Regrowth dynamics of <Emphasis Type="Italic">Calamagrostis epigejos</Emphasis> after defoliation as affected by nitrogen availability

Young plants of a rhizomatous grass Calamagrostis epigejos (L.) Roth were grown from seed in nutrient solutions containing nitrogen in concentrations 0.1, 1.0, and 10 mM. After six weeks of cultivation the plants were defoliated and changes in growth parameters and in content of storage compounds were measured in the course of regrowth under highly reduced nitrogen availability. Plants grown at higher nitrogen supply before defoliation had higher amount of all types of nitrogen storage compounds (nitrates, free amino acids, soluble proteins), which was beneficial for their regrowth rate, in spite of lower content of storage saccharides. Amino acids and soluble proteins from roots and stubble bases were the most important sources of storage compounds for regrowth of the shoot. Faster growth of plants with higher N content was mediated by greater leaf area expansion and greater number of leaves. In plants with lower contents of N compounds number of green leaves decreased after defoliation significantly and senescing leaves presumably served as N source for other growing organs. Results suggest that internal N reserves can support regrowth of plants after defoliation even under fluctuating external N availability. Faster regrowth of C. epigejos with more reserves was mediated mainly by changes in plant morphogenesis.     

Seasonal Changes of Nitrogen Storage Compounds in a Rhizomatous Grass Calamagrostis epigeios

The seasonal dynamics in content and distribution of N-rich compounds between overwintering organs of Calamagrostis epigeios were examined. Samples were taken both from plants grown in natural conditions and in containers with controlled nutrient supply. There were significant changes in content of nitrate, free amino acids and soluble protein in all investigated plant parts during the course of a year. Amino acids showed both the highest maximum and seasonal fluctuation among the all N compounds observed and, therefore, appear to have a central role in N storage. Their content rises in the autumn, remains stable during winter and declines quickly at the beginning of spring. The most abundant amino acids in the end of winter storage period - asparagine, arginine and glutamine - constituted about 90 % of N in fraction of free amino acids. The portion of N stored in soluble proteins, however, was considerably smaller compare to both amino acids and nitrate. The amount of N stored in rhizomes of C. epigeios was smaller than in roots and stubble base before the onset of spring re-growth. This indicates that roots and stubble base are particularly important for winter N storage in this species.     

P effects on N uptake and remobilization during regrowth of Italian ryegrass (Lolium multiflorum)

The influence of P deficiency on the utilization of two sources of N, mineral N (exogenous N) and reserved N (endogenous N), for regrowth of Italian ryegrass (Lolium multiflorum) was studied. P-sufficient (+P) or P-free (−P) nutrition solution was applied from 7 days before defoliation to 24 days of regrowth and the N flows derived from two different N sources within the plant were quantified by ¹⁵N pulse-chase labeling. Shoot regrowth significantly reduced by 12 days of regrowth, while root growth was more in −P plants. Inorganic P (P_i) concentration was highly reduced by P deprivation more in the stubble and regrowing shoots and less in the roots. At defoliation, P deprivation had induced a higher accumulation for all N compounds in the stubble and for amino acids in the roots. The previously incorporated ¹⁵N in stubble and roots as nitrate and amino acids was much decreased in −P plants especially for the first 6 days of regrowth. Total N content in the regrowing leaves was not significantly different between +P and −P plants, but percentage contribution of remobilized N for total leaf N formation was significantly higher in −P plants (78%) than in +P plants (69%) at 6 days of regrowth. From day 12, the utilization of both endogenous and exogenous N was apparently inhibited in −P plants.     

Changes in the amide and amino acid composition of xylem exudate from perennial ryegrass (Lolium perenne L.) during regrowth after defoliation

The paper presents the results of amino acid analyses in xylem sap during leaf regrowth of ryegrass plants defoliated firstly at the 8th and secondly at the 12th week of culture. The free amino acid composition of leaves, stubble and roots was also determined and some of the results are reported. Prior to defoliation, xylem sap contained a high proportion of amides, particularly glutamine. During regrowth after defoliation, the proportion of asparagine in the xylem sap increased until the third day when the highest ratios of asparagine/glutamine appeared. The results are compared with relative amounts of free amino acids in the different plant parts and discussed in relation to source-sink nitrogen transfer.     

