem, leaves, and phloem, as well as leaf water potential and photosynthetic quantum yield in northern Greece during 2003. Tissue sampling was performed in May, July, and September, while field measurements were conducted regularly. Climatic conditions for the 2003 growing season fall within the typical range of the studied area. The N- and C-related parameters displayed distinct seasonal courses. TSNN was highest in May in all tissues, and asparagine (Asn) was then the most abundant compound. Thereafter, TSNN decreased significantly in all tissues and both its concentration and composition remained constant in July and September. In both months, glutamate (Glu) prevailed in leaves, gamma-aminobutyric acid (GABA) in phloem exudates from twigs and trunks, and arginine (Arg) in the xylem sap, where loading with amino acids was rather low during that period, amounting to only 0.8 micromol N ml-1 in September. Highest total foliar N and C contents were detected in May, and the elevated abundance of nutrients as well as an increased foliar delta13C signature at the beginning of the growing season is attributed to remobilization processes. The signatures of delta18O, quantum yield and leaf water potentials varied only slightly throughout the growing season. Although summer precipitation at the study site was considerably lower compared to what is usual for typical central European beech forests, no intensive drought responses of the physiological apparatus were detected in the studied beech trees. This suggests efficient internal regulation mechanisms, constantly ensuring a favourable physiological status under the relatively dry Mediterranean climate.     

Daily changes in uptake, reduction and storage of nitrate in spinach grown at low light intensity

Under poor light conditions, as normally used during winter production of greenhouse vegetables, the nitrate concentration in the shoot of spinach ( Spinacia oleracea L. cv. Vroeg Reuzenblad) showed a diurnal rhythm. This rhythm was mainly caused by a decrease during the day, followed by an increase during the night in the leaf blade nitrate concentration. Nitrate was mainly located in the vacuoles of the leaf blades. A strong correlation was found between net uptake of nitrate by the roots and the nitrate concentration in the leaf blade vacuoles. The nitrate concentration in the leaf blades increased during the initial hours of the night. This increase was caused by a marked increase in the net uptake rate of nitrate by the roots during the first hours of the dark period. During the second part of the night both net uptake rate of nitrate by the roots and the vacuolar nitrate concentration in the leaf blades remained constant.
We conclude that nitrate is taken up for osmotic purposes when light conditions are poor because of a lack of organic solutes. During the night, nitrate influx into the vacuole is needed for replacement of organic solutes, which are metabolized during the night, and possibly also for leaf elongation growth. During the day, vacuolar nitrate may be exchanged for newly synthesized organic solutes and be metabolized in the cytoplasm. A strong diurnal rhythm in nitrate reductase (NR; EC 1.6.6.1.) activity was absent, due to the poor light conditions, and in vitro NR activity was not correlated with nitrate flux from the roots. In vivo NR activity also lacked a strong diurnal rhythm, but it was calculated that in situ nitrate reduction was much lower during the night, so that the major nitrate assimilation took place during the day.     

Diurnal variation of root respiration rates and nitrate uptake as influenced by nitrogen supply

Root respiration rates of Lolium multiflorum supplied with nitrate or ammonium were measured continuously during several days (Exp. A). Net uptake rate of nitrate was similarly measured by an ion selective nitrate electrode in a system of flowing nutrient solution (Exp. B). Diurnal variation of in vitro nitrate reductase activity and nitrate content of tops and roots were determined (Exp. C). Two levels of irradiance were applied throughout, with day:night of 16:8 h. Root respiration rates showed diurnal patterns, most pronounced in the nitrate treatment, with two peaks appearing about 6 and 16 h after commencement of the photoperiod. Respiration rates were highest in the nitrate treatment and at high irradiance. Respiration rates fell after removal of nitrogen, particularly in the nitrate supplied plant and at high irradiance. Net uptake rate of nitrate exhibited diurnal patterns, often with two peaks occurring at the same times as those of respiration rates. In vitro nitrate reductase activity of tops increased steeply 16 h after commencement of the photoperiod and remained at the high level during the following 8 h of darkness. Nitrate content of tops was highest during the 8 h dark period and fell at the start of the photoperiod. Possible controlling systems of the apparent coincidences of diurnal variation rates, net nitrate uptake and nitrate reduction are discussed.     