Origin of Foliar Nitrogen and Changes in Free Amino-Acid Composition and Content of Leaves, Stubble, and Roots of Perennial Ryegrass During Re-growth after Defoliation

Nitrogen re-mobilization and changes in free amino acids werestudied as a function of time in leaves, stubble, and rootsduring ryegrass (Lolium perenne L.) re-growth. Experiments with¹⁵N labelling clearly showed that during the first days nearlyall the nitrogen in new leaves came from organic nitrogen re-mobilizedfrom roots and stubble. On the days of defoliation, stubblehad the highest content of free amino acids with 23 mg per gdry weight against 15 mg and 14 mg in leaves and roots, respectively.The major amino acids in leaves were asparagine (23% of totalcontent in free amino acids), aminobutyrate, serine, glutamine,and glutamate (between 7% and 15%) whereas in roots and stubblethe contribution of amides was high, especially asparagine (about50%). Re-growth after cutting was associated with a rapid increaseof the free amino acid content in leaves, with a progressivedecrease in roots while stubble content remained virtually unchanged.In leaves, asparagine increased from the first day of re-growth,while the aspartate level remained unchanged and glutamine increasedstrongly on the first day but decreased steadily during thenext few days of re-growth. Asparagine in stubble and rootschanged in opposite directions: in stubble it tended to increasewhereas in roots it clearly decreased. In contrast, stubbleand roots showed a similar decrease in glutamine. In these twoplant parts, as in leaves, aspartate remained at a low level.Results concerning free amino acids are discussed with referenceto nitrogen re-mobilization from source organs (stubble androots) to the sink organ (regrowing leaves). Key words: Lolium perenne L, re-growth, nitrogen, free amino acids, glutamine, asparagine     

Contribution of vegetative storage proteins to seasonal nitrogen variations in the young shoots of peach trees (Prunus persica L. Batsch)

Qualitative and quantitative variations in the level of two low molecular weight vegetative storage proteins (VSP 19 kDa and 16.5 kDa) in peach shoots were compared with annual variations in total nitrogen and total soluble proteins. Protein patterns were obtained by SDS-PAGE and silver staining on each of the 12 kinetic samples collected between October 1995 and November 1996. VSP 16.5 kDa and 19 kDa exhibited typical annual VSP variations in both parenchyma and phloem. In wood, VSP 16.5 kDa was only present in November. All N compounds tested were stored in the autumn and their levels fell in the spring. Parenchyma was the principal stem storage tissue for all N compounds tested, even if proteins were more often highly concentrated in phloem and even if wood was the major shoot constituent. In winter, the two VSP accounted for 13% of bark proteins and 11% of wood proteins. Their storage yield, given by the winter/summer (W/S) ratio was higher (18.5) than that of total proteins (4). Between August to March, i.e. during the storage phase, N fractions obtained from VSP (N3) and total soluble proteins minus VSP (N2) accounted, respectively, for only 3% and 21% of total N accumulation in the bark, the remainder being due to the fraction not extracted (N1). A marked drop in all N compound levels characterized the mobilization phase (March to April), particularly for N3 (-84% between March and April) which were mobilized slightly before other N compounds. Although N3 exhibited the best mobilization yield, it represented only 5% of the total N mobilized. So, in spite of a similarity between VSP and N annual variation patterns, there was no tight correlation between their contents in bark. N2 supplied a high proportion of the N used for spring regrowth (40%), but the larger share (55%) came from N1 which was probably made up of free amino acids. Very tight positive correlations have been observed between these two N fractions and the N status. The lower bark total N content measured in August (6.4 mg N g(-1 )DW) during the assimilation phase (April to August) was equal to the unavailable N fraction, and the bark N mobilization potential (between March and August) was estimated at 6.35 mg N g(-1) DW. VSP did not quantitatively represent the main stored N pool. But, because of their high W/S ratio and their early remobilization, they seemed to play an important role in spring regrowth initiation.     