Seasonal courses of key parameters of nitrogen,carbon and water balance in European beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) grown on four different study sites along a European North–South climate gradient during the 2003 drought

During the growing season of the exceptionally dry and warm year 2003, we assessed seasonal changes in nitrogen, carbon and water balance related parameters of mature naturally grown European beech (Fagus sylvatica L.) along a North–South transect in Europe that included a beech forest stand in central Germany, two in southern Germany and one in southern France. Indicators for N balance assessed at all four sites were foliar N contents and total soluble non-protein nitrogen compounds (TSNN) in xylem sap, leaves and phloem exudates; C and water balance related parameters determined were foliar C contents, δ¹³C and δ¹⁸O signatures. Tissue sampling was performed in May, July and September. The N related parameters displayed seasonal courses with highest concentrations during N remobilization in May. Decreased total foliar N contents as well as higher C/N ratios in the stands in central Germany and southern France compared to the other study sites point to an impaired N nutrition status due to lower soil N contents and precipitation perception. TSNN concentrations in leaves and phloem exudates of all study sites were in ranges previously reported, but xylem sap content of amino compounds in July was lower at all study sites when compared to literature data (c. 1 μmol N mL⁻¹). In September, TSNN concentrations increased again at the two study sites in southern Germany after a rain event, whereas they remained constant at sites in central Germany and southern France which hardly perceived precipitation during that time. Thus, TSNN concentrations in the xylem sap might be indicative for water balance related N supply in the beech trees. TSNN profiles at all study sites, however, did not indicate drought stress. Foliar δ¹³C, but not foliar C and δ¹⁸O followed a seasonal trend at all study sites with highest values in May. Differences in foliar δ¹³C and δ¹⁸O did not reflect climatic differences between the sites, and are attributed to differences in altitude, photosynthesis and δ¹⁸O signatures of the water sources. Except of low TSNN concentrations in the xylem sap, no physiological indications of drought stress were detected in the trees analysed. We suppose that the other parameters assessed might not have been sensitive to the drought events because of efficient regulation mechanisms that provide a suitable physiological setting even under conditions of prolonged water limitation. The uniform performance of the trees from southern France and central Germany under comparably dry climate conditions denotes that the metabolic plasticity of mature beech from the different sites studied might be similar.     

Estimating the relative nutrient uptake from different soil depths in Quercus robur, Fagus sylvatica and Picea abies

The distribution of fine roots and external ectomycorrhizal mycelium of three species of trees was determined down to a soil depth of 55 cm to estimate the relative nutrient uptake capacity of the trees from different soil layers. In addition, a root bioassay was performed to estimate the nutrient uptake capacity of Rb⁺ and NH₄⁺ by these fine roots under standardized conditions in the laboratory. The study was performed in monocultures of oak (Quercus robur L.), European beech (Fagus sylvatica L.) and Norway spruce [Picea abies (L.) Karst.] on sandy soil in a tree species trial in Denmark. The distribution of spruce roots was found to be more concentrated to the top layer (0–11 cm) than that of oak and beech roots, and the amount of external ectomycorrhizal mycelia was correlated to the distribution of the roots. The uptake rate of [⁸⁶Rb⁺] by oak roots declined with soil depth, while that of beech or spruce roots was not influenced by soil depth. In modelling the nutrient sustainability of forest soils, the utilization of nutrient resources in deep soil layers has been found to be a key factor. The present study shows that the more shallow-rooted spruce can have a similar capacity to take up nutrients from deeper soil layers than the more deeply rooted oak. The distribution of roots and mycelia may therefore not be a reliable parameter for describing nutrient uptake capacity by tree roots at different soil depths.     

Diurnal changes in nitrogen assimilation of tobacco roots.

To gain an insight into the diurnal changes of nitrogen assimilation in roots the in vitro activities of cytosolic and plasma membrane-bound nitrate reductase (EC 1.6.6.1), nitrite reductase (EC 1.7.7.1) and cytosolic and plastidic glutamine synthetase (EC 6.3.1.2) were studied. Simultaneously, changes in the contents of total protein, nitrate, nitrite, and ammonium were followed. Roots of intact tobacco plants (Nicotiana tabacum cv. Samsun) were extracted every 3 h during a diurnal cycle. Nitrate reductase, nitrite reductase and glutamine synthetase were active throughout the day-night cycle. Two temporarily distinct peaks of nitrate reductase were detected: during the day a peak of soluble nitrate reductase in the cytosol, in the dark phase a peak of plasma membrane-bound nitrate reductase in the apoplast. The total activities of nitrate reduction were similar by day and night. High activities of nitrite reductase prevented the accumulation of toxic amounts of nitrite throughout the entire diurnal cycle. The resulting ammonium was assimilated by cytosolic glutamine synthetase whose two activity peaks, one in the light period and one in the dark, closely followed those of nitrate reductase. The contribution of plastidic glutamine synthetase was negligible. These results strongly indicate that nitrate assimilation in roots takes place at similar rates day and night and is thus differently regulated from that in leaves.     

Rhythmic aspects of estuarine migration of hatchery-reared Atlantic salmon (Salmo salar) smolts

Seasonal, diel and tidal rhythmic activity of hatchery-reared Atlantic salmon ( Salmo salar ) smolts migrating through a large estuary was studied by ultrasonic tracking of 46 individuals during two seasons. Prior to 10 May each year most smolts were inactive and remained near shore in shallow water. After 10 May nearly all smolts moved away from the release point into swift water and made rapid seaward progress. Initiation of migration each year occurred when river and hatchery water temperatures rose above 9°C. Migration in the estuary was largely passive drift, and as a result there were distinct tidal rhythms of ground ('swimming') speed and net seaward travel. There were no diel rhythms in ground speed or net seaward travel; smolts drifted seaward on the tides during both day and night. Smolts may be slightly deeper during day than night.     

Increase in nitrate uptake by soybean plants during interruption of the dark period with low intensity light

Diurnal patterns of net NO⁻₃ uptake by nonnodulated soybean [ Glycine max (L.) Merr. cv. Ransom] plants growing in flowing hydroponic culture at 26 and 16°C root temperatures were measured at hourly intervals during alternate days of a 12-day growth period. Ion chromatography was used to determine removal of NO⁻₃ from the culture solution. Day and night periods of 9 and 15 h were used during growth. The night period included two 6-h dark periods and an intervening 3-h period of night interruption by incandescent lamps to effect a long-day photoperiod and repress floral initiation. At both root temperatures, the average specific rates of NO⁻₃ uptake were twice as great during the night interruption period as during the day period; they were greater during the day period than during the dark periods; and they were greater during the dark period immediately following the day period than during the later dark period that followed the night interruption. While these average patterns were repetitious among days, measured rates of uptake varied hourly and included intervals of net efflux scattered through the day period and more frequently through the 2 dark periods. Root temperature did not affect the average daily specific rates of uptake or the qualitative relationships among day, dark and night interruption periods of the diurnal cycle.     

Quantitative traits associated with adaptation of three barley (Hordeum vulgare L) cultivars to suboptimal iron supply

A laboratory method was developed that allows determination of in situ net nitrification with high sensitivity and at high temporal resolution. Nitrate in soils is quantitatively converted into nitrous oxide under strictly anaerobic conditions in the presence of 10 kPa acetylene by the soil endogenous denitrifier population, with the N₂O detected by a gas chromatograph equipped with a ⁶³Ni electron capture detector. Thus, even low net nitrification rates, i.e. small net increases in soil nitrate concentrations can easily be detected. Comparison of results using this method with results obtained using the classical in situ incubation method (buried bag soil incubation) revealed excellent agreement. Application of the new method allowed both determination of the seasonal pattern of net nitrification as well as correlation analysis between in situ NO and N₂O flux rates and in situ net nitrification rates of the forest soils studied. Regardless of the forest site studied (spruce, spruce limed, beech), and during each year of a 3 years period (1995–1997), net nitrification varied strongly with season and was least during winter and greatest during summer. The long-term annual, mean rate of net nitrification for the untreated spruce site, the limed spruce site and the beech site were 1.54 ± 0.27 mg N kg^–1 sdw d^–1, 1.92 ± 0.23 mg N kg^–1 sdw d^–1 and 1.31 ± 0.23 mg N kg^–1 sdw d^–1, respectively. In situ rates of nitrification and NO and N₂O emission were strongly correlated for all sites suggesting that nitrification was the dominate source of NO as well as N₂O.     