Mobilization of nitrogen reserves during regrowth of defoliated Trifolium repens L. and identification of potential vegetative storage proteins

Although it is well established that carbon reserves contributeto shoot regrowth of leguminous forage species, little informationis available on nitrogen reserves except in Medicaqo sativaL. and Trifolium subterraneum L. In this study, reserves werelabelled with ¹⁵N to demonstrate the mobilization of endogenousnitrogen from roots and stolons to regrowing leaves and newstolons during 24 d of regrowth in white clover (Thfolium repensL.). About 55% and 70%, respectively, of the nitrogen contentsof these organs were mobilized to support the regrowth of leaves.During the first 6 d, nitrogen in regrowing leaves came mainlyfrom N reserves of organs remaining after defoliation. Afterthese first 6 d of regrowth, most of the shoot nitrogen wasderived from exogenous nitrogen taken up while the contributionof nitrogen reserves decreased. After defoliation, the buffer-solubleprotein content of roots and stolons decreased by 32% duringthe first 6 d of regrowth. To identify putative vegetative storageproteins, soluble proteins were separated using SDS-PAGE ortwo-dimensional electrophoresis. One protein of 17.3 kDa instolons and two proteins of 15 kDa in roots seemed to behaveas vegetative storage proteins. These three polypeptides, initiallyfound at high concentrations, decreased in relative abundanceto a large extent during early regrowth and then were accumulatedagain in roots and stolons once normal growth was re-established. Key words: White clover, regrowth, ¹⁵N-labelled, vegetative storage proteins, electrophoresis     

A role for nitrogen reserves in forage regrowth and stress tolerance

Carbohydrate accumulation and utilization during shoot regrowth after defoliation and winter has been studied extensively in most species used as forage. However, recent work suggests that N reserves found in vegetative tissues also are important for defoliation tolerance and winter hardiness. Results suggest that these N reserves constitute an alternative N source used when N₂ fixation and/or mineral N uptake are reduced. ¹⁵N labelling experiments indicate that a large proportion of herbage N is derived from N reserves mobilized from stem bases or roots to developing leaves and shoots. Amino acids and specific proteins (i.e. vegetative storage proteins, VSPs) are deposited in roots and stem bases and, in the case of VSPs, are degraded rapidly after defoliation. Identification and characterization of VSPs will increase our understanding of the role N reserves play in stress tolerance and may lead to innovative approaches for improving forage persistence and productivity.     

Influence of initial organic N reserves and residual leaf area on growth, N uptake, N partitioning and N storage in alfalfa (Medicago sativa) during post-cutting regrowth

BACKGROUND AND AIMS: The influence of initial residual leaf area and initial N reserves on N uptake, final N distribution, and yield in alfalfa regrowing after cutting, were studied. METHODS: The effects of two levels of initial residual leaf area (plants cut to 15 cm, with (L+) or without (L-) their leaves) and two initial levels of N status [high N (HN) or low N (LN)] on growth, N uptake and N partitioning, allocation and storage after 29 d of post-cutting regrowth were analysed. KEY RESULTS: During most of the regrowth period (8-29 d after the initial harvest), HN and L+ plants had higher net N uptake rates than LN and L- plants, respectively, resulting in a greater final mineral N uptake for these treatments. However, the final partitioning of exogenous N to the regrowing shoots was the same for all treatments (67 % of total exogenous N on average). Final shoot growth, total plant N content, and N allocation to the different taproot N pools were significantly lower in plants with reduced initial leaf area and initial N reserve status. CONCLUSIONS: Although both initial residual leaf area and initial N reserves influenced alfalfa regrowth, the residual leaf area had a greater effect on final forage production and N composition in the taproot, whereas the N uptake rate and final total N content in plant were more affected by the initial N reserve status than by the residual leaf area. Moreover, N storage as proteins (especially as vegetative storage proteins, rather than nitrate or amino acids) in the taproot allowed nitrate uptake to occur at significant rates. This suggests that protein storage is not only a means of sequestering N in a tissue for further mobilization, utilization for growth or tissue maintenance, but may also indirectly influence both N acquisition and reduction capacities.     

Morphological pattern of development affects the contribution of nitrogen reserves to regrowth of defoliated white clover (Trifolium repens L.)

The contribution of nitrogen reserves to regrowth following defoliation was studied in white clover plants (Trifolium repens cv. Huia). This was found to be closely linked to the morphological pattern of development of the aerial parts during the same period. Low temperature (6 degrees C) and short day exposure (8 h photoperiod) were used to induce dwarf development, i.e. to increase branching rate and to enhance new sites of leaf production during a period of regrowth. Treated plants exhibited a large reduction in leaf area and a large increase in leaf pool size for the first 10 d of a subsequent regrowth under standard culture conditions (16 h daylight; 22/18 degrees C day/night). The contribution of nitrogen from storage compounds in organs remaining after defoliation (sources) to regrowing tissues (sinks) was assessed by 15N pulse-chase labelling during regrowth following shoot removal. The mobilization of nitrogen reserves from storage tissues of regrowing clover was closely linked to the pattern of differentiation of the newly developed organs. It appeared that regrowth was supported less by endogenous N for the first 10 d after defoliation in treated plants, compared with control plants grown continuously in standard conditions. It is assumed that dwarf plants exhibit a lower dependence upon the mobilization of soluble proteins previously accumulated in roots and uncut stolons. The relationship between leaf development rate and N-uptake recovery following defoliation is discussed.     

缺氮和复氮对菘蓝幼苗生长及氮代谢的影响

对基质育苗后水培的菘蓝进行缺氮与复氮处理,分析其生长情况及氮代谢产物含量的变化,探讨缺氮和复氮对菘蓝幼苗生长及氮代谢的影响,以提高菘蓝产量和品质以及栽培过程中的氮素利用效率。结果显示:(1)正常供氮条件下,菘蓝幼苗的叶绿素含量、谷氨酰胺合成酶(GS)活性、硝态氮含量、靛玉红含量为最高,而其株高、主根直径、根的鲜重与干重、叶的鲜重与干重、根系活力均最小。(2)缺氮处理增加了菘蓝幼苗的主根直径和根干重,提高其根系活力和硝酸还原酶(NR)活性,促进游离氨基酸在叶中的积累;但降低了GS的活性,也降低了叶中硝态氮、可溶性蛋白、靛玉红及根中游离氨基酸的含量;缺氮对叶中靛蓝的含量无明显影响。(3)复氮处理增加了菘蓝幼苗的株高、主根长、根鲜重、叶鲜重、叶干重,提高了其根系活力,降低了NR和GS的活性;与对照相比,复氮降低了叶中硝态氮含量,提高了叶中可溶性蛋白、靛蓝及根中游离氨基酸的含量,但对叶中游离氨基酸和靛玉红含量影响较小。研究表明,缺氮后再复氮有利于菘蓝幼苗叶的生长,同时有利于增加其叶内靛蓝含量,从而提高其产量和品质。     

Osmoregulation and role of nitrate during regrowth after cutting of ryegrass (Lolium perenne)

The osmotic role of nitrate during aftermath growth of Lolium perenne L. cv. Réveille was investigated. Plants were grown from seed in a controlled environment using a liquid medium with 1.0 m M NH₄NO₃ as nitrogen source.
Eight-week-old plants were cut 4.0 cm above the root system and then harvested over a 14-day period of regrowth on the same initial nutrient solution, except that nitrate was ¹⁵N labelled. Throughout the experimental period, nitrate storage and reduction in roots were low. In stubble and especially in leaves, nitrate accumulated during the first 6 days of regrowth whereas nitrate reduction mainly occurred after this period. Analyses of carbohydrate, chloride and potassium contents in stubble and leaves showed that the accumulation of nitrate osmotically compensated for the decrease in soluble sugars during the first 6 days of regrowth.
The cumulative osmotic potential of sugars, chloride and nitrate in differently treated plants was studied in stubble and leaves. Compared with uncut plants, the lower carbohydrate concentrations found in cut plants regrowing on 1.0 m M NH₄NO₃ were compensated for by an accumulation of nitrate. During aftermath growth on low nitrogen nutrition (0.2 m M NH₄NO₃), chloride replaced nitrate, supporting the proposed osmotic function of nitrate.
It is concluded that nitrate is involved in the osmotic adjustment of ryegrass during regrowth after cutting.     