Diurnal changes in net uptake rate of nitrate are associated with changes in estimated export of carbohydrates to roots

The rate of NO3- uptake by soybean (Glycine max [L.] Merrill) roots generally declines during the night in association with progressive depletion of the nonstructural carbohydrate pool in the shoot as well as the concentration of carbohydrates in roots. To determine if NO3- uptake rate changes in response to variations in translocation rate of carbohydrates from shoot to roots per se or to carbohydrate status of the roots, the night period was interrupted with a low light level from incandescent lamps to alter the diurnal pattern of NO3- uptake by roots and export of carbohydrate from shoots of nonnodulated soybean. Depletion of NO3- from replenished, complete nutrient solutions containing 1 mM NO3- was measured by ion chromatography and rates of NO3- uptake were calculated. Changes in export of carbohydrates from shoot to roots during intervals of the night period were calculated as the differences between rates of disappearance in contents of nonstructural carbohydrates and their estimated rates of utilization in shoot respiration and growth. A positive, significant correlation occurred between changes in calculated rates of carbohydrate export from shoots and NO3- uptake rates. Conversely, there was no significant correlation between concentrations of nonstructural carbohydrates in roots and NO3- uptake rates. These results support the hypothesis that carbohydrate flux from shoot to roots has a direct role in regulation of nitrogen uptake by the whole plant.     

Occurrence of Crassulacean acid metabolism in Cissus trifoliata L. (Vitaceae)

Summary Evidence for the operation of CAM in the deciduous climber, Cissus trifoliata L., was obtained in field and laboratory studies. Under natural conditions, diurnal oscillations of titratable acidity and colorimetric measurements of night CO₂ fixation, determined for a period of two and a half years, suggested that acid accumulation was related to plant water status, assessed through the daily courses of stomatal resistance and xylem water potential during dry and rainy seasons. These findings were confirmed by gas exchange studies under controlled conditions which showed that the plant fixed all its CO₂ during the day when it was well irrigated; as water stress increased, dark CO₂ uptake gradually replaced fixation during the day until the plant only performed dark fixation. In severe water stress, even the rate of the latter process decreased until leaves fell.Abbreviations CAM Crassulacean acid metabolism - FW leaf fresh weight - SWC relative soil water content - PAR photosynthetically active radiation - TR total radiation; r, leaf diffusive resistance - WSD water saturation deficit (leaf-air vapour concentration difference) - RWC relative water content of leaves     

Seasonal dynamics of δ13C of C‐rich fractions from Picea abies (Norway spruce) and Fagus sylvatica (European beech) fine roots

The ^13/12C ratio in plant roots is likely dynamic depending on root function (storage versus uptake), but to date, little is known about the effect of season and root order (an indicator of root function) on the isotopic composition of C‐rich fractions in roots. To address this, we monitored the stable isotopic composition of one evergreen (Picea abies) and one deciduous (Fagus sylvatica), tree species' roots by measuring δ¹³C of bulk, respired and labile C, and starch from first/second and third/fourth order roots during spring and fall root production periods. In both species, root order differences in δ¹³C were observed in bulk organic matter, labile, and respired C fractions. Beech exhibited distinct seasonal trends in δ¹³C of respired C, while spruce did not. In fall, first/second order beech roots were significantly depleted in ¹³C, whereas spruce roots were enriched compared to higher order roots. Species variation in δ ¹³C of respired C may be partially explained by seasonal shifts from enriched to depleted C substrates in deciduous beech roots. Regardless of species identity, differences in stable C isotopic composition of at least two root order groupings (first/second, third/fourth) were apparent, and should hereafter be separated in belowground C‐supply‐chain inquiry.     

Diurnal and Seasonal Variations in Organic and Keto Acids in the Leaves of Rice (Oryza sativa L.)

Photo-sensitive and photo-insensitive cultivars of rice weregrown in pot culture during wet (monsoon) and dry (winter) seasonsto study diurnal and seasonal variations of organic and ketoacid content and respiration rates of the leaves at differentstages of growth. Organic acids accumulate with a large concentration of oxalicand tartaric acids in the photo-sensitive cultivar, Latisailin the dry, and in the photo-insensitive, IR-8 in wet seasons.Malic acid, being the most prominent, wasconcentrated in darknessand consumed largely in light in the following day. RQ valuesshowthat diurnal variations which are highest in morning declineduring day and rise again at night.