The consequences of lower nitrogen availability in autumn for internal nitrogen reserves and spring growth of <Emphasis Type="Italic">Calamagrostis epigejos</Emphasis>

The building and use of internal N stores in the grass Calamagrostis epigejos was investigated in context of complex ecological study focused on mechanisms underlying competitive ability of this highly successful invasive species. Induced changes in nitrogen availability in the course of two subsequent vegetation seasons were used as a tool for finding (i) to what extent high N availability in substrate is important for building N reserves in autumn that support spring regrowth and, (ii) if contrasting contents of N storage compounds may result in differences in growth in the next season. Plants were grown in solely inorganic substrate and received a nutrient solution containing 5 mol m⁻³ of NH₄NO₃. The nitrogen supply was reduced in a low nitrogen (LN) treatment to 0.25 mol m⁻³ in August whereas in high nitrogen (HN) treatment remained high till December. During the following growing season were plants from both treatments grown at the low N supply (0.25 mol m⁻³). An increase in the content of N storage compounds was observed from September to December in both treatments. Plants in the LN treatment showed significantly lower total N content and also N allocated to mobilizable reserves (20–50% of HN plants), namely due to a smaller accumulation of amino acids and soluble protein in autumn. External nitrogen availability in autumn is hence highly important for building N reserves in this species. A major portion of the nitrogen stored in HN plants during winter was taken up from growth medium in late autumn, whereas translocation from senescing shoots dominated in LN treatment. During the winter about 50% of N in plants was permanently present in shoots bearing several frost resistant green leaves. Spring regrowth was accompanied by a fast decrease of both total N and the content of N storage compounds in both treatments. Amino acids were identified as the most prominent source of mobilizable N during spring regrowth. Development of leaf area in LN plants was significantly slower in March and April than in HN plants namely due to smaller number of tillers and green leaves per plant. Low N availability in autumn, therefore, may result in restrictions of plant growth and development in the following season.     

Effects of nitrogen on leaves,dry matter allocation and regrowth dynamics in Trifolium repens L. and Lolium perenne L. in pure and mixed swards

The effects of applied nitrogen (N) on dynamics of regrowth, dry matter (DM) allocation and leaf characteristics of grass and clover were investigated. Binary mixtures and monocultures of the diploid perennial ryegrass cultivars Barlet (erect) and Heraut (prostrate) and the white clovers cvs. Alice (large-leaved) and Gwenda (small-leaved) were established in a field experiment. Grass monocultures received three levels of N application (0, 140 or 280 kg N ha^–1), and mixtures 150 kg N ha^–1 (+N) or no N (–N). N was applied split over the season. Application of N reduced the average clover content in the DM of the mixtures from 43 to 12%. Due to defoliation, clover lost relatively more leaf area and less DM than grass, leading to a lower clover fraction in the leaf area index (LAI) of the stubble at the start of the next regrowth. In the –N mixtures, the clover fraction of the biomass and of the LAI increased within successive regrowth periods. In the +N mixtures, large-leaved Alice maintained its content during summer, mainly due to its greater petiole length which increased in response to N. The opposite was observed for Gwenda. At each harvest, the content of small-leaved Gwenda in the LAI and DM was lower than in the stubble at the start of regrowth. The allocation of DM to the petioles of Alice led to a decrease in the leaf weight ratio (LWR) in the +N mixtures, while Gwenda had a higher LWR and specific leaf area (SLA) in the +N mixtures than in the –N mixtures. There was little or no effect of ryegrass cultivar on competition with white clover.     

Seasonal variation in nitrogen storage reserves in the roots of leafy spurge (Euphorbia esula) and responses to decapitation and defoliation

Seasonal fluctuations of carbohydrates and nitrogenous components in the roots of the noxious perennial leafy spurge ( Euphorbia esula L.) are strongly associated with overwintering strategy, Amino acids and distinct soluble proteins accumulate during fall and remain at elevated levels throughout winter. The formation of carbohydrate reserves in roots was not significantly affected by decapitation or selective defoliation; however, maximum amino acid and soluble protein contents were markedly reduced. In particular, the accumulation pattern of a 26 kDa protein was altered. This protein may play a role in plant conditioning and regenerative potential.     

Effect of Previous Defoliation Regime and Mineral Nitrogen on Regrowth in White Clover Swards: Photosynthesis, Respiration, Nitrogenase Activity and Growth

Small swards of white clover (Trifolium repens L.) cv. Haifawere grown in solution culture in a controlled environment at24 °C day/18 °C night and receiving 500 µE m^-2S^–1 PAR during a 14-h photoperiod. The swards were cuteither frequently (10-d regrowth periods) or infrequently (40-dregrowth) over 40 d before being cut to 2 cm in height. Halfof the swards received high levels of nitrate (2–6 mMN in solution every 2 d) after defoliation while the othersreceived none. Changes in d. wt, leaf area and growing pointnumbers were recorded over the following 10 d. CO₂ exchangewas measured independently on shoots and roots and nitrogenase-linkedrespiration was estimated by measuring nodulated root respirationat 21% and 3% oxygen in the root atmosphere. There was a general pattern in all treatments consisting ofan initial d. wt loss from roots and stubble and reallocationto new leaves, followed by a period of total d. wt gain andrecovery, to a greater or lesser extent, of weight in non-photosyntheticparts. Frequently cut swards had a smaller proportion of theirshoot d. wt. removed by cutting and had a greater shoot d. wt,growing point number and leaf area at the start of the regrowthperiod. As a result of these differences, and also because ofdifferences in relative growth rates, frequently cut swardsmade more regrowth than infrequently cut. Initial photosyntheticrates were higher in frequently cut swards, although the laminaarea index was very low, and it was concluded that stolons andcut petioles made a significant contribution to carbon uptakeduring the first few d. Infrequently cut swards continued toallocate carbon to new and thinner leaves at the expense ofroots and stubble for longer than frequently cut swards andas a result achieved a similar lamina area index after 10 d. Nitrogenase-linked respiration was low in all treatments immediatelyafter cutting: frequently cut swards receiving no nitrate maintainedhigh nitrogenase activity, whereas recovery took at least 5d in infrequently cut swards. Swards which received nitrateafter cutting maintained only low rates of nitrogenase-linkedrespiration and their total nodulated root respiration overthe period was lower than those receiving no nitrogen: greaterregrowth in nitrate fed swards over the 10 d compared to N₂-fixingswards was in proportion to this lower respiratory burden. White clover (Trifolium repens L.), defoliation, regrowth, nitrogen, photosynthesis, respiration, nitrogenase-activity     

Changes in the activities of nitrogen assimilation enzymes ofLolium perenne L. during regrowth after cutting

The activities of key enzymes involved in N assimilation were investigated after defoliation of 6-week-old ryegrass plants grown in water culture conditions. In a first experiment, nitrate reductase, glutamine synthetase and glutamate dehydrogenase activities were measured in roots, stubble and leaves on the day of cutting and at 7-day intervals over the following 5-week period of regrowth. Ammonia assimilation enzymes showed little change whereas the nitrate reductase activity sharply decreased 2 weeks after clipping. In a second experiment, the nitrate reductase activity was measured at 2- or 3-day intervals 1 week before and 3 weeks after clipping.In vivo andin vitro assays both showed an increasing activity in leaves up to 8 days after cutting while root activity decreased. The opposite changes then occurred and both organs recovered their initial nitrate reductase activity levels after 12–14 days of regrowth. These fluctuations in nitrate reductase activity were considered to be related to the capacity for C assimilation and the nitrate availability.